Constitutive Modelling

In this paper a typical soil–structure interaction problem is considered, the case of a vertical pile installed in sand and submitted to an axial compression loading. Results from two full scale pile tests are analysed and the tests are reproduced by numerical simulations via finite ...

In this work, a macroelement for shallow foundations on sands has been developed based on the theory of hypoplasticity. The incrementally nonlinear constitutive equations of the macromodel are defined in terms of generalized forces and displacements and are constructed based on the general approach proposed by Niemunis (Extended Hypoplastic Models for Soils. Habilitation Thesis, Bochum University, 2002). A suitable vectorial internal variable—mimicking the concept of intergranular strain introduced by Niemunis and Herle (Mech Cohes Frict Mater 2:279–299, 1997) for continuum hypoplasticity—is employed to provide the model sufficient memory of past displacement history to be able to reproduce the observed behavior under cyclic loading paths. As compared to similar macroelements formulated within the framework of the theory of elastoplasticity, the proposed approach has the advantage of a much simpler mathematical structure, which allows a straightforward implementation in existing structural analysis FE codes. The model performance has then been evaluated by comparing the model predictions with available experimental results from a series of small-scale model tests reported by Nova and Montrasio (Géotechnique 41:243–256, 1991). Overall, the model captures reasonably well the observed response under nonproportional, complex loading paths. A series of simulations reproducing the tests performed by di Prisco et al. (Shallow footing under cyclic loading: experimental behaviour and constitutive modelling. Patron, Bologna, 2003) has also demonstrated the potential of the proposed model to simulate the observed behavior of footings under cyclic loading paths, at least from a qualitative point of view.

The study developed in this work is focused on constitutive modelling of saturated and unsaturated granular soils from the state parameters point of view. The Generalized Plasticity constitutive equation has been extended in order to reproduce stress-strain behaviour of granular soils with a single set of intrinsic model constants for different densities, confining pressures and saturation conditions.

The non-linear mechanical behaviour of porcine brain tissue in large shear deformations is determined. An improved method for rotational shear experiments is used, producing an approximately homogeneous strain field and leading to an enhanced accuracy. Results from oscillatory shear experiments with a strain amplitude of 0.01 and frequencies ranging from 0.04 to 16 Hz are given. The immediate loss of structural integrity, due to large deformations, influencing the mechanical behaviour of brain tissue, at the time scale of loading, is investigated. No significant immediate mechanical damage is observed for these shear deformations up to strains of 0.45. Moreover, the material behaviour during complex loading histories (loading-unloading) is investigated. Stress relaxation experiments for strains up to 0.2 and constant strain rate experiments for shear rates from 0.01 to 1 s(-1) and strains up to 0.15 are presented. A new differential viscoelastic model is used to describe the mechani...

Constitutive models based on the multilaminate framework provide an alternative framework to more common formulations in terms of stress invariants. The basic feature of multilaminate models, namely the consideration of various integration planes to formulate the stress-strain relationship has the advantage that yield criteria, flow and hardening rules are formulated on planes rather than in three-dimensional stress space making the mathematics simpler. Stress or strain induced anisotropy is an intrinsic feature of these models and inherent anisotropy can be introduced by means of a structural tensor which allows the input of orientation dependent strength properties. A number of different constitutive equations for modelling clays and sands based on the multilaminate framework have been presented in the literature and more recently advanced formulations including small strain stiffness effects or non-local softening behaviour have been developed. In this paper only a very brief sum...