Combined mode I-mode III fracture of a high strength low-alloy steel
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INTRODUCTION
DEVELOPMENTS in elastic-plastic fracture mechanics have concentrated on mode I, or opening, loadings. Common failure modes of tough ductile materials, however, include crack propagation under combined mode I-mode III (transverse shear) conditions. Manifestations of such combined mode failure include slant fracture, often associated with plane stress conditions, and the formation of shear lips at free surfaces of growing mode I cracks. The tendency toward shear failure is quite pronounced in some materials, and mode I cracks can reorient to oblique planes during growth and continue propagation under combined mode I-mode III conditions (e.g., Reference 1). Only limited experimental data describing fracture behavior under monotonic mode I-mode III loading have been reported. Despite the fact that such combined mode fracture commonly accompanies failure in high toughness materials, much of the reported literature data has been confined to materials having low fracture resistance. It has been generally concluded that an imposed mode III load contribution lowers the mode I component required to initiate fracture, 2'3'4 although Pook has reported that mode I toughness is insensitive to transverse shear in several high strength alloys. 5 Reported resistances to fracture under pure mode I loading vs pure mode III loading are also conflicting. In terms of J integral values, the ratio Jmc :Jic has been reported as greater than unity for two high strength steels,2'6 equal to unity for plain carbon steels 7 and a high strength aluminum alloy,4 and less than unity for two medium strength steels. 8.9 While it is likely that no simple correlation exists between Jic and Jtn~ for the diverse chemistries, microstructures, and mechanical properties represented in the investigations refJ.G. SCHROTH, formerly Graduate Research Assistant, The Ohio State University, is Senior Research Engineer, Metallurgy Department, General Motors Research Laboratories, Warren, MI 48090. J.P. HIRTH is Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. R.G. HOAGLAND is Professor, Department of Materials Science and Engineering, Washington State University, Pullman, WA 99164. A.R. ROSENFIELD is Research Leader, Physical Metallurgy Section, Battelle Memorial Institute, Columbus, OH 43201. Manuscript submitted April 23, 1986.
METALLURGICALTRANSACTIONS A
erenced, the data suggest that Ji~ and Jm~ have comparable magnitudes with neither being uniformly greater than the other. The lack of a standard practice for either Kiilc or Jm~ determination contributes to the apparent scatter in reported data since a variety of specimen sizes and geometries, and different experimental crack monitoring techniques were used in obtaining the above results. Theoretical analyses of combined mode fracture have been confined to linear elastic and small scale yielding conditions. Energy release rate ~~ and strain energy density u't2'u criteria based on linear elastic behavior both predict that combined mode fractu
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