ABSTRACT
When investigating the flow properties of a cohesive granular material, the steady state flow is a central issue (Schwedes 2002). This state is defined as a permanent deformation of the material without volume change (i.e. possessing a nonzero, trace-less tensor of strain rate) while the stresses at the specimen’s boundaries remain constant.This state reflects different reactions of the system, depending on the type of boundary conditions. For these we will use the following terms: “Strain control” refers to prescribing the (normal or shear) strain rate, and obtaining the corresponding stress as the system’s response. “Stress control”, on the other hand, means prescribing the stress (e.g. by means of a control loop for the strain(rate) or by a force, applied on a well defined area) to which the system reacts in form of the corresponding strain(rate). According to these definitions, “pure strain control” denotes prescribing only the tensor of strain(rate) while “pure stress control” amounts to prescribing only the stress tensor. Of course, combinations of partial stress and partial strain control are possible, as found e.g. in the ring shear tester (cf. section 2.2 for details). We will call such cases “mixed control”.
