Composite Laminates Research Articles

Influence of a hydrogen/oxygen flame on the fire-behaviour and the tensile properties of hybrid Carbon Glass fibers reinforced PEEK composite laminates

This study investigates the residual tensile behaviour of hybrid Carbon Glass fibers reinforced thermoplastic PEEK laminates after they were exposed for 5 min to a hydrogen/oxygen flame. This flame results in a severe thermal aggression characterized by a wall temperature ranging from 900 to 1270 °C and with different heat fluxes (from 200 to 800 kW/m2). The thermally-induced damages were examined by means of microscopic observations and micro CT analyses. The results show that the mass loss linearly depends on the measured heat flux for a 5 min exposure. Depending on the fire testing conditions, the mechanical properties in tension (stiffness and strength) are totally degraded after exposure to the highest heat fluxes (600 and 800 kW/m2) but the retention of the tensile properties is moderate (about −35 to −60% decrease in strength and stiffness, respectively) after exposure to a 200 kW/m2 heat flux. The residual tensile properties of CG/PEEK laminates follow master curves representing the correlations between the mass loss and the changes in the tensile properties regardless the heat flux. These master curves provide a relevant design rule for composite parts to be used under critical service conditions (H2/O2 flame exposure).

Experimental investigation of buckling and post-buckling behaviour of repaired composite laminates under in-plane shear loading

The performance of Ramped Scarf (RS) and Stepped Scarf (SS) repair schemes for highly loaded composite structures has been extensively studied under compressive, tensile, and flexural loadings. However, there is a lack of research on the buckling and post-buckling behaviour of RS and SS repairs under in-plane shear loading, which is critical for aerostructures. This paper addresses this gap by investigating the buckling and post-buckling performance of these repair schemes. Pristine (P) laminates, RS repairs with a scarf angle of 3°, and SS repairs with an overlap step length of 1/60 were manufactured and tested. The quality of the scarf repairs was inspected using artificial intelligence-based machine vision. Mechanical in-plane shear testing was conducted to evaluate the buckling and post-buckling behaviour of P, RS, and SS laminates. The experimental results indicate that RS repairs demonstrated an average of 35 % higher maximum displacement and 12 % higher failure load compared to pristine laminates. SS repairs showed 19 % higher maximum displacement, and 5 % higher failure load compared to pristine laminates. RS repairs excelled in all key mechanical performance indicators, including buckling load, Hooke's stiffness, maximum displacement, and failure load, compared to SS repairs. The failure modes of P, RS, and SS laminates were similar. However, RS repairs exhibited greater susceptibility to repair patch detachment compared to SS repairs.

Damage investigation of hybrid flax-glass/epoxy composites subjected to impact fatigue under water ageing

The aim of this study is to investigate the fatigue behaviour of hybrid flax-glass/epoxy composites under repeated impact loading subsequent to water ageing. Different plates of these composite materials were fabricated using the vacuum infusion technique. Five stacking sequences were considered: [F8], [G/F3]S, [G2/F2]S, [G3/F]S, and [G8], where F and G stand for flax/epoxy and glass/epoxy plies, respectively. Water ageing was conducted by immersing the composite specimens in tap water at room temperature for various durations, and until saturation was reached. Fatigue impact tests were carried out using three impact energies: 3, 4, and 5 J. An advanced high-resolution camera was used to monitor the evolution of damage mechanisms occurring on the non-impacted surfaces, while a laser thermometer was considered to track the temperature variations within each composite specimen. The obtained results show that flax-glass hybridization reduces the mass of absorbed water in flax/epoxy composite by up to 70%. Furthermore, there is a more pronounced decrease in longitudinal modulus and maximum stress in aged composites, with reductions of up to 70% compared to unaged ones. Additionally, visible damage occurs even at low energy levels, manifesting from the initial impacts in both aged and unaged composite laminates. Moreover, a correlation between the number of impacts to failure and the cumulative energy is revealed. Ultimately, water aging reduces the strength of the studied composite laminates and their resistance to impact fatigue. Furthermore, the hybrid laminates with high proportion of flax layers are particularly susceptible to water ageing effects.

Mechanical characteristics of kenaf-glass fiber reinforced hybrid composites by varying the stacking sequences

Fiber composite materials are preferred for their lightweight, low-cost, and commercial uses. As part of this study, laminate materials consisting of two different fiber materials as their reinforcement materials are produced using the hand layup method. This study investigates the mechanical properties of hybrid composite laminates fabricated using kenaf and glass fibers. Six stacking arrangements of the fibers are examined, alongside two reference laminates with individual reinforcements. Epoxy resins HY951 and LY556 serve as matrix materials. ASTM standards guide the mechanical testing of the composites. Results indicate varied tensile strengths based on stacking sequence, with laminate L2 (KKGKK) featuring a single glass fiber core at 75 MPa, and increasing strengths in laminates with additional glass layers: L1 (GKKKG) at 123 MPa, L5 (KGKGK) at 110 MPa, L3 (GKGKG) at 150 MPa, L6 (KGGGK) at 118 MPa, and L4 (GGKGG) at 159 MPa, the highest among all. It was observed that adding one layer of glass fiber with kenaf fiber boosts strength and modulus by 9.52% and 12.19% respectively, compared to pure kenaf fiber composites. Morphological analysis via Scanning Electron Microscopy (SEM) confirms failure due to initial crack propagation in the matrix and fibers. This study offers insights into hybrid composite laminate behavior, pertinent for various industrial applications.

Advancing structural health monitoring: Deep learning-enhanced quantitative analysis of damage in composite laminates using surface strain field

Composite materials have been widely used as critical components in aerospace applications due to their excellent performance characteristics. The real-time accurate identification and quantification of various types of damage within composite material structures pose a significant challenge. This study introduces an innovative damage detection method based on strain fields, which centrally employs deep learning techniques. Utilizing the Res-Mask R–CNN, this study accurately detects and categorizes various forms of damage within composite laminates, including open holes, subsurface holes, and delamination. Moreover, this method also enables precise localization and quantification of damaged areas. A series of experiments and simulations have validated the accuracy and robustness of the network model. Damage inversion experiments demonstrate that the area error of the damaged regions has been reduced to 7.4 %, and the positional error does not exceed 3.31 mm. In simulated scenarios, the shape context distance for complex damage contours does not exceed 0.21, indicating that the critical geometric features of the damage have been successfully preserved. This study provides an effective new approach for damage detection and real-time structural health monitoring of composite laminates.

Fatigue Characteristics Analysis of Carbon Fiber Laminates with Multiple Initial Cracks

In the entire wind turbine system, the blade acts as the central load-bearing element, with its stability and reliability being essential for the safe and effective operation of the wind power unit. Carbon fiber, known for its high strength-to-weight ratio, high modulus, and lightweight characteristics, is extensively utilized in blade manufacturing due to its superior attributes. Despite these advantages, carbon fiber composites are frequently subjected to cyclic loading, which often results in fatigue issues. The presence of internal manufacturing defects further intensifies these fatigue challenges. Considering this, the current study focuses on carbon fiber composites with multiple pre-existing cracks, conducting both static and fatigue experiments by varying the crack length, the angle between cracks, and the distance among them to understand their influence on the fatigue life under various conditions. Furthermore, this study leverages the advantages of Paris theory combined with the Extended Finite Element Method (XFEM) to simulate cracks of arbitrary shapes, introducing a fatigue simulation method for carbon fiber composite laminates with multiple cracks to analyze their fatigue characteristics. Concurrently, the Particle Swarm Optimization (PSO) algorithm is employed to determine the optimal weight configuration, and the Backpropagation neural network (BP) is used to train and adjust the weights and thresholds to minimize network errors. Building on this foundation, a surrogate model for predicting the fatigue life of carbon fiber composite laminates with multiple cracks under conditions of physical parameter uncertainty has been constructed, achieving modeling and assessment of fatigue reliability. This research offers theoretical insights and methodological guidance for the utilization of carbon fiber-reinforced composites in wind turbine blade applications.

Periodic free vibrations of composite laminates with curvilinear fibres and CNTs

This article addresses the combined effect of using curvilinear fibres and carbon nanotubes (CNTs) reinforcements in the non-linear modes of vibration of laminated composite plates. To arrive at the material properties of the three-phase composite material, a two-step hierarchic procedure is followed. A modified version of the Halpin–Tsai model is employed to predict the Young’s modulus of the CNT enriched resin and expressions, deduced from equilibria of a unit cell where a fibre is embedded in resin, are applied to obtain the diverse elasticity moduli of the three-phase composite. Moderately large displacements are considered, with von Kármán strain–displacement relations. Although the presented model is an equivalent single layer one, it applies to thick plates, because a Third-order Shear Deformation Theory (TSDT) is followed. The set of autonomous non-linear equations of motion is reduced using static condensation and a modal basis with selected modes, chosen after a convergence analysis. The reduced set of equations of motion is solved by the shooting method. Numerical tests considering plates with diverse curvilinear fibre paths, CNT contents and thicknesses are carried out. The results obtained are thoroughly analysed.

Normalization and Processing of Rotational Eddy Current Scans for Layup Characterization of CFRP Laminates

ABSTRACT The verification of fiber orientation and layup in carbon fiber-reinforced polymer (CFRP) composite laminates is essential to guarantee the performance of the material. This work reports the background, methodology, and results of a rotational eddy current testing (ECT) system for determining fiber orientation and ply layup of unidirectional CFRP components. The system presented mechanically rotates a 15 MHz directional transmit-receive ECT probe, which poses several speed and consistency advantages. The paper presents a theoretical discussion of how these scans are produced, a unique preprocessing normalization method, and an optimization process for determining the layup from the captured datasets. Results show a high level of accuracy for identifying the unique directions present in a laminate, and the theory-supported model enables the extraction of layup order information of multilayer parts, even where repeated lamina are present within the laminate.

Enhanced mean-field modelling for impact response of composite laminates incorporating strain rate-dependent matrix behaviour and 3D failure criteria

Enhanced mean-field modelling for impact response of composite laminates incorporating strain rate-dependent matrix behaviour and 3D failure criteria

An assessment of the composite laminate veneer videos on YouTube

An assessment of the composite laminate veneer videos on YouTube

Automatic modeling and optimization of tapered laminates with ply drops

Ply-drop (PD) is the termination of specific plies for laminated composite structures to obtain continuous thickness changes. It brings flexibility to the design of tapered composite laminates. However, as a structural defect, ply drops could have an impact on performance. Considering the impact of ply drop during stacking sequence design can provide more accurate performance analysis, but this will bring challenges in modeling and optimization. To consider the PD impact and achieve convenience in optimization, this paper proposes a high-fidelity finite element automatic modelling method of tapered laminates and corresponding optimization framework. By parameterizing the PD information and defining the basic elements and nodes of start stacking surface of the structure, the entire finite element model is layer-wisely constructed and controllable. Subsequently, based on the genetic algorithm framework, a repair strategy and its genetic operations are proposed to ensure that the design variables satisfy the ply-drop design guidelines. And a detailed optimization process is provided. Finally, the strength and deflection performance optimization problem of a tapered laminate with PD from 28 layers to 16 layers under three-point bending test is introduced for illustration of the proposed automatic modeling and optimization method. Comparisons between simulation results and experimental data of the obtained optimization solution verify the effectiveness of the proposed modeling and optimization method.

Damage detection of composite laminates based on deep learnings

Damage detection of composite laminates based on deep learnings

Damage monitoring of glass/epoxy-curved laminates with different stacking sequences using acoustic emission

Delamination is a major concern in the curvature area of a composite laminate due to the flexural stresses occurring between the laminates. This research focuses on the respective merits of different stacking sequences obtained by layering the glass fiber reinforcement at various orientations in an epoxy resin to manufacture a curved laminate. In particular, laminates with stacking sequences, such as Unidirectional [0°]24, Cross-ply [0°/90°]6S, Angle ply [±45°]6S, and Quasi-isotropic laminates [0°/±45°/90°]3S, were tested by performing four-point bending tests. Acoustic emission (AE) monitoring and experimental analysis characterized damage modes, including delamination. AE parameters, such as amplitude, energy, duration, cumulative count, and peak frequency, were utilized to characterize the damage mechanisms of the various stacking sequences. Experimental AE data confirmed that fiber orientation influences delamination resistance and failure mechanisms. Furthermore, compared to other layup sequences, the peak load of cross-ply (Layup B) was found to be greater by 48.7%, 32.87%, and 50.14%. Cross-ply curved laminates also have higher interlaminar tensile strength and curved beam strength than other sequences. They also failed less often due to delamination than other sequences. Finally, scanning electron microscope characterization verified and validated all damage processes found in the AE data collection. As a result, these findings play a significant role in predicting the lifetime, delamination failure, and strength of curved composite laminates.

Investigation of a secondary bonded pultrusion composite laminate containing a ply-drop, Part 2: A novel two-dimensional hybrid fracture discrete element for producing debond failure

This is the second of two papers in which a novel numerical method to predict the debond failure of a secondary bonded pultrusion laminate is presented. In Part 1 of this study, experimental work was described which is used here in the development, calibration and validation of the numerical model. The structure investigated in this work may be represented by a bonded composite laminate with external ply-drops (PDs).In this part of the study, a fully coupled mixed mode cohesive zone model is developed which relies upon a traction-separation relation and the virtual crack closure technique to obtain the mode mixity. These are used to develop a hybrid fracture discrete element (HFDE). Fracture toughness tests, as well as tests on PD specimens are used to calibrate the model. Based on these tests, run-arrest fracture criteria are defined. The model is validated by comparing test results for two other PD specimen types to those obtained with the HFDE and finite element analyses.

Effect of ply misalignment on the notched strength of composite laminates

Predicting the notched strength of carbon fiber-reinforced polymers is a crucial aspect of composite structure design, particularly when considering the uncertainties stemming from geometric features, material variability and defects. This study focuses on the influence of ply misalignment at the meso-scale level. The research employs a comprehensive methodology to establish notched strength allowables, integrating analytical (low fidelity), which have limitations in the representation of stacking sequence effects and in the generation of ply misalignments, and computational tools (high fidelity), employing a finite element model (FEM). The investigation emphasizes the need for a holistic understanding of these factors to enhance the accuracy of predictions. The results predicted by the analytical and, specially, the numerical models are in good agreement with experimental results. Both models underscore the decrease of the notched strength due to ply misalignment. Finally, a hybrid approach is proposed given that FEM predictions offer more accuracy and a detailed comprehension of the failure mechanisms, while fast analytical models can be used to propagate the uncertainties and to determine the allowables.

Failure behavior study of repaired bismaleimide resin matrix composite laminates with considering repairing process

Failure behavior study of repaired bismaleimide resin matrix composite laminates with considering repairing process

A stacking sequence optimization strategy for process‐induced deformation of multi‐layer thick asymmetric laminates

AbstractThe occurrence of process‐induced deformations of composites laminates is challenging for assembly accuracy and may lead to a service life reduction of parts. However, it can be obviously mitigated through different optimization strategies on the basis of the accurate curing process simulation. In this study, a stacking sequence optimization strategy is proposed and applied to multi‐layer thick asymmetric laminates. The shapes of deformed laminate plates are experimentally investigated in virtue of the three‐dimensional coordinate measuring machine. The thermo‐chemical–mechanical behaviors of plates are first verified through the comparisons of model predicted and experimental process‐induced deformations. Then the nonlinear control formula achieved through the regression model is proposed for the direct relationship between stacking sequences and process‐induced deformations. Finally, the required solutions are generated by solving the control formula. With the comparisons between the average deformations before and after optimizations, it is found that the magnitudes of deformations are significantly reduced, especially when the unoptimizated deformations are large.Highlights The thermo‐chemical–mechanical behavior can be described in the curing process simulation model. The deformed shapes of multi‐layer thick asymmetric laminate plates are measured by a three‐dimensional coordinate measuring machine. The stacking sequence optimization strategy is implemented by introducing a nonlinear control formula.

Analyzing the back-face deformation of curved UHMWPE composite laminate under high-speed impact

Ballistic protection extensively employs curved ultra-high-molecular-weight polyethylene (UHMWPE) laminates to conform to protective targets. However, ballistic tests have indicated that the curvature of laminates increases back-face deformation, diminishing ballistic performance, while the mechanism behind this curvature effect on back-face deformation remains unclear. In this paper, the back-face deformation of curved UHMWPE laminates, including apex displacement and the boundary of the deformation region, are systematically studied through numerical simulation and theoretical analysis. Firstly, a numerical model of curved UHMWPE laminates under the high-speed impact is established. The numerical results indicate that as the curvature increases, the deformation region becomes more concentrated, resulting in a larger apex displacement. Secondly, as the curvature increases from zero, the deformation mode of curved laminates changes from membrane stretching dominated to a combination of membrane stretching and bending. Finally, considering the change in the deformation mode, a theoretical analysis for the propagation of bending waves in an orthotropic curved plate is conducted to reveal the relationship between curvature and back-face deformation. The theoretical analysis shows that increasing curvature slows bending wave speed, reducing in-plane deformation region movement, thus increasing apex displacement. This study is expected to help design curved UHMWPE laminates with better ballistic performance.

Low‐velocity impact resistance of the Z‐pin reinforced carbon fiber composite laminates

AbstractA voronoi user material subroutine (VUMAT) was developed using the three‐dimensional Hashin damage criterion and exponential nonlinear damage evolution method. An interlayer damage model based on the quadratic nominal stress (QUADS) criterion and B‐K fracture criterion was introduced, and a finite element model of Z‐pin reinforced composite laminates under low‐velocity impact was established. The low‐velocity impact behavior of Z‐pin reinforced composite laminates with different impact velocities (0.6 m/s, 0.4 m/s, and 0.3 m/s), different layup forms ([0°/90°]4 and [0°/45°/90°/−45°]2), and different Z‐pin spacing (4 mm, 8 mm, and 16 mm) was studied using ABAQUS. The results indicate that different layup forms have little effect on the low‐velocity impact behavior of Z‐pin reinforced composite laminates. The Z‐pin spacing has a significant influence on the low‐velocity impact behavior of Z‐pin reinforced composite laminates. When the impact velocity is 0.4 m/s, the specific energy absorption of composite laminates with Z‐pin spacing of 16 mm is 85.93% and 87.7% lower than that of composite laminates with Z‐pin spacing of 4 mm and 8 mm. As the Z‐pin spacing decreases (Z‐pin density increases), the impact resistance of Z‐pin reinforced composite laminates first increases and then decreases.Highlights• The low‐velocity impact of Z‐pin reinforced composite laminates was studied.• Analyzed the effect of layup forms of laminates on their impact behavior.• Explored the influence mechanism of Z‐pin spacing on the impact behavior.• Studied the energy absorption of different Z‐pin spacing under impact velocity.

Acoustic emission and multiscale computation-guided tensile damage identification in woven composite laminates at cryogenic temperatures as low as 20 K

For laminated composite structures as key components of storage tanks serving at cryogenic temperature, it is crucial to identify the damage mechanisms for evaluating their mechanical properties and guiding structural design. In this work, the cryogenic tensile damage behavior of a thin-walled woven composite laminate was investigated through the acoustic emission (AE) monitoring and multiscale finite element (FE) computation at typical low temperatures of 153 K, 77 K and 20 K. We first established temperature-dependent constitutive laws for the microscale and mesoscale constituents of such composites based on experimental data, followed by the development of a hierarchical computational framework for modeling multiscale damage characteristics at different low temperatures. A fiber-optic acoustic emission measurement system was constructed to provide online monitoring of tensile damage of the woven composite laminates at cryogenic temperatures as low as 20 K. The comparations were made between the predicted cryogenic damage characteristics and in-situ AE signal analysis, assisted by scanning electron microscope (SEM) observations. The computed cryogenic damage evolution closely matched the AE signal identification results. The results indicate that fiber breakage and matrix cracking are the dominant cryogenic damage modes, and that the different low temperatures exert significant effects on the properties of the epoxy matrix, yarns and composite laminates. The combination of the AE monitoring system and the computational scheme provides a valuable tool for evaluating structural integrity and guiding the microstructural design of composite laminates used in cryogenic environments.