Damage In Composite Laminates Research Articles

Detection of edge delamination in composite laminates using edge waves

Detecting near-edge damage in composite structural elements using guided wave-based techniques can be challenging, primarily due to the complexity of wave analysis arising from material anisotropy. Furthermore, scattered waves containing damage information can be contaminated by waves reflected from the edges, which makes detecting near-edge damage difficult to implement. In the literature, studies showed that in elastic materials, the edge of a structure can serve as a waveguide, enabling the existence of typical edge modes with concentrated energy at the edge. However, studies regarding edge waves in composite structures have received limited attention. This paper aims to explore the potential of detecting edge delamination damage in composite laminates using edge waves. The modal properties of edge waves in [(0/90)2]s composite laminates are investigated using the Semi-Analytical Finite Element (SAFE) method. Additionally, dispersion curves for quasi-isotropic composite laminates are calculated. Following this, numerical and experimental studies were conducted to investigate the sensitivity of edge waves in detecting edge delamination in the [(0/90)2]s composite laminates. The outcomes of this study offer physical insights into the modal properties of edge waves and confirm their effectiveness in detecting damage near the edges.

A novel strategical approach to mitigate low velocity impact damage in composite laminates

AbstractThis paper describes a new interleaving methodology to improve composite laminates' low velocity impact (LVI) damage resistance. The approach relies on analyzing interlaminar stress in a conventional aircraft laminate to determine the appropriate interleaving strategy. A model was built to identify the larges interlaminar stress, and two interleaving strategies were defined, normal and shear stress. Four thin interleaving veils were used to validate strategies. Non‐ and interleaved laminates were submitted to LVI tests at 13.5, 25 and 40 (J) of energy. Despite interleaved laminates revealed an increase in thickness and resin volume fraction compared to the reference, they were limited to 10% and 7.6% on shear stress interleaved layups. Interleaved laminates also demonstrated an improved external and internal LVI damage resistance, especially when shear stress interleaving strategy was adopted, preventing external damage and mitigating internal damage up to 59%. No correlation between the veil's type and impact damage demonstrates the importance of interleaving strategies over the veil's characteristics.Highlights Simplified quasi‐static elastic Abaqus model identified the largest in‐plane normal (axial and transversal) and shear interlaminar stresses; Four thin veils were used to validate interleaving strategies adopted; All laminates were produced using the same conditions to minimize process influence on experimental tests; Under low velocity impact (LVI) tests, interleaving laminates in the largest shear stress interlaminar can prevent external damage and mitigate internal damage up to 59%, comparing to the reference; No correlation between the veils and damage demonstrates the relevance of the interleaving strategy over the veil type.

A fragile points method with a numerical-flux-based interface debonding model to simulate the delamination migration in composite laminates

Predicting damage in composite laminates is essential for a better utilization of the material, but it is challenging especially when the inter- and intra-ply damages are coupled. In this paper, a Fragile Points Method (FPM) with a numerical-flux-based interface debonding model is proposed to simulate the laminates’ damage. The FPM is a novel meshless method using the discontinuous Galerkin weak formulations and the point-based discontinuous polynomial trial functions. In this work, an orthotropic interface debonding model has been developed based on the numerical fluxes to simulate the mixed-mode inter- and intra-ply damage. For any interior interface in an FPM model, the interface damage is triggered when the damage criterion is satisfied and the interface is separated when the fracture energy is reached. Due to the discontinuous characteristics of the FPM, the inter- and intra-ply damage modes are introduced explicitly, and thus the coupling between them can be naturally captured. In this paper, the proposed FPM is verified by four benchmark examples for Mode I, Mode II and Mixed Mode delamination. Then the proposed method is applied to simulate the delamination migration, used as an example for the inter- and intra-ply damage. The FPM results show that this method is able to provide reliable prediction on the load–displacement curves as well as the damage and crack patterns, and the mechanism governing the delamination migration is also discussed based on the FPM results.

On quantifying uncertainty in lightning strike damage of composite laminates: A hybrid stochastic framework of coupled transient thermal-electrical simulations

Lightning strike damage can severely affect the thermo-mechanical performance of composite laminates. It is essential to quantify the effect of lightning strikes considering the inevitable influence of material and geometric uncertainties for ensuring the operational safety of aircraft. This paper presents an efficient support vector machine (SVM)-based surrogate approach coupled with computationally intensive transient thermal-electrical finite element simulations to quantify the uncertainty in lightning strike damage. The uncertainty in epoxy matrix thermal damage and electrical responses of unprotected carbon/epoxy composite laminates is probabilistically quantified considering the stochasticity in temperature-dependent multi-physical material properties and ply orientations. Further, the SVM models are exploited for variance-based global sensitivity analysis to investigate the input parameters' relative influence on the lightning strike-induced damage behavior. Due to the adoption of a coupled SVM-based simulation approach here, it has become possible to carry out a comprehensive uncertainty quantification leading to complete probabilistic descriptions of the electrical and lightning damage parameters despite the requirement of performing a large number of computationally intensive function evaluations. The results reveal that source-uncertainty of the unprotected laminates significantly influences the epoxy matrix decomposition, electrical current density and electric potential, wherein longitudinal electrical conductivity is most sensitive to stochastic variations followed by other electrical, thermal and geometric parameters.

Multi-fidelity machine learning based uncertainty quantification of progressive damage in composite laminates through optimal data fusion

Recently machine learning (ML) based approaches have gained significant attention in dealing with computationally intensive analyses such as uncertainty quantification of composite laminates. However, high-fidelity ML model construction is computationally demanding for such high-dimensional problems due to the required large amount of high-fidelity training data. We propose to address this issue effectively through multi-fidelity ML based surrogates which can use a training dataset consisting of optimally distributed high- and low-fidelity simulations. For forming multi-fidelity surrogates of progressive damage in composite laminates, we combine low-fidelity finite element analysis data obtained using Matzenmiller damage model with Hasin failure criteria and high-fidelity finite element analysis data obtained using three-dimensional continuum damage mechanics based model with P Linde’s failure criteria. It is shown that there is a significant computational advantage to using the multi-fidelity surrogate approach as compared to conventional single-fidelity surrogates. Such computational advantage through optimal data fusion without compromising accuracy becomes crucial for the subsequent data-driven uncertainty quantification and sensitivity analysis of composites involving thousands of realizations. Ply orientations come out to be the most sensitive parameters to matrix damage, fibre damage and reaction force in composite laminates. The degree of uncertainty in the output quantities depend on the input-level stochastic variations. For example, a combined stochastic variation of ±10% in material properties and ±10° in ply orientations lead to 1.85%, 16.98% and 11.24% coefficient of variation in the matrix damage, fibre damage and reaction force respectively. In general, the numerical results obtained based on the efficient data-driven approach strongly suggest that source-uncertainty of composites significantly influences the progressive damage evolution and global mechanical behaviour, leading to the realization of the importance of adopting an inclusive analysis framework considering such inevitable random variabilities.

Bayesian parameter estimation for the inclusion of uncertainty in progressive damage simulation of composites

Despite gradual progress over the past decades, the simulation of progressive damage in composite laminates remains a challenging task, in part due to inherent uncertainties of material properties. This paper combines three computational methods — finite element analysis (FEA), machine learning and Markov Chain Monte Carlo — to estimate the probability density of FEA input parameters while accounting for the variation of mechanical properties. First, 15,000 FEA simulations of open-hole tension tests are carried out with randomly varying input parameters by applying continuum damage mechanics material models. This synthetically-generated data is then used to train and validate a neural network consisting of five hidden layers and 32 nodes per layer to develop a highly efficient surrogate model. With this surrogate model and the incorporation of statistical test data from experiments, the application of Markov Chain Monte Carlo algorithms enables Bayesian parameter estimation to learn the probability density of input parameters for the simulation of progressive damage evolution in fibre reinforced composites. This methodology is validated against various open-hole tension test geometries enabling the determination of virtual design allowables.

Towards assessment of fatigue damage in composite laminates using thermoelastic stress analysis

A new approach that utilizes Thermoelastic Stress Analysis (TSA) is proposed to investigate fatigue-induced material degradation in laminated fiber-reinforced polymer composites (FRP). The proposed model accounts for non-adiabatic conditions, the effects of the material temperature on the material properties, and the effects of stiffness material degradation due to damage. Experimental data from the literature is used to validate the part of the model that simulates the heat transfer, which results in a non-adiabatic contribution to the thermoelastic response. Specimens made from E-glass FRP representative of those used in wind turbine blade manufacture are used in the study, which make a challenging proposition for TSA. The evolution of tunneling cracks caused by cyclic loading causes stiffness degradation and changes in the thermoelastic response. The added features of the proposed model are shown to be necessary to interpret the thermoelastic response. The model improves correspondence with experimental data compared to previous TSA methods. Hence a generalized framework is proposed for incorporating the mechanisms that affect the thermoelastic response as materials degrade due to fatigue loading.

Quantitative monitoring of impact damage to composite structures using blade coated MXene sensors

Low velocity impacts usually cause invisible damage of composite structure，which is hard to be found and will cause catastrophic failure. In this work, the blade-coated MXene sensor was used to monitor impact damage of composite laminates. For the purpose of impact monitoring, an experiment was designed where composite laminates were subjected to controlled energy impacts. A circular sensor with a diameter of 100 mm was arranged on the back of the laminate. And ultrasonic C-scan was used to examine delamination damage of laminates after each impact. The experimental results show that the sensor identifies the onset of impact and the generation of damage. And the relative resistance changes of the sensor have a directly linear proportional relationship to the delamination size. There is also a functional relationship between the electrical response of the sensor and the impact energy. The findings are of great significance for the application of composite structure, and can effectively avoid the safety hazards caused by invisible impact delamination damage.

Cumulative Fatigue Damage of Composite Laminates: Engineering Rule and Life Prediction Aspect.

The analysis of cumulative fatigue damage is an important factor in predicting the life of composite elements and structures that are exposed to field load histories. A method for predicting the fatigue life of composite laminates under varying loads is suggested in this paper. A new theory of cumulative fatigue damage is introduced grounded on the Continuum Damage Mechanics approach that links the damage rate to cyclic loading through the damage function. A new damage function is examined with respect to hyperbolic isodamage curves and remaining life characteristics. The nonlinear damage accumulation rule that is presented in this study utilizes only one material property and overcomes the limitations of other rules while maintaining implementation simplicity. The benefits of the proposed model and its correlation with other relevant techniques are demonstrated, and a broad range of independent fatigue data from the literature is used for comparison to investigate its performance and validate its reliability.

A Bond-Based Peridynamic Model with Matrix Plasticity for Impact Damage Analysis of Composite Materials

The prediction of damage and failure to fiber-reinforced polymer composites in extreme environments, particularly when subjected to impact loading, is a crucial issue for the application and design of protective structures. In this paper, based on the prototype microelastic brittle (PMB) model and the LaRC05 composite materials failure model, we proposed a bond-based peridynamic (BB-PD) model with the introduction of plastic hardening of the resin matrix for fiber-reinforced polymer composites. The PD constitutive relationships of the matrix bond and interlayer bond under compressive loading are considered to include two stages of linear elasticity and plastic hardening, according to the stress-strain relationship of the resin matrix in the LaRC05 failure model. The proposed PD model is employed to simulate the damage behaviors of laminated composites subjected to impact loading. The corresponding ballistic impact tests of composite laminates were carried out to observe their damage behaviors. The PD prediction results are in good agreement with the ballistic experimental results, which can verify the correctness and accuracy of the PD model developed in this study in describing the impact damage behaviors of composite materials. In addition, the characteristics and degree of damage in composite laminates are analyzed and discussed based on this PD model. The difference in the impact resistance of composite laminates with different stacking sequences is also studied using the numerical simulation results.

Microscale modelling of lightning damage in fibre-reinforced composites

In this work, three-dimensional (3D) finite element simulations were undertaken to study the effects of lightning strikes on the microscale behaviour of continuous fibre-reinforced composite materials and to predict and understand complex lightning damage mechanisms. This approach is different from the conventional mesoscale or macroscale level of analysis, that predicts the overall lightning damage in composite laminates, thus providing better understanding of lightning-induced thermo-mechanical damage at a fundamental level. Micromechanical representative volume element (RVE) models of a UD composite laminate were created with circular carbon fibres randomly distributed in an epoxy matrix. The effects of various grounding conditions (one-, two-, and four-side grounding), fibre volume fractions ( V f = 55, 60%, and 65%), and peak current amplitudes (10, 20, and 40 kA) on microscopic damage in RVE models were characterised to understand fibre, epoxy matrix, and fibre-matrix interfacial damage associated with a lightning strike. Thermal damage, estimated based on epoxy matrix decomposition temperature, was largely constant up to 20 kA before increasing significantly at 40 kA. Severe thermal damage steadily decreased with increasing V f. Thermo-mechanical damage was predicted using a ductile plasticity model with Drucker–Prager yield criterion for epoxy matrix failure, and cohesive surfaces for fibre-matrix interface debonding. Thermal strain had the largest contribution to thermo-mechanical damage, while dynamic pressure loading was negligible in all RVE models. The RVE model proposed in this work, to the best of the authors’ knowledge, is the first model predicting lightning-induced fibre-matrix interfacial damage.

Rapid prediction of compression after impact properties of composite structures: An equivalent strategy for impact damage

Combined with the results of finite element simulation and experimental ultrasonic C-scan, a phenomenological-based equivalent impact damage finite element model was proposed and established to rapidly predict the damage evolution and buckling deformation of composite structures in compression after impact. An image recognition technology was adopted to divide the impact damage of composite laminates and stiffened panels into different regions, and a damage equivalent was conducted based on the measured impact damage size and morphology. Additionally, the material stiffness and strength parameters in different damage equivalent regions were reduced appropriately by different degrees. The impact pit deformation was equivalent to the element node offset in the corresponding area of the model, and the interface delamination and debonding were equivalent to the voids without elements. The equivalent impact damage model was applied to evaluate the buckling behavior and compressive strength after impact for laminates and stiffened panels using ABAQUS software. Moreover, a three-dimensional damage model considering the interaction of interlaminar delamination damage and interlaminar matrix and fiber damage was introduced to characterize the damage initiation, evolution, and extension behaviors of different damage modes. The comparative analysis indicated that the buckling and failure modes and compression after the impact strength simulated using the equivalent model were identical to the experimental results and had high correlation. This confirmed the accuracy, effectiveness, and applicability of the established equivalent impact damage model in predicting the compression buckling as well as residual strength and fracture modes for composite laminate and stiffened panel with simultaneous multi-zone impact damage.

Influence of neighbouring damage on delamination growth in multiple indented composites

To improve current design approaches for composite structures, it is required to further investigate the damage interaction effects of composite materials under multiple out-of-plane concentrated loads. It is first necessary to comprehend the dependence of the effective delamination threshold, which describes whether a delamination grows on pre-existing delamination damage in composite laminates. A combined experimental and numerical study is presented, in which two sequential out-of-plane quasi-static indentations are applied to fully clamped composite laminated panels, with changing distances between the two indentation locations changing. The results show that the second indentation delamination is more likely to propagate, particularly in the straight-line direction from the second indentation site to the first one, which can be interpreted as a decrease in the effective delamination threshold associated with microcracks ahead of the delamination front. The relevant percentage reduction is 37% and is independent of the imposed indentation load. As a crucial take-away, designers should be mindful that the damage interaction effects could result in greater damage than the sum of the individual cases.

Classification and Characterization of Damage in Composite Laminates Using Electrical Resistance Tomography and Supervised Machine Learning

Electrical resistance tomography (ERT) is a nondestructive evaluation technique that uses the internal conductivity variations of materials to assess structural integrity. Due to the low instrumentation required, the widespread use of ERT in the aerospace industry for monitoring the accumulation of damage in aircraft components can lead to significant reductions in inspections and maintenance costs. However, implementing the ERT method for mapping the damage state of structural components made of carbon fiber reinforced polymeric (CFRP) composites is challenging due to the inability of this method to distinguish between damage modes such as delamination and matrix cracking. This article explores the combined use of ERT and machine learning algorithms such as neural networks, random forests, k-nearest neighbors, and support vector machines to classify and characterize delamination and matrix cracking damage in CFRP laminates. Results show that the proposed classification algorithms can successfully estimate the damage severity of delaminated composites in the presence of matrix cracking. Similarly, the classification algorithms can characterize these independent damage modes with an accuracy of 95%. The algorithms showed robustness to predict the electrical resistance variations of damaged composites and characterize delamination and matrix cracking damage even when intrinsic noise was considered. Although neural networks characterized damage with the highest accuracy, these algorithms were also the most sensitive to noise. For applications where instrumentation noise cannot be completely removed from the ERT signals, the use of nearest neighbors is thus recommended.

Classification of barely visible impact damage in composite laminates using deep learning and pulsed thermographic inspection

With the increasingly comprehensive utilisation of Carbon Fibre-Reinforced Polymers (CFRP) in modern industry, defects detection and characterisation of these materials have become very important and draw significant research attention. During the past 10 years, Artificial Intelligence (AI) technologies have been attractive in this area due to their outstanding ability in complex data analysis tasks. Most current AI-based studies on damage characterisation in this field focus on damage segmentation and depth measurement, which also faces the bottleneck of lacking adequate experimental data for model training. This paper proposes a new framework to understand the relationship between Barely Visible Impact Damage features occurring in typical CFRP laminates to their corresponding controlled drop-test impact energy using a Deep Learning approach. A parametric study consisting of one hundred CFRP laminates with known material specification and identical geometric dimensions were subjected to drop-impact tests using five different impact energy levels. Then Pulsed Thermography was adopted to reveal the subsurface impact damage in these specimens and recorded damage patterns in temporal sequences of thermal images. A convolutional neural network was then employed to train models that aim to classify captured thermal photos into different groups according to their corresponding impact energy levels. Testing results of models trained from different time windows and lengths were evaluated, and the best classification accuracy of 99.75% was achieved. Finally, to increase the transparency of the proposed solution, a salience map is introduced to understand the learning source of the produced models.

Meshfree simulation of progressive damage in composite laminates using discrete element analysis

This study presents a Discrete Element Method (DEM) for a meshfree progressive failure analysis of IM7/8552 carbon fibre reinforced polymer laminates at the macroscale. By implementing a cohesive contact model to describe progressive damage, experimental results from over-height compact tension (OCT) tests are used to calibrate the required input parameters in the DEM contact model. The simulation of various open-hole tension (OHT) test specimens validates the presented DEM model with minimal errors in strength predictions of up to 4%. Discrete Element Method results are directly compared to those obtained from finite element (FE) models. It is demonstrated that DEM is able to seemingly incorporate damage evolution leading to realistic macroscopic damage patterns, however the computational cost is increased by a factor of 100 compared to FE simulations.

Comparing crack density and dissipated energy as measures for off-axis damage in composite laminates

To investigate off-axis cracks in composite laminates and their effect on stiffness and hysteretic energy dissipation, fatigue tests were instrumented with automated crack detection to test damage onset and progression. It was found that the onset of stiffness loss correlates well with the onset of matrix cracks, but not with the dissipated energy. To study the transition between experimentally observed damage modes occurring at the micro-scale, the experimentally obtained dissipated energy was compared with Carraro’s model, which predicts this transition based on local stresses. The results show a correlation between the absolute level of dissipated energy and the micro-damage modes.

A rapid technique for detecting and localizing damage in composite laminates

Composite materials are prone to impact damage, which affects structural stability and causes safety concerns. Due to the complexity and diversity of them, the damage localization in composite materials is very challenging and is getting more attention in the research community. This paper presents a rapid technique for composite damage localization without any prior knowledge of direction-dependent velocity profile. The proposed solution technique uses the time difference of arrival (TDOA) of Lamb waves. The TDOA is obtained by the cross-correlation technique applied to the received signals at different sensor pairs in two “L” shaped sensor clusters (LSSCs). With the help of the LSSC, only seven sensors are required to quickly predict the delamination damage location for quasi-isotropic and anisotropic composite materials (QACM). A simulation model was established to study the influence of damage location and different layup directions on the localization results. The experimental verification was carried out on a T300 carbon fiber board. The localization results of simulation and experiment showed good matching, which proves that the technique is suitable for QACM.

Analysis of lightning ablation damage of composite laminates with multiple fasteners

Composite materials have been widely used in civil aviation aircraft, but a large area of ablation damage occurs after lightning strikes, which threatens the operation safety of civil aviation aircraft. The composite material is fixed by multiple fasteners. The different installation positions of fasteners have a great influence on the result of lightning damage of composite. Based on these, a lightning ablation damage model of laminates with multiple fasteners is established, and the accuracy of the model is verified by comparing with the results in the references. In addition, the effect of different installation methods of multiple fasteners on the lightning ablation damage of laminates is studied. The results show that the number of fasteners is not the decisive factor among the factors that affect the ablation damage of laminates with multiple fasteners, and the installation layout of fasteners has an important impact on the damage results. When the fiber direction and aspect ratio of the laminate are the same, the maximum ablation damage area of terrible fasteners layout is 1.27 times the minimum ablation damage area of best fasteners layout under the action of lightning peak current of 200 kA.

Impact damage of composite laminates with high-speed waterjet

Rain erosion may cause substantial damage to aircrafts during supersonic flight. Such event is investigated here via high-speed waterjet impact on composite laminates. An experimental setup is developed to produce waterjets with the speed up to 700m/s and a finite element model of the waterjet-composite impact event is established. The consistency of experiment and simulation results validates the adopted numerical methods. The distribution of the water-hammer pressure is non-uniform and the maximum pressure occurs near the contact periphery when the water is about to eject laterally. After a high-speed (300∼560m/s) waterjet impacts a composite laminate, the impacted surface depression is observed, and the typical surface damage presents a central region with no visible surface damage surrounded by a faded “failure ring” with resin removal, matrix cracking and minor fiber fracture. Delamination occurs at the interfaces of adjacent layers with unequal dimensions and longitudinal matrix cracking appears on the back surface. Both the velocity and the diameter of waterjets are crucial factors on CFRP damage extents. Water-hammer pressure, the stagnation pressure and propagation of stress waves are failure mechanisms for most matrix damage in CFRP impacted by waterjets.