Change in the fracture surface pattern in the fracture process of SUS631 stainless steel fatigued by repeated bending

Fractal dimension of the fracture surface has been calculated and displayed as a fracture surface pattern map (FPM) to examine the fracture process of SUS631 stainless steel fatigued by repeated bending. SUS631 steel plates of about 1 mm thickness with thermo mechanical heat treatment (ACH) treatment were used that had martensite structure with a small of retained austenite and delta ferrite. Quasi-cleavage fracture with microcracks was dominant in the fatigue fracture of the steel, while ductile fracture surface with dimples was also observed near the fracture surface. It was observed that the fracture surface near specimen surface formed in the earlier stage of fatigue was subjected to more damage by cyclic compressive load, and the fracture surface geometry became simpler than the fracture surface in the interior of specimen formed in the later stage. The geometrical analysis was found suitable to examine the fracture process of materials numerically and visually.