The failure envelope reveals other useful information besides the stress state that causes failure in a material. Where the failure envelope intersects the shear stress axis the normal stress is zero. The magnitude of the shear stress that causes failure in a material subjected to no normal stress is related to the materials cohesive strength. The cohesive strength of a material is the strength of bonding between the particles or surfaces that make up that material (Keary 1996). In rock mechanics the cohesive strength is more specifically the inherent shear strength of a plane across which there is no normal stress (Keary 1996). The slope of the Mohr failure envelope is the ratio of the shear stress to the normal stress at failure. This ratio is termed the coefficient of internal friction.