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The safety assessment of short-fibre-reinforced (SFR) composites, commonly used in structural applications involving repeated loads, requires to evaluate the degrading phenomena taking place in the matrix and at the fibre–matrix interface. The mechanical behaviour under both static and cyclic loading can be simulated applying damage degradation to the matrix mechanical characteristics, and employing fracture mechanics concepts to examine the fibre–matrix detachment as a 3D growing crack with degrading interface properties. In the present paper, a micromechanical model for unidirectional or random SFR materials under fatigue is developed. Some applications related to SFR polymeric composites found in the literature are presented.

In the present paper, the mechanical behaviour of fibre-reinforced brittle-matrix composites, with emphasis to cementitious composites, is examined by adopting both a discontinuous-like FE approach and a lattice model. The main phenomena involved, such as crack formation and propagation, crack fibre bridging, fibre debonding, fibre breaking, are taken into account. The basic assumptions and theoretical background of such approaches are outlined, and some experimental data related to plain and fibre-reinforced concrete specimens under Mode I and Mode I + II loading are analysed. The comparison of the numerical simulation results shows that the lattice model allows us a very detailed description of the fracture pattern, whereas the discontinuous FE approach mainly gives us only global information in terms of both crack path and stress–strain response curve. Nevertheless, the FE approach is computationally convenient and a useful tool for studying problems which do not require a detailed description of the fracture process.

This paper presents a new micromechanics based ductile fracture initiation criterion for ASTM A992 steels. The proposed criterion is developed using micromechanical study of microvoid growth using computational cell simulations. Parametric study is performed on cylindrical computational cells with an embedded spherical microvoid to describe the effects of stress state, matrix hardening properties and initial void volume fraction on microvoid growth. A micromechanical void growth model and a ductile fracture initiation criterion is proposed for ASTM A992 structural steels based on the parametric study. The model is calibrated and validated using experimental testing on eight notched ASTM A992 steel specimens. It is shown that the predicted ductile fracture initiation strains and fracture initiation locations from the proposed model matched fairly well with the experimental results.

International audience; Coupling between initial and damage-induced anisotropies in 3D elastic damaged materials has been so far addressed by homogenization techniques only for particular microcracks configurations. The main difficulty in developing a general 3D micromechanical model lies in the lack of a closed-form solution of the Eshelby tensor corresponding to cracks in an elastic anisotropic medium. In this study, we begin to present closed form expressions of the Eshelby tensor $\tensq{S}$ that we derived for a cylindrical crack embedded in an orthotropic material. The 3D obtained expressions reduce to existing results in the 2D case or in particular 3D cracks configurations. The effective compliance of an orthotropic medium containing arbitrarily oriented cracks is then derived by using the new Eshelby tensor. Finally, a damage model is formulated by combining the above results with classical thermodynamics approach. The ability of the model to capture coupling between initial orthotropy and damage induced anisotropy is demonstrated through a comparison with experimental data available for a ceramic matrix composite (unidirectional SiC-SiC).

The fracture process of progressively drawn steels is studied. Results show that drawing affects the phenomenon, so that heavily drawn steels exhibit anisotropic fracture behaviour. A micromechanical model of fracture is proposed to rationalize the results on the basis of the microstructure of the steels after drawing. In slightly drawn steels, the Miller–Smith model of shear cracking in pearlite may be adequate to describe the fracture process. In heavily drawn steels, a fracture propagation step appears, and it may be caused by extremely slender pearlitic pseudocolonies aligned in the drawing direction, with anomalous (very high) local interlamellar spacing which makes them preferential fracture paths with minimum local resistance.

Experimental studies have shown that fractures often exhibit shear resistance softening and dilatation under shearing loads. From a mechanistic viewpoint, these phenomena are a consequence of the fracture surface roughness and the material mechanical properties. Consequently, this paper utilizes a micromechanical model of fractures that explicitly considers asperity interactions on fracture surfaces. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. A modified spherical harmonic expansion is used to model the orientation distribution of asperity contacts. Evolution laws for asperity heights and asperity contact orientations are introduced to account for the change in surface roughness resulting from asperity damage under shear. Results obtained from the model show that the asperity contact orientation evolution law is essential for correctly modeling the softening and dilatation behavior of fractures. The model results are compared with experimental data culled from the literature.

This paper presents a study of a crack in a semi-infinite brittle matrix reinforced with continuous fiber by incorporating a micromechanical model. In general, fibers sustain the extension of a crack in three different phenomena: bridging, debonding, and sliding along the fiber-matrix interface. The spacings between successively developed cracks are derived by the maximum stress criterion in the matrix; in the meantime, the bridging stress and the factor of sliding are determined by using the inclusion method. The stability and growth of matrix cracking in a semi-infinite, fiber-reinforced composite are described by means of the energy approach. The responses of multiple fracture corresponding to stress-strain curves have also been presented. Finally, the external stress at the theoretical ending point, beyond which multiple matrix cracking ceases to develop, can be predicted from the theoretical model.