Blunt Crack Band Propagation in Finite Element Analysis

A propagating smeared crack band of blunt front is much simpler to model by finite elements than a sharp interelement crack, especially when the propagation direction is unknown. For concrete or rock, a smeared crack band is also more realistic. A strength criterion is generally used for the propagation, but this is not objective because of a strong spurious dependence of results on the chosen element size, Three objective methods, which avoid the use of singularity elements, are proposed. Method A is based on the rate of energy release by the blunt crack band. In method B, the usual strength criterion is used but an adjustment of the strength value according to the element size is proposed. In method C, the propagation direction and crack advance are determined by fitting the Mode I asymptotic series to nodal displacements around the crack front, using an optimization subroutine. Special merits of each method are analyzed and solutions of example problems are compared with exact ones.