Kinetics of stored and dissipated energies associated with cyclic loadings of dry polyamide 6.6 specimens

Abstract An experimental protocol was developed to achieve complete energy balances associated with the low cycle fatigue (LCF) of dry polyamide 6.6 (PA6.6) matrix. The protocol uses two quantitative imaging techniques, infrared thermography (IRT) and digital image correlation (DIC). The first technique provides a direct estimate of heat sources, especially intrinsic dissipation and thermoelastic sources, using the local heat diffusion equation. The second technique gives access to the deformation energy by means of strain and stress assessments. Stresses were derived from strain measurements using a simplified local form of equilibrium equations. Both techniques were then successfully combined with the aim of quantifying various energies involved in the energy balance (e.g. deformation, dissipated and stored energies) and then to obtain an estimate of the Taylor-Quinney ratio. From a thermomechanical modeling standpoint, the experimental results exhibit some interesting findings during the first few cycles. It was found that there was neither mechanical nor thermodynamic cyclic stability. From a mechanical standpoint, a significant ratcheting phenomenon characterized by accumulation of cyclic strain is classically observed. From a thermodynamic viewpoint, it was shown that the dissipated energy per cycle was always less than the mechanical energy that could be associated with the area of the hysteresis loop. This energy difference reflects the significant contribution of the stored energy associated, cycle by cycle, with the microstructural changes. Moreover, a 2d full-field measurement analysis highlighted hot spots occurring in the dissipation fields. The surface detection of these spots was thus correlated with those of the thermoelastic source with the aim of monitoring the fatigue damage accumulation in the region where the crack finally occurred.