To precisely describe the strength of rock materials under a variety of complex stress conditions

we developed a new nonlinear tri-parameter strength criterion based on the theory of twin T2 strength. Compared with experimental results of five types of rock

the rationality and applicability of the proposed criterion were discussed separately under five stress states

i.e.

?1>?2 =?3

?1>?2 =?3

?3=0 and ?1>0

?2>0

?3>0 and . The results shows that the parameters

A

B and C

can be determined by simple tests

though their physical meanings are reasonable. The different ultimate stress ratios and correspond to different strength theories. The limit line in the plane of the proposed criterion is among the single shear strength theory

Mises criterion

and the twin shear strength theory. Furthermore

the criterion conform with the triaxial symmetry and the convexity of yield surface of the isotropic material. The theoretical prediction curve can reflect the interval characteristics of the principal stress and the hydrostatic pressure effect in rock materials. Moreover

it is in good agreement with the strength trends of some materials at complex stress states.