Abstract :
An extension of the Neuber-Novozhilov structural fracture propagation criterion is presented
for mode I (tensile) and mode II (shear) propagation under compressive loads. In addition to allowing
numerical simulation of crack growth, the criterion can be used to model change of propagation
mode, crack branching, and coalescence. The criterion can be applied effectively when the SIF is calculated
accurately (at least three significant digits). A numerical method is suggested for this purpose
that consists of complementing the complex variable hypersingular boundary element method (CVHBEM)
with special procedures for automatically tracing crack propagation and coalescence. The CVHBEM
code with the structural criterion has been used to investigate crack propagation in compression
for both small and non-small fracture process zone (FPZ). The results of numerical experiments are in
agreement with the analytical conclusions available for the case of small FPZ that indicates the possibility
of three distinct patterns of crack propagation under external compressive loads. These are: (i)
smooth curvilinear tensile (wing) cracks, (ii) stair-step propagation pattern with changing modes, and
(iii) in plane shear propagation. The numerical study also indicates that when the critical size of the
FPZ is large enough, the non-singular terms in the expansion of the stress functions strongly influence
the crack trajectories. Specifically, this occurs when the size of the FPZ approaches a quarter of the
half-length of the initial crack. Calculations for a closed initial crack in a half-space under compression
illustrate the general features of crack propagation. Although the dominant direction of crack
growth is that of the applied compressive stress, the pattern of propagation strongly depends on the
particular geometry, critical size of the FPZ, and the ratio of shear-to-tensile microscopic strength
