Measurement of the Mode I Delamination Fracture Toughness of Composite Materials Based on the J-integral Approach

In this research, an experimental characterization of the interlaminar fracture behavior of unidirectional and multidirectional carbon/epoxy laminates under pure mode I loading is presented. The experimental data is analyzed by means of two different data reduction methods based on the LEFM and the J-integral approaches. The J-integral approach, which has been gaining increased popularity in fracture characterization of composite materials in recent years, has great advantages over the standardized LEFM-based approach. It makes possible the full automation of data acquisition by simply using the load and slope angle of the DCB specimen legs to calculate J. In particular, it removes the need for measurement of the crack length, thus eliminating the inaccuracies and difficulties associated with visual detection of the crack tip. Moreover, it is not subjected to the strictly limited scope of linear elastic fracture mechanics, the validity of which is not guaranteed in the presence of damage mechanisms. The results obtained by the LEFM-based data reduction method are shown to agree well with the J-integral-based results in the case of unidirectional layups, implying that the two methods are equivalent, so one may be used in place of the other. For multidirectional laminates, however, for which large amounts of damage was observed in the experiments, the LEFM predictions deviate from the J-integral results, indicating the limitations of the standard LEFM-based data reduction procedure.