Strength of composite flysch samples under uniaxial compression

This paper presents the results of uniaxial compression strength tests performed on samples composed of sandstone and siltstone discs. Samples are loaded until failure, with the aim of determining the dependency between uniaxial compressive strength and the volumetric percentage of siltstone. In order to precisely determine the moment and mechanism of failure of each composite sample, the process was recorded with a high-speed camera (120 fps). Uniaxial compressive strength decreases exponentially with an increase of siltstone volumetric participation. The critical ratio at which the uniaxial compressive strength of the composite sample equals the strength of the uniform siltstone sample was obtained at a siltstone percentage of 60%. The failure mechanism is highly dependent on the siltstone percentage, and occurs as shear or tensile failure, or combined shear-tensile failure in individual discs or throughout the entire composite sample. Comparison of the obtained exponential equation with empirical suggestions shows a good correspondence. It is suspected that the disagreement between the particular conclusions of this study and conclusions of other laboratory studies is due mainly to sample micro-heterogeneity.