This document discusses concepts of failure in materials including tensile failure, shear failure, and failure criteria. It specifically examines the Mohr-Coulomb failure criterion, which states that failure depends on the material's cohesion and internal friction angle. The criterion can be represented on a Mohr's circle diagram, where failure occurs if the circle contacts the linear failure envelope line. Pore fluid pressure is also accounted for using effective stress. Triaxial tests are described that apply different confining pressures to measure failure properties over a range of stress conditions.