Résumé
Polymeric materials reinforced with short glass fibers are becoming ubiquitous in various industrial areas, such as the automotive industry. These composites are of particular interest because of their remarkable advantages, notably regarding the high strength and stiffness, the lightweight and the long fatigue life. Even if academic and industrial researches led to a better knowledge of the fatigue mechanisms in these heterogeneous thermo-hygro-sensitive materials, some issues concerning the understanding of i) the dissipative and stored energy changes, ii) the influence of the loading frequency and iii) the fiber orientation effect still need to be clarified. One of the promising approaches for addressing these issues is based on energy considerations. A combined description of the mechanical and energy phenomena occurring during the deformation process may contribute to a deep knowledge of the behavior. When a material is subjected to inelastic transformations, a part of the mechanical energy expended in the deformation process is converted into heat, but the remainder part remains stored in the material, thereby modifying its internal energy. Many interesting surveys about specific aspects of the stored energy can be found in literature. The most significant developments that have taken place in the computation and interpretation of the stored energy were greatly related to the calorimetric procedures. Most of researches have focused on temperature rise measurements in the aim to estimate the evolution of this energy, using different experimental techniques. In this survey, an experimental protocol was developed to draw up complete energy balances associated with the low cycle fatigue of PA6.6 reinforced with 30% of short glass fibers- the following fiber orientations are systematically considered: 0°, 45° and 90°- and conditioned at the equilibrium with an air containing 50% of the relative humidity. The protocol uses two quantitative imaging techniques, namely Infrared Thermography (IRT) and Digital Image Correlation (DIC). The former technique provides a direct estimate of heat sources, especially intrinsic dissipation and thermoelastic source, using the local heat diffusion equation. The second technique gives access to the deformation energy by means of strain and stress assessments. Both techniques are then successfully correlated in the aim to quantify the mechanical energy rate converted into heat, using the Taylor-Quinney coefficient as indicator. The first results exhibit some very interesting findings since the first few cycles. It is observed that there is neither cyclic mechanical nor thermodynamic stability. A significant ratcheting phenomenon characterized by an accumulation of cyclic strain at each cycle is classically observed. From a thermodynamic point of view, it is shown that the dissipated energy per cycle is always less than the mechanical energy that can be associated with the area of the hysteresis loop. This energy difference reflects the significant contribution of the stored energy associated, cycle by cycle, to the microstructural changes. Moreover, a 2d full-field measurement analysis emphasizes the existence of hot spots occurring in dissipation fields. These hot spots change with the degree of fiber orientation. Their surface detection is thus correlated with those of thermoelastic sources in the aim to follow the fatigue damage accumulation in where the crack may finally occur.