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 andmultidirectional carbon/epoxy laminates under pure mode I loading is presented. The experimental data is analyzed bymeans of two different data reduction methods based on the LEFM and the J-integral approaches. The J-integralapproach, which has been gaining increased popularity in fracture characterization of composite materials in recentyears, has great advantages over the standardized LEFM-based approach. It makes possible the full automation of dataacquisition by simply using the load and slope angle of the DCB specimen legs to calculate J. In particular, it removesthe need for measurement of the crack length, thus eliminating the inaccuracies and difficulties associated with visualdetection 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 LEFMbaseddata reduction method are shown to agree well with the J-integral-based results in the case of unidirectionallayups, implying that the two methods are equivalent, so one may be used in place of the other. For multidirectionallaminates, however, for which large amounts of damage was observed in the experiments, the LEFM predictionsdeviate from the J-integral results, indicating the limitations of the standard LEFM-based data reduction procedure.

Keywords:

Delamination , interlaminar fracture toughness , double cantilever beam (DCB) , energy release rate , J-integral