Failure Of Composite Structures Research Articles

Imaging and reconstruction of asymmetric wrinkles in carbon fibre composites using high-frequency eddy current and full matrix capture-based ultrasound

Ply wrinkles are a common defect found in Carbon Fiber Reinforced Polymer (CFRP) laminates that can lead to failure in composite structures if not correctly evaluated during manufacture stages. The mechanical performance of a structure depends on multiple parameters of a wrinkle, such as amplitude, wavelength, and asymmetry, which ideally need to all be measured to characterise the wrinkle properly. To address this issue, the authors designed and manufactured a set of complex wrinkles with various asymmetry offsets and amplitude and obtained raw data by testing them with a high-frequency eddy-current system. A unique trajectory in the complex Nyquist plot was found on asymmetric wrinkles, demonstrating the dominant influence of wrinkle asymmetry on electrical resistance. The authors also used full matrix capture ultrasound to characterise the wrinkles and measured the profiles by extracting texture phase randomness. The study revealed the superior capability of FMC UT for characterising and retrieving the profile in wrinkle coupons, while HF ECT was more adaptable for identifying the region of interest effectively. This work also implements a state-of-the-art deep fusion strategy to demonstrate a lightweight architecture can be easily used to combine multiple sources of wrinkle profiles and generate a more accurate prediction than traditional methods. Overall, the demonstrated work can be further applied to CFRPs with increasing geometric complexity and defects to facilitate the design efficiency and testing of CFRP components.

Isogeometric multilayer thin-shell analysis of failure in composite structures with hygrothermal effects

Isogeometric multilayer thin-shell analysis of failure in composite structures with hygrothermal effects

Composite structure failure analysis post Lithium-Ion battery fire

The use of composite materials has expanded significantly in a variety of sectors. In road transport,lithium-ion batteries (LIB) are the most commonly used.It is standard practice for batteriesto be housed within a metal enclosure, whichprotects and enables extinguishment in the event ofThermal Runaway (TR). Composite materials have been shown to contribute to lightweighting in many vehicle structures and their use inbattery enclosures has been growing in recent years - with the aim of reducing the weight of the battery assembly and positively impacting vehicle range.This work develops Finite Element (FE) models to assess thermal and mechanical damage and failure mechanisms during a TR event considering a section of a composite battery enclosure. Experimental data for a cylindrical 18650 lithium-ion battery fire is studied and used to define representative thermal loading. This validated loading profile is applied to a composite specimen and material temperature data is used to appraise damage. Finally, the predicted damage isused to predict and quantify the residual failure mechanism and strength of the specimen post battery fire. Results have shown the presence of damage from a single cell runaway can potentially reduce the strength of the specimen by 20% while multi-cell runaway can potentially reduce the strength by 56%. The predictive simulation capability herein could be used as a design tool for battery fire protection of composite enclosures, potentially reducing the need for corrective action, minimising the number of physical tests to support design and certification, as well as aiding in the interpretation of physical test results.

Hygrothermal effects on fatigue delamination behavior in composite laminates

Fatigue delamination growth (FDG) is an important failure in composite structures during their long-term operations. Hygrothermal aging can have significant effects on interlaminar resistance. It is therefore really necessary to explore FDG behavior in composite laminates with hygrothermal aging. Dynamic mechanical thermal analysis (DMTA), mode I FDG experiments and fractographic examinations were conducted to fully investigate hygrothermal aging effects and the corresponding mechanisms on FDG behavior. The DMTA results indicated that environmental aging can induce obvious Tg decrease. Mode I experimental fatigue data interpreted via different Paris-type correlations demonstrated that: Bridging has obvious retardation effects on FDG behavior via the Paris interpretations; The modified Paris relation can well characterize the intrinsic FDG behavior around the crack front; The use of the two-parameter Paris-type relation can appropriately account for R-ratio effects, contributing to a master resistance curve in determining mode I FDG behavior. According to these interpretations, it can be concluded that hygrothermal aging can have adverse effects on mode I FDG behavior. SEM examinations demonstrated that moisture absorption can cause fibre/matrix debonding and resin matrix pores/voids in the composite. However, no obvious difference in damage mechanisms was identified in mode I fatigue delamination for composite with/without environmental conditioning. Both fibre/matrix debonding and matrix brittle fracture were identified on fatigue fracture surfaces. Accordingly, it was concluded that fibre/matrix interface and matrix degradation induced by water absorption were the main reasons for a faster mode I fatigue crack growth in environmental aged composite.

Advances in Automated Eddy-Current Characterisation of Carbon Fibre Composites

Ply wrinkles in CFRP laminates are common and can lead to significant failure in composite structures if not properly assessed during the design, manufacturing, and service phases. Evaluating wrinkle geometries can be difficult as they are influenced by multiple factors, such as asymmetry, wavelength, and amplitude, which impact the structure's mechanical performance. High-frequency eddy current testing has proven effective for detecting in-plane waviness and out-of-plane wrinkles. However, the complex nature of CFRP structures and electrical anisotropy may hinder the effectiveness of an eddy current in-line monitoring system if not taken into account. In this study, an advanced modelling approach is employed to simulate the structural variation of the conductivity tensor. A designated wrinkle area, characterized by reduced electrical conductivity, is integrated with structural modelling to replicate a realistic coupon sample containing a wrinkle defect. Four proposed configurations are used to virtually test the samples, and their detection capabilities are analyzed quantitatively.

Progressive damage analysis and experiments of open-hole composite laminates subjected to compression loads

Due to the stress concentration caused by the hole, failure of composite structures is prone to start in the vicinity of the hole edge. Thereby, it is very necessary to investigate the failure of open-hole composite laminates. In the present work, a user subroutine element in the commercial software ABAQUS is proposed for the progressive damage analysis of open-hole composite laminates subjected to compression loads. The proposed element is constructed by making use of the third-order shear deformation theory in conjunction with a two-dimensional progressive damage model. The fiber kink model and Puck criterion are introduced for failure analysis of fiber and matrix, respectively. The element weakening method is also utilized to predict the damage evolution of open-hole composite laminates subjected to compression loads. To verify the reliability of the proposed model, the compression tests of open-hole composite laminates have been carried out. The percentage error of failure load predicted by the proposed element relative to the experimental result is 6.1%. Furthermore, the proposed element presents the damage evolution of fiber and matrix for the lamina with different stacking angles. It is observed that the fiber and matrix damage occurs firstly at the hole edge for the 45 and 0 degree plies, and the damage propagates along the 45 degree direction. This method can contribute to exploring the failure mechanism of open-hole composite laminates, which can also serve as a reference for the design of composite structures.

Post‐curing and fiber hybridization effects on mode‐II interlaminar fracture toughness of glass/carbon/epoxy composites

AbstractFailure by delamination is one of the primary concerns in laminated composites. The development and progression of the delamination results in the degradation of composite stiffness affecting its structural integrity and may ultimately result in composite structure failure. Therefore, knowledge of the evolution of delamination resistance and the factors which affect the delamination in composites is of utmost importance for the selection of materials. This study investigates the effect of fiber hybridization and post‐curing on mode‐II interlaminar fracture toughness (GIIC) of Glass/Carbon/Epoxy (GCE) composites. Carbon/Epoxy (CE), Glass/Epoxy (GE), and GCE composite laminates were molded employing the process of hand layup technique and post‐cured at different temperatures. GE laminates exhibited a superior load for delamination and deformation compared to CE and GCE laminates. The hybrid GCE laminates exhibited higher interlaminar fracture toughness compared to non‐hybrid GE and CE laminates, which can be attributed to the synergetic effect of Carbon and Glass fibers. The laminates post‐cured at 180°C exhibited higher fracture toughness in their corresponding groups. The delaminated surfaces of CE, GE, and GCE laminates were further examined under Scanning electron microscopy (SEM) to understand the effect of post‐curing and fiber hybridization on mode‐II interlaminar fracture toughness.

Influence of abrasive water jet and conventional drilling on the fatigue performance of hybrid carbon/flax laminates

Most of the research on hybrid natural fiber composites has been focused on non-structural applications. The aim of this research is to improve the understanding of how to use hybrid natural fiber composites for structural applications, such as in the aerospace and automotive industries. The joining of subassemblies with rivets, bolts, or screws is a prominent feature in these industries, which implies that the presence of holes in the structural members is unavoidable. The study of the effect of holes on the mechanical behavior of these members is therefore imperative, especially given that holes are stress risers. An important factor in the design of open-hole hybrid flax composites is their fatigue behavior under cyclic loading because it accounts for most failures in composite structures. The investigation of this behavior is reported in this study where three types of 16-ply symmetric hybrid composites that were manufactured using woven carbon (C) and unidirectional flax (F), namely, unidirectional (CF[0]), cross-ply (CF[0/90]) and angle-ply (CF[[Formula: see text] 45]) are employed. A circular hole was made in the center of each specimen using either an abrasive water jet or conventional drilling methods. The observed machining-induced critical defect was delamination, which manifested in three forms: peel-up, push-out, and secondary. Stepwise fatigue tests were conducted to determine the damage evolution and high-cycle fatigue strength of each layup. The damage evolution of the material was assessed via stiffness damage, thermographic methods, and dissipated energy per cycle. It was observed that CF[0] layups are unaffected by the machining process and they have an identical high-cycle fatigue strength. For the CF[0/90] and CF[[Formula: see text] 45] layups, conventional drilling specimens have a higher high-cycle fatigue strength than their abrasive water jet counterparts.

The effect of rock hardness and integrity on the failure mechanism of mortar bolt composite structure in a jointed rock mass

The appearance of joints in the rock tunnels makes the rock mass show obvious anisotropy and discontinuity in physical and mechanical properties, which has a significant influence on the rock mass stability. As one of the normally used support structures in tunnel construction, Mortar bolts will inevitably pass through the joints during the process. Both rock hardness and rock mass integrity will directly affect the anchorage performance of mortar bolts in joint rock. Considering this, this paper designs a mortar bolt pull-out experiment to investigate the joint rock failure modes and bolt-grout composite structure failure modes under different conditions of rock hardness and rock mass integrity. By taking the two influencing factors of rock hardness and integrity into account, the calculation formulas of ultimate pull-out load and ultimate displacement of mortar bolts in joint rock are established and proposed.

Fatigue performance of bolted shear connectors

ABSTRACT A computer-aided engineering (CAE) fatigue life prediction technique is developed in this paper to determine the fatigue strength of bolted shear connectors in composite structures. A relatively new initiative in the composite construction industry is the use of the blind boltshear connector, which provides a sustainable and practical solution to the main limitation of using traditional welded stud regarding reuse of building components. Furthermore, fatigue is one of the major causes involved in fatal mechanical failures of composite structures. However, limited research has been currently undertaken to assess fatigue life of composite structure. Therefore, the fatigue performance of the blind bolt under constant amplitudes cyclic loading has been investigated using ABAQUS/explicit and FE-SAFE programs. First, the dynamic responses of steel-concrete composite structures under sinusoidal load cycles were simulated using ABAQUS/explicit. Then, the stress–strain time history based on the response law of the composite structure was introduced into the FE-SAFE software to obtain a good prediction on the fatigue life of the blind bolt shear connector. As a result, the logarithmic life distributions of the bolted shear connector were calculated using different constant amplitudes.

Impact response of nanoparticle reinforced 3D woven spacer/epoxy composites at cryogenic temperatures

Fiber-reinforced polymer composites serving in harsh conditions must maintain their performance during their entire service. The cryogenic impact is one of the most unpredictable loading types, leading to catastrophic failures of composite structures. This study aims to examine the low-velocity impact (LVI) performance of 3D woven spacer glass-epoxy composite experimentally under cryogenic temperatures. LVI tests were conducted under various temperatures ranging from room temperature (RT) to −196°C. Experimental results reveal that the 3D composites gradually absorbed higher impact energies with decreasing temperature. Besides, the effect of multi-walled carbon nanotube and SiO2 nanofiller reinforcements of the matrix on the impact performance and the damage characteristics were further assessed. Nanofiller modification enhanced the impact resistance up to 30%, especially at RT. However, the nanofiller efficiency declined with decreasing temperature. The apparent damages were visually examined by scanning electron microscopy to address the damage formation. Significant outcomes have been achieved with the nanofiller modification regarding the new usage areas of 3D woven composites.

A review of phase-field models, fundamentals and their applications to composite laminates

Innovations of advanced materials have been at the core of most engineering technologies, devices and systems. Composite laminates in particular have been extensively employed in many major and advanced industries including aircraft, maritime transport, and automobiles. Failure in composite structures is critical and often complicated, and that is the consequence of the evolution of and interactions among the constitutive fracture events (e.g., matrix cracking, fibre/matrix debonding, delamination, fibre breakage, fibre kinking). Accurate prediction of progressive failure in composite laminates is crucial for the development and design of most lightweight components and structures. This paper, for the first time, outlines a critical review on the developments and recent applications of regularized phase field models for failure problems in composite laminates and structures. Fundamentals of the phase field models are introduced and the developments of the regularized model from different aspects for the modeling are reviewed. Phase field models as well as the modeling strategies from different levels of idealization, i.e., microscopic, mesoscopic and macroscopic length scales are presented. The realization of existing widely used models such as cohesive zone model, plasticity, and damage initiation criterion and energy decomposition into the phase field model is also addressed. Typical applications of the phase field models in composite laminate modeling are discussed.

Parametrically homogenized continuum damage mechanics (PHCDM) models for unidirectional composites with nonuniform microstructural distributions

This paper develops a parametrically homogenized continuum damage mechanics (PHCDM) model for multiscale analysis of damage and failure in composite structures with nonuniform microstructures. Unidirectional glass fibers are nonuniformly dispersed in the microstructures of these epoxy matrix composites. The PHCDM models are thermodynamically consistent, reduced order constitutive models with coefficients that are explicit functions of microstructural descriptors and evolving material variables. Damage anisotropy is represented through a second order damage tensor that contributes to the evolution of a damage surface in the space of damage work conjugate, which characterizes the initiation and evolution of damage. The nonuniform microstructural morphology descriptors are optimally expressed in terms of representative aggregated microstructural parameters or RAMPs for incorporation in the PHCDM coefficients. Optimal expressions for RAMPs are determined through principal component analysis of the two-point correlation functions. The functional forms of RAMPs in PHCDM coefficients are determined using machine learning tools operating on data generated by micromechanical analysis. It is shown that PHCDM models accounting for the fiber distribution information yield a much higher accuracy than those only accounting for fiber volume fractions. The developed PHCDM model is incorporated in a commercial finite element code and structural analysis of structural composites is executed for understanding concurrent damage and failure at multiple scales. The paper successfully demonstrates the accuracy and significant efficiency of the resulting PHCDM model in analyzing deformation and damage in nonuniform composites across length scales for various loading conditions.

Capability analysis of Puck damage model in predicting the damage behavior of unidirectional composite laminates under different scenarios

Composite materials have been broadly used in many engineering fields because of their distinctive properties. They have advantages over metals as their properties offer significant advantages such as very good strength to weight ratio. However, failure in composite structures due to interlaminar and intralaminar raises a maintenance concern because it can lead to invisible damage. Therefore, this paper presents the capability of three-dimensional Puck failure criteria with gradual degradation rule to predict the structural responses, as well as the onset and propagation of failure due to variation of the situation in a composite laminate. The proposed damage model is achieved via the implementation of user-subroutines both in statics and explicit analysis using Abaqus software. Three different scenarios are investigated which were open-hole tension, low-velocity impact, and multi-bolt double-lap joint. The force-displacement and force-time curves are recorded and compared with experimental data taken from literature to measure the performance of such damage model. It is found out that the model adopted here responses well with test curves and demonstrates the high capability of predicting the damage in the direction of in-plane and out-of-plane in a composite laminate.

Failure analysis of fiber-reinforced composites subjected to coupled thermo-mechanical loading

Structures made of fiber reinforced composites have captured extensive attentions in the scientific and engineering communities due to their excellent performance and applicability. When these structures are in service, they are likely exposed to variations of ambient temperature that may have an impact on their strength. To study this effect, a coupled thermo-mechanical model is required. This paper develops a microscopic mechanical model to investigate failure of composite structures subjected to a coupled thermo-mechanical condition. Stiffness degradations of composite laminates are first investigated. A comparison between experimental data and theoretical results under the quasi-static loadings are presented to validate the proposed method. The method provides detailed microscopic stress distribution of the composites under the coupled thermo-mechanical loading for failure analysis, which shows that a higher ambient temperature variation will generally cause stiffness degradation and failure strength for both uniaxially and biaxially loaded laminates.

Geometrically nonlinear finite element model for predicting failure in composite structures

Composite structures are extensively used in many industries, where they are subjected to a variety of loads and may undergo large deformations. Reliable utilisation of such structures requires prior knowledge of their failure response. In order to predict failure loads and modes, accurate, yet computationally efficient, evaluation of three-dimensional (3D) stress fields becomes important. In this paper, we present a modelling approach, based on the Unified Formulation, that accounts for geometric nonlinearity in laminated composites and predicts 3D stress fields for subsequent failure analysis. The approach builds upon the hierarchical Serendipity Lagrange finite elements and is able to capture high-order shear deformation, as well as local cross-sectional warping. A total Lagrangian approach is adopted and the classic Newton-Raphson method is employed to solve the nonlinear governing equations. A key novelty of the proposed formulation is its completeness and its applicability to fully anisotropic structures. In other words, using the Green-Lagrange strain components within the Unified Formulation framework, the explicit form of the tangent stiffness matrix is derived including general stiffness properties. This new model is benchmarked against 3D finite element solution, as well as other formulations available in the literature, by means of static analyses of highly nonlinear, laminated composite beam-like structures. Significant computational efficiency gains over 3D finite elements are observed for similar levels of accuracy. Furthermore, to show the enhanced capabilities of the present formulation, the postbuckling response of a composite stiffened panel is compared with experimental results from the literature. The 3D stress fields computed in the postbuckling regime are used to detect failure of the stiffened panel. The corresponding failure mode, as obtained by the new model, is shown to match with the experiment.

A multiscale modelling procedure for predicting failure in composite textiles using an enhancement approach

Composite materials have largely been analysed for failure and structural performance from the perspective of an anisotropic homogeneous material despite their evident hierarchical nature. Although such an assumption does substantially facilitate the analysis procedure in a computationally efficient manner, predictions on the occurrence of failure differ from reality as the failure mechanisms within composite materials are different from a constituent perspective. This paper demonstrates the ability of a multiscale dehomogenisation procedure that provides failure information at different length scales, namely: the macro- (continuum), meso- (textile) and micro-scale (constituent). This is achieved by adopting an enhancement approach performing simple matrix operations to study the strain distribution. The hierarchical dehomogenisation procedure using Strain Invariant Failure Theory is implemented in Abaqus UMAT subroutine and verified using simple sanity checks and a ‘manual’ sub-modelling technique utilising an open hole tension coupon as an example. A comparison of the two modelling techniques indicate the similar failure predictions at the meso and micro levels with the hierarchical approach presented in this paper being far more computationally efficient. The failure analysis procedure presented in this paper is subsequently demonstrated on a composite I-beam component allowing failure in composite structures to be observed from a constituent perspective where fibre and matrix modes of failure can be identified and examined from an engineering point of view.

A technique for in-situ detection of random failure in composite structures under cyclic loading

A technique to detect the random failure in composite structures is presented. The epoxy matrix material is made conductive by the incorporation of carbon nanotubes. The modified matrix is used to fabricate glass fiber/epoxy/carbon nanotubes composite panels. Conductive grid points made from silver-epoxy paste are attached on the surface of the composite panels, so that electrical resistances in the regions between the grid points can be measured. The increase in electrical resistance between grid points is used to determine the increase in deformation (and possibly cracks) at the region between the grid points. It is found that the location of maximum increase in electrical resistance jumps from point to point as the number of cycles during fatigue loading is increased. This shows the random nature of the development of damage in the composites. The technique can detect the occurrence of early failure, usually in the matrix materials. The result of this work brings out the random nature of the early failure in composites. This also sheds light into whether the concept of crack initiation in composites is valid, since the early cracks jump around. When the paths of the final cracks reach stability, there is rapid crack propagation leading to fast final failure.

Explicit modeling of matrix cracking and delamination in laminated composites with discontinuous solid-shell elements

Composite structural failure usually entails complicated interactions between intralaminar and interlaminar damage. In this paper, a novel discontinuous solid-shell finite element combined with a 3D enriched cohesive element is formulated for the analysis of matrix cracking and delamination in multilayered composites. The proposed formulations are computationally attractive in modeling thin shell structures while retaining the advantages of solid elements. The bending performance was improved by using the enhanced assumed strain method and the assumed natural strain method, which alleviates potential locking problems. A linear elastic orthotropic law was used for layered shells and cracks therein were modeled with a cohesive zone model. A discrete crack method with floating nodes enriching the developed elements enables a mesh-independent description of ply cracks and their interaction with interface cracks. The applicability of the current formulation was verified with numerical examples including large deformations of thin shell problems and delamination growth in post-buckled laminates. Finally, the explicit and totally discrete modeling of matrix and delamination cracks in low velocity impact of cross-ply and angle-ply laminates was demonstrated. Numerical predictions of structural responses and damage patterns agree well with experimental results.

Comparison of NMR and microwave NDE methods for defect detection in composite structures

Abstract New advanced non-destructive evaluation (NDE) methods such as nuclear magnetic resonance (NMR) imaging and microwave NDE methods are being developed for their advantages and high resolution for defect detection in composite structures. Glass/epoxy composite structures are being used for many applications in industry. Often, glass/epoxy composite structures are used along with rubber lining in oil and gas pipe industry. Rubber is adhesively bonded to composite inner surface to meet the requirements. The interface of glass/epoxy composite and rubber lining is critical. Any de-bond (separation of adhesively bonded liner from composite surface) at the interface may lead to failure of composite structure due to corrosive gases and liquids. The present paper reports application of proton single sided NMR method to evaluate the de-bonds of rubber liner with composite. Results are compared with microwave NDE technique which is an advanced non-contact inspection method. Results indicate application of both these techniques for evaluation of bonded interfaces. Advantages of both the techniques over conventional techniques have been discussed. De-bonds within rubber lining were also detected and correlated.