Issue 30

L. Bing et alii, Frattura ed Integrità Strutturale, 30 (2014) 526-536; DOI: 10.3221/IGF-ESIS.30.63 528        1 1 int  n ext damp a M f f f (4)    1 2 1 2    n / n- / n a a Δta (5)      1 2 1 2 1  n n n n a a Δt a (6)              1 2 1 2 1 2 1 2 n / n- / n / n n t t t / , t t t (7) In the equation,  1 n n a ,a are respectively displacement of  1 n n t ,t ;   1 2 1    n n / n a ,a ,a are respectively velocity of   1 2 1 n n / n t ,t ,t ;  n a ,  1  n a is respectively accelerated speed of  1 n n t ,t . The explicit central difference method calculation process can be described as follows: (1) According to the initial condition 0 a , 0 a , we can calculate that       0 1 0 1 2 0 ,    damp n- / a M f f a a . (2) For each time step:  1 2 n / a is calculated by Eq. (5); displacement  1 n a of  1 n t is calculated by Eq. (6); accelerated speed  1  n a is calculated by Eq. (4). Finite element explicit calculation is a conditional stability algorithm which is often constrained by the solution stability and calculation time consuming. This is because to ensure the stability of calculation, we generally need to select the smaller calculation time step  t to increase system cycle count and extend the calculation time. Therefore, on the premise of ensuring stability calculation, we need to choose the larger  t . Considering the physical meaning of the wave finite element,  t can be regarded as the minimum duration in a computational time step, that wave propagation distance is not more than any one unit size. It can be calculated as follows:       min 0 80 0 98 e e e l t α . α . C (8) In the equation, e C is the unit wave velocity; e l is the ratio of the largest area between element volume and unit 6 surface. The system time step of Eq. (8) only considers the seismic wave transmission stability in calculation model rather than artificial boundary conditions and computational stability requirements of supporting measures such as anchor bars to the system time step. Therefore, in the practical engineering calculation, according to different situations, we need to make adjustments of  t . Node Internal Force Solution In the explicit solution of type (1), the key is the solution to node internal force f int . In incremental variational type calculation, we first calculate unit stress increment and update the unit stress. Then the updated nodes stress is integral to the unit nodes to obtain internal forces. Deformation description In the Delta Lagrange explicit calculation, deformation is described by strain rate ij e :                              1 1 2 2 i i j i j ij ij ij j j i i i v v v v v e Ω D x x x x x (9) In the equation, ij Ω is the spin rate tensor; ij D is the deformation rate tensor; i v is the velocity component; j x is the weight coordinates, , i j =1, 2, 3.