Effect of hydrogen on four-point bend tests of U-notched AISI 1090 steel
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I.
INTRODUCTION
HYDROGEN has been shown to influence localized plastic flow and thereby degrade the mechanical properties of steel.l': Extensive work demonstrating this effect has been performed on spheroidized plain-carbon steels. 2-9 The enhancement of plastic instability by hydrogen is pronounced under plane-strain conditions 2-6 and the present work focuses on this case. A series of developments 1~ has led to a continuum mechanical description of plastic instability. The theories predict, as observed experimentally, that the tendency for plastic instability is greater under plane strain conditions 12'13'17-2~and indicate that the onset and growth of instabilities can be associated either with global or local material flow properties or with geometrical softening associated with void initiation and growth. 15'16'22The specific forms of the theory that predict critical strains for the onset of plastic instability in the range of those observed experimentally are those involving the concept of a vertex or corner forming on the yield surface in principal stress space. 12.13.17-20,22Stable plastic deformation tends to result if, as would be the case for a solid yielding according to the von Mises criterion, the yield surface is convex. Then the normality rule would be followed: i.e., the incremental strain vector would be parallel to an outward pointing normal to the yield surface. 23 If a comer develops and the deformation path is such that the stress state enters and coincides with the comer, a set of strain paths is possible; in particular, a new strain vector could more nearly point in the same direction as the stress vector in the yield surface space, and the possibility of unstable local flow is enhanced. The reasons for the generation of a comer in the yield surface could arise at the crystal plasticity level, for example, because of cross slip, rotation of slip systems relative to loading directions, or the generation of local defects. 14.24 In plane strain the instability is manifested by the generation of local shear bands that propagate along the characteristic slip traces of plasticity theory. Hutchinson and Tvergaard 18'19give specific predictions for the critical strain for the onset of such plastic instability for a solid deforming in a rate-independent manner according to the so-called J2 V. B. RAJAN is Research Associate, Nuclear Engineering Department, University of Michigan, Ann Arbor, MI 48109. J. P. HIRTH is Professor, Department of Metallurgical Engineering, The Ohio State University, 116 West 19th Avenue, Columbus, OH 43210. Manuscript submitted December 11, 1985. METALLURGICALTRANSACTIONS A
deformation theory of plasticity; that is, a theory wherein the stress deviator J2 = 76 [(trl - ~2)2 + (or2 - or3)2 + (or3 - tyl)2] is a function of the total effective strain ~ = (V'2/3) [(el - e 2)2 + (e2 - e3)2 + (e3 - el )211/2, where o'i and ei are the principal stresses and strains, respectively. For plane-strain tension, their result for the critical strain e~ for bulk shear instability can be co
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