Some improvements in the analysis of fatigue cracks using thermoelasticity

Thermoelastic stress analysis (TSA) has been developed in recent years as a technique for the direct measurement of crack tip stresses in structural components. The technique provides full-field stress maps from the surface of a component subjected to a cyclic load. An improvement in the methodology for monitoring fatigue crack growth and inferring the stress intensity factor from thermoelastic data is presented. The approach is based on a multipoint over-deterministic method (MPODM) [Sanford JR. A general method for determining mix-mode stress intensity factors from isochromatic fringe pattern. Engineering Fracture Mechanics 1979;11:621–33], where experimental data collected from thermoelastic images are fitted to Muskhelishvili’s equations describing the stress field around the crack tip. The fitting algorithm employed is based on the Downhill Simplex Method and includes the crack tip position as a variable to be optimized. Results obtained from artificially generated images and from tests performed using stress relieved and as-welded single edge notched specimens tested at different R-ratios are reported and compared. It is shown that TSA is a sensitive technique for examining the influence of phenomena such as crack closure and residual stresses during fatigue crack growth. Information obtained from the map of phase difference between the forcing signal and the thermal response is presented and discussed. It appears that the phase map may contain information about heat generation at the crack tip due to plastic work and contact of the crack forces.