On the influence of strain-path changes on fracture
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I.
INTRODUCTION
THEinfluence of strain-path changes on the flow behavior of metals, especially with regard to strain-hardening transients, has been the subject of several recent studies. ~-7 These transients can in turn have several effects on the subsequent flow behavior; for example, recently published finite element modeling indicates that the transients can influence strain localization behavior at strains well beyond the transient region. 8 Thus, formability of sheets can be affected significantly even by strain-path changes at comparatively small strains. In contrast to the above, the effect of strain-path changes on ductile fracture (wherein failure is a result of void nucleation, growth, and linking) has not been examined in detail. It is very likely that the macroscopic strain localization phenomena examined in the formability studies will translate to microscopic flow localization effects influencing the fracture process. In addition, studies based on proportional loading clearly show that the fracture strain for a ductile, microvoid fracture process is a function of both stress state and strain path (for example, see References 9 through 11). Thus, it would be reasonable to expect that ductile fracture should be a function of strain-path history. The purpose of this communication is to present unique data which indicate that ductile fracture, at least in certain cases, is strongly dependent on changes in strain paths imposed prior to failure. In particular, a multi-stage deformation sequence involving uniaxial and equibiaxial tension can result in a significant ductility enhancement when compared to that achieved by proportional straining (p -- de2/de~ = constant, where el and e2 are the major and minor principal strains in the plane of the sheet) to the same final strain state. The effect occurs regardless of whether void nucleation is difficult (pure Ti), or relatively easy (a Ti-H alloy in which hydrides provide sites for void nucleation). It should be noted that the ductile, microvoid fracture behavior of Ti and Ti-H alloys has been characterized for proportional loading over a range of stress states. 10The results presented here are a part of and consistent with more extensive data to be presented later.
S. C. KESTNER, Graduate Research Assistant, and D. A. KOSS, Chairman, Metallurgy Program, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted June 11, 1986. METALLURGICALTRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
Inclusion-free, commercially pure Ti sheet (1.0 mm thick) with a grain size of 0.016 mm and containing 0.196 wt pct oxygen was used for this study. The material was tested in an annealed condition (700 °C/1 hr at 4 × 10-3 Pa) and after thermally charging with hydrogen, also at 700 °C for 1 hour, and helium-gas quenching. Thus, material containing two levels of hydrogen was tested: (a) Ti-30 wt ppm H, and (b) Ti-650 wt ppm H. In the latter case, both inter- and intragranular titanium hydrides ar
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