Abstract
Fatigue crack nucleation and initiation at particles or defects in materials have been the interest of many researchers over the past twenty years. This is because the understanding of these phenomena would provide a better understanding of production of new fatigue resistance materials. In this study a test was developed to test the fatigue properties and to allow microscopic observation of fatigue crack nucleation and initiation in sintered steel. The fatigue test was based on plate bending which gives balanced biaxial tension where an optical finished polished specimens were tested. The results of the experimentation showed that there were mixed mode of crack nucleation and initiation such that most cracks initiated at the interface of particles and matrix especially at high stress levels. Cracks were also observed to initiate at voids and beneath the surface. Quenched specimens and tempered between temperatures ranges of 300℃ and 600℃ showed a similar behaviour of crack initiation mechanism. The significant difference between the as received and tempered specimen was the number of cycles to failure due to differences in material hardness and internal defects. The results of the experimentation are discussed in the light of possible micro crack toughening at the crack tip and presence of residual stresses due to temperature changes. It is believed that some effects of residual stresses at the surface and beneath play a major role in the nucleation and initiation and thus the fatigue behaviour of sintered steel.
