INTERLAMINAR DYNAMIC CRACK-PROPAGATION

Effective computational procedures are established and used to characterize dynamic crack propagation along material interfaces between isotropic and orthotropic materials. We first simulate a dynamic fracture experiment in which crack propagation occurs along an interface between PMMA and steel in a dynamically loaded bend specimen. In the analysis, the dynamic energy release rate and mixed-mode stress intensity factors are extracted from finite element field solutions using suitably formulated conservation integrals. These variables form the basis for defining an interface fracture criterion under dynamic conditions. In order to propagate the crack according to such a criterion, an iterative procedure is utilized to determine the correct crack tip velocity history. An important computed result, which is consistent with experimental observation, is that the energy release rate decreases as the crack propagation accelerates. The physical interpretation of this result is that less energy is absorbed by the moving crack as its velocity increases. In the second part of our study, the computational procedures are modified For the dynamic fracture analysis of a thin composite panel consisting of differently oriented orthotropic laminae. Here we investigate the interplay between delamination and buckling of the more complex structure. It is assumed that the panel contains an initial finite interlaminar crack and is subjected to a uniaxial compressive load. Without any crack extension, two buckling modes are observed under quasistatic conditions. One is characterized by an overall panel buckling and the other is dominated by a local ligament buckling near the crack. Coupling of the two modes produces not only a lower critical buckling load but also an unstable post-buckling behavior. Therefore, an embedded delamination is seen to create imperfection sensitivity, leading to limit load type behavior. Such a condition is potentially troublesome for compression loaded thin composite panels. In addition, the large compressive load may also trigger a dynamic propagation of the embedded delamination. We have simulated this unstable dynamic crack growth using the iterative method and a simplified delamination criterion. In the simulation, the energy release rate, the mixed-mode stress intensity factors and the resulting crack tip speed are obtained. These results predict that an existing embedded delamination can weaken and significantly alter the post-buckling behavior of composite panel.