Acta Mechanica 110, 151-171 (1995) ACTA MECHANICA 9 Springer-Verlag 1995 Constitutive modelling for granular media using an anisotropic distortional yield model G. Z. Voyiadjis and G. Thiagarajan, Baton Rouge, Louisiana, and E. Petrakis, Newark, New Jersey (Received September 14, 1992; revised April 6, 1993) Summary. It has been attempted here to model the mechanical response of granular media through a combination of analytical, numerical and experimental techniques. Based on the experimental evi- dence obtained from a series of triaxial laboratory experiments on specimens made of glass beads, the model attempts to simulate the movement of yield surfaces and the stress-strain relationships. It is found that the yield surface distorts in the direction of loading in a manner analogous to that of metals. An anisotropic distortional yield model is formulated in order to describe the experimental behavior exhibited by these granular media. From the experimental yield surfaces the parameters of the model have been evaluated and a hardening rule, based on the Phillips rule, has been determined. Associativity on the K-plane has been observed experimentally. Using these concepts the constitutive formulation has been presented and the stress-strain curves have been generated. From the compari- sons of these curves we observe a good correlation between the model and the experimental observa- tions. 1 Introduction It is attempted here to combine the results obtained from experiments (Petrakis et al. [10], [11]) on granular media with a plasticity model based on an anisotropic distortional yield model proposed by Voyiadjis et al. [16] and formulate the overall constitutive relationship. Petrakis et al. [10], [11] have conducted a number of experiments on samples composed of glass beads, to study of the stress-strain response of the media and also to obtain yield surfaces after prestraining the media in specific directions upto specific levels of prestrain. These experiments have been conducted primarily for proportional loading. The objective was to experimentally determine the parameters necessary for a constitutive model, namely, the hardening characteristics under various levels of prestrain, the observed normality and the flow rule. To identify the above parameters a number of experiments has been performed under specific stress-paths on hollow cylindrical samples of glass beads. An initial yield surface was first obtained, after which cyclic tests on the same specimen at different levels of prestrain have been performed in order to obtain the location and shape of the yield surfaces. Based on the experimental observations of the work by Petrakis et al. [10], [11], a plasticity model for granular media is proposed herein, that is based on the anisotropic distortional yield model proposed earlier by Voyiadjis and Foroozesh [15] and modified later by Voyiadjis et al. [16]. The distortional model attempts to describe the yield surface in terms of three parameters, that are determined from experimentally obtained data. For the constitutive formulation we further need a flow and hardening rule based on experimental data. The hardening rule proposed