Three-dimensional crack surface evolution in mode iii delamination toughness tests

A study is conducted to determine the manner in which the coupled system of a delamination and multiple transverse cracks evolves and advances in mode III split shear torsion and edge crack torsion delamination toughness tests. Ultrasonic inspection, X-ray computed tomography and microscopy are utilized to view the fracture surface evolution both before and after the onset of planar delamination growth. The three-dimensional fracture surfaces from the two test types are compared to each other and to analogous results from homogeneous and geologic materials. Due to the existence of, and preferential fracture path along, the interlaminar interface in laminated composites, clear differences are found in the behavior of the composites versus that of other materials. Different behaviors in the two types of composite test specimens are also observed and are caused by the proximity of the preimplanted insert to other distinct interfaces: that of the free surfaces in split shear torsion specimens, and of the 90/45 interfaces in edge crack torsion specimens. These interfaces are shown to bound the size of the transverse cracks. This causes differences in the fracture surface evolution between the two composite specimen types and further differentiates the behavior in the composite specimens from that of homogeneous and geologic materials. The application and importance of these observations to mode III delamination toughness testing and delamination growth prediction is described.