Effects of friction and high torque on fatigue crack propagation in Mode III
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I.
INTRODUCTION
R O T A T I N G equipment, such as turbo-generator rotors utilized for electrical power generator and transmission shafts for automotive use, are often subjected to transient high amplitude torsional oscillations which may severely limit the useful life of the structure through subcritical crack growth of undetected flaws. High strain amplitudes, approaching full-scale yielding of the shaft, can arise following electrical transients from particular line switching events in electric power generation and transmission systems. ~'2'3 At such high torques, fatigue growth occurs in anti-plane shear (Mode III) along transverse and/or longitudinal shear planes. 4'5 However, methodology to predict the Mode III growth of such flaws and hence to estimate the loss in fatigue life due to transient torsional oscillations is currently lacking for variable amplitude (spectrum) loading. There is a distinct lack of engineering data relating the growth rate of Mode III cracks to relevant loading parameters and the various modes of torsional fractures are poorly understood, in contrast to the wealth of information on fatigue crack propagation under Mode I (tensile opening) conditions. In both
H. NAYEB-HASHEMI, Postdoctoral Research Associate, and F.A. McCLINTOCK, Professor, are both with the Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. R.O. RITCHIE, formerly with Massachusetts Institute of Technology, is Professor, Department of Materials Science and Mineral Engineering, and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. Manuscript submitted February 26, 1982. METALLURGICALTRANSACTIONS A
cases fundamental, quantitative understanding based on growth mechanisms is not available. The objective of the present paper is to provide an experimental and theoretical basis for characterizing Mode III fatigue crack growth under both small-scale yielding and elastic-plastic conditions in a low strength, low alloy steel. The approach combines continuum fracture mechanics and preliminary mechanistic modeling to serve as a framework for the development of defect-tolerant life estimation procedures for components loaded to high torques. II.
BACKGROUND
Despite the relative scarcity of experimental studies on Mode III crack growth, several analytical models have been developed over the years. The problem of fully plastic fatigue of longitudinal shear cracks was first analyzed by McClintock, 6 and extended by Hult 7 for elastic-plastic conditions. A damage-accumulation model was later proposed by Kayan s where the crack was considered to propagate at a rate consistent with the damage, or fraction of life expended, remaining constant at unity at a structural distance p ahead of the crack. Since crack advance was modeled as hole growth from inclusions or shear localization on specific slip planes, the value of p could be related to the mean inclusion spacing or representative slip band length. Later studies 9 ge
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