A METALLOGRAPHIC EXAMINATION INTO FATIGUE-CRACK INITIATION AND GROWTH IN FERROUS PM MATERIALS

Porous materials behave differently under fatigue loading compared with pore-free materials due to their internal free surfaces, which often act as crack-initiation sites. Depending on the type of fatigue loading, crack initiation may occur simultaneously at multiple sites inside the part until the critical crack length is reached and fast fracture takes place. Crack initiation and growth will be explored using tested axial-fatigue specimens in the failed and pre-failure conditions. The samples are pearlitic and in the as-sintered condition, having differences in both chemical composition and pearlite spacing. Furthermore, an additive was utilized to create larger pores in the sintered samples and allow the effect of pore size to be studied as it related to cyclic loading. Both light optical (LOM) and scanning electron microscopy (SEM) were used to examine the pore-size distributions, interlamellar pearlite spacings, fracture-surface details, and cross sections through stressed regions in failed and pre-failure specimens.