Issue 30

L. Bing et alii, Frattura ed Integrità Strutturale, 30 (2014) 526-536; DOI: 10.3221/IGF-ESIS.30.63 530     P is the strain rate function greater than 1. The Mohr-Coulomb yield criterion is adopted and assumes that the compressive stress is negative and tensile stress is positive. Line AB is the shear yield criterion  0 s f , line BC is the tensile yield criterion  0 t f , that is:                 * * 1 3 * 3 2 ( )  s t t f N TQ c N f (12a) Among which, N       1 sin 1 sin (12b) In the equation,  , c are respectively cohesion and internal friction angle; * 1 σ , * 3 σ are respectively first and third effective principal stress;  t is ultimate tensile strength,    max / tan t c ; T is the damage effect coefficient. Under the second order tensor form,     2 2 2 1 2 3 1 T D D D , 1 2 3 , , D D D are damage coefficient of principal stress direction, solved by the damage evolution equation. Within every calculation step,    ,  T Q is constant. According to the non-correlation of surrounding rock, corresponding potential function to yield criterion is as follows:        * $ 1 3 * 3 s ψ t g σ σ N g σ (13a) Among which,       1 sin 1 sin N (13b) In the equation,  is the surrounding rock dilatancy angle. In the calculation, element stress applies the effective force form considering damage:                         1 1 2 2 3 3 1 0 0 0 1 0 0 0 1 * σ D h σ σ D h σ D h (14) Under the tension cases, assume  1.0 h ; in press conditions,  0.2 h . The damage process of surrounding rock is along with the micro fissure development and micro porosity increase and decrease of strength of materials. In the larger sense, damage of surrounding rock is caused by tension. According to some practical observation results, most of the surrounding rock tensile damage is caused by the actual strain exceeding the limit tensile strain of surrounding rock. For the three-dimensional situation, it can be considered that element stress enters plasticity and first principal tensile strain exceeds ultimate tensile strain. Then damage will occur, that is:                        1 1 0 0 0 s s ij or f D h and f (15) In the equation,    ij h is the three-dimensional damage evolution equation,