Based on the experimental and theoretic studies on the frictional materials in literatures

a generalized nonlinear strength theory is presented. The generalized strength theory has a form of Drucker-Prager strength criterion

in which a unified mathematical expression is used to describe the strength characteristic of various materials. The theory covers or approximates the classical nonlinear strength theories in existence. The generalized strength criterion is smooth curve between the Drucker-Prager criterion and the external contour line of Mohr-Coulomb criterion in deviator-plane

and which is linear function in meridian-plane. The hydrostatic pressure and the deviator stress are separated in stress space; so it can be easily integrated with the concrete constitutive model.