ANALYSIS ON PENETRATION DEPTH OF PROJECTILES INTO ROCK TARGETS BASED ON THE IMPROVED TWIN SHEAR STRENGTH THEORY

In order further to investigate the penetration mechanism of projectiles into a rock target, the property of nonlinear dependence of the octahedral shear stress on an average principal stress state was considered, and the traditional Twin Shear United Strength criterion was improved by adding the nonlinear item of the average principal stress state. The dynamic Spherical Cavity Expansion compressive elasto-plastic penetration model of the rock target is established by the basis of Twin Shear United Strength Theory and Improved Twin Shear Strength Theory respectively, and the resistance calculation expression of the projectile during normal penetration is proposed with the consideration of both vertical and shear friction of projectiles, the normal penetration depth of the projectile into the rock target is obtained. By comparison of the calculated results of theoretical model and classical empirical penetration formula with the data of field penetration tests on granite targets, it indicates that the calculated results based on the Improved Twin Shear Strength Theory are most close to the test data, the main reason of which lies in the consideration of the octahedral shear stress in the strength criterion of the target material.