ABSTRACT RELATIONSHIP BETWEEN DUCTILE CRACK INITIATION AND VOID VOLUME FRACTION Akio Otsuka, Keiichiro Tohgo and Yoshihide Okamoto Department of Iron and Steel Engineering Nagoya University, Nagoya 464, Japan An investigation on ductile crack initiation in structural steel has been made, based on the concept of Gurson's yield function for porous material. First, the condition of ductile crack initiation in the uniform stress field has been investi- gated. The condition of ductile crack initiation under various stress triaxiality obtained from the tests on axisymmetric notched tensile specimens is well expressed by the condition of constant void .volume fraction analytically obtained from the Gurson's model. This result means that the condition of constant void volume fraction may be used as the criterion of ductile crack initiation. Secondly, the behavior of void growth and ductile crack initiation in the area near the notch tip under mode I and mode II loading has been investigated. Under mode I loading, the increase in void volume fraction around the notch with an increase in applied load agrees well with the behavior of porous material predicted by the finite element analysis based on Gurson's yield function, and the ductile crack initiation can be predicted by the concept of critical void volume fraction as in the case of uniform stress-strain field given above. The same criterion is not applicable to the crack initiation under mode II loading and further study is needed. INTRODUCTION Ductile fracture occurs by growth and coalescence of microvoids nucleating from the inclusions and the second-phase particles[l-11). The void nucleation from a particle has been investigated by Gurland and Plateau [2] and Broek [6] based on the dislocation mechanism, and by Argon et al.[1O] and Thomson and Hancock[ll] based on the continuum plasticity. Theoretical investigations on the growth of a single void in infinite plastic body have been made by McClintock[3] for a cylindri- cal void and by Rice and Tracey[S] for a spherical void. The mechanism of coales- cence of voids by interna! necking has been discussed by Thomason[4]. McClintock and Thomason have proposed the expressions for the condition of ductile fracture based on their own models, respectively. These investigations were concerned with a single inclusion and a single void in infinite body except for Thomason's coales- cence model. Recently, the analytical estimation of ductile fracture has been carried out by Rudnicki and Rice[l2], Yamamoto[l3], Tvergaard[l4], and Needleman and Tvergaard[l5], based on the continuum mechanics approach. In the continuum mechanics approach to 309
RELATIONSHIP BETWEEN DUCTILE CRACK INITIATION AND VOID VOLUME FRACTION
May 28, 2023
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