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I.

INTRODUCTION

W I T H the damage tolerant design concept, the early detection of crack initiation and the monitoring of its subsequent growth are vital tasks to ensure the reliability and predict the remaining life of the structural component subjected to cyclic loading. To date, this has not been satisfactorily achieved because of many complex problems. One major problem is the fact that a large part of the fatigue life is spent before the precursor evolves to a detectable crack size with the conventional inspection techniques. This incubation period may vary significantly, especially in low-cycle fatigue, and is unlikely to be predictable on a theoretical basis. Another problem is associated with the small crack effect. El-61 When the crack size is comparable with the grain size, it may grow much faster than expected from the large crack data; and as it grows, it may be slowed down (or arrested). These anomalies are the products of the interaction of the crack tip microplastic zone with the local inhomogeneity due to the alloy microstructure. This includes the grain boundaries, crystallographic orientation, inclusions, and residual stress. After the crack grows large enough, say, 1-mm deep, the crack extention becomes stable and the growth behavior is well described by the Paris power law, where the stress intensity factor range, 2urf, is the key parameter. Extensive research efforts have been devoted to developing techniques which allow the detection of cracks as early in their lives as possible and monitor in situ the irregular growth behavior of small cracks.t2.7] Direct observation, but on the specimen surfaces, is available with microscopies (optical, electron, acoustic) and replications. Information about the depth is contained in the indirect measurements of compliance, potential drop, eddy current, and ultrasonic scattering. Each technique M. HIRAO, Associate Professor, and H. FUKUOKA, Professor, are with the Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan. K. TOJO, formerly Graduate Student, Osaka University, is with the Keihanna Research Laboratory, Shimazu Corporation, Seika, Kyoto 6t9-02, Japan. Manuscript submitted November 3, 1992. METALLURGICAL TRANSACTIONS A

has both advantages and disadvantages and is not compatible with automated, continuous monitoring systems of fatigue crack behavior extending from the earliest stage to the macroscopic flaw. There is a desire in developing such a system to apply it not only to the laboratory tests for investigating the fatigue mechanisms, but also to the real components and structures subjected to the complicated history of fatigue loading. The present approach makes use of the reflected Rayleigh wave from the fatigue crack. Among many ultrasonic modes, the Rayleigh wave is particularly well suited for the purpose because it travels along the solid free surface where the fatigue cracks nucleate owing to a reduced constraint on deformation. It then shows a high sensitivity to surface irregularities. The elastic vibration decays