Mixed mode I/III fracture toughness of an experimental rotor steel
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I.
INTRODUCTION
THERE has
been growing interest in the toughness of materials in shear modes II and III as well as in mixed mode fracture criteria encompassing all three modes. This paper discusses combinations of modes I and III. The relative magnitudes of mode I and mode III toughnesses remain unclear, with reports of Jlc = Jmc,tl] J1c > Jtltc, [2'3'4l and Jmr > Jlc .[5"6'7] TO some extent, as discussed by Tschegg and Suresh, t4} the latter case may result from frictional contact effects with the "intrinsic" mode III toughness, that associated solely with events at the crack tip, always being less than the mode I toughness. From a practical viewpoint, of course, the total Jmc, including friction, is the parameter of interest. For mixed mode I/III testing, the trends appear to fall into two classes is] depending on the overall level of toughness. For materials that tend to have low toughnesses and a limited plastic zone, the addition of a mode III component has either no effect[91 or a small effect, mainly at higher mode III/mode I loading ratios. ~sa~ For tougher steels with larger ratios of total toughness, J, to yield stress, %, the addition of a mode HI loading component tends to lower the mode I toughness component at failure./2,5,7'121 For one steel, [5'7] the effect of mode III was so great that measures of the total toughness were less under mixed mode I/III conditions than in either pure mode I or pure mode III. This latter steel had a Kk/crr ratio that significantly exceeded the typical trend line for steels, tl3] Accordingly, the present experiments were undertaken on a "superclean" rotor steel, which also has a very large K~c/o-y ratio, to determine whether a trend emerged similar to that in the earlier study. H.
EXPERIMENTAL PROCEDURE
The chemical composition of the steel that was studied is listed in Table I. The material was cut in the form of a plate from the center of a large rotor forging, with the longitudinal (L) direction of the plate parallel to the rotor axis and the transverse (T) direction parallel to the rotor radius. S. RAGHAVACHARY, Graduate Adminiswative Assistant, is with the Computer Science Department, The Ohio State University, Columbus, OH 43210. A.R. ROSENFIELD, Research leader, is with Battelle Memorial Institute, Columbus, OH 43201-2693. J.P. HIRTH, Professor, is with the Department of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99163. Manuscript submitted November 6, 1989. METALLURGICALTRANSACTIONSA
The heat treatment of the steel was as follows: 900 ~ 19 hours, air cool; 870 ~ 19 hours, air cool; 650 ~ 22 hours, furnace cool; 840 ~ 42 hours, water spray quench; 595 ~ 54 hours, furnace cool; and 590 ~ 50 hours, furnace cool. The tensile and Charpy V-notch properties suppliedtl41for the as-received material are listed in Table II. Since there is no standard procedure for determining mixed mode values of J, we have followed a procedure/15j which is a somewhat modified version of techniques discussed earlier, t5'71 Figure 1 illustrates the
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