Delamination Fracture Criteria for Composite Laminates

Abstract

Due to the singularity nature of delamination, the fundamental concept of fracture mechanics was applied. However, the oscillatory characteristics near the delamination tip prohibited the use of conventional definition assuming the crack embedded into a homogeneous solid. A proper definition for the stress intensity factors and energy release rates of bimaterial interface cracks was introduced in this paper to study the fracture criterion for delamination. To measure the delamination fracture toughness, the test specimen usually used for the unidirectional composites was redesigned to suit for the cases that the delamination lies between two laminae with different fiber orientations. The test results show that the double cantilever beam (DCB) test is near to pure Mode I and the end-notched flexural (ENF) test is near to pure Mode II. Therefore, like the cracks in homogeneous materials, the test methods DCB and ENF are useful for the measurement of delamination fracture toughness GC and G11,. Based upon this conclusion, a series of cracked-lap shear (CLS) and modified end-notched flexural (MENF) tests and finite element simulation are conducted in this paper to establish the mixed-mode delamination fracture criterion. To validate the proposed fracture criterion, two commonly encountered problems are predicted and tested. One is the prediction of the most probable interface for the onset of delamination and its possible minimum load, in which no delamination was embedded prior to loading. The other is the prediction for the test with delamination existing prior to loading. The results show that the prediction by using this proposed criterion is well matched with the experiment.