Crack tip position evaluation and Paris' law assessment of a propagating crack by means of temperature-based approaches

This paper investigates the possibility to evaluate the crack tip position, crack growth rate and stress intensity factor (SIF), in order to derive the Paris law by starting from the temperature distribution measured by means of an infrared camera, with high geometrical resolution. In more detail, constant amplitude, fully reversed, stress controlled, crack propagation fatigue tests were carried out on two single edge notch tension specimens machined from 4-mm-thick, hot-rolled AISI 304L stainless steel sheets. The temperature maps were then processed by using a signal reconstruction algorithm based on the least square method in order to extract the maps of the amplitude and phase signal components of the first and second harmonics. In particular, the amplitude maps of the thermoelastic signal allowed the estimation of the stress intensity factor (SIF) using well-established methods, while the thermoelastic phase and second harmonic phase maps were used for estimating the crack-tip position. As for the determination of the crack tip using the phase maps, an experimental approach based on the evaluation of the inversion point of a phase profile (parallel to the crack propagation direction) was adopted. Such a point would represent the beginning of the reverse plasticity zone in the first and second harmonics phase maps. The crack tip position based on temperature maps was systematically compared to that measured by means of a digital microscope. Similarly, experimental SIF values derived from the thermoelastic signals were compared with the relevant numerical values from linear elastic FE analyses. The resulting Paris laws were critically compared. The good agreement between the preliminary data suggest further investigations regarding the applicability of these methods in all load conditions.