Influence of different manufacturing techniques on GFRP flat-joggle-flat composite joints using multi-scale reinforcements for enhancing shear properties

Composite materials, particularly glass fibre-reinforced polymers, or GFRP are being used far more frequently. Airframes have been manufactured utilizing reinforced composites, including struts, frames, and flaps, employing raised epoxy-based co-cure technology. The current research describes a multi-scale approach to fortifying graphene nanoparticles (GNP) and carbon fibre Z-pins in order to strengthen the flat-joggle-flat composite joints with different manufacturing technique. Shear investigation showed that by adding GNPs and putting carbon fibre pins (Z-pins) in a crosswise position (perpendicular to the plane) to the joint's surface, concurrent reinforcement gives rise to greater shear characteristics with quasi-static loads. Specifically, there was a 45.6% improvement in shear resistance when contrasted with unreinforced co-cured FJF joints. The FESEM has been utilized to demonstrate the failure analysis of the specimens, which shows the clear failure mechanism of the FJF joint specimens. The FJF joint with multiscale reinforcement has a very high natural frequency of 685.1 Hz as compared to other configurations, according to the vibration analysis.


Introduction
Adhesively bonded composite materials are used a lot in technical applications because they are light, distribute stress well, have a larger assembly area, and can combine different parts [1][2][3].However, the abrupt breakdown in the bond line due to shear as well as fatigue loading is a persistent problem in adhesive junctions made with fibre-reinforced composites [4,5].Experts employ Nano fillers like carbon nanotubes, aluminium oxide, silica, and microfibers to strengthen joints, as well as through-thickness strengthening techniques like sewing, spinning, and pins [6][7][8].
Guntac et al [9] stated that 0.5 wt% of GNP enhances the tensile load as compared to other wt%.Similarly, Yanyu Song et al [10] suggested that, using 1 wt% of GNP improves 138% of shear strength in AgCuTi/GNP joints.The incorporation of 0.75 wt% of GNP in adhesive improves the failure load by 34.73% in FJF joints [11].The utilization of Z-pinning has been identified as a viable technique for enhancing the tensile strength of joints [12][13][14].Pinning increased composite junction shear as well as fatigue characteristics by 40% [15].The research revealed that pins changed instability, adhesion, and joint breakdown to a stable condition.The investigators invented multi-scale strengthening strategies to improve the strength of fibre-reinforced laminated composites [16][17][18][19].When z-pins and nanofillers are applied simultaneously, it increases durability under quasi-static load conditions by a 400% rise in fracture energy, and a substantial increase in resistance to fatigue delamination progression is demonstrated by the multi-scale laminates over mode I interlaminar cyclic-fatigue loads by Ladani.et al [16].An analysis examining how multi-scale carbon reinforcing can synergistically improve the mode-I interlaminar fracture resistance of fibre-polymer laminates made of composites.The outcome states that the rate of Integrating carbon nanofillers plus z-pins-two strengthening methods that have historically been applied separately-can increase the interlaminar fracture-resistant qualities of composite components.Utilizing z-pins, these small carbon fibers enhance their resilience to delamination through several intrinsic as well as external strengthening mechanisms, such as interfacial debonding and void development [20][21][22][23].
Previously, composite materials were joined together using mechanical fasteners like bolts, screws, and rivets.Simple setup and removal are made possible by mechanical attachment, which also makes it possible to verify the strength of each connection.However, perforating composite components breaks down the components of the matrix and produces discontinuities in the fibers [24][25][26][27].Consequently, it enhances the laminates' stress stability and lightness.The benefits of lighter FRP matrix composites were diminished.In order to address the problems associated with hand attachment, professionals embraced the adhesive bonding method [28][29][30].Golmakani et al [31] Examined the effects of layer density and organization on the physical and mechanical features of composites with ten distinct compositions.The results disclose that Young's modulus as well as the material's tensile strength are influenced by the laminate layer structure, according to the outcomes.Comparing the laminating material strengthened with glass fibers to the reference samples, the impact measurements show a considerable improvement in impact resistance as a consequence of the addition of glass fibers to the structure.
Since adhesives provide consistent stress transfer across the overlapped region, they are an excellent method of attaching materials despite physical connection challenges.It also reduced the overall weight, stress concentration, and cost of assembly of the composite construction.Epoxy adhesive is used in adhesive bonding and offers a number of benefits, including the ability to combine various components, homogeneous stress transmission, a better coating, and a lighter combination [32,33].However, they have some drawbacks as well, such as a longer curing time, lower humidity, and less tolerance to temperature changes.Additionally, the adhesives and substrate continue to experience high levels of stress due to the structural discontinuities at the connection site [34].Nanoparticle-augmented adhesives are one of the most active areas of materials technology and engineering study at the moment.It has recently been discovered that the strength and rigidity of resins can be increased by strengthening them with various nano-fillers [35,36].
Based on a review of the research, most studies looked at how different reinforcements affected the shear behavior of composite junctions [37][38][39][40].In order to strengthen joints, the present work intends to investigate the combined influence of reinforcements GNP and Z-pins to enhance the shear strength in the FJF composite joints utilizing various bonding procedures.To investigate the combination influence on the tensile performance along with the failure mechanism by using the FESEM.And a new approach to investigate the natural frequencies by using a model analysis of composite joints.0.75 wt% GNP has been incorporated into resin glue as reinforcements, and 2 vol% of carbon fibre pins (z-pins) were inserted within the Z-direction of the composite laminates.And finally the multi-scale reinforcements such as GNP+Z-pins on the joint surfaces enhance shear properties.

Fabrication materials
The bi-directional glass fibres (400 gsm) have been used to prepare the laminates with resin (Ly-556) along with binder (Hy-951) with a volume ratio of 10:1.The research-grade graphene nanoparticles (GNP) and carbon fibre pin (2 mm) Z-pin are used as reinforced material for FJF joints.

Preparation of adhesive
Initially graphene nano particles with different weight proportions are mixed with the pure acetone and ultrasonicated for one hour for dispersion of the nano-particles in the pure acetone, then the solution is mixed with epoxy and again ultra-sonicated for 45 min to completely mix of nanoparticles with epoxy, later the solution is kept on the electric heater and stirred continuously up to one hour to removal of excess acetone from the mixture, and finally got the GNP based adhesive material.The overall process of graphene based epoxy material is shown in figure 1.

Preparation of Z-pins
A new technique to prepare Z-pins as shown in figure 2 includes that, initially epoxy resin was mixed with carbon fibre yarns, which were dragged more tightly before being hooked at both ends under the tube.After that, the fibres were shrunk, and excess glue was removed from the hot-deflate tube by heating it at 75 °C for 30 s using a heat blower.Then blades were used to rip out the whole length of tubing.The pins then underwent post treatment in an oven at 70 °C for 48 h.The overall pin exterior diameter is about 2 mm.

Manufacturing methods of FJF joints
GFRP composites were fabricated utilizing the traditional hand lay-up method in conjunction with a vacuumbagged process, with a 2 mm thickness, and wooden mould is used to provide the joggle angle.Different manufacturing technique has been used to prepare the joint specimens like co-curing, co-bonding and secondary bonding as per the geometry as shown in the figure 3. Initially neat FJF joint specimens has been prepared using all the different manufacturing technique.Here in co-curing method, two uncured laminates have been joined together with epoxy adhesive, and allow them to dry for 24 h.And in co-bonding process, one laminate is already pre cured and another is uncured and joined with epoxy adhesive, and allow them to dry for 24 h.Finally in secondary-bonding both the laminates are already cured than joined together with epoxy adhesive material and allowed them to dry for 24 h.
Later, the GNP-based adhesive material was used for preparing the FJF joints with three different manufacturing techniques.And then the carbon fibre pins (Z-pins) are utilised as a reinforcement material in the specimens.Finally, multi-scale reinforcement (GNP+ Z-pins) has been used as a reinforcement material using different manufacturing techniques.Three samples for each configuration has been tested and average valve has been considered.The Instron 8801 equipment was used to conduct a tensile experiment at an interval of 1 mm min −1 , as indicated in the figure 4.

Free vibrational analysis
An analysis was conducted to determine the effects of the production process on multiscale reinforced free vibrational features, neat FJF, GNP-based, Z-pin-based, and free initial conditions.By applying modal analysis, the frequency response was found.A Dytran 5800SL compact impulse sledgehammer with modally adjusted tuning (sensitivities of 104.5) was used to stimulate the co-cured FJF joints.The displacement data was obtained via a compact, small ceramic shear ICP accelerometer weighing 0.5 g.
The acquired signals were fed into a four-channel data gathering system, which converted the time-domain signals into frequency response signals using the Fast Fourier Transformation (FFT) technique.Using a circle fit method, the DEWE program assesses the effective modal damping connected to a specific mode.Figure 5 shows the schematic depiction of the vibrational analysis.

Shear analysis
The average load and shear strength of co-cured neat FJF joint has been increased as compared to co-bonding and secondary-bonding as a result of lower peeling stress around the bonding line, because cured epoxy resins  are brittle and have a low resistance to the beginning and progression of fractures and also epoxies' poorer resistance against shear force resulted from their lower fracture resistance.0.75 wt% of GNP increases the shear strength of CC FJF joint by 54.06% and 31.44% as compared to co-bonding and secondary-bonding as shown in table 1.Because of a textured interlayer improved adhesion because it enhanced the interactions within the glue and the interface.This led to improved resistance to shear stress and higher strength in the joints.The use of Z-pins as reinforcing material has improved the joints efficiency.The resilience to shear was improved through a co-curing technique by using 2% pin reinforcement, which also improved composite co-cured junctions' energy-absorbing ability and force.While the pinning technique stopped the bonding layer from cracking and the tension from tearing away.Nevertheless, secondary-bonding joints are significantly weaker in tensile stress than co-curing and co-bonding bonds since pins damage the adhesive layer, weakening the material's adherent bonding and accelerating the spread of cracks.
The load versus displacement and stress versus strain curves of co-cured FJF composite joints are shown in figure 6, which indicates that the multi-scaled co-cured joints have the highest tensile load as compared to all other combinations of the joint.This is due to the extrinsic and intrinsic strengthening approaches utilizing    GNP and z-pins on the joint area.GNP creates the coarser surface in the bonding area, whereas the z-pins act like the bridging between the adherend surfaces, and they hold the maximum loads.

Failure analysis
Uneven crack development was the root cause of the adhesive failure of co-bonding and secondary-bonding joints that occurred with unreinforced FJF joints as shown in figure 7(a).Co-cured neat FJF joints have mostly failed in cohesive pattern because of strong bonding between adhesive and adherend materials.Consequences that have ensued inadequate ability to withstand shear stress, the failure happened at a lesser load.Additionally, the uniform dispersion of GNP improved adhesion and surface roughness.For this reason, overall co-cure joints experienced failure in a cohesive manner that actually increased their resilience to failure as shown in figure 7(b).
The coarser surface created the interaction between adhesive and adherend therefore interfacial bonding between them was good.Through-thickness-strengthened joints with 2% pins experienced a stable bond-line breakdown.This was because the better bridging grip provided by pins slowed down the crack's onset and spread.Multi reinforcements like 0.75 wt% GNPs as well as 2 vol% carbon fibre Z-pin enhanced joint strength through multiple toughening, preventing early failure from increased tensile load sensitivity.It increased adhesion and formed an exterior crack-spanning region within the bond layer.The inclusion of GNP increased adhesion across adherents by roughening the bond line and bridging crevices at overlapping ends.According to FESEM analysis, pin separation and shear fracture contributed to the failure of a 2 vol% pin-enhanced co-cured joint.The pins extended through the bond line, whereas the opposite face exhibited an open hole as shown in figures 7(c) and (d).The destruction caused by the opposed adhering surface indicates strong friction, preventing pins pull-out.Multiple strengthening methods, such as shearing stripes, GNP bridging, crack expansion variation, and pin-created intrinsic crack-interlacing zones, have delayed fracture beginning and development.

Vibration analysis of FJF joint specimens
The effects of GNP addition, Z-pins, and multi-scale reinforcement and the manufacturing technique on the vibration analysis of FJF joints with free end boundary conditions, including the natural frequency responses with all three modes as illustrated in the table 2. Due to the GNPs' uniform distribution within the adhesive and created coarser surface within the bond line region, entire natural frequencies have been increased.This resulted in a strong interaction between the adherends and the adhesive.As compared to neat FJF and GNP enhanced adhesive, 2% volume of Z-pin based specimens have more natural frequency, due to the strengthened junction allowed the lowest possible level of tension to occur within the adhesive and adherend while carrying the highest load.
The co-cured specimens have the better interaction within the bonding area as compared to co-bonding and secondary-bonding, because of cured adhesive are fragile in nature and leads to crack initiation.The multi-scale reinforced specimens have the highest natural frequency as compared to all other configurations of the samples.This variation is due to GNP's creating a rough surface on the bonding area and the Z-pins creating the interlocking effect among the laminates.This multiscale reinforcement provides the continuity of the fibres in the laminates, hence the overall natural frequency of the specimens are high in this configuration.

Conclusion
The current investigation of the work shows that reinforcing FJF joints with GNPs, carbon fibre pins (Z-pins), and multi-scale greatly improves their shear characteristics, toughness, and natural frequency of the FJF joints.Results demonstrated that, as compared to neat FJF joint specimens, GNP addition as well as pin-enhanced cocured FJF joints were more resistant to failure under tensile loading and improved the load-carrying ability and strength of the joints by 77.37%, 54.06%, and 46.06%, respectively, compared to neat FJF co-cured joints.Multireinforcing of 0.75 wt% GNP and 2 vol% pins improved co-cured FJF joint stiffness by 15.12% and 21.43%, respectively, as compared to other configurations of joints and different bonding techniques like co-bonding and secondary-bonding, relating to distinct reinforcement.The FESEM results disclose that the presence of GNP in the adhesive created a coarser surface and improved the interaction among the laminates.Multi-scale reinforcing has the ability to build the bridging between the joints.Free vibrational analysis showed that the multi-scale reinforced with co-curing technique had the highest natural frequency (685.1 Hz) as compared to all other manufacturing methods and configurations.

Figure 5 .
Figure 5. Schematic representation of the free vibrational analysis.

Table 1 .
Tensile and shear properties of FJF joints.

Table 2 .
Free vibrational characteristics of the FJF specimens.