Fatigue crack propagation and closure behavior of modified 1070 steel: Experimental results

Abstract

A combined experimental and finite element study of fatigue crack propagation and crack closure behavior, in a modified 1070 steel, has been conducted. In this paper, the experimental aspects of this study are presented and discussed. A comparison of crack closure measurement techniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesses of compact tension specimens, a series of constant maximum stress intensity and constant load ratio, constant load and constant load ratio, constant maximum stress intensity and increasing load ratio, single tensile overload, and conventional fatigue crack propagation tests were conducted. Implications of the influence of specimen thickness, crack length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture surfaces were performed to assess the relative importance of crack path meandering, oxide-induced ind surface roughness-induced crack closure mechanisms. In the domain of stable crack growth, closure is dictated by the mechanism of plasticity-induced crack closure; while near-threshold behavior was found to be dominated by the conjoint and mutually interactive mechanisms of oxide-induced and surface roughness-induced crack closure. Test results reveal a significant influence of specimen thickness on closure and overall growth rate behavior. The salient advantages of the new surface strain gage method are elucidated and the conformance of experimentally measured and calculated growth rates is highlighted.