Monotonic and cyclic damage in metallic sheets

Abstract: This paper aims to provide an account of some interesting features of damage in metallic sheets under monotonic and cyclic loading using the information and understanding developed through a series of experimental investigations conducted on interstitial free steels. The experiments primarily consisted of damage evaluation in un-notched and notched sheets vis-à-vis that in thick specimens under monotonic loading, and that on sheets by interrupted or continuous cyclic loading. Some salient observations indicate that: (i) void nucleation occurs in two different stages, originating from non-metallic inclusions and precipitates. The critical strain for void nucleation at precipitates (ɛ_c) is lower for sheet metals than that in thick specimens. (ii) ɛ_c is a function of notch length in sheets, and the function assists to estimate the strength of particle/matrix interface, (iii) under cyclic loading, steel sheets exhibit non propagating microcracks below the endurance limit. Above the endurance limit slip bands promote formation of larger fatigue cracks primarily at ferrite grain boundaries. A series of grain boundary cracks link up to form meso-cracks, one of which grows to cause final failure and (iv) the growth of the macro-crack initially occurs in opening mode followed by its propagation in mixed mode through striations, intergranular cracking and through thickness necking prior to failure.