Comparison of different inspection techniques for fatigue cracks

For qualifying non-destructive inspection techniques, test blocks with reproducible cracks are necessary. Austenitic stainless steel samples with reference reflectors were created and compared with artificial fatigue cracks produced by a resonance testing machine. The samples were inspected by eddy current and ultrasonic testing to estimate their length and depth. Additionally, inductive thermography measurements were carried out. This method can be used to detect surface flaws in metallic materials. As the flaw depth influences the eddy current distribution and the heat diffusion, the temperature difference caused by a flaw correlates with its depth. The fatigue cracks become visible by evaluating the IR image sequence and by observing the typical 'butterfly' pattern. FEM simulations were used to model the thermography experiments. The signals of short and long cracks were compared to examine how the depth estimation derived for long cracks can also be applied for short fatigue cracks.