DFG project number: 254872581 (follow-up of project HE2933/8-1) The majority of constitutive models, that are used nowadays to describe the behaviour of granular materials such as sands, are continuum models based on phenomenological approaches. In order to describe some of the phenomena occurring on the macroscopic scale, e.g. an abrupt change of stiffness due to a load reversal, these 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 μ-computed tomography (CT) allows for a 3D imaging of natural soil samples in various loading conditions and is 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 is evaluated with respect to the limited resolution. Mechanical experiments in the x-ray CT scanner have been carried out on natural sands in the running project. During a macroscopic loading the sand specimens were scanned using a laboratory x-ray scanner in order to assess the grain-scale behaviour in-situ and link it with the macroscopic observations. 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.

This project is open access and publicly accessible.

 

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