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The fatigue damage of a material generally goes through of an incident or a heterogeneity producing a local stress concentration. The growth of crack is related to the existence of a plastic zone at the crack tip and accompanied by an energy dissipation. The objective of this study is to analyze the evolution of the energy at the crack tip during the propagation of a crack in a welded joint. In order to determine the energy nearest the end of the crack in the plastic zone, we used microhardness measurements. The estimate of the total strain in each measurement point was determined by a polynomial relation between Hv and ε t /2. After the determination of the average strain, the total energy in each zone was deducted. The higher value of this energy is in the Weld Metal, it is more lower in the Base Metal and even lower in the Heat Affected Zone.

In this work, we focus on a new generation of polymer named Polybutylene terephthalate (PBT). In order to analyse and determine true behaviour of this polymer, a special experimental method was used. Hence, the true stress/strain responses are investigated under a large plastic deformation in different stress triaxiality frameworks with a particular attention on the volumetric strain evolution, with their decomposition to an elastic volumetric strain, plastic volumetric strain and the pure shear. Moreover, the effect of stress triaxiality on the plastic instability and the fracture strain is also examined. With the plastic instability analysis, it was found that plastic strain hardening increases gradually with the triaxiality. Finally, in order to evaluate the damage of this polymer, a theoretical damage formula is proposed.