Hypoplastic constitutive models are based on nonlinear tensor functions and are characterized by simple formulation and few parameters. In its early stage, mainly basic hypoplastic constitutive equations were concerned, where the stress tensor is assumed as the only state variable. There followed some enhanced models based on the basic constitutive equation by including void ratio as an additional state variable. In this paper, we first show that the widely used hypoplastic model by Wolffersdorff is seriously flawed because the underlying basic equation does not perform properly. We proceed to develop a basic hypoplastic constitutive equation by introducing a new tensorial term, which preserves the critical state at large strain. The model performance is demonstrated by parameter study for some element tests. This simple and robust basic equation is well suited to build more sophisticated models.

Persistent shear bands in heterogeneous materials develop progressively, rather than instantaneously. For a meaningful capture of this deformation pattern, the band must develop naturally and freely up to the peak load and well into the softening regime. We use a new micropolar hypoplastic framework to capture the development and propagation of a persistent shear band in a rectangular specimen of silica-concrete sand with spatially varying density. The constitutive framework includes a characteristic length as a regularization parameter, as well as a couple stress representing the angular momentum generated by particle spins. We use computed tomography (CT) imaging and digital image processing to quantify the spatial density variation of the sand, and a void ratio-dependent constitutive model to capture the effect of variable density. Results of the numerical simulations demonstrate the capability of the proposed micropolar hypoplastic framework to track the development and propagation of a persistent shear band in a heterogeneous sand up to the peak load and well into the softening regime.

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Gas hydrate-bearing sediments (GHBSs) have been considered as a potential energy resource. In this paper, the mechanical properties of GHBS are firstly investigated by the integrated test apparatus for synthesis of GHBS using silty sand as skeleton. Triaxial tests indicate an obvious transition of stress-strain relationship from strain hardening under low hydrate saturation and strain softening under high hydrate saturation. The hypoplastic models coupled with Drucker-Prager criterion and the Mohr-Coulomb criterion are proposed to analyze the stress-strain relationship of GHBS with considering the effective porosity because of the hydrate filling in the pores of GHBS. The strain hardening and softening behaviors are well predicted with less material parameters compared with the classical models. Compared with the test results, the proposed hypoplastic models are verified to be capable of capturing the salient features of the mechanical behaviors of GHBS under the conditions of little temperature change and no hydrate dissociation. KEYWORDS effective porosity, gas hydrate-bearing sediment, hypo-plastic model, strain hardening, strain softening 1 | INTRODUCTIONAs the most common naturally occurring gas hydrate, gas hydrate-bearing sediments (GHBSs) have been considered as a potential energy resource, and the related exploration and exploitation technologies have been extensively investigated in many countries. Gas hydrate-bearing sediment, a crystalline solid composed of natural gas and hydrogen-bonded water