Micromechanical analysis of state variables for phenomenological constitutive models of soils

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In order to describe some of the phenomena in soils occurring on the macroscopic scale, e.g. an abrupt change of stiffness due to a load reversal, constitutive models use phenomenological state variables (e.g. back stress in elasto-plasticity or the intergranular strain concept for hypoplasticity) which often lack a clear physical meaning. The mechanisms that control the macroscopic behaviour and, as such, different phenomena, that can be observed on the continuum scale, must be sought at the grain-scale with the interactions of individual particles playing the key-role. X-Ray-computed tomography (CT) allows for a 3D imaging of natural soil samples in various loading conditions and, thus, is and will be the tool used in this project. In order to extract information on the structure of the granular material, different image analysis approaches can be used and their accuracy should be evaluated with respect to the limited resolution. In the first part of the currently running project the ability as well as the accuracy of different approaches to extract fabric descriptors, i.e. in this case particle- and contact-based entities, from 3D images is assessed and enhancements are proposed and validated. Artificial as well as high resolution images coming from x-ray nano-CT serve as the basis of this analysis which shows that the standard approaches, implemented in any commercial software, strongly suffer in accuracy and often introduce huge artefacts. Furthermore, real mechanical experiments in the x-ray CT scanner have been carried out on natural sands in the running project. The main focus of the follow-up project is to exploit the obtained experimental database. For this purpose, the individual fabric entities will be extracted from the 3D images at different steps of the loading and captured using fabric tensors. The evolution of various fabric tensors, defined using different variables and different orders, is then assessed and linked to the evolution of the macroscopic stresses and strains. The evolution of the microstructure can be linked to the evolution of the phenomenological variables, e.g. the intergranular strain for hypoplasticity for changes in loading direction, leading to a possible micromechanical enhancement of these concepts. Establishing a link between micromechanical variables, such as the fabric tensors describing the stucture, and the macromechanical observations cannot only enhance our understanding of different phenomena occurring on the continuum scale, but also enable an incorporation of these effects into phenomenological approaches in a more straight-forward and reliable way. In the study on high-resolution images of natural grains it wasfound, that the fractality of the shape of natural grains makes a geometrical definition of inter-particle contacts highly complicated. In cooperation with University of Syndey a part of this project will investigate the influence of the scaling of the contacts.