PHYSICAL MEANING OF THE FATIGUE LIMIT IN SURFACE-FINISHED SPECIMENS.

Rotating bending fatigue tests were made on specimens which were polished longitudinally or circumferentially with two kinds of silicon carbide papers. The material used was a quenched and tempered SCM 435 steel. The specimen surfaces were observed by the two stage plastic replica method. The effects of surface finishes on the fatigue strengths of the specimens were investigated. The main results obtained are as follows: (1) Non-propagating micro-cracks are observed on the surfaces of the specimens after 10**7 cycles of the fatigue limit. In specimens polished under different conditions, the physical meaning of the fatigue limit can be defined as the limiting stress for the propagation of a micro-crack. (2) The geometric features of the starting points of cracks affect the fatigue strengths of surface-finished specimens.