Mode III fracture of 4340 steel: Effects of tempering temperature and fracture surface interference
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INTRODUCTION
T O R S I O N A L fracture, also referred to as Mode II1 or anti-plane strain failure, is a common type of failure mode in a number of engineering applications. Some examples of practical situations where this fracture mode is of particular significance include springs and transmission components in the automotive industry and turbo generator rotors in the power plant industry. Numerous analyses of crack-tip stress and deformation fields in anti-plane strain are available in the literature. However, very little experimental information exists on the Mode III fracture of ductile solids, t~-t3x and the micromechanics of torsional failure under quasistatic loads have not been investigated in detail. Tsangarakis tt'2~ studied the influence of microstructure on the Mode III fracture response of AISI 4340 steel and a hypereutectoid steel (the composition of which, in weight percent, is 1 pct C, 1.66 pct Mn, 0.65 pct S, 0.167 pct Cr, and 0.11 pct Ni). In the former material, his experiments showed that an increase in the (shear) strength of the material, achieved through variations in tempering temperature, also leads to an increase in the fracture toughness in Mode III, Kinc.I~ In the latter steel, an increase in Kinc resulted from an initial increase in strength and subsequently was independent of strength.12] A closer inspection of the results in References 1 and 2 reveals that the plastic zone size at fracture initiation was too large to permit a "valid" characterization of the fracture behavior using linear elastic fracture
E.K. TSCHEGG, formerly Visiting Associate Professor at Brown University, is with the Institute of Technical and Applied Physics, Technical University of Vienna, Karlsplatz 13, A-1040, Vienna, Austria. S. SURESH is Associate Professor, Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted November 6, 1987.
METALLURGICAL TRANSACTIONS A
mechanics (LEFM). Conditions for the validity of LEFM analyses mandate that the radius of the uncracked ligament be substantially greater than (at least 12 times) the size of the plastic zone. As compliance with the LEFM requirements is often difficult to achieve in normal, laboratorysize torsional fracture specimens, Kmc values derived from LEFM characterization can often lead to misleading interpretations of the torsional fracture resistance. An additional difficulty in interpreting the intrinsic resistance of alloys to Mode III fracture stems from the interference and frictional sliding between the fractured faces. H4'15] As crack closure due to fracture surface abrasion severely reduces the crack-tip "driving force", the apparent resistance to Mode III failure is observed to be a strong function of crack length and specimen geometry. The role of frictional sliding in influencing Mode III fatigue crack growth in steels has been investigated in detail, t14-1sJAlthough the effects of frictional sliding between the crack faces on quasistatic Mode III fracture have been the subject of recent studies in ceramics, t~gj
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