Failure Of Composite Laminates Research Articles

Micromechanical progressive damage analysis of inter- and intra-layer failures in fiber-reinforced composite laminates

We analyze progressive damage and failure of composite laminates by using a micromechanical bridging model and compare numerical and experimental results for three fracture problems, namely, the four-point bending, the simple tension, and the simple tension of a laminate with an open hole at its centroid. These problems involve fiber–matrix interface debonding, constituents’ damage, interlayer delamination, and localized damage due to stress concentration. Macroscopic constitutive equations of unidirectional lamina, derived from those of the fiber and the matrix by using the bridging model with the fiber material assumed to be linearly elastic and the matrix to be elasto-plastic obeying the Drucker–Prager yield criterion, are employed. Strains in each constituent of the composite are assumed to be infinitesimal for the additive decomposition of strains into elastic and plastic parts to be valid, and the incremental plasticity theory is used. Stresses in the two constituents are found from their values in the homogenized material by using a dehomogenization technique. The intra-layer damage is assumed to initiate at a material point when the failure criterion for either the fiber or the matrix is satisfied. Young’s modulus of a constituent is degraded by following a Weibull distribution. A finite element is deleted when an energy-based failure criterion is satisfied in it, and the analysis is continued till the structure fails. The delamination between adjacent plies is simulated by including a thin resin layer at the interface and studying failure initiation and propagation in it. The computed reaction force versus the displacement curves and the failure patterns in the three problems are found to agree with the corresponding experimental data.

Numerical simulation of bolted joint composite laminates under low-velocity impact

In the present study, a three-dimensional finite element model of the bullet impact composite laminates is proposed. The user material subroutine VUMAT is used in the proposed three-dimensional model. The failure of composite laminates under different impact energies is analyzed, and the progressive failure process of the composite laminates is further analyzed to predict the failure strength of composite laminates. Then, based on this method, a three-dimensional finite element model of bolted joint composite laminates under low-velocity impact is established. The effects of bullet impact on the bolted joint composite laminates under different boundary conditions are analyzed, and the failure characteristics of the composite structures are investigated when the bullets impacted different positions.

Optimal Design of a Spar Cross-Section for Fabric-Covered Wind Turbine Blades

Wind turbine blades and aircrafts have grown in size to improve energy efficiency and reduce cost. The weight of these structures has also increased due to their larger sizes. Reducing the weight of these structures can increase efficiency and lower fuel consumption, and decrease the structural load. Fabricating structures using composite materials is currently the most popular way to reduce weight. Several optimization algorithms are employed to achieve optimal composite structure design. A combination of these composite materials and a well-developed optimization algorithm can be effectively applied to improve the design of these larger composite structures. This study investigated the design of a composite spar and its feasibility by analyzing static conditions under extreme loads. Structural properties and stress analyses were calculated using the VABS program. Pre-processing and post-processing were performed by applying in-house MATLAB-based codes. The maximum stress failure theory was applied to check and confirm the composite laminate failure. A particle swarm optimization algorithm was also applied to analyze the design of the composite spar. The analysis results show that the proposed design method in this study is feasible and applicable to the design of composite spars for wind turbine blades and aircraft wings.

Simulation of failure in laminated polymer composites: Building-block validation

The development of numerical tools to complement the experimental determination of structural design parameters is of key importance to hasten the certification process of new materials and structures. In this work, a methodology to simulate elastic and inelastic deformation of composite laminates at the subcomponent level based on finite element analysis is proposed. A modified version of a continuum damage model proposed in the literature combined with a frictional cohesive zone model is used to capture the intralaminar and interlaminar damage and failure of composite laminates in general loading conditions. The methodology is validated for three aerospace-grade carbon fibre reinforced (epoxy) polymer composite material systems and coupon configurations with increasing level of complexity, including unnotched tension/compression, open-hole tension/compression and filled hole compression. The predictions obtained are in good agreement with the experimental results for all the test cases, oftentimes within the standard error of the tests, with a maximum relative error of 13%.

IGFEM-based shape sensitivity analysis of the transverse failure of a composite laminate

This manuscript presents a shape sensitivity analysis method based on an Interface-enriched Generalized Finite Element Method (IGFEM) formulation and its application to the sensitivity of the transverse failure of a fiber-reinforced composite laminate with respect to the geometrical parameters that define its microstructure. The analytical sensitivities with respect to individual fiber radius and placement are first derived within the context of a cohesive IGFEM solver specially developed to simulate the fiber/matrix debonding observed in the transverse failure of composite laminates with high fiber volume fraction. The IGFEM solver utilizes $$C^{-1}$$ continuous enrichment functions and a cohesive failure model to capture the transverse cracking associated primarily with fiber/matrix interface debonding. In addition to the sensitivities with respect to individual geometrical parameters such as the radius of individual fibers, the sensitivities of the transverse stress–strain response with respect to the parameters that define the distributions of the geometrical parameters such as the mean and standard deviation of the fiber radius and nearest-neighbor distance distributions are also derived. The sensitivity analysis is performed on realistic microstructures composed of hundreds of fibers to characterize the influence of the geometrical parameters and their distributions on the transverse failure response of the composite laminate.

Understanding progressive failure mechanisms of a wind turbine blade trailing edge section through subcomponent tests and nonlinear FE analysis

This paper presents a comprehensive study on structural failure of a trailing edge section cut from a composite wind turbine blade. The focus is placed on understanding progressive failure behavior of the trailing edge section in subcomponent testing during its entire failure sequence. Digital Image Correlation (DIC) is used to capture buckling deformation and strain distributions of the specimen. Detailed post-test inspection is performed to identify failure modes and failure characteristics. A nonlinear Finite Element (FE) model that accounts for all observed failure modes is developed based on continuum damage mechanics and progressive failure analysis techniques. Multiple structural nonlinearities due to buckling, contact and material failures are included in the model to predict the failure process. The study shows that in addition to the buckling-driven failure phenomenon, the surface contact of sandwich panels contributes to the failure process of the trailing edge section. Foam materials start to fail before the ultimate load-carrying capacity of the specimen is reached, while both composite materials and adhesive materials fail in the post-peak regime. The matrix-dominant failure and delamination develop before the fiber-dominant failure in composite laminates. The proposed FE model captures the progressive failure process of the trailing edge section reasonably well.

Mathematical modelling and simulation of delamination crack growth in glass fiber reinforced plastic (GFRP) composite laminates

Delamination crack growth is a major source of failure in composite laminates under static and fatigue loading conditions. In the present study, damage mechanics based failure models for both static and fatigue loadings are evaluated via UMAT subroutine to study the dela- mination crack growth phenomenon in Glass Fiber Reinforced Plastic (GFRP) composite laminates. A static local damage model proposed by Allix and Ladev`eze is modified to an non-local damage model in order to simulate the crack growth behavior due to static loading. Next, the same classical damage model is modified to simulate fatigue delamination crack growth. The finite element analysis results obtained by the proposed models are successfully compared with the available experimental data on the delamination crack growth for GFRP composite laminates.

A Damage Model Reflecting the Interaction between Delamination and Intralaminar Crack for Failure Analysis of FRP Laminates

In this paper, a progressive damage model reflecting the interaction between delamination and intralaminar crack is developed to predict fracture behaviors and the ultimate load-bearing ability of the fiber-reinforced polymer laminates subject to quasi-static load. Initiation and evolution of intralaminar crack in composites are modeled using a continuum damage mechanics model, which has the capability to reliably predict the discrete crack direction by introducing the crack direction parameter while analyzing the multi-failure of FRP composites. Delamination is modeled using a cohesive zone method with the mixed bilinear law. When the continuum damage model and cohesive zone model are used together, the interactive behavior between multiple failure mechanisms such as delamination induced by matrix cracking often seen in the failure of composite laminates is not generally captured. Interaction between delamination and intralaminar crack in FRP composite structures is investigated in detail and reflected in a finite element analysis in order to eliminate the drawbacks of using both models together. Good agreements between numerical results and experimental data are obtained.

Mesoscale modelling of damage in half-hole pin bearing composite laminate specimens

This paper presents the development and validation of a mesoscale numerical model for predicting the bearing damage and failure of composite laminates reinforced by unidirectional fibers. Firstly, half-hole pin bearing tests were carried out on composite laminates with both quasi-isotropic and soft lay-ups, as well as in specimens with variations in ply thickness and stacking sequence, using multiple measurement and inspection tools for a comprehensive characterization of the damage mechanisms. Then, three dimensional (3D) finite element models with a fiber-aligned mesh for composite plies and considering various frictional contacts were built to simulate the bearing tests using a commercial explicit solver. The embedded material model incorporated 3D phenomenological invariant-based failure criteria using in situ ply strengths, together with mechanism-based continuum damage models (longitudinal bi-linear damage model and transverse 3D smeared crack model) for intralaminar damage, and the cohesive zone model for interlaminar damage. Finally, detailed analyses and comparisons of the experimental and numerical results were performed in both macroscopic mechanical behavior and mesoscale failure mechanisms, where a good correlation was observed. Sensitivity studies on the effect of the modelling parameters on the post-peak response prediction were also conducted, providing relevant guidelines to identify future research directions.

Characterization of cohesive model and bridging laws in mode I and II fracture in nano composite laminates

The cohesive model and traction-separation curves have brought a considerable possibility for researchers and fracture engineers to assess and simulate failure in composite laminates. Reliable determination of the traction–separation laws is very pivotal to the success of this approach in finite element methods. The objective of this paper is assessment of nanoparticles effect on bridging laws, cohesive mechanism and traction-separation parameters of nanocomposites mode I and II fracture. To do this analyzing of the experimental data from double cantilever beam, and end notched flexure tests including construction of the R-curves (energy release rate versus crack length), reconstruction of these curves in terms of the pre-crack tip opening and sliding displacement, and calculation of the corresponding bridging and traction-separation laws through the J-integral approach were carried out. For the calculation of the energy release rate in Mode I, three corresponding data reduction schemes namely Corrected Beam Theory, Experimental Compliance Method and Modified Compliance Calibration are utilized, while Compliance Calibration Method, Corrected Beam Theory and Compliance-Based Beam and II fracture are applied for that of mode II. The main concern of this research is introduction of critical parameters of two modified models to simulate mode I and II fracture. Adding 0.43 wt% nanoparticles to composite DCB samples leads to increase of 116%, 68% and 70% in GI,0 calculated by CBT, ECM and MCC respectively, and a 72% increase in GI,ss is measured by CBT, while this value for ECM and MCC is 110% and 48%. Adding 0.2 wt% nanoparticles to the composite samples results in 86% (50 MPa) increase in critical stress in mode II fracture calculated by method CBBM. This method presented the lowest value for critical displacement, fluctuated between 0.08-.11 mm in mode II.

Internal damage evaluation of composite structures using phased array ultrasonic technique: Impact damage assessment in CFRP and 3D printed reinforced composites

The application of non-destructive techniques for the evaluation of internal damage in composite materials is a challenge due to their complexity. The aim of this study is to analyse the ability of ultrasonic technique to identify and evaluate manufacturing defects and internal damage in composite laminates. Artificial object inclusions of different shapes, sizes and materials were embedded in carbon fibre reinforced epoxy (CFRP) laminates. Comprehensive investigation of non-destructive evaluation using phased array ultrasonic testing to trace and characterize the embedded defects is presented. Phased array ultrasonic technique was able to precisely locate most of the artificial inclusion in the composite laminates. Nerveless, the shape and size of the inclusions were not accurately determined due to the high signal attenuation and distortion characteristics of the carbon fibre epoxy composite.Furthermore, a major concern affecting the efficient use of composite laminates is their vulnerability to low velocity impact damage. The dynamic behaviour of composite laminates is very complex as there are many concurrent phenomena during composite laminate failure under impact loading. Thus, the practicality of the previous results is demonstrated by the evaluation of impacted carbon fibre reinforced epoxy laminates using ultrasonic testing. The influence of laminate thickness and impact energy on impact damage is also investigated. The performance of phased array ultrasonic testing for the inspection of composite laminates with barely visible impact damage is evaluated. In addition, fibre-reinforced thermoplastic composites are becoming more significant in industrial applications due to their excellent mechanical performance and potential recycling. In this study, impact damage in 3D printed continuous glass fibre reinforced thermoplastic laminates is also analysed. Phased array ultrasonic testing is performed and the amount of damage is quantified in terms of delaminated area.

Effect of delamination on the flexural response of [+45/−45/0]2s carbon fibre reinforced polymer laminates

The damage arising in the manufacturing or service operation can result in the degradation in mechanical properties or even structural failure in composite laminates. This work investigated the flexural behaviour of [+45/−45/0]2s carbon fibre reinforced polymer laminates with the artificially embedded delamination (pre-delamination) at different interfaces. After static flexural experiments, the internal 3D damage including various failure modes was characterised and quantified in the X-ray microtomography. It was found that regardless of the pre-delamination, similar in-ply (fibre failure, matrix cracking and fibre/matrix debonding) and interlaminar (delamination) failure modes occur dominantly in the outer ply group of the compression zone in all the laminates. However, the pre-delamination and its location have the influence on both the distribution and size of the 3D damage, and thus on the flexural properties. The flexural strength that is reduced by pre-delamination is the most (least) sensitive to the pre-delamination embedded at the third (ninth) interface.

Simulation of the Mechanical Response of Thin-Ply Composites: From Computational Micro-Mechanics to Structural Analysis

This paper provides an overview of the current approaches to predict damage and failure of composite laminates at the micro-(constituent), meso-(ply), and macro-(structural) levels, and their application to understand the underlying physical phenomena that govern the mechanical response of thin-ply composites. In this context, computational micro-mechanics is used in the analysis of ply thickness effects, with focus on the prediction of in-situ strengths. At the mesoscale, to account for ply thickness effects, theoretical results are presented related with the implementation of failure criteria that account for the in-situ strengths. Finally, at the structural level, analytical and computational fracture approaches are proposed to predict the strength of composite structures made of thin plies. While computational mechanics models at the lower (micro- and meso-) length-scales already show a sufficient level of maturity, the strength prediction of thin-ply composite structures subjected to complex loading scenarios is still a challenge. The former (micro- and meso-models) provide already interesting bases for in-silico material design and virtual testing procedures, with most of current and future research focused on reducing the computational cost of such strategies. In the latter (structural level), analytical Finite Fracture Mechanics models—when closed-form solutions can be used, or the phase field approach to brittle fracture seem to be the most promising techniques to predict structural failure of thin-ply composite structures.

Multi-scale modelling of dynamic progressive failure in composite laminates subjected to low velocity impact

This paper aims to investigate the structural mechanical responses and progressive damage behaviors of composite laminates subjected to low velocity impact. First, a three-dimensional multi-scale model based on micromechanics of failure (MMF) criterion with new damage evolution laws is introduced for intralaminar damage, which considers the different mechanical behaviors of the microscopic constituents for fiber and matrix. The macroscopic damage variables of the new damage evolution laws are directly evaluated by the degraded elastic parameters of the microscopic constituents calculated from representative volume elements (RVEs). Second, by using this multi-scale approach, the relationship between macroscopic stress and microscopic stress is established by stress amplification factors (SAFs) through the RVEs using ABAQUS-PYTHON scripting language. The proposed model is implemented by the user-defined material subroutine VUMAT built on ABAQUS/Explicit platform, and the bilinear cohesive model in ABAQUS is employed to capture the onset and progression of interlaminar delamination. Finally, impact numerical simulation for the impact force-time/central displacement curves and energy dissipation is performed on three composite laminates with different layup patterns. Relatively good agreements are achieved between the experimental and numerical results, which validates the effectiveness of the multi-scale model.

Assessment and propagation of mechanical property uncertainties in fatigue life prediction of composite laminates

A probabilistic model for estimating the fatigue life of laminated composite materials considering the uncertainty in their mechanical properties is developed. The uncertainty in the material properties is determined from fatigue coupon tests. Based on this uncertainty, probabilistic constant life diagrams are developed which can efficiently estimate probabilistic ɛ-N curves at any load level and stress ratio. The probabilistic ɛ-N curve information is used in a reliability analysis for fatigue limit state proposed for estimating the probability of failure of composite laminates under variable amplitude loading cycles. Fatigue life predictions of unidirectional and multi-directional glass/epoxy laminates are carried out to validate the proposed model against experimental data. The probabilistic fatigue behavior of laminates is analyzed under constant amplitude loading conditions as well as under both repeated block tests and spectral fatigue using the WISPER, WISPERX, and NEW WISPER load sequences for wind turbine blades.

An improved delamination fatigue cohesive interface model for complex three-dimensional multi-interface cases

This work presents a cohesive interface model for predicting interlaminar failure of composite laminates under tension-tension fatigue loading. The model features improvements on previous formulations and utilizes four-integration-point elements, which offer several new advantages, while maintaining the merits of the previous single-integration-point elements. An element-based crack tip tracking algorithm is incorporated to confine fatigue damage to crack-tip elements only. A new local rate approach is proposed to ensure accurate integration of strain energy release rate from local elements. Furthermore, a dynamic fatigue characteristic length is proposed to offer a more accurate estimation of fatigue characteristic length in complex three-dimensional cases. Fatigue initiation is incorporated by using a strength reduction method, without changing the propagation characteristics. The numerical approach has been verified and validated using multiple cases and was then applied to fatigue damage development in open-hole laminates, where a good agreement between numerical analysis and experimental results was obtained.

A separable cohesive element for modelling coupled failure in laminated composite materials

A three-dimensional separable cohesive element (SCE) is proposed to enable the modelling of interaction between matrix cracking and interfacial delamination in laminated fibre-reinforced composite materials. It is demonstrated that traditional cohesive elements are incapable of modelling the coupled failure mechanisms accurately if partitioning is not allowed. The SCE may be partitioned according to the configuration and geometry of matrix cracks in adjacent plies, thus maintaining appropriate connection between plies. Physically, the original interface is split and new interfaces are formed to bond the homologous cracked solids during fracturing process. The stress concentration induced by matrix cracks and the load transfer from cracked solid elements to interface cohesive element are effectively modelled. A comprehensive set of cases of multiple matrix crack configurations from plies of different fiber angles is considered. The proposed SCE is applied to model progressive failure in composite laminates and the results are found to agree with experiments.

Strain-Rate-Dependent Yield Criteria for Progressive Failure Analysis of Composite Laminates Based on the Northwestern Failure Theory

The strain-rate-dependent failure of a fiber-reinforced toughened-matrix composite (IM7/8552) was experimentally characterized over the range of quasi-static (10−4 s−1) to dynamic (103 s−1) strain rates by testing off-axis lamina and angle-ply laminate specimens. A progressive failure framework was proposed to describe the matrix-dominated transition from linear elastic to non-linear material behavior as determined from the experimentally measured stress-strain material response, and the Northwestern Failure Theory was adapted to provide a set of apparent yield criteria for predicting the matrix-dominated yielding of composites using the lamina-based transverse tension (F2ty), transverse compression (F2cy), and in-plane shear (F6y) yield strengths. The underlying theory was validated by determining the applicability of the new damage-mode-based yield criteria. Starting with the lamina, the proposed criteria accurately predicted the matrix-dominated yielding. Angle-ply laminates were then investigated to isolate the matrix-dominated laminate behavior based on fiber orientation, and the predictions were found to be in superior agreement with the experimental results compared to the classical failure theories. The results indicate the potential of using the Northwestern Yield and Failure Criteria to provide the predictive baseline for damage propagation from yield to ultimate lamina failure in composite laminates.

Progressive Analysis of Bearing Failure in Pin-Loaded Composite Laminates Using an Elasto-Plastic Damage Model

Bearing failure of composite laminate is very complicated due to the complexity of different failure mechanisms and their interactions. In this paper, an elasto-plastic damage model is built up to describe the process of failure in composite laminates subjected to bearing load. Non-linear behavior of composite before failure is taken into consideration by using a modified Sun-Chen one parameter plasticity model. LaRC05 failure criteria are employed to predict the initiation of failure and the evolution of failure is described by a CDM based stiffness degradation model. Both theory and some application issues like parameter determination are discussed according to phenomenon of experiments. The model is firstly validated by several experiment results of unidirectional laminate and then applicated into the progressive analysis of bearing failure in pin-loaded multidirectional laminates, both intralaminar and interlaminar damage are taken into consideration. The result of finite element analysis is compared with experiment results; it shows good agreements in both mechanical response and progress of failure, so the model can be evaluated to be effective and practical in bearing failure analysis of composite laminates.

Mode I and Mode II Delamination of Flax/Epoxy Composite Laminate

In recent decades, natural fibres are getting their attention as reinforcement in composite materials. This is because natural fibres are environmental friendly. However, delamination is commonly recognised as one of the earliest failures in composite laminates. The objective of the present work is to investigate mode I and mode II delamination behaviour of flax fabrics reinforced epoxy composite. The delamination characterisation was carried out using double cantilever beam (DCB) and three point end notched flexure (ENF) tests. The fracture toughness were calculated using experimental calibration method (ECM). Results showed that the average fracture toughness was 485 N/m and 962 N/m, respectively. Finally, through scanning electron micrographs, it was observed that the ply/ply debonding and fibre/matrix debonding were the major fracture mechanisms in DCB specimen. As for ENF specimen, shear fracture dominated the energy dissipation process.