Composite materials in wind energy: design, manufacturing, operation, and end-of-life

Wind energy has consistently grown over recent decades and has grown in many aspects including in terms of installed capacities, turbine size, blade length, and grid penetration. Along with this, wind energy is one of the largest producers of composite structures, and as a result is one of the largest users of composite materials. For wind energy applications, composite materials require high reliability, low-cost, and near-term and future industry goals are to reuse or recycle composite materials. These requirements are quite challenging as wind energy faces a challenging operating environment, which puts great pressure on wind blade materials over their entire life cycle. This paper aims to examine the challenges of composite materials for wind energy applications, and to highlight a few research studies that offer potential new solutions and new insights across the entire life cycle of composites for wind energy systems ranging from the design phase, to manufacturing, to operation, and finally to the end-of-life phase.


Introduction
The growth in both wind turbine size and volume of wind turbine deployments has driven very high usage of composite materials, and as a result the wind energy industry has been the largest industrial consumer of composite materials worldwide [1] for many years.This growth in turbine and blade size over the past decades [2] has put greater pressure on technical requirements for materials and their associated manufacturing processes for wind turbine blades, and this has translated to pressures in all life-cycle phases encountered by blade designers, manufacturers, and wind farm owners and operators.
Wind turbine blades are a true composite of multiple materials including various fiber types, resins, core materials, adhesives, and protective coatings [3].Fiber reinforced composites in blades are based on glass fibers or carbon fibers, with various fiber orientations (uniaxial, biaxial, triaxial, etc.), and infused with different resins (epoxy, thermoplastics, etc.).Core types typically include balsa, PVC foam core, and PET foam core.All these materials are strategically placed by designers in the blade structure to meet strict and challenging structural requirements, while minimizing weight and cost [4]- [6].
As wind turbines and blade grow larger, a number of trends have emerged.With longer blades, manufacturing and logistical challenges have grown [7].To restrict blade mass and improve structural performance, greater use of carbon fiber has been seen in blades [8].Longer and more slender, flexible blades are also more susceptible to aero-elastic instability [9], [10].To explore new markets and to address these technical, logistical, and manufacturing challenges, several new turbine concepts have IOP Publishing doi:10.1088/1757-899X/1293/1/012002 2 been investigated (downwind [11], two-bladed [12], vertical axis wind turbines (VAWTs) [13]) and new material systems (pultrusion [8], among others) are being investigated.For operating machines, digitization has grown in the wind energy field including the use of digital twins [14]- [16] for operations.Further, new materials systems are also under investigation to specifically address end-oflife decisions and offer more recycling options for blade materials [17].
This paper aims to examine the challenges of composite materials for wind energy applications, and to highlight some studies that offer potential new solutions and new insights across the entire life cycle of composites for wind energy systems ranging from the design phase, to manufacturing, to operation, and finally to end-of-life (Fig. 1).
The outline of the paper is as follows.Section 2 presents an overview of materials selections for wind energy and examines material selection and design for a series of 100meter blade designs exploring different material choices.In Section 3, manufacturing is examined with a focus on resin uptake effects on the skin/core interface of blade materials explored via experimental and numerical methods.Operational aspects are the focus of Section 4 where materials issues related to digital twins are examined.Section 5 concludes the life cycle with a look at some end-of-life studies for more sustainable materials and manufacturing of recyclable blades by fusion joining.

Material Selection and Design Considerations
Composite wind turbine blades are designed to meet a wide range of structural requirements, in addition to constraints on cost, manufacturability, transportability, inspectability, etc.The structural requirements primarily include: 1) strength evaluation, 2) tower clearance (deflection evaluation), 3) fatigue evaluation, 4) buckling evaluation, and 5) assessment of blade dynamics for resonance and aero-elastic stability.These five primary structural requirements [6] are assessed in a design loop by structural designers [4], [5] in a typical blade design process until all structural requirements are met, and additional requirements on cost, manufacturability, and other factors are also met.
Meeting the structural requirements and manufacturability constraints in blade designs relies on selection of suitable candidate materials as well as strategic placement of these materials into the blade structure, which we now discuss.The inboard region of the blade, starting at the blade root and extending to roughly maximum chord (referred to as the root buildup) is mostly comprised of fiberglass laminates (e.g., triax).For blade outboard sections, from roughly maximum chord to the blade tip, a representative blade cross-section in shown in Fig. 2, where the principal areas of these outboard cross-sections are labeled: 1) spar caps (uniaxial laminates), 2) panels (sandwich composites of normally double bias and core materials), 3) shear webs (also sandwich composites), and 4) reinforcement of the leading edge and trailing edge (various laminates).A designer is thus faced with selection of materials and placement of those materials in layers into these various regions of the blade structure (root buildup, spar caps, core panels, shear webs, leading edge reinforcement, and trailing edge reinforcement, as well as the blade skins, adhesive bonds, and overlays).
The mainstream type of wind turbine in the market today is a three-bladed, upwind horizontal axis wind turbine (HAWT) configuration.However, several different, and to some extent new, turbine types are being investigated to either address the challenges of modern wind blades (e.g., technical, manufacturing, logistical, or others) or to develop machines tailored for new environments such as extreme-scale designs, shallow-water offshore, and design for floating offshore conditions.As a result, designers and developers are investigating a number of new turbine rotor types including two-bladed designs [12], downwind architectures [11], [18], and vertical axis rotors [13], [19], in contrast to the conventional three-bladed, upwind, horizontal axis rotor.Fig. 3 shows a comparison of these different rotor configurations, where from left to right are upwind, downwind, two-bladed, three-bladed, and then a vertical axis wind turbine rotor type.

SNL100 100-meter Blade Series: Materials and Manufacturing Outlook
A series of 100-meter blade designs for a 13.2 MW horizontal axis wind turbine were developed and made publicly available as reference models in the 2011-2014 timeframe [5], [20]- [22].The SNL100 blade series provided an opportunity to examine material choices for, at the time, future large blades; however, today blades of 100-meter length have become a reality.In this series of four 100-meter blades, first an all-glass baseline was designed [20], referred to as SNL100-00 (the baseline blade), then a change from glass to carbon spar was designed [21], referred to as SNL100-01 (carbon spar blade), next a new core material strategy was studied [22], referred to as SNL100-02 (core strategy blade) and in the fourth and final design, a new design incorporating the prior new materials plus new flatback airfoils was designed [5], referred to as SNL100-03 (final, optimized design).
Through this 100-meter blade study, several key design drivers for large blades were identified and documented including gravitational fatigue loading, weight growth, panel buckling, and aero-elastic stability.The mass of the baseline blade (SNL100-00) was 114 tons, while successive blade masses were 74 tons, 59 tons, and finally 49 tons, for the SNL100-01, SNL100-02, and final SNL100-03, blade designs, respectively, which demonstrated the mass and cost reduction benefits of new materials and progressively more innovative designs.
This SNL100 series study demonstrated the feasibility of large blade structures, and the potential to design large blades that meet structural requirements while being lightweight, manufacturable, and aero-elastically stable at 100-meter scale.Design drivers for large blades were identified, as briefly summarized above, however, more details on all the findings and recommendations for the SNL100 series can be found in [5], for the interested reader.
Regarding material challenges in large blades, the findings of the SNL100 series studies provided an early look at carbon spar caps in blades (SNL100-01, SNL100-02, and SNL100-03) and showed that they offer significant weight reduction (but with a cost penalty).In addition, a new core material strategy for large blades using advanced core materials (SNL100-02 study) was found to be advantageous as utilizing balsa in critical buckling areas and PET foam in the non-critical buckling areas can provide both weight reduction as well as a sustainable core material solution in that balsa core is regrowable and PET foam is recyclable.Further, the growing significance of gravitational fatigue loading in 100-meter scale blades pointed to greater need to reinforce the blade in the edgewise direction.

Manufacturing: experimental mechanics of skin/core interphase for blade materials
Sandwich composite is a composite material that is fabricated by attaching two thin and stiff skins to a lightweight and thick core with high thermal, and acoustic insulation, and a high strength-to-weight ratio.Due to the many advantages sandwich composites possess compared with other structural materials, this material was intensively used in the wind blades [23], [24].Skin/core interphase is always the first place of fracture initiated in the sandwich composites; it is necessary to fully understand the mechanism behind it.This section contains three subsections and focuses on the mechanism of skin/core interphase and guides researchers on how to design sandwich structures properly.

Nanoindentation measurement of core-skin interphase viscoelastic properties in a sandwich glass composite
3.1.1.Introduction.Low debonding toughness in the skin/core interphase region of the sandwich composites is always considered one of the shortcomings [25]- [27].Furthermore, it is an understanding of the mechanical properties of the skin/core interphase region in the nanoscale, which can illustrate how a low debonding toughness formed in the skin/core interphase [28]- [30].To address this issue, this work uses the viscoelastic-nanoindentation technique through the Berkovich nanoindenter tip to visualize Young's modulus spatial distribution at the interphase region.A glass skin/polyvinyl chloride (PVC) foam core sandwich composite was prepared by vacuum-assisted resin transfer molding (VARTM).Under a loading rate of 1 mN s−1, the time-averaged Young's modulus change from higher values (1.4 GPa) close to the glass skin region to the smaller values (0.8 GPa) close to the foam core was captured.This work proves the viscoelastic-nanoindentation technique is a sufficient and high-resolution way to benefit the analysis of the interphase, such as stress distribution calculations, which can eventually determine the bonding quality between skin and core and be beneficial to the design process of such sandwich composites.

Results and Discussion
. The fabrication procedure, glass skin/PVC foam core sandwich composite, and spatial distribution of stiffness in the skin/core interphase region are shown in Fig. 4. Details of the sample preparation, viscoelastic-nanoindentation technique, and viscoelastic properties of the skin/core interphase region can be found in the previous work [31].
The spatial distribution of the time-averaged Young's modulus on the indented area against its optical micrograph is shown in Figs.4(e) and 4(f).The viscoelastic analysis of nanoindentation data gives a better accuracy than Oliver and Pharr's approach owing to the nature of epoxy resin [32]- [34].Compared with the mechanical data sheet, the interphase region was softened by the PVC foam core instead of stiffened by the glass skin [35].The reason for the stiffness softening in the indented area is a relatively larger contact area between the cured epoxy and PVC cell walls than glass skin.Furthermore, the indented area is not close enough to the glass skin, and a semisphere void on the left edge of the indented area can be other reasons for stiffness softening in the interphase.Overall, a significant change in modulus across a short distance (240 μm) proves that nanoindentation is a highresolution approach compared with other mechanical characterization approaches for sandwich composites.

Conclusion.
The time-averaged Young's modulus was computed through the nanoindentationviscoelastic technique.The captured stiffness gradients through the skin/core interphase prove the effectiveness of this approach.A high correlation between the time-averaged Young's modulus mapping and the optical micrograph of the same indented area was obtained (softening due to imperfection and the larger contact area between PVC foam core and skin/core interphase).This work GPa suggested that strengthening the debonding toughness of the sandwich composites should target the foam core side of the skin/core interphase region.

3.2.
The effect of resin uptake on the flexural properties of compression molded sandwich composites 3.2.1.Introduction.A high stiffness-to-weight ratio is desired in composite structures [36]- [38] such as wind blades.Modifying the resin uptake of sandwich composites through compression molding (CM) is an easy and convenient way to maintain a high stiffness-to-weight ratio.This work investigates the flexural properties of sandwich composites made with glass skins/PVC foam cores (H60 and H80; smooth (PSC) and grooved and perforated (GPC)) at several resin uptakes.When the resin uptake of H80 GPC sandwich composites reduces by 11.0%, the specific flexural strength and modulus increase from 82.04 to 90.70 kN • m/kg and 6.03 to 7.13 MN • m/kg, respectively, which benefit most from resin uptake reduction comparing with the rest three types of core sandwich composites.Adequate ranges of resin uptakes (32% to 38% and 40% to 45%) are determined for the H80 PSC and GPC sandwich composites when reaching a high stiffness-to-weight ratio while preventing resin starvation, respectively.The findings suggested that applying the low-resin uptake within the appropriate range of sandwich composites in the wind blades leads to lighter blades and thus lower cyclic gravitational loads.

Results and Discussion
. The schematic of the cross-section view of the wind blade; images of glass skin/PVC foam sandwich composites, digital image correlation (DIC), and failure modes; the effect of resin uptake on flexural properties of sandwich composites are shown in Fig. 5. Details of the sample preparation, experimental techniques (like three-point bending, etc.), mechanical properties of the PVC foam core, and finite element modelling (FEM) of sandwich composites under three-point bending can be found in the previous work [39].Figure 5(a) shows a schematic diagram of sections of a wind turbine blade where sandwich composites are used (leading edge panel, trailing edge panel, and shear web) [3].Core molding conditions are shown in Fig. 5(b).The grooves and perforations facilitated a fast flow and a proper saturation of resin in the skin/core interphase and inside of the core, which secured a good bonding and high flexibility between the skin and core.The smooth core is chosen as the benchmark for specimens with low resin uptake considerations.Figure 5(c) proves the size of the grooves of the sandwich composite is inversely correlated to mold compression.Figure 5(d) reveals resin starvation in sandwich composites due to two reasons: 1. non-uniform spatial distribution; 2. the resin precipitate to the bottom mold owing to gravity.Figures 5(e)-5(h) show the flexural and specific flexural strength and modulus as a function of resin uptake for four types of core sandwich composites.Compared with glass skins, resin contributes much less to stiffness than weight gain.As a result, lower resin uptake is desired in the adequate resin uptake range to maximize the strength-to-weight ratio of the sandwich structure.A sharp discontinuity in the flexural strength and modulus of the H60 GPC core sandwich composites as a function of resin uptake curves is noted.This phenomenon is due to the cellular structure shrinkage of cores by a high mold compression concentration during the CM process.In Figs.5(i)-5(l), a high first principal strain concentrates on the bottom section of the core, and early structure failure (core shearing crack and bottom skin-core debonding) was observed in the H60 GPC sandwich composites.Furthermore, for the H80 PSC sandwich composites, the first principal strain is uniformly distributed on the core, leading to top skin crack and core crush failure.We can conclude that when the GPC core is used in the sandwich composite construction, a high core stiffness is needed to encounter the stress concentration issue.

Conclusion.
The effects of resin uptake on the flexural properties of sandwich composites were investigated under bending conditions.A proper resin uptake regime has been found for four types of core sandwich composites with a high stiffness-to-weight ratio and without resin starvation exists.The results reported in this work could be beneficial to the design of large wind turbine blades to reach a  ) and H80 PSC core (resin uptake: 39.66%) sandwich composite under DIC configurations at fracture initiation stage; (j) and (l) Failure modes of sandwich composites; core type: 1. H60 GPC core (core shearing crack and bottom skin-core debonding) and 2. H80 PSC core (top skin crack and core crush).

Bending and shear improvements in 3D
-printed core sandwich composites through modification of resin uptake in the skin/core interphase region 3.3.1.Introduction.Fused deposition modelling (FDM) printed polymers are rarely used as a structural material.As a result, a new manufacturing routine is needed to improve the incorporation of FDM-printed polymers in composite structures.In this work, glass skin/FDM printed polylactic-acid (PLA) core sandwich composites are prepared by the CM process, which provides a good manufacturing strategy for skin/core interphase modification.A significant improvement was found compared with the optimized resin uptake (optimized resin uptake range: 20.43%-22.86 wt%) 3Dprinted PLA core sandwich composite and lowest performance sandwich composite (Improvement: inplane shear strength (∼34%)/modulus (∼29%), out-of-plane shear strength (∼25%)/modulus (∼31%), specific peak bending load (∼19%)).The 3D-printed cores are suitable for replacing balsa cores in sandwich structures for many applications with a satisfactory strength-to-weight ratio.
IOP Publishing doi:10.1088/1757-899X/1293/1/0120028 3.3.2.Results and Discussion.Flexural properties of the 3D-printed beams, in-plane shear properties, DIC images, and failure mode of 3D-printed core sandwich composites are shown in Fig. 6.Details of the sample preparation, experimental techniques (like four-point bending, etc.), 3D-printing parameters, shear analysis of 3D-printed core sandwich composites, and comparison of published works about sandwich composites can be found in the previous work [40].A proper thermoplastic polymer should be chosen for the shear properties of sandwich composites investigation.In Figs.6(a) and 6(b), PLA materials stand out compared with the rest of the 3D-printed beams and most of the conventional core materials (except aluminum core) due to their high strength and stiffness-to-weight ratio, and cost-effectiveness.The in-plane shear strength and stiffness of balsa core sandwich composites are considered as 2.72 and 186 MPa with 0.152 g/cm 3 nominal core density [46], [47].The 3D-printed core sandwich composites have 2-3 times higher in-plane shear strength and similar in-plane shear stiffness than balsa core sandwich composites, and 3-3.5 times higher specific in-plane shear strength and similar specific in-plane shear stiffness than balsa core sandwich composites.Furthermore, the 3D-printed core could be more cost-effective because this core could save resin during manufacture compared with the balsa core.In Figs.6(g) and 6(h), a low resin uptake in sandwich composites could lead to strain concentration in the core due to an insufficient amount of resin in the skin/core interphase, and an adequate level of resin uptake could introduce a proper amount of resin to achieve a sufficient shear stress transfer from the skin to the core.From images of failure modes, we can conclude that the PLA core possesses a higher stiffness than the H60 PVC foam core.However, the PVC foam core can sustain a larger energy absorption due to better ductility.In general, the PVC foam core should be stiff enough to transfer the shear loads from one skin to the other skin.The resin uptake in the skin/core interphase of the 3D-printed core sandwich composites should be properly modified to prevent catastrophic structural failure.

Conclusion.
The shear properties of two types of core sandwich composites fabricated by the FDM/CM process were investigated by three-point bending and four-point bending tests and derived through the first-order shear deformation theory.In the 3D-printed core sandwich composites, the optimized resin uptake regime of PLA core sandwich composites is determined (20.43% to 22.86%).
Based on our investigation, the bending and shear properties of 3D-printed core sandwich composites are sensitive to resin uptake in the skin/core interphase due to a different chemical bonding mechanism from conventional core sandwich composites.For sustainability and recyclability of material selection, although balsa is a naturally grown material, balsa will be impregnated by resin owing to its open cell structure leads to a low recycling efficiency.Overall, a suitable strategy of skin/core interphase enhancement for 3D-printed core sandwich composites is needed to achieve a continuous shear stress flow between the skin and core.

Operation: digital twin models of blade and turbines
Accurate numerical tools (models or codes) are, of course, essential for wind turbine design, to simulate inflow and operating conditions, compute loads, evaluate structural/material designs, etc.However, these tools can also be quite useful for wind farm operations -namely for asset management to support decision-making throughout each turbine's lifetime [16], [48], [49].These numerical design tools can be used to develop a model that represents the physical asset (the operating wind turbine), and such a model is referred to as a "digital twin".Digital twin models of operating wind turbines offer one very promising technology for decision support in wind farm management.
Wind turbine aero-elastic models (such as OpenFAST) can be used to support decision-making in wind farm operations, which includes a wide range of possible use cases including [49]: • RCA (root cause analysis) of material damage identified during inspection, • life-extension studies for aging components to assess the remaining life of a component, • assessing the integrity of a structural repair, and • assessment of potential component upgrades, among many other uses.A digital twin model provides information and insight to support decision-making for "what-if scenarios" and uses cases, such as those listed above, for fleet operations and maintenance in a virtual setting.

Turbine-level digital twin models
Figure 7 illustrates the concept of a turbine-level digital twin model, where all major components are modeled including the tower, blades, drivetrain, etc.Such a model is multidisciplinary as aerodynamics, structures, structural dynamics, control systems, wind inflow conditions, etc. are all captured in a single model.Some recent research has focused on how to create the digital twin model based on technical specifications and wind turbine data streams [49].For example, aerodynamics models were developed in [49] for blades based on power curve measurements and coefficient of thrust data or specifications, while structural models of blades, for example, were created based on mass properties and blade bending frequencies.These Figure 7: Digital twin concept for a wind turbine structural models were developed in such a way as to represent the mass properties and structural dynamics properties of the actual wind turbine by modifying the material properties within the model.

Multi-fidelity digital twin models
While turbine level models are quite useful for decision support tasks such as loads analysis, and lifeextension studies, for tasks such as evaluation of the severity of structural damage or evaluation of structural integrity (or value of doing) a repair a high-fidelity model is needed.This need for highfidelity models within a digital twin framework motivated development of a multi-fidelity digital twin model [14], where at the turbine-level mid-to low-fidelity models are used to model the entire system (similar to the prior section) and at the component-level high-fidelity models are used for more detailed analysis.As an example, consider a multi-fidelity digital twin model for a wind turbine blade (as illustrated in Fig. 8).
The multi-fidelity digital twin model has the capability to address components at the material level, and in the case of a wind turbine blade, can address each material in the blade composite at the layer or constituent level.For a blade, the multifidelity approach starts with a high-fidelity design representation of the blade geometry and internal structure (composite layup) (e.g., using a blade modeling or CAD program such as NuMAD [50]).This highfidelity model can be analyzed in finite element programs to perform any number of high-fidelity analyses: static (strength and deflection), modal, buckling, fatigue, etc., to provide digital twin-based decision support.A low-fidelity model (such as a beam model) can be developed based on the highfidelity specification, and in this way both fidelities of the model are consistent in the multi-fidelity digital twin model.
The multi-fidelity digital twin method presented in [14] is based on following a blade throughout its lifetime starting with the design definition (design basis), gathering information in the manufacturing phase, further gathering data from lab-scale or proof testing, and also collecting data in the operational phase, all to further refine and update the model.This process of data gathering and model refinement for the Figure 8: Multi-fidelity digital twin of a wind blade [14] Figure 9: Process for creation of a multi-fidelity digital twin: Design basis, Manufacturing data, Lab testing data, and Operational data.[14] IOP Publishing doi:10.1088/1757-899X/1293/1/01200211 various stages is detailed in the flowchart of Fig. 9.
A case study of developing a multi-fidelity digital twin for a prototype-scale wind turbine blade (called, SUMR-D [6]) is presented in [14] that starts with a design-stage blade digital twin (with lowand high-fidelity models), then these models are updated with new data at it comes available during the manufacturing stage (based on any deviations in the as-built blade), then a post-manufacturing labscale structural testing phase where both static load-deflection and modal testing data are collected for model refinement, and ultimately then into service in the operational phase.As noted, this multifidelity structural model offers all the benefits of the low-fidelity turbine-level model, with the additional capabilities of the high-fidelity model, to improve certainty regarding the state of health of the wind turbine component, in this case the blade.In the case of the SUMR-D blade [6], [14], the development and validation of the multi-fidelity digital twin for the blade pre-operation (i.e., post labscale testing) provided an updated, and accurate model that was then used to design more accurate control systems and to improve safety in operating this wind turbine.

Fusion-joining of additive-manufactured PLA segments for recyclable and sustainable structures application: Introduction
The demand to involve thermoplastic polymers in engineering structures (like wind blades) has been increasing due to a growing issue of environmental pollution from composite wastes and the advantages of thermoplastic polymers (like segmentation, structural repair, fusion-joining, etc.) [51]- [53].Fusion-joining is a potential solution for three cases: 1. upscaling the 3D printed parts limited by the building volume; 2. structural repair to reduce labor cost; 3. on-site fabrication of segmented structure to reduce the transportation cost [54]- [57].However, each fusion-joining method possesses pros and cons and researchers still have not confined it to one universally accepted process for thermoplastic structures [58]- [60].
In this work, novel displacement-controlled resistance welding and hot plate welding processes were used.Three types of metal mesh, namely 30% (open area fraction)/ 0.11 mm (open size) Ni-Cu, 34%/0.07mm Ni-Cu, and 36%/0.25 mm Co-Ni, were used as the heating elements in the displacement-controlled resistance welding.A pair of heating molds under the shaft and socket shape were coupled with a heating pan to achieve a uniform heating region for two 3D-printed segments for the hot plate welding method.The optical micrographs and X-ray micro-computed tomography of the fusion-joined PLA slender beams show that there are no voids formed in the bonding region.After modification of the fusion-joining process, displacement-controlled resistance welding and hot plate welding can achieve 94% and 114 % welding strength efficiency (compared with the 3D-printed single continuous beams), respectively.The two processes possess a great potential to be dominant in fusion joining the thermoplastic polymers with fine surface finishing and good internal bonding quality to reduce plastic wastes, achieve easy and rapid structural repair and easy transportation, hence increasing sustainability, recyclability, and economic efficiency of a broad range of polymer and composite structures.

Results and Discussion
Manufacturing process; images of the optical micrograph, X-ray micro-computed tomography, failure modes; mechanical properties of the fusion-joining process (displacement-controlled resistance welding, hot plate welding) are shown in Fig. 10.Details of the sample preparation, experimental techniques (like three-point bending, etc.), 3D-printing parameters, analysis of the fusion-joining process, annealing, and comparison of published works about fusion-joining processes can be found in these literature [61], [62].Figures 10(a)-10(c) show the displacement-controlled resistance-welding and hot plate welding setup for the PLA samples.Micrographs and X-ray micro-computed tomography in the bonding region for displacement-controlled resistance welded and hot plate welded beams under different magnifications were investigated, respectively.No voids were detected, and a relatively good bonding was achieved.Based on the previous investigation, the failure mode of the slender beam possesses a strong relationship with the failure modes [63], [64].The rank of welding strength efficiency will correlate to the following failure modes: substrate failure > cohesive & substrate failure > adhesive & substrate failure > cohesive failure > adhesive failure.Figure 10(g) proves that a relatively good bonding quality is achieved for the displacement-controlled resistance welded beams with a 94% IOP Publishing doi:10.1088/1757-899X/1293/1/01200213 welding strength efficiency.In the case of the hot-plate-welded beams, no annealing was required to maximize the strength-to-weight ratio but to maximize the modulus-to-weight ratio of hot-platewelded beams.A 114% welding strength efficiency was achieved for the hot plate welded beams.

Conclusion
The bonding quality of displacement-controlled resistance-welded and hot plate welded slender PLA beams was investigated by three-point bending experiments.An investigation of the parameters of the resistance welding process showed that the power output, resistance welding time, initial compressive pressure, displacement rate in the displacement-controlled process, and total travelled displacement play a critical role in resistance welding.PLA is worthy to bring into civil engineering structures due to many advantages compared with a thermoset resin, especially in recyclability and processability.A few subjects remain to be studied in the future regarding the displacement-controlled resistancewelding process, especially on the economic efficiency when applied to a broader range of composite or 3D-printed structures.
As for the hot plate welding process, we proved that hot plate welding is a fast, high dimensional accuracy, and low polymer deterioration fusion-joining process for 3D-printed polymer structures through X-Ray micro-computed tomography.The effect of the annealing time on the flexural properties of the 3D-printed short beams was investigated using three-point bending experiments.In a constant-temperature annealing, 70 °C is an adequate temperature as it causes a relatively small shrinkage below 30 mins.However, annealing under 70 °C beyond 30 mins was not beneficial to the 3D-printed PLA structure owing to the high level of linear shrinkage.Furthermore, thermal buckling and edge wrapping can easily occur owing to a low stiffness-to-length ratio of the 3D-printed beams.The hot plate welding process remains uncertain regarding the economic efficiency during the engineering structure construction process.

Concluding Remarks
In summary, wind energy has consistently grown over recent decades and has grown in many aspects including in terms of installed capacities, turbine size, blade length, and grid penetration.As a result, the wind energy field is today, and will continue to be, one of the largest industrial users of composite materials.Thus, composite materials for wind energy systems requires a great deal of attention, and new research is needed across the entire life cycle from the design phase, to manufacturing, to operation, and finally to end-of-life.This paper examined the challenges of composite materials for wind energy applications, and highlighted recent research studies that offer new solutions and new insights across this life cycle of composite materials from design, to manufacturing, to operation, and to end-of-life.
Design: Composite wind turbine blades are designed to meet a wide range of structural requirements, in addition to constraints on cost, manufacturability, transportability, etc.Thus, material considerations in large blade designs were the focus of Section 2. Several key design drivers for large blades were identified, based on studies of a series of four, progressively designed, 100-meter blades (the SNL100 100-meter series), which include gravitational fatigue loading, weight growth, panel buckling, and aero-elastic stability.Regarding material challenges in large blades, the findings of the SNL100 series studies provided an early look at carbon spar caps in blades offering weight reduction with a cost penalty.Further, a new core material strategy was found to be advantageous for large blades where balsa was utilized in critical buckling areas and PET foam in the non-critical buckling areas, as a result this provides both weight reduction as well as a sustainable core material solution in that balsa core is regrowable and PET foam is recyclable.
Manufacturing: In Section 3, manufacturing is examined with a focus on the skin/core interphase of sandwich composites, which are used extensively in panels and shear webs of wind turbine blades.Three studies were presented, and in the first study, the nanoindentation technique was shown to be a sufficient and high-resolution technique to measure bonding quality in the skin/core interphase.In the second study, the effect of resin uptake in various PVC core perforation and groove types was examined.Here, optimal levels of resin uptake were identified to maximize mechanical properties while minimizing extra resin mass and cost.In the third study, bending and shear improvements in 3D-printed core sandwich composites were examined with focus on modification of resin uptake in the skin/core interphase region.Here, the potential for 3D-printed core to replace traditional core materials was shown based on strong mechanical performance of the sandwich composites, high recyclability, and potential for lower resin uptake and less resin usage with 3D-printed core.
Operation: Digital twins for blades and turbines were presented in Section 4, where the digital twins offer decision support for wind farm operations and maintenance, for example, to support RCA (root cause analysis) of material damage identified during inspection, life-extension studies for aging components to assess the remaining life of a component, or integrity of a structural repair.A novel multi-fidelity digital twin framework, encompassing high-and low-fidelity component models, was presented to improve certainty in structural analysis based on a digital twin with addition of highfidelity models.The multi-fidelity digital was demonstrated for a subscale 21-meter prototype blade following the blade from design, through manufacturing and lab-scale testing, and ultimately into operation.
End-of-Life: Section 5 concludes the life cycle with an examination of design for recycling based on fusion joining of additively-manufacturing PLA segments for wind blades.Here, two joining processes were investigated including resistance welding with metal implants and hot plate welding (no implants).The two joining processes both possess a great potential to be dominant in fusion joining thermoplastic polymers with fine surface finishing and good internal bonding quality while reducing plastic wastes and increasing sustainability, recyclability, and economic efficiency of a broad range of polymer and composite structures.

Figure 1 :Figure 2 :
Figure 1: Life cycle Loop for Wind Blade Composite Materials: Design, Manufacturing, Operation, and Endof-Life Phases

Figure 4 :
Figure 4: The fabrication procedure, glass skin/PVC foam core sandwich composite, and spatial distribution of stiffness in the skin/core interphase region: (a) schematic of the VARTM setup; (b) the cross-section of the glass skin/PVC foam sandwich composite; (c) a 5× optical micrograph of the skin/core interphase region of the sandwich composite; spatial distribution of the average Young's modulus in the interphase region: (d) a 5× optical micrograph of residual indents from 8×10 nanoindentations by Berkovich tip on the skin/core interphase region of the sandwich composite; (e) spatial distribution of time-averaged Young's modulus in the same area shown in (d); (f) timeaveraged Young's modulus at different interphase grid rows from nanoindentation measurements in the same area shown in (d).
.1088/1757-899X/1293/1/012002 7 higher stiffness-to-weight ratio and lower material cost than the current state-of-the-art fabrication routine by reducing resin consumption.

Figure 5 :
Figure 5: The schematic of the cross-section view of the wind blade; images of glass skin/PVC foam sandwich composites, DIC, and failure modes; the effect of resin uptake on flexural properties of sandwich composites: (a) cross-sectional view of the wind turbine blade (note: the red circle is indicating the sandwich composites zone.)(adapted from Griffith et al. [20]); (b) H80 PSC and GPC core with the same density (ρ = 0.08 g/cm 3 ) (from top to bottom: smooth core; grooved and perforated core top and bottom view); (c) a front view of the sandwich composites; (d) resin starved sandwich composites; (e)-(h) plots of flexural strengths and modulus; specific flexural strength and modulus as a function of resin uptake for the four types of core sandwich composites (note: units of specific flexural modulus: E/ρ and specific flexural strength: σ/ρ are kN • m/kg); (i) and (k) Three-point bending of a H60 GPC (resin uptake: 41.30%) and H80 PSC core (resin uptake: 39.66%) sandwich composite under DIC configurations at fracture initiation stage; (j) and (l) Failure modes of sandwich composites; core type: 1. H60 GPC core (core shearing crack and bottom skin-core debonding) and 2. H80 PSC core (top skin crack and core crush).

Figure 6 :
Figure 6: Flexural properties of the 3D-printed beams, in-plane shear properties, DIC images, and failure mode of 3D-printed core sandwich composites: (a) and (b) specific flexural properties as a function of the density of 3D-printed polymer beams, foam, and honeycomb structures [41]-[45], note: the red dashed line zone is indicating the conventional material, the blue dashed line zone is indicating the 3D-printed material; (c)-(f) in-plane shear and specific in-plane shear properties as a function of resin uptake of compression-molded sandwich composites; (g) and (h) first order principal strain distribution of the PLA lattice core sandwich composites ((g) 19.31% resin uptake, (h) 23.97% resin uptake, respectively) under four-point bending loads at fracture initiation stage; (i) and (j) failure modes of the PLA lattice and PVC foam core sandwich composites under four-point bending tests; note: (i) skin/core debonding and core shearing failure (PLA), (j) skin/core debonding, core shearing, and core crushing failure (PVC foam).

Figure 10 :
Figure 10: Manufacturing process; images of the optical micrograph, X-ray micro-computed tomography, failure modes; mechanical properties of the fusion-joining process (displacementcontrolled resistance welding, hot plate welding): (a) displacement-controlled resistance welding setup; (b) and (c) hot plate welding setup; (d) and (e) micrographs of the resistance-welding region; (f) X-ray micro-computed tomography of hot plate welded beam; (g) failure modes of displacementcontrolled resistance-welded 3D-printed beams under three-point bending: substrate failure (S): blue; cohesive & substrate failure (C&S): red; adhesive & substrate failure (A&S): black; cohesive failure (C): yellow; adhesive failure (A): green; (h) and (i) welding strength & modulus efficiency as a function of averaged power rate of the displacement-controlled resistance-welded PLA beams by three types of metal meshes; (j) and (k) flexural modulus and strength as a function of annealing time of three types of the hot plate welded beams.