Issue 30

L. Bing et alii, Frattura ed Integrità Strutturale, 30 (2014) 526-536; DOI: 10.3221/IGF-ESIS.30.63 531  1 is the first principal tensile strain of the surrounding rock,    is the ultimate tensile strain of the surrounding rock. For the situation of the ultimate tensile strain of untested surrounding rock in practical engineering, we adopt specific value of rock tensile strength s R and elasticity modulus E , that is:       s R kE (16) In the equation, k is the safety coefficient. In the damaged area, surrounding rock stress and physical parameters decrease with the increase of slant plastic strain. The greater the cumulative plastic deformation is, the higher the damage degree of surrounding rock will be. In the adjacent area in front of the ultimate strength of surrounding rock material, micro crack damage increase is very rapid. This kind of change rate of damage can usually be described by exponential function. Therefore, three-dimensional damage evolution equation can be expressed as follows:           1 exp 1, 2, 3 p p i i i D R i (17) In the equation,  p i is the slant plastic strain in the principal strain direction, R is damage constant of the material. For the purposes of showing the state of surrounding rock damage in the post-processing, second order damage tensor is converted to a first-order damage scalar when output the results file, expressed as    2 2 2 1 2 3 D D D D . Stress Correction and Damage Calculation Unit stress calculation is the core of the explicit finite element method. In the elastic and plastic damage calculation of the tn+1 moment, the gauss point elastic stress is calculated using unit material initial parameters and whether unit is in a state of damage is judged (when    0 D tn , unit damage coefficient of the moment is calculated). If the unit is in a state of damage, each gaussian point stress is revised as the effective stress. On this basis, considering damage effect on yield surface and carry out yield judgment. In case of yield, plastic stress correction is required. Also, we need to judge whether principal tensile strain exceeds the ultimate tensile strain. If there is exceeding, it's considered unit damage and damage coefficient is calculated. This stress correction method is applicable to the situation that yield function    n f is the linear function. Based on the principle that plastic stress should flow on the yield surface, it is required that plastic stress of tn + 1 moment as follows:      0 n n f σ σ (18) For the linear function, Eq. (18) can be converted to:        0 n n * f σ f σ (19a)           0 n * f f f (19b) In the equation, * (·) f is calculated by yield function minuses to the constant term   0 n f . Based on this transformation, linear yield function plastic stress correction formula was deduced by service manual [20] as follows:              N I i i i n g S (20) In the equation,  is the plastic flow factor; i S is the elastic constitutive;