Laminate Lay-up Research Articles

Carbon contrast agents for terahertz spectroscopic NDT of impacted glass fibre reinforced plastics

Impact damage poses a significant challenge for composite materials since it is often subsurface and hard to detect, necessitating effective assessment approaches. Traditional NDT methods face challenges in identifying this specific type of damage. In this study, a departure from traditional NDT approaches was taken by utilizing THz spectroscopy to detect impact damage. Three different lay-ups of GFRP laminates were studied; GFRP with non-woven carbon fibre veil, GFRP coated with MWCNTs, and plain GFRP. Impact tests were conducted ranging from 5, 12 and 20 J in energy and 2D images were then acquired It was found that the inclusion of the MWCNTs and carbon veil gave superior image contrast compared to plain GFRP. Furthermore, the damaged area observed in the images increased with impact energy across all configurations. The outcomes derived from THz spectroscopy were benchmarked against those from ultrasound testing. THz spectroscopy demonstrates greater promise as a highly effective technique for detecting impact damage using carbon materials, offering advantages over traditional NDT methods due to its distinct wavelength, portability, non-contact capability, capacity to depict fibre alignment, lack of additional calibration specimens and coupling agents, and its ability to partially uncover the scope of impact damage.

Optimization of composite aeronautical components by Re-designing with double-double laminates

This paper introduces a novel approach to optimize aircraft structural design for enhanced performance and environmental sustainability. Traditional constraints on composite laminate stacking sequences, such as the symmetrical and balanced nature of classical quasi-isotropic laminates, often lead to over-dimensioning of aeronautical components. To overcome these limitations, the adoption of a new class of laminates known as Double-Double laminates, originally introduced by Prof. S.W. Tsai, is proposed. Unlike traditional laminates, Double-Double laminates offer flexibility in layer orientations, small building blocks, easy homogenization, and tapering capability.This study presents various case studies of increasing complexity to assess the effectiveness of the proposed methodology. First, a multicopter frame is redesigned to reveal the methodology's ability to reduce component weight while ensuring strength requirements under typical service loads. Then, the “DD Automated Design Tool”, a newly developed Finite Element Method tool interfacing with the Lamsearch engine, is introduced. This tool facilitates the rapid identification of optimal composite double-double laminate layups based on material properties, applied loads, and boundary conditions. By iteratively interacting between the Finite Element Method software and the Lamsearch engine, the tool efficiently identifies the best laminate angles to meet specific strength requirements.To validate the weight-saving capability of the proposed methodology, the optimizations on a composite fuselage section and a real composite wing-box under realistic loading conditions have been carried out. Both cases resulted in significant weight reductions compared to initial configurations, demonstrating the effectiveness of Double-Double laminates in achieving structural efficiency and meeting performance demands.

Characterization of relaxation behaviour of CF/PEKK aerospace composites using the time-temperature-crystallinity superposition principle

In this study, the Time-Temperature-Crystallinity Superposition Principle (TTCSP) was applied to determine the viscoelastic behavior of Thermo-rheological Complex Materials (TCM), specifically Carbon fibre/Poly-Ether-Ketone-Ketone (CF/PEKK) composites. The study investigated the effects of various parameters on the viscoelastic behavior of the composites, such as the degree of crystallinity after different melting temperatures, relaxation, and crystallization times. The TTCSP was utilized on the relaxation data to generate great-grand master curves for the degree of crystallinity for different laminate lay-ups. Hot press forming was employed to manufacture samples under different processing conditions, including various melting and cold crystallization temperatures. Differential Scanning Calorimetry (DSC) was employed to calculate the degree of crystallinity of CF/PEKK composites, while the Dynamic Mechanical Analyzer (DMA) was used to obtain the relaxation data. The generated great-grand master curves proved effective in predicting the relaxation behavior of the composites consolidated using single and double hold cycles at different melting temperatures and crystallization times, respectively. The great-grand master curves presented in this study can serve as valuable tool to calibrate key viscoelastic and/or thermo-viscoelastic material models for aerospace-grade CF/PEKK composites. These models are crucial for simulations aimed at predicting residual stresses and process-induced deformations during the thermoforming process.

Bolt-Hole Elongation of Woven Carbon-Epoxy Composite Plates and Joints Using the Digital Image Correlation Technique

The elongation of the bolt hole is an important parameter for assessing the failure of bolted joints. However, direct experimental measurement using strain gauges and extensometers is difficult. This article shows that digital image correlation (DIC) can overcome the difficulties and provide important indications of the failure mechanisms of bolted joints. Hole elongation was measured using DIC in the following carbon/epoxy composite configurations: standard open-hole tensile (OHT) and filled-hole tensile (FHT), single-lap shear only-bolted (OB), and single-lap shear hybrid-bolted/bonded (HBB) joints. For each configuration, the hole-elongation changes were tracked for cross-ply (CP) and quasi-isotropic (QI) stacking sequences with two thicknesses. In the tensile load direction for OHT and FHT cases, CP showed a greater hole elongation than QI. However, the opposite trend was observed in the transverse direction. In OB joints, bypass loads contributed more to the hole elongation than bearing action. In HBB joints, it has been observed that the adhesive significantly reduces hole elongation, particularly for CP configurations. Moreover, it was found that in HBB joints, hole elongation was independent of laminate lay-up, while it was very determinative in OB joints.

Improving the impact performance of natural fiber reinforced laminate through hybridization and layup design

There is growing interest in Natural Fiber Reinforced Polymer (NFRP) composites for structural components despite their lower mechanical performance than synthetic composites such as Glass Fiber Reinforced Polymer (GFRP) composite. This study demonstrates significant improvement in impact resistance of NFRP-GFRP hybrid laminates can be achieved through the integration of laminate layup design with interlaminar hybridization. By placing more GFRP plies near the top and bottom of the hybrid laminate while reducing the occurrence of neighbouring GFRP-GFRP plies and neighbouring NFRP-NFRP plies around the centre of the laminate, the impact resistance of the hybrid laminate can be significantly improved and even surpass GFRP laminates. On top of stacking sequence of plies, the impact resistance of the hybrid laminates can be further improved by introducing a bio-inspired helicoidal layup. The result shows that with 30 wt% GFRP plies and 70 wt% NFRP plies, the perforation energy of NFRP-GFRP hybrid helicoidal laminate outperforms both the plain GFRP and plain NFRP laminates by 11.7% and 143.8%, respectively, whereas the peak load of the hybrid helicoidal laminate subjected to impact is also comparable to the plain GFRP laminate.

Study on projectile impact resistance of carbon-glass hybrid bioinspired helical composite laminate

Mantis shrimp dactyl clubs have excellent impact resistance due to their special helical structure. Meanwhile, hybrid fibre has become increasingly popular to improve the mechanical properties of composite materials. Based on the helical structure of the periodic region of mantis shrimp dactyl clubs and hybrid fibres, in this study, a hybrid fibre helical lay-up laminate (HHL) is designed. The laminates are prepared using unidirectional carbon fibre prepregs and unidirectional glass fibre prepregs. To study the protective performance and damage process of HHL, a light gas gun impact test and numerical simulation calculations were carried out on the specimens. The harvested protective capability of the HHL specimen was then compared with those of carbon fibre unidirectional lay-up laminate (CUL), carbon fibre helical lay-up laminate (CHL), and hybrid fibre unidirectional lay-up laminate (HUL) specimens. The ballistic protection capability of HHL at different helical lay-up angles was also explored. The testing results show that HHL has the best ballistic protection performance. HHL’s ballistic limiting velocity is 228.5 m/s, which is 49.5%, 30.8%, and 19.9% higher than those of CUL (152.8 m/s), CHL (174.7 m/s), and HUL (190.5 m/s), respectively. Finally, hybrid fibres with a 90° interlaminar helix angle were found to have the highest ballistic limiting velocity. The reason is that the larger interlaminar angle allowed more fibres to be involved in the protection, increasing the energy loss of the projectile and enhancing the protection of the laminate.

Non-linear stability of isotropic stiffened composite channels under compression

In this study, compressed laminated channels with isotropic stiffeners on their free longitudinal edges are investigated. The use of edge stiffeners in the form of clamps or angles made of an isotropic material significantly affects the behaviour of the structures under study. Two cases of laminate layup are examined, assuming that the columns are freely supported. It is assumed that there are strong interactions between the eigenmodes. In addition to that, in the two analysed cases, the lowest eigenmode is local and does not cause structural failure. Thus, the expected ultimate load-carrying capacity values will be greater than the lowest eigenmode. The perturbation method and the finite element method are employed to solve an eigenproblem and a problem of nonlinear stability loss. The adequacy of the models and boundary conditions used in both methods is validated by comparing local, global, symmetric and antisymmetric as well as distortional eigenmodes. Postbuckling equilibrium paths and ultimate load-carrying capacity values are determined for all analysed cases. In the perturbation analysis, results obtained with the 4- and 5-mode approaches are compared for low pre-deflection amplitudes. Results obtained by all methods showed a very high qualitative agreement. The geometric imperfections used in numerical simulations were assumed to be identical to the eigenmodes with low amplitudes. The FEM results show that this has no effect on the postbuckling equilibrium paths and ultimate load-carrying capacity of the columns. A low amplitude value is enough to disturb the lowest eigenmode.

Investigating the impacts of processing variability on tool-part interaction for interply-toughened aerospace composites using a novel shear technique

This study aims to investigate uncertainties in tool-part interaction during manufacturing of advanced aerospace composites. To achieve this goal, a custom-built shear test was developed using a Dynamic Mechanical Analyzer (DMA) to directly characterize tool-part interfacial stresses during composites processing. This novel method was used to quantify tool-part stresses during processing of interply-toughened Toray T800S/3900-2B on a steel tool with various conditions, accounting for variables such as the number of release coats on the tool, cure pressure and temperature, laminate layup, and strain rate. The observed trends in results were correlated with tool and part surface microstructures, investigated by laser scanning digital microscopy. This study’s findings underscored that the number of release coats, cure pressure, and temperature significantly influence tool-part interaction due to altered surface conditions and viscoelastic behaviors at the tool-part interface.

Effect of hygrothermal aging on compression behavior of CFRP material with different layups

This study investigates the mechanical degradation mechanism and statistical analysis of residual compressive strength of three lay-up laminates under more than one-year seawater environment. Fick and Langmuir model were used to describe the moisture absorption process with a maximum moisture equilibrium content of 3.78% and various characterization methods were used to reveal the microstructural evolution of CFRP during aging. Experimental results demonstrate that the multiple fiber directions in the MD specimens provide additional moisture absorption channels, leading to a stronger wicking effect and a higher equilibrium moisture content. The layups can significantly affect the compression failure mode after long-term aging. The compressive strength of the cross-ply and multidirectional laminates decreased by 50% while that of unidirectional laminates decreased by 28%, and surface cracking is easily observed in the aged specimens. An empirical prediction model based on the Langmuir model and experimental data was proposed and verified by the data from the literature, with at least a 10% improvement in prediction accuracy. Meanwhile the three-parameter Weibull distribution was used to describe the residual compressive strength after hygrothermal aging and tested by Kolmogorov-Smirnov test. It is expected to provide insight for the application of CFRP in seawater environment.

Ultrasonic Attenuation of Carbon-Fiber Reinforced Composites

Ultrasonic attenuation measurements were conducted on cross-ply and quasi-isotropic lay-ups of eight types of carbon-fiber reinforced composites (CFRPs) using through-transmission methods with diffraction correction. Attenuation values were substantially higher than those of unidirectional composites and other structural materials. Wave modes, fiber distributions, matrix resins, and consolidation methods affected total attenuation. Transverse mode, quasi-isotropic lay-up, and polyimide and thermoplastic resins generally produced higher attenuation. No clear trends from the fiber distribution were revealed, indicating that it is not feasible presently to predict the attenuation of various lay-ups from the unidirectional values. That is, direct attenuation tests for different laminate lay-ups are needed. This work expanded the existing attenuation database by properly determining the attenuation coefficients of two additional layup types of CFRP laminates. Results showed the merit of ultrasonic attenuation measurements for quality control and structural health monitoring applications. A crucial benefit of the through-transmission methods is that they enable the prediction of Lamb wave attenuation in combination with software like Disperse (ver. 2.0.20a, Imperial College, London, UK, 2013).

Impact Testing and Modelling of Composite Laminate Panels for Use in Off-Road Racing Vehicle Belly Guards

Off-road racing vehicles require protection on the underside of their chassis in order to protect vital components from impact damage. The use of composites in thin laminate form to achieve this protection is widespread, although failure due to impact from foreign objects still occurs. The use of UHMWPE (Ultra High-Molecular Weight Polyethylene) fibres, which have superior mechanical properties to aramid fibres in vehicle belly guards, is not prevalent and, hence, could prove useful in this application. A comprehensive Finite Element Analysis (FEA) is performed in order to determine suitable laminate panel layups that can be tested, analysed, and compared to the original laminate layup, which comprises six layers of aramid and two layers of carbon fibre fabrics. This provides initial insight into the comparison of the new proposed laminates and reveals if improvements have been made. The laminates found using FEA are manufactured into panels that represent the fixture and loading cases seen in racing vehicles. Experimental testing is carried out on the various panels, and the results are compared to those of the mathematical modelling. Substituting the currently used carbon fibres with more aramid fibres increases the impact resistance of the panel. Using UHMWPE fibres greatly increases the impact resistance of the panel; however, fibre delamination becomes more prevalent. This is due to the poor fibre wettability of UHMWPE fibres and the large strain before failure of the fibres. The modelled results show good agreement with the experimental results in terms of the locations at which damage occurred.

Wing Optimization Design Based on Composite Global Hawk Unmanned Aerial Vehicle

Composite materials have become the approach to solve the high stiffness and light weight of unmanned aerial vehicle structures. The wing had an extremely important influence on the flight of the unmanned aerial vehicle. The optimal composite wing design aroused widespread attention since it enhanced the aerodynamic performance of unmanned aerial vehicles. This article was intended to optimize and design the unmanned aerial vehicle with advantages in aerodynamic performance. According to the parameters of the military-civilian integrated unmanned aerial vehicle, a three-dimensional model of the overall structure of the composite Global Hawk unmanned aerial vehicle was designed. The effect of composite materials on the wing and the optimization of the laminate layup structure were studied. XFLR5 software was utilized to analyze the aerodynamic performance of the wing. The original NACA0012 airfoil and the optimized NACA3412 airfoil of the composite Global Hawk unmanned aerial vehicle were analyzed respectively for structure and performance. XFLR5 software was utilized to conduct flight simulation under the set parameters, and the effect of changing the angle of attack on the wing performance was analyzed. The results demonstrated that the optimized wing outperformed the original wing in terms of the lift, drag, torque, and lift-drag ratio.

Model of electromagnetic interference shielding effectiveness for a multifunctional composite containing carbon-fiber-reinforced polymer and copper mesh layers

A multifunctional composite structure consisting of a carbon-fiber-reinforced polymer (CFRP) laminate with additional Cu mesh layers was investigated for use as an electromagnetic interference (EMI) shield in aerospace applications. The Cu mesh layers were added to increase EMI shielding effectiveness (SE) of the structure. A finite element model of EMI SE was constructed to predict SE in the radiofrequency to microwave range for different laminate lay-ups. It was found that the SE of the CFRP layers was improved by including plies with different relative fiber orientations. Further, the combination of CFRP layers, which primarily shield by absorption, and Cu mesh, which predominantly shields by reflection, provided adequate SE over a wider frequency range than the individual components alone due to the opposing trends of increasing absorption loss (CFRP component) and decreasing reflection loss (copper mesh) with increasing frequency.

Effect of low temperature on mode I and mode II interlaminar fracture toughness of CFRP-steel hybrid laminates

Delamination is the dominant failure type in fiber metal laminates (FML), particularly when combining carbon fiber reinforced plastics (CFRP) with steel. Their interface behavior is frequently studied using the double cantilever beam (DCB) and end-notched flexure (ENF) setup for mode I and mode II delamination, respectively. By nature, analysis of hybrid interfaces requires asymmetric laminate layups. Thus, thermal residual stresses (TRS) are acting on the interfaces. A framework for the correction of the apparent fracture toughness from experimental testing and for accurate numerical modeling is provided. The approach is validated using DCB and ENF test results at -55 °C and 23 °C. The results demonstrate the necessity of incorporating TRS in the analysis of asymmetric CFRP-steel FMLs, i.e. including a temperature step and using the true fracture toughness value as simulation input. Otherwise, delamination onset is mispredicted by up to 29 %.

Lay-up effect on the open-hole shear strength of composite laminates

The effect of lay-up on the open-hole shear (OHS) strength of quasi-isotropic carbon fibre/epoxy composite laminates was investigated. It was found that laminates with equivalent specific stiffness but different laminate lay-up, i.e. with different arrangements of plies and ply thicknesses, exhibit significant differences in ultimate OHS strength of up to 16.3% and 27.9% for the prepreg material systems IM7/8552 and T800/epoxy, respectively. Using Digital Image Correlation (DIC), X-ray Computed Tomography (CT), and Finite Element Analysis (FEA), it is shown that the lay-up dependent OHS strengths were associated with distinctly different failure modes. These lay-up dependent failure modes are challenging to predict. It is therefore proposed that the OHS test could be used for material screening, and as a new benchmark case for the development and validation of advanced progressive damage modelling frameworks for laminated composites, complementing the commonly used open-hole tensile (OHT) test.

Low velocity impact behavior of auxetic CFRP composite laminates with in-plane negative Poisson’s ratio

Introducing auxeticity or negative Poisson’s ratio is one potential solution to mitigate the low velocity impact damage of fiber reinforced polymer matrix composites, which can be achieved by tailoring the layup of an anisotropic composite laminate. This study aims to investigate the effect of laminate-level in-plane negative Poisson’s ratio on the low velocity impact behavior of carbon fiber reinforced polymer (CFRP) matrix composites using numerical simulations. The layups of the auxetic composites that allow them to produce negative Poisson’s ratios are identified based on the Classical Lamination Theory and verified through fundamental coupon-level experimental tests. To ensure meaningful comparisons, the non-auxetic counterpart composites are designed by allowing them to produce positive in-plane Poisson’s ratio while closely matching the longitudinal effective modulus of the auxetic laminate. The simulation results indicate that the auxetic laminates suffer smaller (12.6% on average) delamination area in top and bottom interfaces, much smaller (38% on average) matrix compressive damage in the top and bottom plies, and smaller (14.6% on average) fiber tensile damage area in each ply of the laminate at relatively higher impact energies (5 and 8 J).

A novel parallel method for layup optimization of composite structures with ply drop-offs

This paper presents a novel parallel optimization method for obtaining blended layups of composite laminates which closely match target lamination parameters. Firstly, a guide-based adaptive genetic algorithm (GAGA) which stochastically searches the layups is developed. Then the parallel optimization method DLBB-GAGA is developed by combining GAGA and a dummy layerwise branch and bound method (DLBB) which performs a logic-based search in a parallel computation, during which optimization information is shared between the two methods. The combination of these two different methods gives the parallel DLBB-GAGA method the advantages of both, enhancing the searching ability for the blended layup optimization. The superiority of the parallel DLBB-GAGA method is demonstrated through comparisons between the three methods, and it is concluded that the method is particularly effective for practical design where more layup design constraints are considered.

Nonlocal modeling of crack-interface interaction in a composite system

The mechanical response and the fracture phenomena in a composite system not only depend on the elastic and fracture properties of individual constituents but also on additional parameters such as fiber alignment, fiber volume fraction, interface properties and laminate layup. Fiber alignment is one such parameter that governs the design of the composite according to the purpose required. In the present work, a nonlocal approach combined with a cohesive zone model is proposed and implemented in finite element framework. The proposed model can capture different failure phenomena in a matrix- fiber system. The influence of various properties of fiber, matrix, and the fiber-matrix interface on the mechanical response of the composite system is studied. An exponential coupled cohesive zone law is considered for modeling fiber-matrix interface. Smeared representation of crack and interface are considered in the analysis. The results are validated with numerical simulations present in the literature.

Development of a unified design buckling curve for fibre reinforced polymer plates subjected to in-plane uniaxial and uniform compression

Presented are 24 non-dimensional buckling curves to estimate the strengths of Fibre-Reinforced Polymer (FRP) square plates having four simply supported edges and subjected to in-plane uniaxial and uniform in-plane compression. The curves are constructed by the authors from a parametric numerical analysis using ABAQUS® software with changing variables for: material properties; initial geometrical imperfections; laminate lay-ups; and plate thicknesses. These strength curves express relationships for the buckling reduction factor with plate slenderness, and account for post-buckling strength. We observe that regardless of the laminate lay-up (except for purely unidirectional), the choice of FRP material and the magnitude of the initial geometrical imperfection the predicted buckling reduction factors display a meaningful correlation with plate slenderness. Presented is a proposed unified buckling design curve, defined as the lower bound to 18 of the 24 ABAQUS®-generated buckling curves. This new curve is benchmarked by the authors against experimental test results extracted from the literature and it is found that there is a reasonable agreement. The authors recommend that the proposed buckling design curve has the potential to be introduced into structural design standards as a procedure to design the buckling strengths of FRP plates.

Parametric Investigation into the Shear Strength of Adhesively Bonded Single-Lap Joints.

In this paper, the shear strength of adhesively bonded single-lap joints were experimentally and numerically investigated. Based on the validated simulation, the effects of lap length, adhesive layer thickness, adhesive layer shape, adhesive layer overflow length, and laminate lay-up on the shear strength of adhesively bonded single-lap joints were studied. The load-displacement curves and shear strength under different parameters were compared. It was shown that the shear strength of single-lap joints gradually decreases with the increase of lap length and adhesive layer thickness, which were 53.83% and 16.15%, respectively. Considering the potential condition in fabrication, the adhesive layer shape and adhesive layer overflow length were also investigated. The adhesive with normal and triangle shape owned the comparable shear strength, which was higher than the arc one. The shear strength increased by 19.37% from 18.43 MPa to 22.00 MPa with increasing the adhesive layer overflow length to 50% of lap length. It was beneficial for shear strength to increase the adhesive layer overflow length to 50% of lap length. Among the selected four lay-ups, [0]16s had the highest shear strength, which was nearly 3 times greater than the one of [90]16s. In the real process preparation, increasing the number of 0° layers, selecting the appropriate lap length and thickness of the adhesive layer, and controlling the shape and length of the adhesive layer overflow are of great help to improve the tensile shear strength of the single-lap glue joint.