The effect of overload on the fatigue crack propagation in metastable beta Ti-V alloys
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INTRODUCTION
MANYstudies have shown that the application of a tensile overload during fatigue crack propagation (FCP) leads to a retardation in crack growth in the overload affected zone ahead of the crack tip. 1-~5The degree of retardation in crack growth depends upon the loading variables, i.e., the stress intensity range, extent of the overload, number of overload cycles, load ratio, frequency of loading, specimen thickness (plane strain or plane stress), etc. Material parameters, such as the yield strength, 5'6 strain hardening exponent, 9'13A5fracture toughness,~2 etc., have been found to have an influence. Micromechanisms involved in overload effects are not clearly understood, and current theories may be broadly divided into five categories: (i) residual compressive stresses at or near the crack tip, 16 (ii) closure effects generated by the crack tip deformation, ]7 (iii) crack tip blunting, TM (iv) crack tip strain hardening, 1 and (v) crack tip branching. ,9 All of these effects are in some way associated with an increased amount of plastic deformation at or near the crack tip during the application of an overload. This research was undertaken to determine whether or not the deformation mode affects the overload response of metastable beta titanium-vanadium alloys. Previous studies 2~ have shown that the deformation modes, low cycle fatigue (LCF), and fatigue crack growth rates (FCGR) of metastable beta titanium-vanadium alloys may be altered substantially by varying the vanadium content. The metastable beta Ti-24 pct V alloy* deforms by *Composition given in weight percent.
coarse twinning and fine wavy slip, whereas the metastable beta Ti-32 pct V alloy deforms by coarse planar slip. The Ti-32 pct V alloy has a higher yield stress, a lower degree of cyclic hardening, and a better cyclic strain-life response when compared with the Ti-24 pct V alloy. No stress-induced transformation to the alpha phase has been E.W. LEE is Postdoctoral Fellow, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA 30332. S.B. CHAKRABORTTY, formerly Research Scientist at Georgia Institute of Technology, is now Resident at Norfolk General Hospital, Norfolk, VA 23507. E.A. STARKE, Jr. is Earnest Oglesby Professor of Materials Science, Department of Materials Science, University of Virginia, Charlottesville, VA 22901. Manuscript submitted March 21, 1983.
METALLURGICALTRANSACTIONS A
observed in either alloy after quenching from below 750 ~ The FCGR in air of the Ti-32 pct V alloy is lower for both low and high stress intensity ranges (AK's) and higher for intermediate AK's, as shown in Figure 1. However, fractographic features are similar for both alloys, suggesting that the FCP modes are similar. At low growth rates (da/dN
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