Mode III fatigue crack propagation in low alloy steel

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I. INTRODUCTION IN electric power generation and transmission systems, electrical transients in the electric power network can excite similar mechanical transients in the mechanical power unit. In this regard, concern has recently been expressed over the possibility that certain of these transients, such as arising from particular line switching events (e.g., "high-speed reclosure ''t'2'3) or accidental fault conditions may result in failure of turbogenerator shafts, due to the excitation of high-amplitude torsional oscillations in the shaft. ~-5 During normal operation, these oscillations are small and quickly damped, and the shaft is subjected simply to a steady-state torque, generating shear stresses typically 20 pct of yield, and smaller rotating bending stresses caused by the supported weight of the turbines. If, however, the resonant frequency of the electrical network approaches the torsional resonant frequency of the mechanical unit, these oscillatory loadings can become significantly larger and less damped, a condition referred to as subsynchronous resonance 5 which, in the worst case, can lead to appreciable yielding of the shaft. ~Fatigue crack initiation and growth to full scale fracture is thus possible. The orientation of such crack growth, however, may be difficult to predict, since it may occur in Mode I (tensile opening) at -+45 deg to the axis of the shaft along planes of maximum principal (tensile) stress, or in Mode III (antiplane shear) along transverse or longitudinal shear planes, or under combined modes along intermediate directions (Figure 1). Methods to account for potential loss in fatigue life of turbine shafts subjected to torsional loading are currently being developed 6-9 and are based on stress-strain/life or defect-tolerant approaches. '~ Whereas stress/strain-life (local strain) models are based largely on crack initiation, l~ R.O. RITCHIE, formerly with Massachusetts Institute of Technology, Cambridge, MA, is currently Associate Professor, Department of Materials Science and Mineral Engineering, and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720; F.A. McCLINTOCK is Professor, Department of Mechanical Engineering, M. I. T., Cambridge, MA 02139; H. NAYEBHASHEMI is Research Assistant, Department of Mechanical Engineering, M. I. T., Cambridge, MA 02139; and M. A. RITTER, formerly with M. I.T., is currently at lnstron Corporation, Canton, MA 02021. Manuscript submitted April 30, 1981. METALLURGICAL TRANSACTIONS A

I MODE I ( tensile )

M O D E TT (shear)

M O D E TIT (anti-plane strain)

Fig. ! - - Possible modes of crack separation.

the approach utilized in the on-going current program 6'7 is modeled on defect-tolerance where lifetime is assessed in terms of the number of cycles to propagate the largest undetected flaw to failure. Unfortunately, studies on thc propagation of torsionally loaded fatigue cracks, particularly under Mode III conditions, are rare in the literature. The objective of the present paper is to provide an experimental and theoretical

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