Co-authors​ :

 Mio SATO (JAPAN), Natsuko KUDO (JAPAN), Jun KOYANAGI (JAPAN) 

In this study, we estimate the residual strength in a material after repeated loading by correlating the amount of entropy generation during material deformation with material degradation. In discussing the lifetime of polymer composites, some studies have been conducted to describe their strength degradation by thermodynamic entropy, recently. Inelastic deformation of the material causes an irreversible structural change inside the material. Such an internal structural change is expressed by a thermodynamic internal variable based on Clausius-Duhem inequality (the second law of thermodynamics); i.e., deterioration of the material proceeds with the disorder of the system. The disorder in the system, which causes deterioration of the characteristics, is raised by the mechanical deformation and the entropy increases accordingly. The disorder in the degraded system continues to increase to the critical state and causes the failure of the materials. Therefore, irreversible entropy in materials can be used as a criterion for evaluating lifetime of materials. Koyanagi et al. introduced a strength degradation coefficient to describe the strength degradation of composites in macro-scale and related it to the amount of entropy generation. It is possible to predict the residual strength of composite materials using this method. Here, the value of strength degradation coefficient should be quantitatively determined by micro-scale finite element analysis (FEA). In this study, the strength degradation coefficient is obtained by micro-scale FEA to quantify the relationship between entropy generation and strength degradation in composite materials. In the micro-scale numerical analysis, a representative volume element (RVE) model consisting of resin and fiber is used, and an entropy damage criterion is introduced into the resin to describe the strength reduction (Fig.1). The strength degradation coefficient in each direction is quantitatively determined by applying cyclic displacements in the normal and shear directions, respectively. Fig. 2 shows the example result of the relationship between entropy generation and strength degradation under cyclic tensile loading. According to this relationship, the strength degradation coefficient can be determined quantitatively.