Didier Baptiste

A statistical micromechanical model of anisotropic damage for S.M.C. composites

Abstract The objective of this paper is to predict the entire stress/ strain curves and the loss of stiffness with damage for sheet-moulding-compound (SMC) composites. This is achieved by developing a statistical micro-macro relationship with the help of the model of Mori and Tanaka. In the case of an SMC composite with a 32% fiber volume fraction, debonding at the fiber/matrix interfaces is the predominant ‘microdamage’ mechanism. In order to relate the microstructure, and its local perturbations, to the macroscopic damaged behaviour, we introduce a statistical local damage criterion and the concept of the equivalent damaged inclusion in the micro-macro relationship of Mori and Tanaka. This paper shows the relationship between the statistical interface failure criterion and the scattering observed on the macroscopic mechanical test results.

Prediction of the effective damage properties and failure properties of nonlinear anisotropic discontinuous reinforced composites

We studied the tensile behavior and damage of two different composites. An A12080 reinforced with different volume fractions of silicon carbide particles and an SMC R (polyester matrix with randomly oriented glass fibres). The main damage mechanism is particle failure (for the MMC) or fibre-matrix interface debonding (for the SMC).

Tensile dynamic behaviour of a quasi-unidirectonal E-glass/polyester composite

Abstract The objective of this study is to determine the tensile behaviour of a unidirectional E-glass/polyester composite in relation to the strain rate. Several orientations of fibres in the composite are tested with an hydraulic tensile machine, which covers a range of loading rate from quasi-static up to approximately 20 m/s. It is shown that experimentally and numerically, high velocity entails specific difficulties. In this study, some tests were improved in order to limit these effects and to obtain directly reliable material properties. The composite studied is quasi-unidirectional, the influence of 5% vol. of weft fibres is evaluated at quasi-static strain rate. It is found that the presence of the weft fibres is the source of the damage development. Longitudinal (0°) and transverse (90°) orientations of fibres in the composite are tested. The effects of strain rate on mechanical properties (maximum stress, Youngs modulus, strain to failure) are noted. Damage mechanisms seem to evolve with the strain rate.

Prediction of the anisotropic damaged behavior of composite materials: introduction of multilocal failure criteria in a micro-macro relationship

Abstract In an SMC composite two damage processes are observed: fiber fracture and interfacial debonding. Local criteria of damage initiation were introduced in a micro-mechanical model to reproduce these phenomena and predict the material behavior. The model is based on the stiffness prediction by the Mori and Tanaka approach. Tensile stress-strain curves until total failure were simulated. The evolution with damage of the elastic constants is compared with ultrasonic and mechanical measurements and with micro-mechanical calculations. In an Al SiCp composite damage was identified to be particle fracture. A statistical local criteria of particle fracture is then introduced into the model. Tensile behaviors including plasticity are simulated with or without damage and compared with experimental results. Moreover, to model the very local anisotropy due to damage, we develop the equivalent anisotropic undamaged inhomogeneity (EAUI) method.

Modelling of Damage Behaviour of a Short-Fiber Reinforced Composite Structure by the Finite Element Analysis Using a Micro-Macro Law

The objective of this paper is composed of two parts: (1) presentation of the damageable elastic law for randomly reinforced composites based on the micro-macro relationship of Mori and Wakashima type (1990); and (2) calculation of composite structures by a finite element code, ABAQUS, using this damage model. Several mechanical tests on the short-fiber reinforced composite, such as bending, torsion, and the combination of bending and torsion, have been performed experimentally and simulated. A good correlation is obtained between experiment and simulation.

Determination of a tridimensional failure criterion at the fibre/matrix interface of an organic-matrix/discontinuous-reinforcement composite

The quality of the fibre/matrix interface is one of the most important factors for obtaining a good composite material. Many experimental procedures have been developed to measure the fibre/matrix interface strength, but these methods can only give a strength in terms of shear energy or shear stress limit. In this paper, we present two complementary methods which try to obtain an interface debonding criterion in which the effect of the local shear stress is coupled with the local normal stress on the interface. Experimental methods are defined and a micromechanical calculation based on the stiffness prediction by the Mori and Tanaka approach is used.

Prediction of the fatigue-damaged behaviour of Al/Al2O3 composites by a micro–macro approach

Abstract The use of very heterogeneous materials in structural components submitted to cyclic loadings, leads us to present an elastoplastic micromechanical model. After some revision of the homogenisation principle based on a mean field theory, a non-linear kinematic and isotropic strain hardening is introduced in the matrix. Validation is made on a Al–3.5% Cu/SiC particles and the case of an A356/Al2O3 fibres is treated as a first application. Damage is introduced in the model by using a fibre failure criterion. It is based on the increase in the volume fraction of broken fibres as a function of the maximum principal stress in the fibre family. The damage law is identified with in situ tensile test performed inside the scanning electronic microscope. The number of broken fibres is determined with the applied load and the number of cycles. The model predicts the fatigue behaviour, the lost of stiffness, the volume fraction of broken fibres for different volume fraction, aspect ratio, distribution of orientation, distribution of strength of the fibres. The effect of the mechanical fatigue properties of the matrix is also studied.

Experimental Study of the Stress Relief in Patched Pluminium Specimens

Composite patches are often used to reinforce or to repair damaged structures, especially aeronautical components [1]. The mechanical properties of the bonded joint between metallic substrate and composite patch clearly influence the quality of the reinforcement. So the objective of this work is to study in detail the mechanical response of such a joint.