Development of an evaluation technique for mode-I stress intensity factor of notched carbon/epoxy orthotropic laminate composites and its application.

摘要

Fracture behavior of orthotropic materials such as fabric-reinforced laminate composites containing discontinuities has been of importance since inception of fracture mechanics. However, methods for evaluating stress intensity factors for such materials have not been as much developed as that for conventional materials. This research project was undertaken to develop a stress-based method for evaluation of mode-I stress intensity factor for a through-the-thickness cracked orthotropic body and, further, to expand its applicability to study the effect of structural yarn locking---known as nesting---on the fracture of fabric-reinforced laminate composites.; The proposed stress method provided critical stress intensity factors for carbon/epoxy fabric-reinforced composites with 50% of fiber volume fraction and various crack lengths. These values were verified by a complementary strain gage method, and it was concluded that the proposed stress method yielded as low as 4.2% in error compared to the results from the strain gage method.; The proposed stress method was employed to investigate the relationship between the critical stress intensity factor and nesting for the same material with differing fiber volume fraction. By introducing a new measure for nesting, the degrees of nesting were quantified. The critical stress intensity factors for different volume-fraction laminates were expressed in terms of the values of quantified nesting. The empirical relationship between the critical stress intensity factor and nesting was found to be an inverse of sigmoid function by means of regression analysis.