Crack-tip behaviors of stationary and growing cracks in Al-Fe-X alloys: Part II. Crack opening profile
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I.
INTRODUCTION
THIS is
Part II of a two-part article which examines the strain distributions and crack opening profiles of stationary and growing cracks in A1-Fe-X alloys. In Part I, [u the experimental strain distribution ahead of stationary cracks subjected to monotonic loading is shown to be described by HRR singularity but by logarithmic singularity when the cracks have extended for a distance greater than 0.1 mm. In this paper (Part II), the crack opening profiles of these same stationary and growing cracks are studied. In theory, the crack-tip singularities derived from the crack opening profile should be identical to those observed in the strain distribution. As indicated in Part I, the near-tip behaviors of stationary and growing cracks in three A1-Fe-X alloys, which are A1-8Fe-7Ce, A1-8Fe-2Mo- 1V, and A1-10.5Fe-2.5V, were studied at 25 ~ and 316 ~ These alloys were produced by ALCOA, Pratt and Whitney, and AlliedSignal Inc., respectively. Summary of the tensile and fracture properties of the AI-Fe-Ce, AI-Fe-Mo-V, and AI-Fe-V-Si alloys, which are required for analyzing the crack-tip strain and crack surface opening displacement data obtained in this investigation, can be found in Reference 2. II.
EXPERIMENTAL PROCEDURES
Single-edge notched specimens with fatigue precracks were used. The fatigue cracks were initiated at a stress intensity factor, K, of 30 to 45 pct of K o the critical stress intensity at fracture. All of the specimens were electropolished and tested under monotonic loading at either 25 ~ or 316 ~ The test procedures involved loading the cracked specimen incrementally to prescribed load levels in the scanning electron microscope (SEM) and photographing the regions lying ahead and behind the crack tip. Small load increments were generally used when the applied K approached the Kic val-
KWAI S. CHAN, Principal Engineer, is with the Department of Materials Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228-0510. Manuscript submitted January 6, 1989. METALLURGICAL TRANSACTIONS A
ues of individual alloys. Crack surface opening displacements were obtained both at the crack (notch) mouth and near the crack tip. Still micrographs of the crack tip in the loaded and unloaded conditions at load levels immediately before or after a small crack extension were then analyzed for displacements and crack surface opening displacements using the stereoimaging technique. {31 The A1-Fe-Mo-V and AI-Fe-Ce alloys exhibited brittle fracture and no stable crack growth at 25 ~ For these cases, the crack-tip conditions prior to unstable fracture were analyzed. Crack-tip strains and opening displacements were obtained for both stationary and growing cracks. The former refers to monotonically loaded cracks prior to crack extension, while the latter refers to those after cracks have extended. A summary of the test conditions and strain-hardening exponent, n (n = 1I N , when N = d In o / d In e p, where tr and e p are stress and plastic strain, respectively), for each of the alloys stud
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