MEASUREMENT OF THE CRITICAL SIZE OF INCLUSIONS INITIATING CONTACT FATIGUE CRACKS AND ITS APPLICATION IN BEARING STEEL

Inclusions which can initiate microcracks in bearing steels under rolling contact were detected before spalling occurred by a special technique. The critical sizes of these inclusions, their shapes and depths below the rim surface were measured. A statistical relationship between the critical size A(c), the amplitude of the orthogonal shear stress (DELTA-tau-xz) and the number of cycles up to microcrack initiation were also established. This relationship coincides with the dislocation theory of fatigue. The material composition parameter in the dislocation accumulation equation, KH, was calculated for bearing steels from the test results. Several curves showing variations between A(c), N(o) and DELTA-tau-xz were developed and a new classification of inclusion sizes is suggested.