Polycrystalline modeling of the behavior of neutron-irradiated recrystallized zirconium alloys during strain path change tests

International audienceDuring normal operating conditions, zirconium alloy nuclear fuel cladding tubes experience various biaxial loadings with complex strain path histories. Experiments have been conducted on neutron-irradiated thin cladding tubes in order to study the response to changes in the loading path. These tests consist of alternate internal pressure tests and axial tensile tests. During the internal pressure test steps, the flow stress exhibits significant cyclic strain softening, while axial tensile tests exhibit a smaller degree of cyclic strain softening. TEM analyses of the tested samples have revealed that the observed mechanical behavior can be attributed to clearing of irradiation-induced defect by gliding dislocations. The cyclic strain softening observed for internal pressure tests is due to clearing of defect by dislocations gliding in the basal planes. The smaller degree of cyclic strain softening in tensile tests is due to clearing of defects by dislocations gliding in the prismatic and pyramidal planes. A polycrystalline model has been developed to simulate these tests. This model is able to reproduce many features of the complex behavior of the material and provides a better understanding of the role of the clearing of defects and the contribution of kinematic hardening on the behavior of neutron-irradiated recrystallized zirconium alloys

HIGH TEMPERATURE ELASTIC‐PLASTIC SMALL CRACK GROWTH BEHAVIOR IN A NICKEL‐BASE SUPERALLOY

Abstract— An experimental study of the elastic‐plastic fatigue behavior of small surface cracks in Alloy 718 at 650°C was conducted under conditions of total strain control. During cycling, the crack growth was continuously monitored using a direct current potential drop technique while the influence of crack closure was monitored using a laser interferometry technique measuring the crack mouth opening displacement. The crack tip plastic zone size was also measured using a post‐test delta phase decoration technique. Results show that the growth rates of the small cracks correlate well with long crack LEFM data when using an appropriate elastic‐plastic driving force parameter. The anomalous crack growth rates observed in some experiments were found to be experimental transients dominated by the crack initiation fracture and do not represent an intrinsic behavior of Alloy 718. No contribution to the anomalous small crack growth could be attributed to either crack closure or enhanced crack tip plasticity. Copyright © 1994, Wiley Blackwell. All rights reserve