Simulation of ductile crack growth using computational cells: numerical aspects

This study explores key computational issues that affect analyses employing the computational cell methodology to predict crack growth in ductile meta ls caused by void growth and coalescence. These issues - computational load step size, procedures to remove cells with high porosity from the analysis , and the porosity for cell deletion can adversely affect predicted crack g rowth resistance (Ii) curves and/or hinder convergence of both local consti tutive and global iterative computations. Strain increments generated by la rge computational load steps introduce errors in the predicted peak stress of computational cells and prevent convergence of stress updates for the Gu rson-Tvergaard constitutive model. An adaptive load control algorithm, whic h limits the maximum porosity over a load step, eliminates this problem. Th e delayed release of remaining forces in newly deleted cells elements eleva tes the stress triaxiality and thus artificially accelerates the rate of cr ack extension. The release of cell forces using a traction-separation model minimizes this effect while maintaining good numerical convergence of the solutions. Crack growth analyses for a moderate strength steel demonstrate that critical porosity values (f(E)) between 0.1 and 0.2 show almost no eff ect on predicted R-curves, while both larger and smaller values lead to low J-Delta a curves, Finally, a parametric study indicates that specimens of low-hardening materials and specimens with high crack-front constraint show a stronger influence of large computational load steps and the delayed rel ease of cell forces. Use of the adaptive load control algorithm and the tra ction-separation model with the controlling parameters described here, mini mize numerical effects on predicted R-curves. (C) 2000 Elsevier Science Ltd . All rights reserved.