Damage Evolution In Composite Materials Research Articles

Hygroscopic phase field fracture modelling of composite materials

This paper investigates the effect of moisture content upon the degradation behaviour of composite materials. A coupled phase field framework considering moisture diffusion, hygroscopic expansion, and fracture behaviour is developed. This multi-physics framework is used to explore the damage evolution of composite materials, spanning the micro-, meso- and macro-scales. The micro-scale unit-cell model shows how the mismatch between the hygroscopic expansion of fibre and matrix leads to interface debonding. From the meso-scale ply-level model, we learn that the distribution of fibres has a minor influence on the material properties, while increasing moisture content facilitates interface debonding. The macro-scale laminate-level model shows that moisture induces a higher degree of damage on the longitudinal ply relative to the transverse ply. This work opens a new avenue to understand and predict environmentally assisted degradation in composite materials.

Low-velocity impact behavior of interlayer hybrid foldcore sandwich structures with carbon/glass fibers

Interlayer hybrid foldcore sandwich structures have been introduced in this study. Both facesheets and foldcores were hybridized in the same ply sequence with two layers of plain woven carbon (C) and two layers of satin woven glass (G) fiber prepregs (Carbon-to-Glass ratio 1:1). Foldcore sandwich structures with six-ply combinations were designed including two symmetric layups (GCCG and CGGC) and four asymmetric layups (CGCG, GCGC, CCGG, and GGCC). The static compression and low-velocity impact performance of foldcore sandwich structures of different ply sequences were investigated. A user-defined material subroutine (VUMAT) was employed to characterize the damage evolution of composite materials based on selected failure criteria. The findings revealed that hybridization was effective in enhancing its energy absorption value of pure carbon fiber foldcore sandwich structures in terms of static compression performance. The damage degree of the interlayer hybrid foldcore sandwich structures increased gradually with the increasing impact energy. Moreover, the damage of facesheet was mainly caused by fiber tensile, matrix tensile damage and delamination, and damage of foldcore was mainly delamination. GCGC sandwich panel with asymmetrical cross-ply sequence has the optimal load bearing capacity, and CGGC sandwich panel with symmetrical layup has a strong energy absorption capacity under high-energy impact load. The foldcore sandwich structure showed smaller peak load, longer crushing distance, and more energy absorption value under larger oblique impact angle. The simulation and experimental results show good agreement in peak force, maximum energy absorption, and damage mode.

A progressive damage based lattice model for dynamic fracture of composite materials

In this paper we present an extension of the linear elastic lattice model for anisotropic materials previously developed by the authors. This approach takes into consideration damage evolution in composite materials by effectively integrating a softening constitutive law into our earlier triangular lattice model. In this framework, the material constitutive law accounts for the fracture energy of the matrix in the fiber direction in a simple and direct way. To validate the proposed methodology, we conducted a dynamic fracture analysis of a unidirectional carbon fiber composite material. In particular, we studied rectangular notched beam specimens under impact loading conditions. Results are presented in terms of crack pattern, crack speed, and crack length evolution. Predictions obtained were in reasonable agreement with previous numerical and experimental observations.

Assessment of Residual Elastic Properties of a Damaged Composite Plate with Combined Damage Index and Finite Element Methods

In structural component applications the use of composite materials is increasing thanks to their optimal mechanical characteristics. However, the complexity of the damage evolution in composite materials significantly limits their widespread diffusion. Non-destructive tests are thus becoming ever more important. The detecting Damage Index ( D I d ) technique has been recently brought in the realm of the non-destructive characterization tests for components made of composite material. In contrast to other techniques, this methodology allows to quantitatively assess local residual properties. In this paper, the D I d technique is adopted in combination with the finite element method. The mechanical response of two composite plates (an 8-layer twill fabric carbon/epoxy) subjected to four-point bending test is firstly used to tune a finite element model of the laminate. Then, an undamaged laminate of the same composite material is progressively damaged through repeated four-point bending tests. Local residual elastic properties are mapped on the plate through the D I d technique. A continuous polynomial curve has been considered to account for the variation of the elastic modulus in the finite element model. The resulting force-displacement curve of the numerical analysis is compared to experimental data of damaged plate, resulting in very good agreement. The combination of the experimental activity and the numerical finite element analysis points out the accuracy of the D I d methodology in assessing local residual elastic properties of composite materials.

Macro-mechanical damage modeling of fibrous composite materials accounting for non-linear material behavior

In the present paper, a new macro-mechanical model for tracing damage-evolution in composite materials is proposed. The present model represents a new extension and a new approach to a previous model (Ghazi-Farid model), which can be applied for a general state of stress. The model is verified by comparing its results with those corresponding to Ghazi-Farid model for different composite materials and it seems to give very close correlations. The proposed model can be applied to both elastic and inelastic materials as well as generally orthotropic fibrous composite nonlinear-materials.It was concluded that shear damage is always higher than any other damage types due to the high nonlinear-material shear behavior, which causes high plastic strain-energy density. Damage in a composite lamina causes a reduction in its stiffness. Therefore, a new quantum-damage variable is proposed: '' tangential quantum-damage variable '' to quantify the overall-damage in a composite lamina. Percentage reduction of composite stiffness depends mainly on the amount of resulting damage irrespective of fiber-orientation angle. So, a new trend for the behavior of composite materials is introduced which states that the relation between damage-evolution and corresponding stiffness-reduction follows a certain behavior and it is independent of fiber-orientation. Each composite material has a unique trend which is verified using three different composite materials; Boron-Epoxy-Narmco 5505, Graphite-Epoxy 4617/Modmore-II, and Carbon-Epoxy AS4/3501–6.A new damage-term is introduced as: "directional damage-variable", to simplify tracing damage in the case of a uniaxial off-axis loading. The new damage-variable was used to predict damage-evolution in the three laminas made of the indicated composite materials. It is concluded that, Graphite-Epoxy 4617/Modmore-II has the minimum damage at all stress levels and Boron-Epoxy-Narmco 5505 has the maximum damage. The importance of the new damage-term makes it easier to predict damage and make preferences between several composite materials subjected to uniaxial off-axis loading.

Characterization of residual strength in transversely loaded glass-polyester composites by acoustic emission and sentry function

This paper addresses damage evolution in composite materials by using acoustic emission (AE) as a non-destructive testing method to evaluate residual strength of specimens. Two composite samples at different lay-ups (Unidirectional and Cross-ply) were used in this investigation. The three-point bending test was applied on composite samples transversely while damage progression was monitored by means of acoustic emission technique. The test was stopped at three different damage levels, one (Low) corresponding to the load value of 10% before maximum load, the second (Medium) corresponding to the first load drop in the load-displacement curve and the third (High) to the complete failure of the specimen. Depend on loading level, the damage value was variant as it caused different residual strength that was related to acoustic emission signals activities. To find this relationship, firstly, the tensile test was carried out on damaged samples to evaluate the residual strength of them. Secondly, a method based on a special purpose function, called Sentry function, which is defined as the logarithm of the ratio between mechanical energy and acoustic energy, was applied to find out the relationship between the tensile residual strength and acoustic emission activates. Results show that the measurement of the strain energy in specimens and of the acoustic emission energy released by fracture events made it possible to estimate the amount of induced damage and can be effective in residual strength estimation and damage evolution.

Study of damage evolution in composite materials based on the Thermoelastic Phase Analysis (TPA) method

Abstract Standards and conventional procedures used for analysing fatigue damage in composite materials involve high experimental campaign costs due to time-consuming tests. This aspect becomes relevant for large structures where the cost of experimental setup tends to rise according to structure dimensions. In this regard, in recent years, efforts to produce fatigue characterisation of materials have made use of several experimental techniques, i.e. thermographic techniques. Most of these, however, refer to Standard specimens and laboratory equipment and set-up. Through the use of Thermography, in this work a new procedure has been developed which is capable of monitoring damage in GFRP composite material. The analysis of thermal signal in the frequency domain allows for the isolation of indexes which are related to the thermoelastic and dissipative heat sources. In particular, the phase of thermoelastic signal, associated with intrinsic dissipation processes occurring in the material, has been used to localize and assess the damaged areas in a quantitative manner. Moreover, the thermoelastic phase analysis leads to an evaluation of the endurance limit of composites. In fact, by comparing the results with those provided by the standard test methods, the potential has been shown of the proposed procedure firstly as a non-destructive technique for continuous monitoring of damage in composite structures undergoing fatigue loadings, and secondly, as a fatigue limit index.

Acoustic emission technique for fracture analysis in wood materials

Understanding the failure mechanisms of construction materials, as well as their damage evolution, constitute two key factors to improving structural design tools. Depending on the failure modes to be highlighted and studied, several test methods and analysis tools have been developed. One such development, the acoustic emission technique (AET), is an experimental tool appropriate for characterizing material behavior by means of monitoring the fracture process. Despite the widespread uses of acoustic emission techniques to characterize and monitor the damage evolution of composite materials, only a few research studies have focused on using AET to characterize the mechanical behavior of wood materials. In the present work, the failure process in Douglas fir under monotonic loading is studied by comparing three experimental methods: force-displacement curve analysis, acoustic emission measurements, and digital image acquisition. First of all, results show good correlation and complementarities among the methods employed. Second, analyzing acoustic emission signals by considering the event number and the cumulative events yields interesting information on crack initiation and growth without the material. Moreover, an additional analysis of acoustic emission data (involving the determination of source locations and the study of amplitude distributions) opens the possibility to characterize the fracture process zone which is a key damage mechanism in wood materials.

Comment on “Simulation of damage evolution in composites: A phase-field model”

Here we reassess the results of Biner and Hu [13] on phase-field simulations of damage evolution in composite materials. In particular we discuss the validity of the results presented therein in the framework of linear elasticity theory.

Dynamic response and damage evolution in composite materials subjected to underwater explosive loading: An experimental and computational study

Abstract The effect of underwater shock loading on an E-Glass/Epoxy composite material has been studied. The work consists of experimental testing, utilizing a water filled conical shock tube and computational simulations, utilizing the commercially available LS-DYNA finite element code. Two test series have been performed and simulated: (1) a reduced energy series which allowed for the use of strain gages and (2) a series with increased energy which imparted material damage. The strain gage data and the computational results show a high level of correlation using the Russell error measure. The finite element models are also shown to be able to simulate the onset of material damage by both in-plane and delamination mechanisms.

A stochastic fatigue damage method for composite materials based on Markov chains and infrared thermography

Stochastic Markov chain methods are applied to model the fatigue damage evolution in composite materials subjected to cyclic mechanical loadings and monitored by infrared thermography (IR-T) techniques. The IR signal from the surface of open-hole S2-glass/E733FR laminates is captured concurrently during constant amplitude fatigue loadings. The IR testing has high thermal sensitivity below 10 mK and the image integration is synchronized with the mechanical loading. A thermoelastic stress analysis (TSA) technique using thermomechanics is used to process the IR fields and relate them to surface stresses and strains. Damage metric is developed for the composite samples based on an area stress reduction threshold. The applicability and validity of the proposed TSA damage index is evaluated by comparing it to the classical overall stiffness reduction measured at select fatigue intervals using an extensometer. The IR-TSA damage index at the last fatigue cycle is used to calibrate the Markov chain models (MCMs). The damage predictions of the MCMs are then examined at different fatigue cycles. A new method is proposed to construct a stochastic S– N curve utilizing the MCMs. The proposed IR-TSA with Markov stochastics is shown to be very effective in predicting the damage evolution and allowed constructing a wide-range of stochastic S– N curves for several composite material systems including experimental results from the literature.

A fatigue damage model of composite materials

The mechanical properties of composite materials degrade progressively with the increasing of the number of cyclic loadings. Based on the stiffness degradation rule of composites, a phenomenological fatigue damage model is presented in this paper, which contains two material parameters. They are proportional to the fatigue life of materials and inversely proportional to the fatigue loading level. Thirteen sets of experimental data of composite stiffness degradation were employed to verify the presented model, and the statistical results showed that this model is capable of describing the damage evolution of composite materials. The characteristics of damage development and accumulation of composite materials subjected to variable loading were studied in this paper. Four sets of two-level loading experimental data were cited to verify the damage model, and the results showed that the predicted life is in good agreement with the experimental ones.

Damage evolution and infrared thermography in woven composite laminates under fatigue loading

An analytical model based on cumulative damage has been used for predicting the damage evolution in composite materials. The model is verified with experimental data from a carbon/epoxy composite fatigued under tension–tension load. Fatigue tests of specimens have been monitored with an infra-red thermography system. By analysing the temperature of the external surface during the application of cyclic loading, it is possible to evaluate the damage evolution. The model agrees well with the experimental data, and it can be used to predict the evolution of damage in composites.

Probabilistic Fatigue–Creep Life Prediction of Composites

In this paper, a continuum damage mechanics-based model is developed to model the damage evolution of composite materials subjected to fatigue and creep loading. A simulation-based procedure of probabilistic creep fatigue life prediction of composite materials is presented with the proposed damage accumulation model. Uncertainties in loading are considered through simulated random loading histories. Material properties are addressed as random variables to consider the uncertainties in materials. The proposed method is capable of considering the interaction effects between different loading levels and the interaction effect between creep and fatigue. The application of the damage accumulation model and the proposed method for probabilistic life prediction are illustrated with a numerical example.

Fatigue damage modelling of composite materials

This paper develops a mathematical model for fatigue damage evolution in composite materials. The characteristics of damage growth in composite materials are studied and compared with those of damage growth in homogeneous materials. Continuum damage mechanics concepts are used to evaluate the degradation of composite materials under cyclic loading. A new damage accumulation model is proposed to capture the unique characteristics of composite materials. The proposed model is found to be more accurate than existing models, both in modelling the rapid damage growth early in life and near the end of fatigue life. The parameters for the proposed model are obtained with experimental data. A numerical example is implemented to illustrate that the proposed model is able to accurately fit several different sets of experimental data.

Transformation field analysis of damage evolution in composite materials

Evolution of distributed damage in heterogeneous solids is modeled using the transformation field analysis method (Proc. Roy. Soc. Lond. A (1992) 437, 311) and selected models of interface debonding in fibrous or particulate composites. In this approach, stress changes caused by local debonding under increasing overall loads are represented by residual stresses generated by damage-equivalent eigenstrains that act together with the applied mechanical loading program and physically based local transformation strains on an undamaged elastic aggregate. Interaction of the actual and equivalent eigenstrains with the mechanical loads at any state of damage is described by certain transformation influence functions; this provides explicit expressions for the local stress averages and the damage-induced enhancement of the overall compliance at any current damage state. Damage rates are then derived from a prescribed probability distribution of interface strength and local potential energy released by debonding. Numerical simulations of damage evolution in a glass/elastomer composite indicate which of these two conditions controls the process at different reinforcement densities and overall stress states. In general, the energy released by a single particle at a given overall stress decreases with increasing reinforcement density, and in proportion to particle size. Therefore, dense reinforcement by small-diameter particles or fibers should enhance resistance to inclusion debonding in composite systems.