Stability issues in uniaxial tensile tests on brittle disordered materials

The stability of crack propagation in brittle disordered materials like concrete and rock depends not only on the structure of the material at the micro- and meso-level, but also on global structural conditions. The resulting softening behaviour is affected by both components. In order to determine stable softening diagrams for all kinds of materials (including high strength, brittle concretes) for specimens of different size a reliable control system should be developed. In this paper results are shown of uniaxial tension tests on sandstone. The tension specimens are mounted between cables of varying size in order to vary the stiffness of the test set-up. A newly developed MAX-RATE control system, which is based on the maximum deformation rate measured for a number of LVDTs mounted on the specimen’s surface, is described. The control LVDT is appointed dependent on the magnitude of the deformation rate. Stable softening curves, even for very low stiffness test machine have been obtained