Damage Development In Composites Research Articles

Identification of current material state in composites using a dielectric state variable

Damage development in composites is a complicated process, unlike metals where the failure is triggered by unstable growth of a single crack; failure in composites is triggered by interaction of multiple local damage events. The ‘beginning of the end’ in composites is well described in terms of ‘criticality’ and, identification of material state is an important step for predicting this criticality. Current non-destructive evaluation techniques can accurately map the damage zone but are incapable of identifying the material state. In the current research, the main intention is to observe and model the variation in dielectric response as a function of defect development and coupling to characterize the current material state (Characteristic Damage State (CDS), damage interaction) in unidirectional, woven glass fiber reinforced polymer composites loaded in quasi-static axial tension. The changes in dielectric response are correlated with change in material state validated by damage patterns obtained using edge replication techniques.

An unsupervised pattern recognition approach for AE data originating from fatigue tests on polymer–composite materials

This work investigates acoustic emission generated during tension fatigue tests carried out on a carbon fiber reinforced polymer (CFRP) composite specimen. Since massive fatigue data processing, especially noise reduction, remains an important challenge in AE data analysis, a Mahalanobis distance-based noise modeling has been proposed in the present work to tackle this problem. A sequential feature selection based on Davies-Bouldin index has been implemented for fast dimensionality reduction. An unsupervised classifier offline-learned from quasi-static data is then used to classify the data to different AE sources with the possibility to dynamically accommodate with unseen ones. With an efficient proposed noise removal and automatic separation of AE events, this pattern discovery procedure provides an insight into fatigue damage development in composites in the presence of millions of AE events.

Fatigue-driven model for mode II interlaminar delamination propagation of fibre/epoxy-reinforced composite laminates under three-point end-notched flexure

This paper deals with mode II interlaminar delamination properties and failure mechanisms of composite laminates in three-point end-notched flexure (3-ENF) tests under fixed cyclic loading. The results for fracture toughness, fatigue driving force and delamination propagation behaviours of three different types of carbon and E-glass fibre/epoxy resin-reinforced composite laminates are evaluated and compared with each other for providing an insight into fatigue damage development in composites and for constituting a fundamental basis for the development of a characterization model. A new characterization formulation is presented for mode II interlaminar delamination propagation based on controlling fatigue stress. Finally, the model is applied to experimental data, demonstrating the practical and effective use of the proposed model. It is shown that fatigue-driven strength for mode II interlaminar delamination propagation can be obtained realistically from experimental data using the new formulae.

Coupled carbon nanotube network and acoustic emission monitoring for sensing of damage development in composites

Material and structural reliability is crucial for the potential use of composite materials in a variety of structural applications. As a result, damage detection and health monitoring of fiber-reinforced composites are attracting progressively more attention in the field of materials research. In this work, small amounts of carbon nanotubes are added into traditional fiber composites to form electrically conductive networks throughout the polymer matrix. The electrical response of the carbon nanotube network to accumulated damage combined with the acoustic emission (AE) technique is utilized to sense damage initiation and evolution in laminated composites. The parameters which correspond to the resistance change of the specimen due to damage in quasi-static and cyclic tests are used to evaluate the damage state in the material. There exists a bi-linear relationship between the resistance change and AE signal cumulative counts which gives insight toward the damage state of the material during quasi-static and cyclic tests.

A strain-based residual strength model of carbon fibre/epoxy composites based on CAI and fatigue residual strength concepts

This paper addresses the static, compressive after impact (CAI) and fatigue properties and the failure mechanisms of T300/QY8911 carbon fibre reinforced composite laminates with different lay-ups to optimize the stacking sequence. Ten different lay-ups have been studied and the results for static strength, CAI and fatigue residual strength under different fatigue stress amplitudes are evaluated and compared with each other. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a strain-based residual strength model. A new formulation is presented for a fatigue-driven residual strength surface based on controlling fatigue strain. The parameter determination formulae of the residual strength surface are established to deal with the test data effectively and easily. Finally, the model is applied to experimental data, demonstrating the practical and effective use of the proposed model. It is shown that fatigue-driven residual strength can be obtained realistically from experimental data using the new formulae.

Mechanical Behaviours for Textile Composites by FEM Based on Damage Mechanics

To make an expansion of composite consumption ever widening with use at demanding and safety critical applications, it is important to establish the evaluation technique of the strength, the safety, etc. To reach the goal with a useful reliable technique, we have developed a numerical simulation program on damage development of composites based on damage mechanics. The following points are the keys to simulate the mechanical behavior; (1) The generation technique of FE model with yarns and matrix, (2) The classification of anisotropic damage mode based on damage mechanics, (3) Multi-scale modeling by Mesh superposition method. In this paper, the numerical technique and the results of multi-scale analysis of woven composites are described.

On static and fatigue strength determination of carbon fibre/epoxy composites Part 1: Experiments

This work seeks to investigate the static and fatigue properties and the failure mechanisms of T300/QY8911 carbon-fibre-reinforced composite laminates with different layups to optimize the stacking sequence effect. Three different layups have been studied, namely, [45/02/-45/902/-45/0/45/90]s, [45/-45/0/-45/0/45/90/45/0/-45]s and [45/0/-45/0/-45/0/45/0/90/0]s. The results for static strength and fatigue residual strength under different fatigue stress amplitudes are evaluated and compared with each other. The damage and failure mechanisms of the composite laminates are discussed. It is observed that all tension-tension fatigue damage patterns of the notched laminates are similar while the tension-tension fatigue properties vary with the laminate layup. The reduction of stress concentration caused by the tension-tension fatigue damage leads to an improvement of the residual strength of the notched specimens in contrast to their static strength properties. The damage mechanics of the notched laminates under compression-compression loading are more complex than those of the tension-tension fatigue specimens; their damage patterns are influenced by the test clamping fixture, the layup, the size of specimens and the diameter of the hole. The residual strengths are lower than those of the specimens without fatigue damage. It is verified that in a high-temperature and moisture environment, there is a decrease in static compression strength. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a strain-based residual strength model.

Cycle-based analysis of damage and failure in advanced composites under fatigue: 1. Experimental observation of damage development within loading cycles

Damage development within loading cycles and the possibility of cycle-based modeling of fatigue damage evolution and failure in advanced composites are addressed in this work. In Part 1 of the work, the fatigue damage development in an unnotched composite laminate is studied by an advanced acoustic emission (AE) technique. The overall time history of the acoustic emission is recorded and separated into the emission from loading and unloading phases of the loading cycles. The source location histories and distributions of the AE events over the fatigue stress range are extracted and analyzed. Qualitatively different time histories and stress distributions for the emission generated during loading and unloading are observed and analyzed, for the first time. It is concluded that most of the emission from the unloading phase is due to internal friction between the crack faces and most of the damage is developed during the loading phase of the cycles. The location distribution of the fatigue damage is found to be fairly random to failure, indicating that the dominating fatigue fracture process in an unnotched laminate is scattered damage nucleation and accumulation. The time history of the overall damage evolution is found to exhibit the classical initial, steady, and final failure development stages. Within the loading cycles, the damage is developed over the entire stress range, with the substantial amount developed at stresses below the maximum fatigue stress. The latter observation sheds light on the reported effects of the loading cycle shape on the fatigue behavior and life of composites. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a cycle-based model in Part 2 of this work.

Damage Formulation of Random Particulate Composites by Combined Micromechanics and Damage Mechanics

Damage evolution of random particulate composites is formulated by the combination of two distinct theories, micromechanics and damage mechanics. Damage mechanics is applied to filler particles to describe interfacial debonding, which in turn incorporated into Mori-Tanaka's mean field theory for stiffness prediction of damaged composites. The model uses an energy balance concept to account for nonlinear behavior due to particle debonding. With this model, the effects of particle volume fraction, size distribution and interfacial strength on damage development of composites are analytically investigated. Emphasis is also placed on the effect of partial debonding which plays a dominant role for damage induced anisotropy. It is found that the model can reproduce not only stress-strain relationship but also variation in Poisson's ratio observed in damage process.

Damage development in composites with large stress gradients

Implementation of a meso-scale damage model into the commercial finite element code ABAQUS via user-defined FORTRAN subroutines is described, and the results of investigations on damage development in structural configurations with large stress gradients are presented for carbon-fiber/polymer-matrix composites. The implemented model, which involves damage in both the meso-scale layer and an interface between the layers, is applied in a study of damage growth in finite-width angle-ply coupons and a composite panel with a terminated stiffener. The definition and evolution of state-dependent variables for layer damage is accomplished by means of a user-defined subroutine and the damageable interface is accomplished by using a user-defined element. It is demonstrated that as the fiber orientation progresses from 10 to 45°, the mode of damage changes from interlaminar damage, (i.e. delamination) to intralaminar damage, consistent with experimental observations. Analysis of damage development in the composite panel subjected to uniform compression shows that interlaminar shear stresses produce delamination at the skin-stiffener interface before failure of the panel due to tensile fiber rupture,— again consistent with experimental observations.

On-line observation of interlaminar damage by ultrasonic inspection

This paper explores an on-line experimental method to highlight the process of internal damage development in composites by taking advantage of ultrasonic inspection. A loading device, which can work together with an ultrasonic inspection system, was designed, and the interlaminar shear damage of a double-sided grooved specimen of composite was examined on-line with the system. A full view of the progressive internal interlaminar damage, seen only with difficulty by common inspection methods, was successfully achieved.

A multiplexed optical fibre-based extrinsic Fabry-Perot sensor system for in-situ strain monitoring in composites

The detection of impact damage in fibre reinforced composites is of significant concern because such damage can reduce the load-bearing ability of the composite. A number of factors can influence the nature and extent of impact damage development in composites including: (a) the type of reinforcing fibre and resin system; (b) the magnitude of the residual (fabrication) stresses; (c) the lay-up sequence; and (d) other factors such as the nature of the impactor, impact velocity, impact energy, temperature, moisture content in the composites, etc. From a structural health monitoring point of view, it is necessary to investigate the distribution of damage through the thickness of the composite. This paper reports on a simple, partially multiplexed optical fibre strain sensor system for in-situ strain and residual strain measurements in a carbon fibre reinforced epoxy composite. An extrinsic Fabry-Perot interferometric (EFPI) sensor design was used along with single-mode fibres. The multiplexing scheme was based on wavelength division via the use of two super luminescent diodes (SLDs) at different wavelengths. A low-cost fibre optic CCD spectrometer was used as the detector. The multiplexing scheme was demonstrated using two EFPI sensors. In principle, a number of EFPI sensors can be multiplexed using the proposed scheme provided that each sensor is illuminated at a specified and different wavelength. The feasibility of detecting the residual strain in the composite was demonstrated successfully at two specified positions within a 16-ply carbon fibre reinforced composite panel. Preliminary results indicated that the sensor system was also capable of detecting the effects of a 3.2 J impact. Excellent correlation was obtained between the EFPI sensor output and that obtained using surface mounted strain gauges.

Damage development in composites: Mechanisms and modelling

This paper presents first a review of the mechanisms of damage and its development in composite materials subjected to mechanical loads. The materials considered are thermosetting polymers reinforced with short or long fibres of glass or carbon. Laminates with woven fabric are also considered. A discussion regarding how the essential features of the damage mechanisms may be incorporated in a modelling approach is then given. Two basically different but complementary approaches, namely, micromechanics modelling and continuum damage modelling, are described. The present author's own approach using the internal variables concept is discussed at some length, and the predictions of elastic property degradation given by it are illustrated by experimental data. This paper is an updated and expanded version of a previous paper written by the author (32).