Measurement of the in situ transverse tensile strength of composite plies by means of the real time monitoring of microcracking

Failure of a ply due to transverse loading is one of the mechanisms that was taken into account in physically-based failure criteria, used in composites design. However, experimental data are scarce and the measurement techniques used in the past are time consuming and involve a lot of specimen handling during testing. While some physical information is currently well consolidated (such as the dependence of the strength on ply thickness, or in situ strength), there still remain relevant open questions. This work presents a methodology, which does not interfere with the tensile test, to detect transverse cracks by optical means. Four different configurations of CFRP are considered. The results show that the in situ strength depends on the thickness of the ply and the orientation of the adjacent layers. In the case of thick transverse plies, the strength is controlled by full-width transverse cracks whereas, in thin plies cracking parallel to the specimen’s mid-plane occurs before transverse matrix cracking.     

Two-dimensional strain-based interactive failure theory for multidirectional composite laminates

A 2-D strain-based interactive failure theory is developed to predict the final failure of composite laminates subjected to multi-axial in-plane loading. The stiffness degradation of a laminate during loading is examined based on the individual failure modes of the maximum strain failure theory, and a piecewise linear incremental approach is employed to describe the nonlinear mechanical behavior of the laminate. In addition, an out-of-plane failure mode normal to the laminate is also investigated to more accurately predict the failure of multidirectional laminates. The theoretical results of the failure model presented are compared with the experimental data provided by the World-Wide Failure Exercise, and the accuracy of the model’s predictive capabilities is investigated.     

The effects of interfacial adhesion strength on the characteristics of an aluminum/CFRP hybrid beam under transverse quasi-static loading

The effects of interfacial adhesion strength on the damage behavior and energy absorption characteristics of an aluminum (Al)/carbon fiber reinforced plastic (CFRP) short square hollow section (SHS) beam under three point bending loading was investigated. An Al SHS beam was wrapped by CFRP with a [0°/+45°/90°/−45°]_n (n = 1 or 2) stacking sequence, and four gradations of interfacial adhesion strength were caused by physical or chemical changes of the Al adherend with different mechanical abrasion and optimal Argon plasma treatment. A different level of appropriate interfacial adhesion strength existed for each hybrid specimen depending on the CFRP laminate thickness to obtain the highest energy absorption capability, and this was verified by detailed observation of the failure mechanism of the hybrid specimen. The specific energy absorbed (SEA) was improved by up to 57.2% in the Al/CFRP [0°/+45°/90°/−45°]₂ SHS beam compared to the Al SHS beam without compromising the crush force efficiency (CFE).     

Compressive static strength model for impact damaged laminates

A simple analytical model for the prediction of the compressive strength of composite structures with Barely Visible Impact Damage (BVID) subject to static loading is presented. The model represents the complex damage morphology using circular approximations of the damage area and determines a critical interface for propagation of BVID. Results are compared with experimental values for static strength of a variety of examples reported in the literature. For impacts on the skin under a stiffener the model is accurate to within 5% of the reported experimental result. It is demonstrated how the model can be manipulated for use in laminate optimisation for improved damage tolerance.     

Damage-based failure theory and its application to 2D-C/SiC composites

Damage evolution is of great importance to determine the final failure of ceramic matrix composites (CMCs). In order to characterize the damage coupling behavior and its effect on failure strength of CMCs, a new methodology for damage coupling analysis was formulated. And, new kind of damage-based failure criteria were established under plane stress state, including maximum damage criterion and quadratic damage criterion. Both the criteria require investigation upon the damage evolution laws. The proposed failure theory was applied to predict the coupled damage behavior and the off-axis strength of a 2D C/SiC composite. The theoretical results agree well with the experimental data.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.     

Residual strength of composite laminates containing scarfed and straight-sided holes

This investigation is motivated by the needs to quantify the load-carrying capacity of composite laminates with scarfed holes, a damage cut-out shape employed to achieve flush repairs of composites. Both experimental testing and analytical modelling were carried out to investigate the damage progression behaviour of composite laminates containing either straight-sided or scarfed holes. Hoop strains were recorded by strain gauges located along the scarf surface and the results indicate a much greater extent of damage progression than specimens containing straight-sided holes. Three different strength-prediction models were employed to quantify the residual strength, including an analytical cohesive zone model developed in this work, an analytical inherent-flaw fracture mechanics method and a finite element-based continuum damage model. Comparisons of the experimental results with the model predictions reveal that the continuum damage model, calibrated using data from coupons with straight-sided holes, provides promising correlation with experimental results.     

Numerical simulation of strain-rate dependent transition of transverse tensile failure mode in fiber-reinforced composites

This study numerically simulates strain-rate dependent transverse tensile failure of unidirectional composites. The authors’ previous study reported that the failure mode depends on the strain rate, with an interface-failure-dominant mode at a relatively high strain rate and a matrix-failure-dominant mode at relatively low strain rate. The present study aims to demonstrate this failure-mode transition by a periodic unit-cell simulation containing 20 fibers located randomly in the matrix. An elasto-viscoplastic constitutive equation that involves continuum damage mechanics regarding yielding and cavitation-induced brittle failure is used for the matrix. A cohesive zone model is employed for the fiber–matrix interface, considering mixed-mode interfacial failure. For the results, the relationship between failure modes and the strain rate is consistent with the authors’ previous studies.     

A novel methodology for the rapid identification of the water diffusion coefficients of composite materials

The paper presents a novel methodology for the rapid identification of the water diffusion coefficients of composite materials. The methodology consists in employing a numerical parametric Proper Generalized Decomposition (PGD) method allowing incorporating the diffusion coefficients among the number of degrees of freedom. Compared to classical identification schemes, often based on Finite Element Method (FEM) iterations, the proposed method allows achieving consistent CPU time gain. The method is general and can be applied when diffusion anomalies take place or when diffusion–reaction coupling must be taken into account, moreover can deal with anisotropic materials. However, for the scope of illustration, in the present case, it is applied to the simple case of “classical diffusion” (Fick’s model with constant boundary conditions) and concerns isotropic materials.     

Preserving the strength of corrugated cardboard under high humidity condition using nano-sized mists

The paper evaluates the adsorption of water vapor and compression strength of three types of commercially made corrugated cardboard boxes for packing strawberry, mizuna and broccoli. The experiments were conducted on the specimens and empty cardboard boxes at constant temperature and 95% relative humidity (RH). The samples were stored under the environments of two types of mists, namely nanomist and ultrasonic-mist over a period of 7 days. Nano-sized mist, which are called nanomists and defined as particles of about 60 nm in diameter, easily evaporate and are considered not to damp the corrugated boxes in comparison with the larger size ultrasonic-mists. The change in moisture content of the samples was first measured at intervals of 6, 12 and 24 h and then daily over 7 days. Compressive strength test was measured by the means of using a tensile and compression testing machine. The results revealed that moisture content of both specimen and cardboard box tests exposed to the nanomist and ultrasonic-mist at the end of experiments was 19.9% d.b. and 30.4% d.b., respectively (dry basis: g-water in material/ g-dry weight) although temperature and relative humidity were almost the same for both cases. Furthermore, the strength of cardboard specimens conditioned with nanomist after 7 days at 5.8 °C and 94.2% RH decreased by 44.3–56.9% whilst under ultrasonic-mist condition it reduced by 66.5–70% depending on the types of cardboards. Similarly, maximum compressive load of corrugated cardboard boxes exposed to nanomist and ultrasonic-mist decreased gradually over the time. It was analytically predicted that the boxes exposed to nanomist maintained its maximum compressive load at 28%, whereas those exposed to ultrasonic-mist remained at 14% after 7 days. The maximum compressive load of corrugated cardboards exponentially decreased with an increase in moisture content.     

Failure response of fiber-epoxy unidirectional laminate under transverse tensile/compressive loading using finite-volume micromechanics

The transverse damage initiation and extension of a unidirectional laminated composite under transverse tensile/compressive loading are evaluated by means of Representative Volume Element (RVE) presented in this paper based on an advanced homogenization model called finite-volume direct averaging micromechanics (FVDAM) theory. Fiber, fiber-matrix interface and matrix phases are considered within the RVE in determining fiber-matrix interface debonding and matrix cracking. The simulated fracture patterns are shown to be in good agreement with experimental observations.     

Comparison of a viscoelastic frictional interface theory with a kinetic crack growth theory in unidirectional composites

In this paper we compare a frictional interface theory for fiber and matrix load sharing with a kinetic crack growth theory as applicable to the failure of unidirectional composites. First we formulate the creep lifetime prediction based on the viscoelastic frictional interface theory, and then we determine a parameter in the kinetic crack growth theory by fitting it to the frictional interface theory in terms of the creep lifetime prediction. Times-to-failure under a constant strain rate condition are then derived by these two models, and they are compared. The residual strengths after interrupted loading are also compared.     

A numerical methodology for optimizing the geometry of composite structural parts with regard to strength

The increased use of composite materials in lightweight structures has generated the need for optimizing the geometry of composite structural parts with regard to strength, weight and cost. Most existing optimization methodologies focus on weight and cost mainly due to the difficulties in predicting strength of composite materials. In this paper, a numerical methodology for optimizing the geometry of composite structural parts with regard to strength by maintaining the initial weight is proposed. The methodology is a combination of the optimization module of the ANSYS FE code and a progressive damage modeling module. Both modules and the interface between them were programmed using the ANSYS programming language, thus enabling the implementation of the methodology in a single step. The parametric design language involves two verifications tests: one of the progressive damage model against experiments and one of the global optimization methodology performed by comparing the strength of the initial and the optimum geometry. There were made two applications of the numerical optimization methodology, both on H-shaped adhesively bonded joints subjected to quasi-static load. In the first application, the H-shaped joining profile was made from non-crimp fabric composite material while in the second from a novel fully interlaced 3D woven composite material. In the optimization of the joint’s geometry, failure in the composite material as well as debonding between the assembled parts was considered. For both cases, the optimization led to a considerable increase in joint’s strength.     

Parametric study of failure mechanisms and optimal configurations of pseudo-ductile thin-ply UD hybrid composites

The effect of different parameters on the gradual failure and pseudo-ductility of thin UD hybrids is studied using an analytical method developed recently. Damage mode maps are proposed to show the effect of different geometric parameters for a specific material combination. This type of map is a novel and efficient method to find the optimum configuration of UD hybrids and also indicates the importance of thin layers to achieve the optimum geometric parameters in practice. The material parametric study reveals that there is always a trade-off between the “yield stress” and the amount of pseudo-ductility; higher yield stresses leads to lower pseudo-ductility and vice versa. However, application of high-stiffness fibres with high strengths as the low strain material can provide both better pseudo-ductility and yield stress.     

2D and 3D imaging of fatigue failure mechanisms of 3D woven composites

A detailed investigation of the failure mechanisms for angle-interlocked (AI) and modified layer-to-layer (MLL) three dimensional (3D) woven composites under tension–tension (T–T) fatigue loading has been conducted using surface optical microscopy, cross-sectional SEM imaging, and non-destructive X-ray computed tomography (CT). X-ray microCT has revealed how cracks including surface matrix cracks, transverse matrix cracks, fibre/matrix interfacial debonding or delamination develop, and has delineated the complex 3D morphology of these cracks in relation to fibre architecture. For both weaves examined, transverse cracks soon become uniformly distributed in the weft yarns. A higher crack density was found in the AI composite than the MLL composite. Transverse cracking initiates in the fibre rich regions of weft yarns rather than the resin rich regions. Delaminations in the failed MLL specimen were more extensive than the AI specimen. It is suggested that for the MLL composite that debonding between the binder yarns and surrounding material is the predominant damage mechanism.     

Computational micromechanics evaluation of the effect of fibre shape on the transverse strength of unidirectional composites: An approach to virtual materials design

Computational micromechanics of composites is an emerging tool required for virtual materials design (VMD) to address the effect of different variables involved before materials are manufactured. This strategy will avoid unnecessary costs, reducing trial-and-error campaigns leading to fast material developments for tailored properties. In this work, the effect of the fibre cross section on the transverse behaviour of unidirectional fibre composites has been evaluated by means of computational micromechanics. To this end, periodic representative volume elements containing uniform and random dispersions of 50% of parallel non-circular fibres with lobular, polygonal and elliptical shapes were generated. Fibre/matrix interface failure as well as matrix plasticity/damage were considered as the fundamental failure mechanisms operating at the microscale under transverse loading. Circular fibres showed the best averaged behaviour although lobular fibres exhibited superior performance in transverse compression mainly due to the higher tensile thermal residual stresses generated during cooling at the fibre/matrix interface.     

Formulation and assessment of an enhanced finite element procedure for the analysis of delamination growth phenomena in composite structures

An existing procedure based on the combined use of the Virtual Crack Closure Technique and of a fail release approach for the analysis of delamination growth phenomena in composite structures has been enhanced with a front-tracing algorithm and suitable expressions for the evaluation of the Strain Energy Release Rate when dealing with non-smoothed delamination fronts. The enhanced procedure has been implemented into a commercial finite element software by means of user subroutines and applied to the analysis of a composite stiffened panel with an embedded delamination under compressive load. The effectiveness and robustness of the enhanced procedure have been assessed by comparing literature experimental data and numerical results obtained by using different mesh densities in the damaged area (global/local approach).     

Simplified ultimate strength analysis of compressed composite plates with linear material degradation

Simply supported, rectangular, composite plates subjected to in-plane compressive load have been investigated for ultimate strength. An efficient, semi-analytical method has been established based on large deflection theory and first order shear deformation theory. After damage initiation, linear degradation of the material properties has been applied to the affected region of a failed ply. Two different displacement fields have been examined for their influence on the strength predictions. The approach is validated against earlier advanced finite element calculations, and can be readily applied in specific design situations or to generate parametric design curves.     

Numerical evaluation for debonding failure phenomenon of adhesively bonded joints at cryogenic temperatures

In the present study, material characteristics, such as inelastic constitutive behaviour and debonding failure, of an adhesively bonded joint (ABJ) at cryogenic temperature have been evaluated using a computational approach. The modified Bodner-Partom model (BP model) has been introduced to describe the material nonlinearities of ABJ. The Gurson-Tvergaard model (GT model) has also been implemented into the constitutive model in order to analyse the phenomenon of debonding failure. An ABAQUS user-defined subroutine UMAT is developed using a damage-coupled constitutive model based on an implicit formulation. The numerical results are compared with a series of lap shear tests of ABJ at cryogenic temperature in order to verify the proposed method.