Crack initiation and propagation under thermal fatigue of austenitic stainless steel

Thermal fatigue tests on AISI 316L(N) austenitic stainless steel samples are performed through pulsed laser on specimen that can be subjected to an additional static mechanical load. These tests are carried out in Helium environment with a dedicated and heavily instrumented set-up. The fatigued surface is monitored by a hybrid multiview system composed of two visible light and one infrared cameras that, through 3D-registration, provides in-situ access to the 3D surface displacement fields and 2D temperature fields. At a fatigue frequency of 1 Hz, the surface temperature range covered per cycle can be varied from 150 degrees C to 250 degrees C, conditions that allow surface damage to be reproduced. The multiview system reveals the time-resolved mechanisms of surface damage, from significant cyclic plasticity with persistent slip bands to microcrack initiation and growth, leading to their quantitative characterization (microcrack density, length of major crack, orientation) all along the test. These observations are confirmed at a few check points where the test is interrupted for optical microscopy inspection of the surface. Finally, the thermal fatigue data are compared to purely mechanical isothermal uniaxial fatigue data through the use of an equivalent strain, and an excellent (and conservative) agreement is obtained.