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So far various criteria on fracture of rocks have been proposed by many investigators. One can find the ambiguous usage of terms in different contexts in the literature dealing with the fracture criterion. In order to study the fracture of rocks on the basis of precise concept, the difinition of terms proposed by Bieniawskii> will be accepted in what follows. When a rock is subjected to a compressive stress in the axial direction under low confining pressure in triaxial compression test, the stress at strength failure tends to take the value close to that at yield, namely, brittle fracture occurs. For this reason, we have often failed to discriminate between the criterion on strength failure and that on yield strictly when rocks are subjected to low confining pressure. On the other hand, when a rock is subjected to a compressive stress in the axial direction under high confining pressure, the stress at strength failure shows the value apparently different from that at yield ; and the behavior of the rock becomes to exhibit ductile fracture in general. Accordingly, a precise distinction must be made concerning yield and fracture criteria in multiaxial stress conditions. Mohr2 introduced a hypothesis for failure of soils and rocks. Drucker and Prager3) generalized the Mohr's criterion in terms of invariants of stress tensor. Griffith4 postulated a fracture criterion based on the presence of small cracks in a material. According to studies of failure of rocks, it has been found that failure criteria applicable to predicting the strength of materials may be those by Mohr and Griffith. Hu and Pae5 took the influence of hydrostatic pressure on the plastic behavior into account to propose a yield criterion. However, if these criteria are applied to the plastic potential in flow rule, it is found that the equations of plastic strain rate derived from foregoing criteria do not adequately describe the plastic deformation of some rocks. This may be because the above-mentioned criteria are not functions of cap's type which means closed form in stress space. Thus, it may be suggested that the four criteria are not relevant to the yield criterion of rocks. The objective of the present paper is to deduce a yield criterion in order to describe the plastic behavior of soft rocks precisely. For this purpose, a yield function is proposed on the basis of invariants of tensors and experimental evidences. Particularly the form of work-hardening parameter is expressed by taking account of plastic work divided into two parts related respectively with the change in volume and with the change in shape. Furthermore the appropriateness of the proposed yield function is investigated through experimental datas>'7>.

PROPOSAL OF A YIELD FUNCTION AND DESCRIPTION OF PLASTIC BEHAVIOR OF SOFT ROCKS