A FINITE-ELEMENT MODEL OF PERSISTENT SLIP BAND INTERACTION WITH STRENGTHENED SURFACE-FILMS DURING LOW-CYCLE FATIGUE

Hardened surface layers such as those produced by ion implantation can inhibit the emergence of persistent slip bands (PSBs) at the free surface during low cycle fatigue, thereby extending crack initiation life. They have little effect, however, on bulk PSB formation. PSBs nucleated in the interior eventually impinge on the underside of the surface layer, producing localized cyclic shear strains in the film which lead to film rupture and rapid crack initiation. As a result, increases in fatigue life are modest. The present finite element model analysis shows that PSBs carrying plastic strains characteristic of fatigued f.c.c. metals produce stresses in an elastic surface layer which may exceed 1% of the surface film shear modulus. The detailed results are consistent with phenomenological data on surface damage accumulation during low cycle fatigue of ion-beam-modified nickel. Implications for the design of surface microalloys for inhibition of fatigue crack initiation are discussed. © 1990.