Effects of hydrogen on notch ductility and fracture in spheroidized AISI 1090 steel
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INTRODUCTION
THE recent hypothesis that
strain localization and plastic instability are involved in hydrogen embrittlement phenomena is discussed by Thompson and Bernstein 1 and indicated directly in the work of Lee et al. 2 The former authors l conclude that our limited understanding of strain localization and plastic instability constitutes "an impediment to understanding of hydrogen fracture." A major complication in the study of strain localization3 is that it could be triggered by geometric softening associated with profuse void formation, resulting in a highly dimpled ductile fracture but giving low toughness to the material. On the other hand, it could occur as a result of some yet unknown mechanism, and then trigger local void profusion, accentuating the instability, and resulting in fracture comprised of quasicleavage, shear and shallow dimples, as well as equiaxed dimples. In both processes, hydrogen conceivably could affect the propensity for a strain localization. Unlike the "hydrogen-induced plasticity" concept, 4"5 the strain localization approach does not require that H (dissolved hydrogen) induce softening6 or hardening 7,s or that H change the degree of slip planarity in order to produce degradation. Also, while either __H-enhanced plasticity or H--induced hardening may prevail over the entire range of deformation between general yielding and fracture, strain localization begins only after some substantial homogeneous plastic deformation, 9:~ usually close to that required for crack initiation. Finally, both strain localization and the presence of H in a metal can induce a significant loss of toughness, with no significant change in the microstructural fracture mode. 1 Strain localization may, therefore, offer some opportunity for rationalizing the apparent controversy concerning observed H effects. For example, while loss of macroscopic strain hardening may be necessary to cause plastic instability, H other factors such as the stress or strain gradient, the presence of a free surface, the noncrystallographic character of the shear bands, and the precise form of the constitutive law governing incremental shear are important considerations as well. 11'12'13 In fact, it has been suggested theoreticallyn and verified experimentally 13 that localization can occur even with positive strain hardening, O.A. ONYEWUENYI, formerly Graduate Student at The Ohio State University, is now a Metallurgist with Shell Development Company, Houston, TX 77001. J. P. HIRTH is Professor, Metallurgical Engineering Department, The Ohio State University, Columbus, OH 43210. Manuscript submitted February 8, 1982. METALLURGICALTRANSACTIONS A
without a need for "an ideal-plastic or strain softening state...or related local degradation in strength. ''12'13 Four aspects of the influence of H on plastic instability and fracture were examined in the present work on spheroidized AISI 1090 steel. First, the effects of precharged hydrogen on the onset of both surface and bulk plastic instability were studied. Blunt U-notch spec
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