Analysis of Crack Initiation Mechanism in Fatigue Reliability of Environmentally Sound Surface Modification SMn 435

Abstract

To improve the fatigue reliability for SMn 435 steel by an environmentally sound surface modification, the influence of the modification on fracture mechanism was studied experimentally. The fatigue properties of SMn 435 are found to be obviously improved by gas soft-nitriding, so that they can no longer be estimated by using a concept of “S-N Globe” which has successfully been applied to many surface-treated materials. The detailed observations have revealed that fatigue crack initiation in a gas soft-nitrided SMn 435 specimen occurs in a hardened region near surface not in a subsurface region with virgin microstructure. The crack initiation is attributable to cyclic plastic deformation in the subsurface region. Such a crack initiation mechanism is different from that previously observed in other surface treated materials, explaining the reason why the S-N Globe concept can not be applied to the present case. © 1998, The Japan Society of Mechanical Engineers. All rights reserved.