Enhancing mechanical properties of cellular core sandwich panels: a review of topological parameters and design improvements

Sandwich panels’ exceptional mechanical properties and low density, owing to their multifunctional characteristics and innovative design, made them a popular choice in numerous industries. Sandwich panels with cellular cores are known for their exceptional energy absorption properties, which make them effective energy absorbers for high-impact scenarios such as accidents or explosions. For advancing research on sandwich panels, it is vital to develop innovative designs that can enhance their energy absorption and flexural stiffness. This review outlines the most essential topological parameters that influence the mechanical properties of cellular core structures. This paper gives insight into recent advancements related to optimizing sandwich panel structures for various engineering applications. The topological parameters investigated by researchers include core structure, thickness, number of layers, and material. The choice of core material governs the overall mechanical behavior of the panel. In this paper, various structures, including foam, honeycomb, lattice, corrugated, bioinspired, and various materials, are compared. Functionally graded structures were also explored in the literature as they can significantly optimize the response of sandwich panels in high and low-velocity impact applications. Similarly, a multi-layered core structure can enhance the total stiffness and specific energy absorption of the panel.


Introduction
Sandwich structures have shown exceptional characteristics in several applications, including automotive, aerospace, and naval industries, due to their unique properties (e.g., high strength-to-weight ratio, high rigidity, and bending strength) [1,2].The panel can have several design configurations, materials, and geometries [3].These configurations can be altered and designed to achieve specific properties for a specific application.Sandwich panels were first used in World War II as Mosquito night bombers in England [4].Since then, the structure of the sandwich panels has undergone significant research to design and investigate its behaviour under various loading conditions.This has led to numerous studies to optimise its structure, manufacturability, and materials for a specific engineering application.
Sandwich panels comprise two lightweight skins with high stiffness that enclose a low-density core.The cores are bonded between the skins, as illustrated in figure 1. Sandwich panels are engineered to meet diverse applications, such as ensuring safety, bearing loads, and absorbing energy.They offer notable advantages in industries such as marine vessel manufacturing, automotive design, and aviation, where a lightweight yet robust configuration is essential.In particular loading scenarios, the mechanical performance of sandwich structures is significantly influenced by the core structure and its design.The face sheets primarily provide the bending strength and stiffness to the panel, while the core transfers the shear forces between the upper and lower face sheets [5][6][7].
Various configurations can be attained by changing the core structure [8], increasing the thickness of the core and face sheets [9], varying the density of the core [10], and using different types of material for both the core and the face sheets [11].Several core designs, like lattice cores [12], honeycomb cores [13], corrugated cores [14], and truss cores [15], were investigated by researchers to improve their dynamic and quasi-static behavior under a particular loading condition.Sun et al [13] investigated the effect of face sheet thickness, core height, cell thickness, and cell size of honeycomb core structures on the impact behavior of sandwich panels.Since the facesheet material constitutes most of the weight of a sandwich panel, it plays a critical role in determining the properties of the panel as a whole.Villanueva and Cantwell [16] examined sandwich panels with plain composite skins and fiber metal-laminate (FML) skins and found that those with FML skins exhibited superior highvelocity impact responses.Similarly, the energy absorption of the panel is affected by the topological characteristics of the cellular core.Ozen et al [17] studied the effect of honeycomb and re-entrant core structures on the energy dissipation and impact strength of sandwich panels subjected to low-velocity impacts.
The influence of the core configuration on the performance of sandwich panels is significant and interdependent with the behaviour of the core design and adhesive materials.Core designs and materials are continuously being developed, with research focused on incorporating hollow core structures to reduce the overall weight of the sandwich panel and improve its energy absorption under various loading conditions.However, this may result in significant failure or complete perforation of the sandwich structure due to reduced stiffness depending on the relative density, constituent material, and loading conditions.Under low-velocity impact applications, sandwich panels can fail due to perforation [18], matrix cracking [19], debonding [20], delamination [21] or shear core failure [22].Nasirzadeh and Sabet [23] investigated the impact of varying rigid foam core densities within a composite sandwich panel under high-velocity impact conditions.Specific energy absorption analysis revealed superior ballistic performance was linked to sandwich panels featuring foam core densities below 70 kg m −13 .Damage assessments conducted in the perforated area revealed significant projectile yawing within the foam core of the highest energy absorption, leading to a side impact of the projectile against the rear composite facing.Notably, the thickness of foam cell walls and struts emerged as crucial factors influencing the crushing behaviour and energy absorption.Additionally, higher foam core density exhibited complete foam collapse in a brittle manner, disintegrating and, consequently low energy absorption.Figure 2 shows different failure modes in sandwich panels [24][25][26][27].
However, if the loading conditions differ, the failure modes change.In the case of the three-point bending test or four-point bending, sandwich panels exhibit face sheet wrinkling [28], Core buckling due to compressive forces [29], matrix cracking [30], core cracking [31], or delamination [32].Morada et al [33], studied the mechanical behaviour of a novel sandwich beam featuring a hybrid core composed of epoxy resin and Alumina trihydrate (ATH).This study examined the interplay between indentation mode and core shear failure mode and the impact of face/core debonding on the overall failure mechanism.The study revealed that core shear failure significantly influenced the ultimate failure of the sandwich beams.Developing novel sandwich structures is an ongoing process as researchers strive to improve their impact resistance and energy absorption capabilities through experimentation with different geometries and selection of materials with high energy-absorbing properties.To achieve the desired properties, various core designs for sandwich panels have been proposed.Researchers have focused on discovering the ideal lightweight core material and geometry.Cellular structures have gained considerable popularity in various fields of engineering due to their unique mechanical and physical properties.Advancements in manufacturing technology have enabled the production of cellular solids with controlled microstructures and properties for multiple functions in low-structural weight applications.Cellular solids consist of interconnected solid struts or plates that create the edges and faces of cells [34].The different topological features of cellular solids offer various mechanical properties that can be tailored depending on the desired application.The mechanical behaviour of the cellular solid is determined by its relative density, which is the ratio of the density of the cellular material to the density of the solid of which the struts are made [35].Moreover, the geometrical features that influence mechanical behaviour include relative density, core geometry, material properties, and unit cell properties [36].The material's porosity is directly related to the relative density of the solid, which can reach a maximum of 0.3; beyond that point, the structure is classified as non-cellular porous [34].Cellular structures can be used as the core in composite structures due to their unique ability to dissipate considerable energy through large plastic deformation [37].
This paper presents a comprehensive review of various core configurations that can enhance the performance of sandwich panels, such as honeycomb, corrugated, foam, truss, and bio-inspired structures.The effects of core height and density are also discussed.The review covers the characteristics of the structure's unit cell, recent efforts to optimise the use of sandwich panels in various industries, and potential areas for future research.The discussion begins with an examination of different core structures, focusing on their unit cell characteristics, followed by a review of functionally graded core topologies.Additionally, the paper addresses the effects of changing core height and using multi-layered cores, as well as material selection for the core of sandwich panels.Finally, the paper concludes with recommendations for future research in this field.

Core material
Sandwich structures are known for their unique flexibility in properties, which is determined by the infinite combinations of core materials and design configurations.Although sandwich structures offer numerous benefits across various industries, there are still unanswered questions that must be addressed before they can be widely applied, such as the selection of appropriate materials.Ongoing research aims to characterize various material properties for energy absorption applications, motivating further investigation.While materials such as metals [38], polymers [5], and smart materials [39] are consistently explored, researchers also strive to optimise their use.Polymeric foam cores, for instance, are less expensive and simpler to manufacture than metallic foams, making them commonly used in applications such as shock waves, low-velocity impact, and crashworthiness [40].Different polymers have been utilized for foam core structures, including polyvinyl chloride (PVC) [41], polyurethane (PU) [42], polyethyleneimine (PEI) [43], and polystyrene (PS) [44], among others.

Metallic foams
When more energy absorption is needed, metallic foams are often used as the core material.The most frequently used metallic foam material is aluminium alloy, which is more desirable than magnesium alloy due to the latter's higher rates of erosion and corrosion [45].Yan et al [46] investigated the effect of adding aluminium foam to corrugated core sandwich panels under compressive loading.The results revealed that adding aluminium foam to corrugated core sandwich panels significantly increased impact load resistance compared to unfilled panels.The foam-filled panels exhibited enhanced stability due to reduced buckling wavelength.The aluminium foam also positively influenced impact force, energy absorption, and overall performance.This was attributed to the foam's energy-absorbing properties and lateral support to corrugated core members, delaying buckling onset.The aluminium foam-filled panels outperformed empty counterparts which experienced early core web buckling.Xu et al [47] emphasized the importance of investigating magnesium alloys, as they hold promising potential to become a vital material for numerous engineering applications worldwide.The research on magnesium alloys has witnessed significant growth, with the number of publications increasing steadily over the past decade.Magnesium alloy research has become a major area of interest in the field of materials science.Research has focused on the microstructural evolution of magnesium alloys during deformation, including dynamic recrystallization behaviour, texture, and twinning.Grain refinement and alloying element regulation within microstructures have also been subjects of interest.Overall, magnesium alloy research is experiencing growth, recognition, and innovation across various areas including microstructure analysis, mechanical properties enhancement, corrosion resistance improvement, composite development, and the exploration of new alloys and processing methods.In a recent study, Faidzi et al [48] investigated the use of magnesium alloy as the core material, which showed a high potential to be utilized as the main core material in sandwich panels for protection purposes.Using magnesium alloy for the core resulted in a 4.5% decrease in projectile penetration compared to using aluminium alloy.Additionally, Zhang et al [49] state that the properties of magnesium alloy can be further improved by the addition of nanomaterial reinforcement, such as carbon nanotubes, during the manufacturing process.The work focused on investigating the impact of phosphating treatment on magnesium alloys and its subsequent effects on surface properties, wettability, bonding performance, and corrosion resistance.The study revealed that the phosphating film, magnesium alloy substrate, and prepreg exhibited strong and uniform bonding.The phosphate magnesium-fibre metal laminate (MgFML) demonstrated improved tensile strength.Lee et al [50] studied the use of nickel, copper, and polypropylene (PP) foams as the core material sandwiched between two polycarbonate face sheets.The panels were compared in terms of shock absorption, and the results revealed that a combination of nickel and copper foam layers had the best performance.Through analyses and experimental studies, the research identified that the composite structure with a core layer made of 3 mm thick nickel foam and surface patterns carved onto the top layer demonstrated the best performance.This configuration exhibited the highest levels of vibration attenuation, shock absorption, and structural stability, all while achieving mass reduction.

Polymeric foams
Metals are typically used in applications where weight is not a major concern.While polymers are significantly lighter than metals, they are also less stiff.However, polymers have high ductility and can undergo substantial deformation compared to metals, which can lead to increased energy absorption through the densification of cellular unit cells within the core of sandwich panels.Naik et al [51] fabricated sandwich panels with steel skin sheets and a polypropylene core, which exhibited relatively high shear strength at the interface.In this study, a novel approach for fabricating high interface strength metal-polymer-metal sandwich panels was developed.Interface failure in the fabricated sandwich panels was attributed to the failure of spot welds between the wire mesh and the steel sheet.Notably, debonding between the wire mesh and the polymer core was not observed, indicating the effectiveness of the wire mesh interface.Overall, this research presents an innovative approach for fabricating high-strength metal-polymer-metal sandwich panels with improved mechanical properties and formability.The findings have implications for various industries.Brekken et al [52] investigated the response of sandwich panels with extruded polystyrene polymeric foam cores when subjected to blast loading.Quasi-static material tests were conducted to characterize the mechanical behaviour of the aluminium skins and polystyrene foam cores.The dynamic response of the foam cores was studied through low-velocity impact tests.They found that the failure load significantly increased for foam-filled configurations, although the foam density and thickness were important factors to consider in terms of panel response.A polymeric core can effectively absorb blast energy.Ashraf et al [53] studied the compressive response of three different polymeric materials (EPS, HDPE, and PU) under quasi-static testing.They found that HDPE and EPS materials showed an increase in crush force efficiency.The unique metal-polymer-metal configuration can be useful for applications such as shipping, automobiles, and aerospace.

Smart materials
Smart materials, which can respond to stimuli such as heat, light, and chemical changes, have emerged as a new family of materials for use in sandwich panels.Stimulus-responsive materials (SRMs) include shape-memory alloys [54], shape-memory polymers [55], and shape-memory hydrogels [56].Shape memory alloys are activated through heating or an alternating or static magnetic field.There are three families of shape memory alloys: copper, nickel, and iron based.For engineering applications, however, copper-based shape memory alloys are the most researched [57].These materials are functional and can directly convert nonmechanical energy to mechanical domains.Serjouei et al [58] used additive manufacturing to 4D print smart sandwich structures with high potentials of energy absorption using shape memory polymers.The study explores shape memory Polylactic Acid (PLA) materials, employing Fused Deposition Modelling (FDM) to fabricate two distinct geometries (hexagonal and square horseshoe shapes).Experimental compression tests were executed to assess energy absorption, deformation, and mechanical traits impacted by design and printing variations.Findings revealed panels reverting to their original shape after deformation via heating.Increasing unit cell count notably enhanced energy absorption, albeit reducing ductility, necessitating optimal 4D printing parameters.This research advances 4D printing and smart materials, underscoring the utility of such sandwich structures for energy absorption.Quadrini et al [59] fabricated shape memory sandwich panels with properties that lead to self-healing where the core was made up of polyurethane foam with PUF/DCPD microcapsules reinforcement.This study pioneers self-repair in composites using shape memory materials and healing agents, aiming for simultaneous recovery and repair.The concept envisions heat-triggered restoration, in contrast to freezing deformation.Results revealed a 98% healing efficiency and 98.5% shape recovery after three-point bending tests.Notably, recovering the core's original shape is vital, as defects arising from recovery can impact mechanical properties.This breakthrough, combining shape memory, self-repair, and self-healing, holds transformative potential for large-scale structures across industries like aerospace and automotive.

Composite cores
To maintain high structural integrity and stiffness of the core, specifically achieving high transverse shear strength, researchers have developed techniques to manufacture composite cores that are both lightweight and have higher shear strength and elastic modulus when compared to polymeric cores.However, the challenge of composite core structures remains in the manufacturing process, where ongoing research suggests that there are possible approaches that could be used to overcome this challenge.A novel manufacturing technique has been developed by George et al [60] to create CFRP sandwich structures, coupled with an extensive exploration of their mechanical attributes.The authors successfully created a CFRP pyramidal lattice using IM7 carbon fibre and pre-machined polymer foam via pressure-assisted SC 1 resin transfer moulding.Hybrid CRP/foam core panels with Kevlar reinforcement were also formed.Results showed core strength and moduli increased proportionally with foam density due to enhanced foam characteristics and retained circular CFRP truss arrangement, reducing elastic buckling risk.Micromechanical models accurately predicted material behaviour using stress decomposition and micro buckling analysis.The hybrid material exhibited exceptional volumetric and gravimetric energy absorption, surpassing constituent materials.Similarly, an innovative approach to fabricating and prototype lattice cores using carbon fibre composites through Laser-Based Cutting (LBC) was introduced by Xiong et al [61].
This method increased bonding area over previous designs.Mechanical properties were studied through compression and shear tests, aligning with predictions.Strength hinged on Euler buckling, delamination, and strut fracture during crushing.No bond failure occurred during loading.Laser-cut cores show potential to boost shear strength limits for higher density pyramidal truss cores.Optimizing properties may involve aligning more fibres parallel to struts for enhanced mechanical performance.This technique offers promise for lightweight multifunctional carbon fibre panels.Numerical methods have also been used by other researchers to optimize core behaviour.The mechanical response of 3D woven glass fibre sandwich composites is investigated by Sadighi et al [62] by the aid of finite element simulation.To achieve this, a 3D finite element model is devised, treating the glass fabric as incapable of withstanding bending moments while the surrounding resin is represented as a homogeneous solid.In order to validate the accuracy of the finite element predictions, an array of mechanical tests, including flatwise compression, shear, three-point bending, four-point bending, and edgewise compression, was conducted, adhering to relevant ASTM test standards.The composite core's volume fraction plays a pivotal role in both the mechanical characteristics and the overall weight of the core.Moreover, the properties are additionally affected by the constituent material of the composite core.According to a study conducted by Xiong et al [63], carbon fibre composite sandwich panels with pyramidal truss cores were subjected to quasi-static compression and low-velocity impact tests.The study used cores with different volume fractions of 1.25%, 1.81%, and 2.27%.The authors investigated uniform compression responses for identical cores and stepwise graded panels.The quasi-static compression tests showed load peaks from truss layer failures due to buckling or crushing, and specific energy absorption was also measured.The impact tests analysed the effects of low-velocity impacts.The study found that two-layer carbon fibre pyramidal truss cores exhibited energy absorption similar to glass fibre cores, suggesting that such structures have the potential to be lightweight and multifunctional.Alternative composite structures have also been studied by Chao et al [64], the investigation delved into the mechanical and dynamic characteristics of a sandwich panel with a jute fibre-reinforced composite periodic core.A finite element (FE) model was established to derive the natural frequencies and loss factors for both single-core and periodic-core sandwich panels, considering varying end conditions.The validity of the model was verified by cross-referencing the outcomes with published literature.The jute fibre-reinforced polymer composite displayed a tensile modulus of 6.47 GPa and a flexural modulus of 4.58 GPa.Notably, the periodic core sandwich panel exhibited the highest natural frequencies when contrasted with single-core composite sandwich panels.The results also indicated that augmenting the core-skin thickness ratio positively impacted the dynamic properties of the periodic core sandwich panel.Moreover, an escalation in the skin-core thickness ratio resulted in heightened natural frequencies and loss factors for the jute fibre-reinforced periodic core sandwich panel, particularly when subjected to CC end conditions.
Zangana et al [65] investigate the impact behaviour of corrugated sandwich panels incorporating a glass fibre core and hybridised with Kevlar and high-performance Zylon fibres, utilising ratios of 75:25.The study reveals insights into impact responses, damage mechanisms, energy dissipation, and post-impact residual load capacity.The panels are tested using four-point bending to examine their behaviour comprehensively.Notably, hybridising with Zylon fibres proves most effective for impact energies exceeding damage thresholds.Replacing a single layer (25% replacement) of the glass fibre core with Kevlar or Zylon significantly reduces core failures.Glass-Glass combinations show challenging-to-detect internal failures, which Glass-Zylon hybridization mitigates through external failure modes due to Zylon's superior energy absorption.This hybridization method also distributes impact forces more widely across the upper face sheet, minimizing stress concentrations and enhancing impact resistance, as confirmed by finite element modelling.The incorporation of high-performance plies within the sandwich core leads to substantial gains in specific energy absorption without adding undue structural weight.Glass-Kevlar and Glass-Zylon combinations surpass traditional core materials.Moreover, Glass-Zylon hybridization minimizes strength and stiffness reductions post-impact compared to Glass-Kevlar and Glass-Glass.Predictive empirical equations achieve residual capacity estimations within a 10% accuracy range.To advance this research, future investigations involve sophisticated finite element analyses exploring diverse stacking sequences of high-performance plies to better understand ply orientation effects on structural responses.In summary, this study introduces an innovative approach to enhance impact performance in sandwich panels, offering valuable insights for practical composite applications.

Summary of surveyed literature
The literature investigates the complex realm of sandwich structures exploring core material selection.Ongoing research is intensely directed towards unveiling material properties that are critical for energy absorption applications.A summary of the different investigated core materials is portrayed in table 1. Significantly, polymeric foam cores emerged as an appealing material due to their cost-effectiveness and ease of production.While aluminium is the most common material used for the core of sandwich panels, researchers have explored the use of polymers and smart materials to decrease weight and increase energy absorption.The literature examined examines various polymers, including PVC, PU, PEI, and PS, which have been employed to create foam core structures with diverse mechanical attributes.Investigating failure modes under scenarios of lowvelocity impact, the research studies a range of failure mechanisms encompassing perforation, matrix cracking, debonding, delamination, and shear core failure.This comprehensive understanding of failure modes forms a robust basis for reinforcing the resilience of sandwich structures built around polymeric foam cores.Metallic foams, particularly those derived from aluminium alloy, are appealing as core materials since they have the ability to absorb higher energy levels.A precise exploration is undertaken to understand the impact of embedding aluminium foam within corrugated core sandwich panels subjected to compressive loading.This investigation reveals a significant increase in impact load resistance and panel stability, attributed primarily to the modulation of buckling wavelength within the foam-filled panels.This strategic incorporation of metallic foams introduces superior robustness, enhancing the practical use of sandwich structures across several sectors.Smart materials, characterized by their responsiveness to external stimuli, emerge as a cutting-edge material in sandwich structure research.Shape memory alloys and polymers gained attention as potential transformative elements for energy absorption.Moreover, the introduction of 4D printed smart sandwich structures, featuring shape memory polymers, opens up new opportunities for reversible energy absorption.These carefully crafted structures exhibit the exceptional ability to revert to their original shape through controlled heating, a characteristic that holds the potential to redefine energy absorption applications.Within the realm of composite cores, the literature focuses on enhancing structural integrity and rigidity.Researchers systematically explore innovative manufacturing techniques, spanning pressure-assisted resin transfer moulding to laser-based cutting, in the creation of lightweight composite cores with superior shear strength and elastic modulus.These pioneering techniques provide potent tools to overcome challenges in composite core production, resulting in the synthesis of composite cores boasting remarkable attributes.A distinct focus emerges on jute and highperformance fibre cores, unravelling the intricate domain of impact behaviour.Sandwich panels resourcefully incorporate jute fibre cores, hybridized with Kevlar and Zylon fibres, undergoing thorough analysis to understand their response to impact loading.This has led to heightened impact resistance, energy absorption capacities, and overall performance, facilitated by the synergistic fusion of diverse fibre materials.
In conclusion, the reviewed literature not only emphasises the interdisciplinary nature of research related to sandwich structures but also highlights the effort that has been taken in comprehending and leveraging their potential across a wide range of applications.By blending experimental investigations with numerical simulations, researchers are shaping a landscape where the field of material science, innovative engineering, and design blend harmoniously.This has resulted in the development of inventive sandwich structures capable of effectively enduring impacts and overcoming multifaceted challenges amongst diverse domains.

Core structure
Designing the structure is one of the primary steps in creating a new sandwich panel.Traditionally, sandwich structures are categorized into honeycomb, truss, foam, and corrugated cores, as depicted in figure 3.However, in recent years, researchers have investigated novel core configurations, leading to an evolution of core structures.The honeycomb, corrugated, and truss cores possess regularly repeated inner structures.
Cellular foam cores are commonly used in sandwich structures to reduce weight in engineering applications.Two types of foam are typically employed: polymeric and metal.The topology of the cells, cell size, cell shape, and relative density are the main characteristics that define foam structures.Polymeric and metal foams are employed as core materials due to their energy absorption capabilities [67].Polymeric foams generally have lower densities than metallic foams, giving them better specific energy absorption properties [68].In a study by Jing et al [69], closed and open cell aluminium foam core sandwich panels were investigated under impulsive loading, and the open cell foam cores demonstrated lower shock resistance performance.The main objective of the study is to understand how different types of cellular metal cores affect the dynamic response and shock resistance performance of sandwich panels when subjected to localized impulsive loading.The researchers conducted experiments involving the impulsive loading of clamped square sandwich panels with three different types of cellular metal cores.Crupi and Montanini [70] conducted three-point bending tests on aluminium foam core sandwich panels under impact loading, and the foam core was able to withstand larger deformations at constant loads, making aluminium foam composite structures ideal candidates for energy absorption.The tests were conducted under both static and impact loading conditions.This allowed for the examination of the mechanical behaviour and collapse modes of the AFS panels under different loading rates.The findings are particularly relevant for applications requiring high energy dissipation capacities, such as in impact-resistant structures.Additionally, Crupi et al [71] investigated the impact behaviour of sandwich composites with various core structures and found that the aluminium foam core had higher post-impact strength than PVC (polyvinyl chloride) foam.The study compared different composite types for their impact performance, focusing on energy absorption and failure modes.It offers insights into selecting materials for impact-resistant applications.Aluminium sandwiches have superior post-impact strength due to their unique failure behaviour.Depending on the application, the constituent material and density can be adjusted.Cellular cores have a high strength-toweight ratio and excel in energy absorption due to their collapse, allowing significant deformation.In sandwich panels, face sheets handle uniaxial loads, while cellular cores deal with flexural loads.Therefore, it's crucial to examine the flexural properties of cellular cores with varying topological features, as it greatly affects their characteristics.
Altering the topology of the cellular core is one approach to tuning the mechanical properties, while the other one is the use of different constituent material.The use of polymeric foams is widely seen in various applications like automotive engineering and military defence because of their superior moisture properties, absorption capabilities, and flammable resistivity [2].Polymeric foams could be either be rigid or flexible depending on the geometry of the cells.Crupi et al [72] experimentally investigated the impact behaviour of sandwich panels with PVC foam.Results reveal that more energy was required for the complete failure of the panel when compared to aluminium foam core.The experimental results are compared to the impact response of Aluminium Foam Sandwich (AFS) panels in terms of energy absorption.AFS specimens exhibited a ductile fracture behaviour with large out-of-plane displacement, contrasting with the predominantly elastic failure mode of PVC foam sandwiches.AFS structures demonstrated better post-impact strength due to their ductile failure mode and larger deformation characteristics.This was in contrast to the more catastrophic and localized fracture observed in PVC foam sandwiches.Compared to metallic foams, polymeric foams exhibit lower fracture toughness and impact resistance but are preferred in applications that require high thermal insulation and strength-to-weight ratios.Structural engineering applications often utilize polymers; however, their usage is limited by hygrothermal conditions and bonding behaviour between face sheets and foam cores.Metallic cores are more advantageous when high thermal conductivity is required due to the high conductivity of metals, whereas polymers act as insulators.However, metallic core fabrication is more expensive than polymer fabrication, especially when complex cell shapes are involved, as metallic cores require additive manufacturing, while polymeric cores can be 3D printed.The effect of material selection of the mechanical properties is further investigated in section 5.

Honeycomb structure
Honeycomb core structures, shown in figure 3(a), are lattice structures with open cells.Typical geometries of the honeycomb cores are triangle, square, re-entrant, hexagonal, circular, and auxetic [17,[73][74][75][76].A major issue in conventional honeycomb structures is their tendency to produce saddle-shaped or anticlastic shaped curvatures which lead to localized damage.To tackle this problem, a revolutionized honeycomb structure with a negative Poisson's ratio unit cell is introduced.The structure is characterized by its transverse shear modulus [77], geometric effects, and energy absorption under impact [78,79].The auxetic topologies mainly reported on reentrant structures are chiral, rotating rigid, and double arrowhead unit cells [80].Hou et al [6] investigated and compared the reliability of conventional honeycomb and re-entrant honeycomb topologies in terms of energy absorption.Results reveal that the re-entrant structure performed better in terms of durability and robustness.The study examines dynamic three-point bending tests on lattice composites, comparing auxetic (negative Poisson's ratio) and non-auxetic (positive Poisson's ratio) structures.The re-entrant core, with folded cell walls for impact load support, is less efficient at energy absorption than other structures.Additionally, it maintains its performance consistently during cyclic impacts, while truss and honeycomb cores experience progressive structural damage.Hedayatian et al [81] studied the behaviour of sandwich beam structures with conventional and re-entrant honeycomb unit cells, under low velocity impact loading, in which re-entrant structures demonstrated ideal characteristics for shielding applications.The study explores sandwich beam behaviour under low-velocity impact.Different cellular cores (auxetic and conventional) with various re-entrant angles were used.Higher re-entrant angles in auxetic samples resulted in lower peak response forces at the same impact energy level.This configuration absorbs more energy with less force, making it suitable for protection.Switching from conventional to auxetic cores doubled the energy needed to damage the structure, due to the negative Poisson's ratio effect strengthening the structure by pulling unit cells into the impact area.Wang et al [82] proposed a novel re-entrant star-shaped honeycomb (RSH) and evaluated the structure under impact resistance.The study focuses on a novel auxetic structure called RSH, which is created by combining two classical auxetic structures, RH (re-entrant hexagon) and SSH (shrinkable square honeycomb).In contrast to the behaviour of RH, the absorbed energy of RSH initially experiences a slight decrease followed by an increase as the impact velocity increases, with the RSH structure's minimum absorbed energy velocity aligning with the first critical impact velocity between low-velocity and medium-velocity modes.Results revealed that re-entrant star-shaped honeycomb have better impact resistance when compared to re-entrant honeycomb and star-shaped honeycomb.For the RSH, the absorbed energy increased by almost 64% as compared to the re-entrant honeycomb under an impact velocity of 1 m s −1 .A comparative study conducted by Ingrole et al [83] explored the failure and deformation modes of four honeycomb structures (regular, re-entrant auxetic, locally reinforced auxetic strut, and regular auxetic strut hybrid honeycomb) under in-plane uniaxial compression loading.The authors found that the hybrid configuration resulted in superior mechanical properties than the auxetic and honeycomb structures.Lowering the Poisson's ratio increases the energy absorption.Modifying honeycomb unit-cell structures can affect mechanical properties.Changing topological features while keeping core density constant can improve some properties but worsen others.For better energy absorption, researchers can transform the honeycomb unit-cell into structures like RSH.However, low-velocity impact tests, dependent on out-of-plane buckling strength, may change when transitioning from traditional honeycomb to RSH.It's important to examine how other mechanical characteristics deteriorate with this shape change while maintaining constant relative density.A study was conducted by Wang et al [84], in which the elastic properties were investigated for various cell shapes at different relative densities.The studied cellular shapes were the traditional shapes that are commercially used which are re-entrant, rectangular, triangular, hexagonal and circular.The results show while the re-entrant has the highest elastic modulus out of all the other shapes, at fixed relative density.The rectangular shape has the highest shear modulus and the re-entrant shape, which had the highest elastic modulus, has the lowest shear modulus out of all shapes.Therefore, a complete comprehensive study of the mechanical properties has to be investigated when considering altering the topological features of cellular solids, and it is important to consider equal relative densities when comparing to the traditional unitcells.

Corrugated core structure
Corrugated cores, illustrated in figure 3(c), are prismatic structures that form open channels in a single direction.The structural behaviour and mechanical performance of the corrugated cores have been extensively investigated for addressing issues that are related to the isotropic design, core-face reinforcement, and dynamic response to external loads.Various innovative designs of corrugated structures have been suggested in recent years, with the aim of broadening their application in diverse fields.Such novel core designs are hierarchal corrugated cores [85], bi-corrugated plate core [86], zig-zag trapezoidal corrugated core [87], and woven corrugated cores [88].Integrated woven corrugated sandwich cores were first developed by Jin et al [89] to increase the resistance of face-core debonding.The failure modes and mechanical properties of the sandwich composite were studied by performing in-plane and out of plane compression, three-point bending, and out of plane shear tests.Results revealed the gradual crushing and core contact with the face sheets resulted in increasing the ductility.Moreover, debonding failure was observed under bending conditions which shows that the debonding resistance of the skin to core was enhanced.In another study, Russel et al [90] investigated stitching the corrugated core to 3D woven S2-glass fibre face sheets under impact load.The impact resistance increased due to high face to core attachment and load transfer capacities.Cai et al [91] examined the enhancement of a sandwich panel with a trapezoidal corrugated core, under the air blast loading condition.The study focused on achieving an optimal TZSP design that balances deformation response and energy absorption response under blast loading conditions The study revealed that the use of corrugated cores improved the overall performance of the plates.Hierarchical corrugated core designs take advantage of the fact that the strength of the low-density core is due to the buckling strength of each individual core member.Comparison of initial and optimized designs demonstrated that optimized designs could prevent back face fracture and maintain similar SEA levels to initial designs under high blast intensity.Optimized designs also exhibited lower MaxD (reduced by 42.1% and 63.1%) and improved SEA (increased by 66.8% and 75.9%) for two different stand-off distances (SoDs).To enhance the buckling strength, a hierarchical approach is employed whereby single corrugation elements are fabricated individually.Yang et al [85] designed hierarchical truncated conical shells with corrugated cores under quasi-static axial compression.The study introduced and investigated highperformance hierarchical truncated conical shells (HTCS) with corrugated cores, focusing on crashworthiness through numerical simulations.Authors compared this configuration with the truncated conical sandwich shells displaying that truncated conical sandwich shells portrayed better energy absorption abilities.The energy absorption capability of the proposed HTCS outperformed the truncated conical sandwich shells (TCSS) structure proposed earlier.This highlighted the promising potential of the HTCS design for energy absorption applications.

Truss cores
Truss cores are among the oldest and most widely used core structures in sandwich panels.Truss cores, shown in figure 3(b), come in different geometries (e.g., tetrahedral, pyramidal, or X-type configurations).There are several patterns derived from the truss structure in the past couple of years [92][93][94][95].Qiu et al [94] proposed a novel design that generalises the standard pyramid truss core through the geometric operations on the open unfolded nets.Authors developed origami-based design strategies while simultaneously controlling the 3D geometries of the unit cells.The study presents an origami-based design approach for creating different types of polyhedral tessellated cellular cores (PTCCs) within truss-core panels (TCPs).Three origami-based design strategies are developed to generate related 2D origami patterns.Two operation parameters are introduced to control the 3D geometry of PTCCs.The method allows for the incorporation of various polyhedral cells, including triangular, rectangular pyramid, pentagonal prism, and hexagonal prism into TCP structures.Based on the pyramidal truss core structures an enhanced novel lattice structure was designed using the transition process referred to as the Hourglass structure [96].This truss core is similar to a pyramidal truss core with two layers and similar inter-node spacing but has more sparse arrangements and decreased slenderness ratio which results in better buckling resistance.The origami-based design reduces manufacturing complexity by utilizing 2D fabrication.Alternatively, methods like origami robots, 3D printing, and 4D printing can be employed to overcome forming challenges and achieve larger aspect ratios.Moreover Ullah et al [97] studied the energy absorption capacities of Titanium based Kagome and compared the structure with an atomic lattice.Results showed that the Kagome structures portrayed superior strength over the atomic lattice structures.The study highlights their potential to sustain higher impact loads within the elastic core deformation range while absorbing substantial energy during plastic impact, all with minimal permanent deformation.This suggests their suitability for low-speed impact applications, making Kagome structures promising for impact-resilient designs.Wang and Hu et al [98] explored the effect of wooden truss lattice sandwich panels by performing three-point bending tests revealing that the use of poplar veneer truss for the lattice structure enhanced bending performance with respect to oriented strand board truss.The study focuses on a novel wooden truss lattice sandwich structure designed using an integrated spatial concept.Bending properties of these lattice sandwich structures are evaluated through three-point bending tests to analyse material selection and structural design for optimal bending performance.Overall, the lattice sandwich structure with poplar truss demonstrates better bending performance compared to the oriented strand board (OSB) truss counterpart.In the same panel material, the incongruous truss lattice sandwich outperforms the synclastic truss in bending.Further studies could compare veneer truss with Kagome lattice for flexural properties.Pure bending and shear tests can assess Kagome lattice and veneer truss stiffness and shear modulus for comparison.

Bio-inspired core structures
To deal with the various challenges in different engineering areas, bio-inspired structures have been explored and tested for the provision of novel sandwich panel designs.In nature, animals and plants extend excellent structures that have low density, high capacities of energy absorption, and high strength.The pomelo fruit has a unique spongy mesocarp layer which has the ability to dissipate energy levels of 80 Joules without leaving any visible traces of outer damage on the peel [99].In addition, the Macadamia integrifolia shell and Cocos nucifera shell nuts have significant puncture and impact resistances [100].Zhang et al [101] investigated the energy dissipation of beetle elytron and honeycomb plates.Results showed that the energy dissipation capacity of the beetle elytron plates was five times higher than that of the honeycomb plates.The beetle elytron core, with closed-section hollow trabeculae at honeycomb intersections, creates a deformation pattern with three halfwaves.This yields higher buckling stress (proportional to wall thickness squared).Despite having 42% more volume than honeycomb plates, this bio-inspired design boosts compressive strength by 2.44 times and energy dissipation by 5.0 times using the same manufacturing method (FIM).The beetle elytron core demonstrates remarkable buffering properties, making it valuable for diverse applications, despite its ancient presence in beetles.Ha et al [102] proposed a novel bio-inspired honeycomb sandwich panel adapted from the microstructure of the woodpecker's beak.In their numerical tests, results showed that when subjected to dynamic crushing the bio-inspired honeycomb sandwich panels portray enhanced energy absorption capabilities as compared to sandwich panels with conventional honeycomb cores.There was a 125% increase in specific energy absorption when the woodpecker's beak microstructure was used.The study also indicated that the energy absorption capacity of the bio-inspired honeycomb sandwich panel was influenced by the wave numbers and amplitude of the bio-inspired core.Larger wave numbers and amplitude in the bio-inspired honeycomb core led to greater SEA.Additionally, increasing the core thickness contributed to improved SEA.Zhang et al [103] assessed the performance of a re-entrant arc-shaped honeycomb structure inspired by the turtle shell, under impact loading.The in-plane crushing behaviour of bio-inspired RAHs is influenced by impact velocity and cell micro-topologies.The incorporation of arc-shaped structures into conventional reentrant honeycombs alters macro-/micro-deformation characteristics during crushing.The introduction of reentrant arc-shaped structures contributes to higher crushing load efficiency and lower average stress fluctuations, resulting in better crushing load uniformity and energy dissipation.The incorporation of arcshaped structures led to improved uniformity in crushing load during the energy dissipation phase, resulting in greater crushing load efficiency.Another promising prospect for the development of bio-inspired sandwich structures with superior energy absorption capabilities is the design of a woodpecker's head.Sabah et al [8] developed a novel honeycomb sandwich beam that was based on the head of a woodpecker and tested it under low velocity impact.A bio-inspired beam of carbon fibre laminated skins with aluminium and rubber honeycomb cores was developed in which numerical and experimental results showed significant improvements in performance of bio-inspired beams compared to conventional ones.Results showed that under repeated impact, configurations based on the woodpecker's head portrayed smaller stress at the bottom skins and significantly lower damage areas and could withstand higher impacts than the conventional honeycomb sandwich panels.Introducing sinusoidal waves to traditional honeycomb cells boosts energy absorption, but the added strut length from frequency and amplitude isn't factored into relative density.Traditional cellular cores plastically fail due to yielding or buckling.Further study is needed on sinusoidal waves' effects via three-point bending, pure bending, and shear tests.While bio-inspired shapes promise higher energy absorption, aluminium honeycombs' manufacturability favours mass production, making them preferable.Aside from the animal-inspired structures, plants have attracted significant interest in recent years.Halder and Bruck [104] designed a sandwich structure bio-inspired from the structure of Palmetto wood made from CFRP sheets and a polymeric foam core that mimics the Palmetto wood structure.The sandwich panel's foam core was reinforced with carbon rods, making it look like the structure of the wood.Results showed that the flexural stiffness of the panels with a bio-inspired core increased by 100 % compared to the conventional foam core.Reinforcing foam cores with carbon rods adds weight while affecting the strength-to-weight ratio of cellular cores.While reinforcement boosts mechanical properties, weight gain relative to enhancement must be weighed.Carbon rod distribution within the core matters.This study examined damage evolution, revealing carbon rods didn't alter it.However, due to their high modulus, carbon rods' reinforcement heavily impacts damage evolution.Carbon rods can efficiently enhance energy absorption and damage evolution if different orientations are explored.3D printed sandwich panels are noted to be less sensitive to failure in the interaction between the face sheets and cellular core during buckling.This is attributed to the absence of adhesion between the cellular core and face-sheets. [66] Out-ofplane: The structures underwent crushing, which is likely to be governed by global buckling behaviour, often described using Euler buckling. [61]

Shear
The experiment illustrated that lasercut cores can notably enhance the maximum shear strength, particularly in the context of pyramidal truss cores with elevated relative densities.
Delamination, or the separation of composite layers, was observed as a failure mechanism, indicating interlayer bond failure.The findings indicate that thinner panels exhibit greater compressive stiffness and experience higher maximum stress when subjected to flatwise compressive loads.
Thinner panels show higher compressive stiffness and maximum stress due to higher bending moments in the body of the piles and at intersections with the face sheets.Thicker panels exhibit lower load-bearing capacity due to these bending moments, leading to lower bearable loads. [62]

Shear 10mm
Bending 12mm The shear test outcomes suggest that panels with reduced thicknesses possess higher yield stress and shear modulus values, based on the experimental observations.CFRP Pyramidal Truss 1550 kg m −3 15mm 2 Compression When compared with a reference glass fibre woven textile truss core, the current two-layer sandwich structures equipped with a continuous carbon fibre truss core exhibit either equivalent or improved energy absorption capacity per unit mass.
The impact damage includes tearing of the face sheets, debonding between face sheets and core, fracturing of truss elements, and delamination within the core. [63] Low velocity Impact

Summary of surveyed literature
The process of designing sandwich panel structures is explored as a fundamental step in their creation.Historically, sandwich structures have been categorized based on core types, including honeycomb, truss, foam, and corrugated cores.However, recent research has led to the investigation of novel core configurations, resulting in an evolution of core structures.Among these, honeycomb, corrugated, and truss cores exhibit regularly repeated inner structures.Different core structures investigated are portrayed in table 2. Cellular foam cores, incorporating polymeric and metallic materials, are commonly employed to reduce weight in engineering applications.The characteristics defining foam structures, such as cell topology, size, shape, and relative density, play a significant role in their performance.Polymeric foams generally offer better specific energy absorption due to their lower density, compared to metallic foams.The impact behaviour of sandwich panels with different cellular metal cores is examined, revealing insights into their dynamic response and shock resistance under localized impulsive loading.Foams are cellular structures with high specific mechanical properties despite having low relative density.The literature survey (summarized in table 3) indicates that open-cell foam is preferable to closed-cell foam.Metallic foam has superior energy absorption abilities and fewer failure modes than polymeric foam in sandwich panel applications.However, polymeric foam outperforms metallic foam in specific mechanical properties for lightweight sandwich structure unit cells.The shape of the cell can be tailored to the intended application and significantly influences the mechanical properties of sandwich panels.
Researchers have found that honeycomb unit cells can increase localized damage, leading to the development of innovative re-entrant honeycomb designs with negative Poisson's ratios.These designs have resulted in improved compressive strength and stiffness, and further studies on star and hybrid-shaped re-entrant configurations have shown even more promising mechanical properties.However, the negative Poisson's ratio effect is only triggered in linear elastic deformation and is neglected in high strain rate applications.To overcome this limitation, researchers have explored bio-inspired unit cell shapes to enhance the mechanical properties of sandwich panels in high strain rate scenarios.Unfortunately, manufacturing cellular structures poses limitations in industrial applications.Corrugated structures have been used due to their ability to be more automated and easily mass-produced, leading to their widespread use in automotive engineering applications.Different core topologies are discussed, including honeycomb, corrugated, truss, and bio-inspired structures.Honeycomb structures are studied extensively, particularly the novel re-entrant honeycomb topology, which displays improved durability and robustness.The impact resistance of corrugated cores is enhanced by various innovative designs, such as hierarchal corrugated cores and woven corrugated cores.Truss cores, characterized by diverse geometries, are well-established in sandwich panels and have undergone innovative advancements, such as origami-based designs and wooden truss lattices.Bio-inspired core structures inspired by nature's efficiency are explored, showcasing remarkable energy absorption capabilities.For example, bio-inspired honeycomb sandwich panels based on the woodpecker's beak and beetle elytron exhibit enhanced energy dissipation and compressive strength.While topological features can heavily enhance the mechanical properties of sandwich panels, specifically the energy absorption capabilities, other mechanical properties should be investigated when considering altering topological features.Considering a fixed relative density, the overall weight of the sandwich panel remains the same, and while energy absorption capabilities are justified, it is essential to consider other mechanical properties that effect specific applications, such as flexural, uniaxial, bending and shear tests.Adding on, while complex shapes can alter the mechanical properties and relatively enhance the mechanical properties for specific applications, the manufacturability of the proposed unit-cells should be taken into account when considering complex shapes.The reason traditional honeycombs are heavily commercialized is due to their ease of production on a mass scale.The enhancement of topological features should not only consider enhancing the mechanical properties, but also ensuring ease of manufacturing on a mass scale without having to adhere to additive manufacturing, which can be costly for mass production.

Functionally graded core
Gradient structures found in nature exhibit variable density by combining several topological structures.In contrast to uniform structures, functionally graded structures have distinct mechanical properties.Functional gradient, across thickness for instance, enables the cellular core to have lower weight than the traditional core while efficiently utilizing high thickness in areas where it is needed most.These configurations were first proposed by Fuchiyama and Noda [106] to tackle the issues of stress concentration in sandwich panels.Functionally graded materials (FGM), shown in figure 4, have the advantage of high performance and improved damage tolerance which tend to have smooth property variations allowing the optimization of structural functions.Even though the technology of such structures was relatively novel in the early 21st century, continuous research has shown that they have immense potential in different applications.FGMs make mechanical property modification possible improving the mechanical and structural performance of sandwich     The majority of the sandwich panels experienced significant overall deformation when subjected to external loads.The sandwich panels exhibited localized failures in the central loading area. [69] Closed-Cell Foam Aluminium AFS Schunk: 0.87 g/cm 3 AFS Alulight: 0.95 g cm −3 AFS Schunk: 10.62 mmAFS Alulight: 10.91 Static and dynamic three-point bending Substantial variations in absorbed energy between Schunk and Alulight panelswith no strain rate sensitivity observed in impact tests for the examined AFS types.Alulight panels exhibited delamination and compromised structural performance, potentially significant for energy dissipation-intensive applications.
Delamination was observed in one skin of the Alulight panels.The three-point bending tests revealed different collapse modes for the aluminium foam sandwiches (Schunk and Alulight).These modes are categorized as Mode I, Mode IIA, and Mode IIB. [70] Foam PVC Aluminium alloy 0.075 g cm −3 15 mm 1 CompressionLow velocity impact PVC foam require more energy to portray complete failure.
The PVC foam sandwiches predominantly exhibit an elastic failure mode. [72] Honeycomb(DSH, PSH, USH) Aluminium alloy 2700 k g /m 3 1 5 mm 1 Low velocity impact-Three-point bending Both the average contact force and peak force during impact were greater for DSH compared to PSH and USH.
During the bending tests of the DSH sandwich panels, the primary mode of damage observed was related to the fibre and resin matrix.The helicoidal arrangement of the DSH caused cracks to deflect between layers along the fibre's crack propagation direction, rather than propagating directly through the transverse section Solid, Honeycomb, Foam When compared to the conventional sandwich structure, the newly introduced novel sandwich structure demonstrates a significantly superior performance, marked by substantial improvements in its properties.
In the flatwise compression direction, the regions of elastic deformation, plastic deformation, and eventual corrugation fracture are identified as the dominant failure modes.The deformation process involves non-linear responses due to slack take-up, elastic behaviour up to peak stress, local buckling of corrugations, compaction of the core, and eventual corrugation fracture.Edgewise compression Load-displacement behaviour exhibits an initial linear stage followed by a strength degradation stage due to progressive shearing failure of adhesive.The peak force and stiffness vary between specimens, indicating debonding between face sheet and unit cell, with the weaker bonded cells initiating failure and propagating to stronger cells. [88]

In-plane compression Out-of-plane compression
The core experiences bendingdominated behaviour under compression, with core strength and stiffness directly proportional to the square of its relative density.The ductile behaviour of the corrugated core is achieved through the gradual expansion of contact regions between the skin and the cellular core.
IWCSC exhibits bending-dominated behaviour in compression.
The ductile mechanism of the corrugated core, with gradually extending contact areas between the skin and core, results in a stable deformation plateau.Shearing in the warp direction leads to a ductile failure mode due to compression and extension of different core walls. [89] Corrugated core E-glass Relative density: 0.33 13.1 mm 2 Compression During dynamic loading, uniform deformation across the core thickness was observed in specimens for impact velocities below approximately 150 m s −1 .At higher impact velocities, deformation became localized near the impacted face.
At lower velocities (50 ms −1 ), the deformation modes resemble those observed under quasi-static conditions.However, at higher velocities (150 ms −1 ), deformation becomes more localized near the impacted face, causing the core to "stub" against the impacted face.This localized deformation mode was   The bioinspired RAHs display a notable reduction in maximum peak stresses under constant impact velocity conditions.The RAHs exhibit superior crushing load efficiency and decreased average stress fluctuations compared to conventional RHHs at equivalent impact velocities.Bio-inspired RAHs offer improved uniformity in crushing load distribution during energy dissipation.
The prominent failure modes in this study involve the in-plane crushing deformation of the bioinspired re-entrant arc-shaped honeycombs (RAHs).These failure modes are characterized by the impact velocity and cell microtopologies [103] Bio-inspired core (Palmetto wood) Polymeric soft foam reinforced with carbon rods 35 kg m −3 10 mm 1 Three-point bending Quasi-static three-point bend assessments of sandwich composite structures with a bioinspired core reveal that substituting the conventional core with a bioinspired one bolsters the mechanical strength of the foam-core sandwich configuration.
The failure modes observed include shear-dominated delamination at the face-sheet and core interface, bending/kinking failure of the face sheet under compression, and reduced transverse compression around reinforced carbon rod due to increased stiffness.sheet.The crack growth of specimens aged for 720 h was more stable than those aged for shorter durations, and the peeling load decreased with increased aging time.
structures [107].In specific, the elastic modulus of these materials can be functionally graded such that they are used for applications that require impact loading [108].In nature, biomaterial usually constantly alter their mechanical properties.For instance, bamboo's density is continuously increasing toward the external surface so it can withhold higher bending loads [109].

Performance of functionally graded materials
Extensive research has explored FGMs for their mechanical properties and energy absorption capabilities, as these properties are highly sought after [24,110].Cellular structures with functionally graded material have unique applications in several industrial applications [111].To predict the performance of composite structures with functionally graded material, Do and Lee [112] proposed a design with high order shear deformation in which the geometrical properties and loading type have a direct effect on the stability of the sandwich panel.The method employed the Reproducing Kernel Particle Intrinsic Method (RPIM) with a higher-order shear deformation theory (HSDT) that accounts for geometric nonlinearity and initial imperfections.FGM sandwich plates under biaxial mechanical edge compression experience larger post-buckling deformations and lower post-buckling strengths compared to plates under uniaxial compression.The post-buckling strength decreases as the load ratio increases.Yaghoobi and Taheri [113] studied different buckling characteristics of FGM sandwich panels where the authors introduced several mathematical formulations to examine the performance of these panels with respect to linear buckling.For sandwich plates with high porosity coefficients, an increase in the core to total thickness ratio generally led to a decrease in critical buckling capacity due to its adverse impact on plate stiffness.Moreover, metal foam cores with lower porosity in the upper and lower regions produced the most effective response.Chen et al [114] analysed the buckling and bending performance of functionally graded sandwich panels with different porosity distributions.In this study, a new type of porous plate with functionally graded (FG) porosity is introduced and its structural performance is compared to that of a conventional sandwich plate with a uniform porous core.Increasing the porosity coefficient corresponds to larger internal pores with higher density, resulting in reduced flexural rigidity.This leads to lower buckling loads and larger bending deflections.Additionally, the introduction of hybrid core designs, such as combining agglomerated cork and PU foam, demonstrates an innovative approach to addressing the limitations of multi-layered configurations.These hybrid cores show significant improvements in flexural toughness, load-bearing capacity, and other mechanical properties.They offer a promising solution to overcome the drawbacks of traditional multi-layered cores while enhancing the overall performance of sandwich panels.Elnasri and Zhao [115] numerically investigated the impact perforation of sandwich structures with graded polymeric hollow sphere cores.The density gradient profiles were varied with different sequencing of the layer density, either from increasing or decreasing order.It was concluded that the absorbed energy of the density graded core sandwich plates is higher under conditions of low-velocity impact and can be enhanced by changing the arrangement of the core layers.Further investigations on sandwich structures proposed that different gradient properties can be designed to obtain distinctive characteristics and functionalities by tuning the design parameters like strut length, cell size, and strut diameter within porous structures [116].One of the most common methods to obtain gradient lattice structures is the Voronoi-tessellation [117], a method based on the space division.Employing the Voronoitessellation for the design of non-uniform structures can achieve anisotropic and lightweight structures with a density gradient.Wang et al [117] suggested a design method based on the Voronoi-tessellation in which a  Bending deformation of the sandwich panels under blast loading leads to core tensile fractures.Tensile fractures occur at the edges of the core compression region for graded panels, and at the lower part of the core layers for ungraded panels. [24] Graded Horizontally156-468 kg m −3 (Arranged in different configurations across layers) Hollow spheres

Polymer 40 mm 4 Impact perforation
For graded core configurations and under low velocity impact, the absorbed energy is enhanced.Increasing energy absorption can be achieved by modifying the core layer arrangement.
The failure modes that occurred include core compression, core tensile fracture, and core shear in the AF core layers of the sandwich panels, as well as top face sheet deformation with local concave-convex patterns and global deformation, and bottom face sheet deformation without damage. [115] Graded VerticallyRelative density: 0.24-0.30(Regulargradient, gradient increasing, gradient decreasing, discrete gradient)

Lattice BCC elementary
Stainless steel

mm 7 Quasi-static and dynamic compression
There is a higher rate of densification for the topology with gradually increasing density.
Deformation under both static and impact compression tests, revealing different behaviours based on lattice structure topologies, with particular attention to gradient topologies exhibiting unique deformation characteristics and mechanical responses. [118] Graded Vertically Irregular porous structure Titanium Alloy Unspecified 1 Compression The way gradient pore structures behave mechanically is quite distinct from uniform porous structures at the beginning.In gradient pore structures, the compressive stress platform tends to go upwards, and this upward angle gets bigger when the porosity gradient is greater.

N/A [119]
Graded VerticallyFunctionally graded cell wall angles Re-entrant honeycomb Titanium Alloy 24 mm 1 Transverse dynamic pressure Functionally graded configurations significantly affect the EPR-deflection curves.
Buckling, yielding of face sheets, core compression, delamination between layers, and local instabilities such as wrinkling [120] Graded VerticallyFunctionally graded thickness

Metalceramic 10 mm 11 Low velocity impact
Functionally graded cores resulted in increased contact forces and decreased contact times.There is better absorbing impact energy for the functionally graded cores.
Capillary cracking in the ceramic-rich layer, intergranular cracking along Al powder particle grain boundaries, crack propagation in brittle layers such as the 70% ceramic-rich layer, and delamination regions where the bond between the layers is weak.Damage progression, crack propagation, and loss of functional integrity were observed in structures with varying material compositions subjected to impact loading, with the lower pure-metal layer playing a significant role in maintaining structural integrity. [121] Graded VerticallyFunctionally graded density Titanium Alloy 24 mm 6 Low velocity impact Due to the impact loading, delamination or debonding occurs at the interfaces between the face sheets and the core  For graded specimens with a smaller cell side length in the first core layer, a localized failure mode (Mode II) was observed.This type of failure involves concentrated deformation in specific regions, possibly leading to local structural damage.Progressive buckling and folding patterns were observed in regions of the specimen subjected to large bending deformations [124] Graded VerticallyFunctionally graded density (0.11, 0.15, and 0.18 in different configurations across layers) Foam Metal 30 mm 3 Blast loading Compared to ungraded and different graded configurations, the graded configuration of low-middle-high density portrays enhanced capabilities of blast resistance.
The top face-sheet gradually compresses the gradient core layer by layer due to its initial velocity.This compression results in localized core deformation, especially in the central region.The sandwich panel continues to deform under its own inertia, characterized by overall plastic bending and stretching.The deformation is governed by the plastic behaviour of the material and results in maximum permanent deflection at the central point. [ Graded Vertically Foam Aluminium 120 mm 1 Compression During compression, the gradual collapse in graded samples began in the less dense area and spread towards the denser region.
The graded porous structure of aluminium foam leads to non-uniform deformation during compression.The compression of graded foam initiates in the upper layers of the sample with lower relative density.The collapsed zone sequentially extends downward through the denser zones, resulting in uneven deformation.[127] multifunctional and density gradient lattice structure was obtained successfully.When subjected to quasi-static compression tests, results showed that the porosity had a profound effect on the material's mechanical properties.The study adjusted porous structure compressive properties to match cortical bone.Elastic moduli ranged widely from 0.14 to 2.37 GPa, and compressive strengths from 1.94 to 116.61 MPa.Functionally graded thickness within cellular cores can be valuable for uneven stress distribution in impact applications.It is an efficient method, but as shape changes, relative density effects must be considered when using this approach.

Functionally graded lattice cored structures
The limitation of proposing a functionally graded thickness remains in the manufacturability sector.Based on different applications, the gradient of the cellular core thickness should be controlled through different manufacturing methods.Sienkiewicz et al [118] evaluated the mechanical response of several gradient lattice structures manufactured using selective laser melting in which the density gradient of the core structure was designed in three different ways: gradient discrete, gradient increasing, and gradient decreasing.The comparison between gradient lattice structures and uniform lattice structures revealed that cores with gradient topologies are more suitable for applications that require high energy absorption.The concept of functionally graded core was first used in lattice metamaterials through the introduction of gradual cell shape variation.This has successfully been applied with the re-entrant honeycomb cores [119] in which numerical results show that functionally graded configurations display unique effects on the structural response of auxetic honeycomb cores.Li et al [120] investigated the bending behaviour of sandwich panels with functionally graded and negative Poisson's ratio honeycomb core in different thermal environments.The study introduces a new concept called Effective Poisson's Ratio (EPR) and investigates the EPR-deflection curves for the first time.These curves reveal how the structural behaviour changes as the beam undergoes deformation Results showed that negative Poisson's ratio and functionally graded density effects the deflection curves and the flexural properties of the sandwich panel.Gunes et al [121] explored the mechanical behaviour of sandwich plates with a functionally graded core subjected to low-velocity impact in which functionally graded material portrayed advantages in the impact energy absorbance.Compared to pure metal plates, functionally graded sandwich plates were found to be advantageous in absorbing impact energy.The layered structure of functionally graded plates, with ductile puremetal layers on the top and bottom surfaces, contributed to protecting the functional integrity of the structure against impact damage.The layered structure of functionally graded plates contributed significantly to protecting the integrity of the structure against impact damage.Li et al [122] analysed low velocity impact behaviour of sandwich panels with functionally graded auxetic lattice cores and a negative Poisson's ratio, in which higher impact resistance was achieved for the plates with functionally graded auxetic cores.Functionally graded 3D lattice cores affect impact response, with graded plates showing less back face sheet displacement and higher contact forces.This indicates that material property gradients in auxetic lattice cores improve impact performance.Ma et al [123] suggested a lattice metamaterial with functionally graded distributions for the purpose of blast protection in which results showed that there is an improvement in the protection ability of sandwich panels with optimum design functionally graded cores.Li et al [124] tested triple layered graded honeycomb core structures under conditions of blast loading and proved that there is significant effect of the geometric configurations on blast resistance and deformation type.More significantly, the authors state that the honeycomb unit cells with higher density arranged closest to the centre performed better.Nonuniform thickness in a sandwich panel can cause uneven mechanical properties, leading to stress concentrations, reduced load capacity, and unpredictable behaviour.Varying cellular core thickness can make some areas buckle while others yield, complicating the determination of failure modes.

Functionally graded foam structure
Jing and Zhao [125] investigated the dynamic response and energy absorption capabilities of sandwich panels with a gradient metallic foam core in which it was concluded that the efficiency of the blast resistance is dependent on the manner in which the core gradient layer is arranged.Results showed that the core density gradient with the best performance is that with increasing density from top to bottom of the core.Since foam properties are significantly dependent on density, they can be tuned through varying the bulk density of the foam core structure [126].The blast resistance performance of sandwich panels can be improved by arranging the stacking sequence and varying the stiffness/strength of core layers.Graded sandwich panels with certain core configurations exhibit superior blast resistance compared to the ungraded panel.The study finds that the blast resistance capability of graded sandwich panels with specific core configurations can be significantly improved compared to ungraded panels.The specific arrangement of core layers and their varying properties play a crucial role in enhancing the structural performance under blast loading conditions.Different foaming methods have the ability to produce complex and continuous density gradients with the potential of increasing the mass efficiency within metallic foams.This, in turn, could aid in improving the interfacial strength in structures of higher order like graded sandwich panels [127].

Summary of surveyed literature
The section discusses the concept of functionally graded structures which can be found in nature that combines several topological structures to achieve varying density and distinct mechanical properties.A summary of studies using functionally graded structures in sandwich panels is presented in table 4.These structures are compared with uniform ones and offer advantages such as efficient material use and improved mechanical performance.Fuchiyama and Noda [106] first proposed such configurations to address stress concentration in sandwich panels.Functionally graded materials (FGM) have smooth property variations and are valuable for applications demanding high performance and damage tolerance.Research has shown their potential in impact scenarios.The surveyed literature suggests that functionally graded materials have distinct mechanical properties due to their variable density and property distribution.Unlike uniform structures, FGMs can have smooth property variations, allowing for the optimization of structural functions.The performance of FGMs has been extensively researched for their mechanical properties and energy absorption capabilities, particularly in sandwich structures with graded material cores.The use of Voronoi tessellation and other design methods have successfully obtained anisotropic and lightweight structures with density gradients.Gradient lattice structures have also been explored, with numerical results showing that functionally graded configurations can have unique effects on structural response.Designing these structures involves factors like geometry and loading type that influence stability.Different arrangements of core layers impact energy absorption, and adjusting porosity improves mechanical properties.The literature does also mention bamboo as an example of a natural material that exhibits a gradient structure.Bamboo's density is continuously increasing towards the external surface, allowing it to withstand higher bending loads.The use of natural biomaterials, such as bamboo, demonstrates a clear attempt to draw parallels between human-engineered functionally graded structures and natural adaptations.The example of bamboo's increasing density towards the external surface showcases nature's efficiency in load-bearing mechanisms.This not only strengthens the argument for functionally graded materials but also highlights the potential for bio-inspired design in engineering While the advantages of functionally graded structures have been extensively investigated, it's important to recognize the existing challenges, particularly in their manufacturing.This recognition introduces practicality to their use in sandwich panels which emphasizes that even such promising concepts encounter obstacles.This pragmatic perspective is crucial for steering researchers and engineers towards the effective implementation of these ideas in real-world applications.

Core height and multi-layered cores
This paragraph discusses the mechanical response of sandwich panels in relation to core height and layering.The study explores the effect of different core heights and layering configurations on the sandwich panel's energy absorption, stiffness, and resistance to deflection.Additionally, the use of core layering to optimize the crush force efficiency and energy absorption of sandwich panels is discussed.Moreover, the paragraph highlights studies that propose multi-layered hybrid cores as a potential solution to the limitations of traditional multilayered core configurations.

Effect of varying core height
Numerous research has been conducted on the response of sandwich panels with varying core thickness or increasing number of core layers.Several configurations are shown in figure 5 in which the core height and/or the number of layers can be increased.In various studies, the effect of changing height of the core on the mechanical response of different core configurations was investigated.Liu et al [128] studied the low velocity impact on aluminium foam core sandwich panels with four different core heights and the energy absorption and impact resistance of these sandwich panels were tested.Specifically, the study aims to understand how combinations of fibre metal laminate (FML) skin and foam core thicknesses influence energy absorption and impact resistance.Results from the study showed that increasing the foam core height by 5 mm increases the energy absorption and stiffness of sandwich structures.For example, the 30 mm core panel absorbs 6.5% more energy than the 25 mm core panel.In a similar study conducted by Liu et al [129] the effect of foam core height on the high velocity impact was investigated and the authors report a significant effect on the energy absorption due to varying core height.The study showed that energy absorption increases by 32.2% when the core height is increased by 5 mm.Increasing the foam core thickness can lead to the disappearance of debonding between the top fibre metal laminate and the Aluminium foam core in panels with FML skins.Energy absorption shifts to foam compaction instead of interfacial debonding.Ren et al [41] showed that increasing the height of PVC foam core resulted in improvements on the sandwich panel's resistance to deflection.The analysis examines the influence of core thickness on the deflection resistance of the sandwich plates.It is observed that increasing core thickness improves deflection resistance, but the impact of core thickness on the deformation of the rear sheet is limited.Sun et al [73] stated that although the core height does not quantify to observable changes in the peak load and elastic limit under different impact energies, it enhances the bending stiffness of the composite structure.The effect of core height is more obvious on the bending deflections of the back face sheets of the panels.As the height of the core increases, the bending deflection of the back face sheets becomes smaller.In addition, He et al [130] observed that the bending stiffness of sandwich panels with honeycomb core was drastically affected by the thickness of the core.The study reveals that modifying the honeycomb core configuration has a significant impact on the overall impact resistance of the structure.Specifically, increasing the cell wall thickness or decreasing the side length of the honeycomb core leads to improvements in impact resistance.However, increasing the core height has a minor effect on the peak load during impact.The authors stated that although the core height does not directly affect the behaviour of the panels under low velocity impact, the effect is more prominent for residual flexural strength.Tran and Peng [131] studied the blast resistance of sandwich panels with TPMS core, results revealed that subjecting different core heights to blast loadings contributed to different reaction forces.The effect of core thickness was studied.Thicker gyroid cores showed increased initial peak reaction forces on the concrete foundation but reduced specific energy absorption.
Increasing the core height by 0.4 mm, the reaction force increases by almost 200% leading to an increase in the stiffness of the sandwich panel.Liu et al [128] reported that an increase in energy absorption due to increase in the foam core height was insignificant at almost 6.5 %.On the other hand, increasing the core thickness to a maximum of 35 mm portrayed a resistance to the full perforation with respect to thinner cores.Sun et al [73] stated that energy absorption was not directly affected by the core height under low velocity impact and the initial response to loaddisplacement does not significantly change.Varying the cellular core height can enhance bending stiffness more than energy absorption.Bending stiffness relies on the second moment of inertia, which increases with core height, significantly boosting bending stiffness.However, the effect on energy absorption is not as pronounced.Energy absorption depends on yielding or buckling, influenced by cell wall thickness.Core height contributes to higher buckling or yielding strength but can decrease energy absorption due to increased buckling risk.It can also weaken shear strength and resistance to transverse loads, causing localized failures or reduced durability in impact situations.Ren et al [41] discussed in their study the effect of core height on the deformation of the rear sheets of a sandwich panel.The behaviour of the panels under impact is mainly dominated by local indentation and is therefore independent of the height of the core in the initial stages of the impact.However, increasing the core height of gyroid core sandwich panels [131] resulted in a reduction of the energy absorption.

Core layering effect
The literature explores the use of core layering to determine the core height of sandwich panels.This involves repeating a specific cell structure horizontally to create a multi-layered sandwich panel.Increasing the number of layers in the core generally improves the crush force efficiency (CFE) but has little effect on energy absorption.However, multi-layering can decrease the stiffness of the panel, limiting its use for anti-collision devices that require good energy absorption and load-bearing capacity.The crush force efficiency (CFE) measures the effectiveness of sandwich structure in absorbing the impact force (the force that travels through the sandwich panel) [132].Higher CFE means that the core of the structure has performed better in absorbing the impact force, making the sandwich panel more effective and better for protection.Energy absorption of sandwich panels refer to the amount of energy that has been absorbed by the structure when subjected to impact [132].Stiffness, on the other hand, measures the panel's ability to resist elastic deformation when loaded.A sandwich panel can have a high CFE but low resistance if the core material is designed to absorb high impact forces but is not as rigid or strong as the face sheets [132].The resultant is a panel that can effectively absorb impact, but prone to failure or deformation when subjected to different types of stresses or loads.Therefore, to design multi-layered or thicker-core sandwich panels, the core configuration should be tuned to meet structural stiffness requirements in various engineering applications.Multi-layering enhances the panel's crush force efficiency [133].Huo et al [134] investigated the crash behaviour of a multi-layered sandwich panel with aluminium foam core both experimentally and numerically.The study aimed to understand how different sandwiching components influence the low-velocity impact response.Results showed that the multi-layer configuration resulted in better crush force efficiency.However, increasing the number of core layers led to a decrease in the overall stiffness of the sandwich panels.The core height was kept constant at 15 mm while the number of layers ranged from 1 to 4. The energy absorption of the sandwich panel was almost constant with increasing numbers of core layers; however, the overall sandwich panel stiffness decreases.Zhu and Sun [135] results agreed with the results of Huo et al [134] when investigating multilayer foam core under low velocity impact.In their study, Zhu and Sun investigated the low-velocity impact response of a multilayer foam core sandwich panel with composite face sheets.A comparison was made between a geometrically symmetric monolayer foam core sandwich panel and a multilayer version with the same mass.The multilayer sandwich panel exhibited superior impact resistance.Specifically, it showed a reduction of over 15% in the maximum contact force and an increase of 15.5% in crush force efficiency.These results indicated that the multilayer configuration effectively enhanced the impact performance of the sandwich panel.The global stiffness of multilayer sandwich panels was less than that of a single layer.In terms of impact resistance, the multilayer sandwich panels showed better performance when both configurations had the same mass.Palomba et al [136] studied the effect of using multilayer honeycomb core showing that both single and multi-layer sandwich structures were subjected to perforation under highest impact energy.The objective was to enhance the crashworthiness of traditional honeycomb sandwich panels by introducing double-layer configurations with different cell dimensions and core arrangements.The study showed that there are higher energy absorption capabilities of multi-layered structures as compared to a single layer.In specific, the structures with multi-layers and larger unit cell size presented the highest specific energy absorption.The impact tests on double-layer structures revealed a differential energy absorption mechanism between the layers based on cell dimensions and layer positions.Structures with a larger cell size in the bottom layer showed uniform compression of the lower core, while others exhibited localized deformation around the impact point.The arrangement of the core could lead to the development of progressive energy-absorbing mechanisms.Metallic pyramidal lattice truss core sandwich panels were fabricated by Qi et al [137] through additive manufacturing and tested for out-of-plane compression, in-plane compression, and shear stiffness.The research included both single-layered and multi-layered configurations.The study involved the design and manufacturing of pyramidal lattice truss core sandwich structures using additive manufacturing.This allowed for the creation of complex geometries and variable cross sections in the lattice truss core.Multi-layered cores showed exceptional load support capability.The thick face-sheets caused the lattice truss columns to fail by plastic yielding, with the cores taking on secondary roles in load bearing.Results showed that the multi-layered configuration had better load support capabilities after collapsing of the first two layers.The pyramidal arrangement was also investigated by Yasui [138] under dynamic crushing and quasi-static behaviour for multilayer honeycomb sandwich panels showing that the pyramidal assembly portrayed better capabilities of energy absorption.Based on the results, the study concluded that multi-layer panels with pyramid built-up types (prismatic streamlined type) and uniform built-up types could offer high-performance characteristics suitable for various practical applications.These panels showed promise as impact energy absorbing buffers across different scenarios.Li et al [124] tested three layered aluminium honeycomb sandwich structures under blast loading showing that as the cell dimensions of the core layer decreased, the deformation mode becomes localized failure.Optimization designs that take into account both the cell structure and layering aspects result in improved deformation and energy absorption.While the mechanical properties are proven to be significantly enhanced with the core layering effects, the drawbacks are interfacial bonding issues between the cores.Proper bonding is crucial in order to maintain structural stability and integrity of the sandwich panel.Delamination due to inadequate bonding will result to disruption in the load transfer within the core, which leads to a reduced mechanical performance.

Hybrid core design
To tackle the disadvantages of using multi-layered core configurations, Najafi and Eslami-Farsani [139] recently proposed a multi-layered sandwich panel with a hybrid core of agglomerated cork and PU foam bonded together and compared it to a panel of PU foam core under compression, three-point bending, and high-velocity impact.The mass of both tested sandwich panels was kept the same and results showed that the flexural toughness, flexural load, and initial flexural toughness of the panels were improved significantly at 145%, 506%, and 816%, respectively.Moreover, the compressive stiffness increased by 1233% through the hybrid multilayer configuration.The absorbed impact energy also increased, though not as significantly as the toughness and stiffness.Cai et al [140] examined the response of sandwich structures that used a multi-layered hybrid core to an air blast.The panels included multi-layered aluminium foam core sandwich panels, multi-layered hybrid aluminium foam/UHMWPE laminates core sandwich panels, and panels with papery UHMWPE laminates (PP).The study experimentally compared how a foam core and a hybrid foam/UHMWPE core failed.Results showed that implementing a foam core with increasing density decreased the deformation of the face sheets.Additionally, adding the UHMWPE laminates further reduced deformation within the face sheets.Placing the UHMWPE layer close to the back face of the panel increased front face indentation and global deformation of the UHMWPE layer.This arrangement also reduced the permanent deformation of the back face.The presence of the hybrid core configuration significantly decreased the centre velocity for the front face sheets.Additionally, including papery UHMWPE laminates prevented shear cracks in the foam core.Panels with these laminates between foam layers performed better in back face deformation than those with laminates between face sheets and foam cores.The top-performing sandwich panels, featuring multi-layered hybrid aluminium foam/ UHMWPE laminate cores, showed only a slightly 6% higher back face deflection compared to the baseline AF panels.Ni et al [141] investigated the performance of single and double-layered pyramidal-ceramic-epoxy hybrid sandwich panels under impact and showed that the double-layer hybrid core configurations outperformed the single-layer and homogenous (non-hybrid) metallic core structures in terms of energy absorption.The presence of ceramic insertions within the hybrid core leads to erosion and mass loss of the impacting projectile.Additionally, the epoxy resin filling the voids in the core contributes to enhancing the overall ballistic resistance by creating an integrated structure.Core layering, in comparison with single layer sandwich panels, can lead to anisotropic behaviour of the cellular core, which can be more complex when during the prediction of the sandwich panel mechanical performance under different loadings.

Summary of surveyed literature
The surveyed literature highlights the significant role of core height in influencing the mechanical response of sandwich panels.Varying the core height has been shown to impact energy absorption, stiffness, and bending behaviour.Increasing the core height tends to enhance the bending stiffness, due to the resulting higher second moment of inertia.However, the effect on energy absorption is not as pronounced, indicating that core height primarily impacts stiffness rather than energy absorption capabilities.Table 5 presents a summary of the existing literature on the effect of varying core height and number of layers on the mechanical properties of sandwich panels.The reviewed studies indicate that altering these parameters has a significant impact on the stiffness and energy absorption capabilities of the panel.The concept of core layering has also been widely explored.While increasing the number of core layers generally improves the crush force efficiency (CFE), it often leads to reduced stiffness.This trade-off between improved impact efficiency and decreased stiffness highlights the complexity of designing multi-layered structures.It's crucial to consider the specific application requirements when designing the core of sandwich panels in terms of number of layers to achieve the desired balance between energy absorption and stiffness.Additionally, the introduction of hybrid core designs, such as combining agglomerated cork and PU foam, demonstrates an innovative approach to addressing the limitations of multi-layered configurations.These hybrid cores show significant improvements in flexural toughness, load-bearing capacity, and other mechanical properties.They offer a promising solution to overcome the drawbacks of traditional multi-layered cores while enhancing the overall performance of sandwich panels.However, it is crucial to maintain an optimal balance between stiffness and energy absorption by careful tuning of these topologies.Furthermore, changing the cell topology while varying height or increasing the number of layers can further enhance the stiffness and mechanical properties of the sandwich panels.Additionally, by designing hybrid cores, researchers can achieve further improvements in the mechanical properties of sandwich panels.Additionally, the introduction of hybrid core designs, such as combining agglomerated cork and PU foam, demonstrates an innovative approach to addressing the limitations of multi-layered configurations.These hybrid cores show significant improvements in flexural toughness, load-bearing capacity, and other mechanical properties.They offer a promising solution to overcome the drawbacks of traditional multi-layered cores while enhancing the overall performance of sandwich panels.

Conclusion and future research
This extensive review focused on examining core designs of sandwich panels and analysing influential factors affecting their mechanical performance, encompassing core height, cell configuration, functionally graded cores, and constituent materials.Key findings and implications from the reviewed literature include:

High velocity impact
Increase in foam thickness does not significantly increase impact resistance but it increases energy absorption.Increase in foam thickness enhances debonding between core and skin.
Under high velocity impact, the layers of the top fibre metal laminate (FML) skin experience delamination, especially in the region surrounding the impact centre where bending occurs.The AL foam core also experiences deformation, primarily in the form of compaction or crack, or a combination of both.

High velocity impact
Increase in the PVC foam core lengthened the response time of the sandwich panel.The influence of using 2 layers in core was slight on the impact resistance.
Dynamic symmetrical deformation of sandwich plates upon impact, with rear face sheets exhibiting slight spring back.The response rate increases with impact loading, while greater core thickness lengthens response time.Core density enhances impact resistance, showing large ductile deformation, tensile-tearing, and transverse shearing in face sheets.Low impact loading results in central compression with incline cracking, medium and high intensity leads to core penetration and fragmentation. [41] 10-20 1 Honeycomb core Aluminium Not specified Low velocity impact Increase in the Al honeycomb core height had little to no effect on the rising slope, peak load, and elastic limit (in which they were almost the same).Increase in core height enhances the bending stiffness of the sandwich panel.The impact behaviour of the sandwich panel is independent on the core height.
Failure mode A involves localized indentation with damage concentrated around the impact region, while failure mode B is characterized by front face sheet wrinkling, back face sheet bulging, and overall core crushing due to global bending.Additionally, plastic buckling of the core is observed.On the other hand, at higher impact energies, the dominant failure patterns include fibre fracture and matrix cracking of the impacted face sheet, along with core crushing and shear.0.2-0.6 1, 2, 3, 4 TPMS Core (Gyroid, Primitive, Diamond, F-K)

Blast Resistance
Increase in TPMS core height significantly increases the peak reaction force.At early stages of the blast events, ticker core absorbed more energy.Larger deformations are observed for sandwich panels with a smaller number of layers.
Higher number of layers resulted in better stiffness.Energy  The EF-type and HT-type structures showed really good reactions to impacts, like having a gentle initial force, a strong and steady middle force, and even bending.
In quasi-static tests, the steel cores showed uniform antisymmetric or symmetric deformation.However, in impact tests, their steel cores exhibited non-uniform, inclined deformation modes.On the other hand, the ectopic frame (EF) and horizontal tube (HT) structures demonstrated consistent and robust deformation modes under different loading rates. [132]

Flatwise compression
As the number of core layers increased, specific energy absorption and the crush force efficiency increased For regular corrugated sandwich panels, the core layers were compressed to the point of overlapping before failing together.In contrast, perpendicular core configurations led to a layerby-layer failure of the core. [133]

Low-velocity impact
As the core layers are increased, the crush force efficiency becomes better.Increasing core layer decrease sandwich panel stiffness.
In the case of higher impacting energy, the top face sheet may be completely penetrated, while the bottom face sheet experiences significant deformation as it bears a substantial portion of the impact load.Increasing the core layer number from 1 to 2 resulted in an increase in the peak force.The unit cell configuration of the core played a major role on the damage of multilayer sandwich panels.
Impacts result in upper face sheet deformation and buckling of honeycomb cells.Smaller cell sizes lead to localized collapse under impact, while larger cells exhibit more extended deformation.Failure of top and intermediate skins occurs at different energy levels in double-layer structures, with upper skin debonding observed due to significant local deformation. [136] 1 & 2 Pyramidal lattice truss core Fine aluminium alloy powder Unspecified CompressionShear Structures with multiple layers inside are really good at holding up heavy loads as compared to a single layer Compression:Initially, the lattice truss column behaves elastically, showing a linear regime in the load-displacement curve.As the load increases, the face sheets undergo plastic yielding, leading to a gradual increase in response until reaching the failure load.Shear:The lattice structure demonstrates elastic behaviour in the initial phase of shear loading, showing a linear increase in the load-displacement curve.Plastic yielding is observed at specific nodes, followed by more widespread node failures as the strain increases.

Low-velocity impactQuasi-static crushing
When the multilayer core configuration was used with two different types of unit cells: uniform and pyramid, the pyramid type has better energy absorption.
In quasi-static compression tests, an initial peak stress is observed due to the elastic behaviour of the material.After reaching the peak stress, the panel experiences collapsible folding, often occurring at about half of the peak stress.This folding behaviour continues until the bottoming-out point.Dynamic axial crushing behaviour in multi-layer panels shows a peak in compression load, followed by a plateau stress with buckled folding deformation.This plateau stress is about half of the initial peak stress.Crushing deformation progresses through the core panel until  The hybrid multilayer core resulted in significant increase on the flexural load (506%), initial flexural stiffness (816%), and flexural toughness (145%).Multilayer hybrid core had increase in compressive stiffness (8%) and compressive load (47%).Absorbed energy increased by 21%.
Improved delamination resistance by reducing shear stress in core-skin interfacial bonding.Enhanced compressive stiffness and strength of the core, delaying foam crushing through through-thickness support.Improved stress distribution and increased load-carrying capacity through connection of top and bottom skins.

Blast loading
The top sandwich panels with a mix of aluminium foam and UHMWPE layers in the middle didn't do as well when it came to how much the back side bent.But these layered cores did a good job of absorbing most of the blast's energy.
Highly localized dishing deformation of the front face and global inelastic deformation of the back face.In high to low gradient, partial tearing appeared at the central area of the front face.The location of the UHMWPE layer influenced deformation and failure modes.Placing the UHMWPE layer near the front face enhanced support and reduced front face deflection but increased back face deflection. [140] 17.522.1 1 & 2 Truss Metallic Unspecified Low velocity impact The double-layer sandwich plate we suggested is better at protecting against ballistic impacts compared to a single layer that weighs the same.The double-layer one can withstand impacts at speeds of around 1850 m/s from regular projectiles, while the single-layer plate can only handle impacts at speeds of about 1300 m s −1 .
Localized deformation and fragmentation of ceramic prisms and lattice truss members upon projectile impact, plastic deformation and perforation fracture of face-sheets, erosion of the projectile due to ceramic fragments, and overall structural deformation and mass loss as projectiles penetrate sandwich plates.Double-layered configurations show increased interfacial complexity, enabling more efficient energy absorption and stress wave deflecting/spreading, with variations in layer thickness affecting ballistic performance. [141] • Sandwich panels offer design flexibility that enables customization and optimization of core structures to enhance mechanical properties.Core geometry, such as honeycombs, truss, corrugated or other bioinspired core structures, significantly influences structural performance, and tailored cores can improve multifunctionality for thermal and mechanical applications.
• Functionally graded cores outperform conventional cores in designing energy-absorbing structures, exhibiting controlled energy absorption and weight reduction under varying loading conditions.Adjusting variables like density, thickness, and yielding stress enhances properties, and density-graded fillers further improve energy absorption.
• Multilayer cores enhance specimen strength, albeit with potential stiffness and weight drawbacks.Optimizing unit cell configuration in multilayer cores affects load distribution, and combining multilayer and functionally graded configurations can yield superior results.
• Choice of core material depends on application requirements.Aluminium offers lightweight, stiffness, and commercial availability, polymers provide high strength-to-weight ratios with low stiffness, and metallic cores excel in high energy absorption applications.Nanomaterial reinforcement and smart materials like shape memory polymers show potential for enhanced mechanical properties.
Based on these conclusions, potential gaps and future research directions are identified: • Further exploration of cellular structure geometries and topologies can lead to improved sandwich panel mechanical properties.
• Developing manufacturing methods overcoming additive manufacturing limitations is crucial.
• Optimization of functionally graded material design, properties, and manufacturing methods is needed.
• Research on enhanced bonding between different materials for improved performance is required Exploration of synergistic effects of multiple parameters on mechanical properties is essential.
• Further assessment of smart materials and the development of novel materials can enhance sandwich panel properties.

Figure 1 .
Figure 1.Schematic of a sandwich structure.

Figure 3 .
Figure 3. Different core configurations by unit cell of (a) honeycomb (b) truss (c) corrugated (d) random cells based on Voronoi tessellations (random foam structure).

20 Mater.
Res. Express 10 (2023) 102001 A Charkaoui et al foam core decreases after 720 h of ageing.In Shear failure of the foam core was not observed, and the core separated cleanly from the inner face [105] 28 Mater.Res.Express 10 (2023) 102001 A Charkaoui et al

Figure 4 .
Figure 4. Functionally graded thickness honeycomb plate (a) graded along the Y-direction (b) graded along the x-direction.

Figure 5 .
Figure 5. Different configuration of varying core (a) honeycomb sandwich panel with 10 mm core height (b) honeycomb sandwich panel with 20 mm core height (c) honeycomb sandwich panel with multilayer honeycomb core (two layers in different colors).

− 3
Low velocity impactThreepoint bending.Increase in the Al hexagonal honeycomb core height has no significant effect on the peak load of the sandwich panel.Increasing core height has For lower impact energy (5 J), the predominant failure patterns involve inter-laminar delamination and matrix damage of the top face sheet.[130] 38 Mater.Res.Express 10 (2023) 102001 A Charkaoui et al

2
Foam core Aluminium Unspecified Low velocity impact As the number of Al foam core layer increased from 1 to 2 the impact resistance improved but Local indentation and deformation of the top face sheet, followed by localized crushing and shearing of the foam cores beneath the impactor, interlayer [135] 39 Mater.Res.Express 10 (2023) 102001 A Charkaoui et al /m 3 60 k g /m 3 90 kg m−3 . Express 10 (2023) 102001 A Charkaoui et al −3 , 450 kg m −3 and 650 kg m−3

Table 1 .
Previous work on changing core material in sandwich panels.

Table 2 .
Various unit cells of core structures.

Table 3 .
Previous work on changing core structure in sandwich panels.

Table 4 .
Previous work on using functionally graded core in sandwich panels.

Table 5 .
Previous work on changing core height or using multilayer core in sandwich panels.