The nature of the two opening levels following an overload in fatigue crack growth
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INTRODUCTION
IN 1973, Paris and Hermann tL21presented a paper which dealt with crack opening behavior of a propagating fatigue crack following a single tensile overload. Of particular significance in their experimental work was the observation of not one, but two, opening events during the period of retarded fatigue crack growth following the application of an overload. Two different interpretations have been placed on the mechanical processes responsible for these two opening events. In one interpretation, t~-g] it is assumed that the two opening events are plane strain in nature, with the first opening event on loading occurring at some distance behind the crack tip and with the second opening event occurring at the crack tip itself. In a second interpretation, [5~ it is assumed that the first opening event occurs along the plane strain portion of the crack front and that the second opening event occurs in the plane stress region of the crack front at the specimen surfaces. The present paper is directed at clarification of this issue, as well as at the determination of the mechanism responsible for the retardation in growth rate subsequent to an overload as a function of the baseline AK level. II. B A C K G R O U N D A typical variation in the crack growth rate at a constant baseline AK level, as influenced by a single, 100 pet overload, is indicated by the solid line in Figure lt6] for a 6.25-mm-thick specimen of the aluminum alloy A.J. McEVILY, Professor, and Z. YANG, Graduate Assistant, are with the Department of Metallurgy, University of Connecticut, Storrs, CT 06269-3136. Manuscript submitted July 26, 1989. METALLURGICAL TRANSACTIONS A
2219-T87, which had been tested at a baseline AK of 16.0 MPa m 1/2 at R = 0.05. It is noted that during the overload a pop-in event occurred, and the plane strain portion of the crack front jumped forward by 1.5 mm. Immediately after the application of the overload, an initial increase in crack growth rate at the surface took place. The rate then decreased to a minimum below the preoverload baseline rate, before increasing and returning to the baseline rate when the fatigue crack had traversed the overload-affected zone. The initial increase in growth rate over the baseline level is associated with blunting of the crack tip by the overload and consequent reduction in closure in the wake of the tip. As the crack penetrates the overload-affected region, a slowing down of the growth process occurs as closure is developed, hence the designation "delayed retardation." The rate of crack growth in the interior of the specimen shown in Figure 1 was determined by measuring the spacing of striations on the fracture surface. Figure 1 also shows the crack opening levels as measured by a crack-mouth clip gage and by a strain gage which was located on the surface of the specimen near the crack tip. The crackmouth gage is more sensitive to through-thickness, mainly plane strain opening behavior, whereas the surface gage is more sensitive to crack opening in the plane stress region. It is s
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