FATIGUE-CRACK PROPAGATION AND CLOSURE BEHAVIOR OF MODIFIED 1070-STEEL- EXPERIMENTAL RESULTS

A combined experimental and finite element study of fatigue crack prop agation 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 tec hniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesse s of compact tension specimens, a series of constant maximum stress in tensity 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 wer e conducted. Implications of the influence of specimen thickness, crac k length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture sur faces were performed to assess the relative importance of crack path m eandering, oxide-induced and surface roughness-induced crack closure m echanisms. In the domain of stable crack growth, closure is dictated b y the mechanism of plasticity-induced crack closure; while near-thresh old behavior was found to be dominated by the conjoint and mutually in teractive mechanisms of oxide-induced and surface roughness-induced cr ack closure. Test results reveal a significant influence of specimen t hickness on closure and overall growth rate behavior. The salient adva ntages of the new surface strain gage method are elucidated and the co nformance of experimentally measured and calculated growth rates is hi ghlighted. Copyright (C) 1996 Elsevier Science Ltd.