On the validity of the J-integral as a measure of the transverse intralaminar fracture energy of glass fiber-reinforced polyurethanes with nonlinear material behavior

The determination of the fracture toughness of fiber-reinforced polymers (FRPs) exhibiting significant nonlinear material behavior requires the use of methods that take into account the contribution of nonlinear phenomena on the computation of the crack driving force. While the J-integral method has been standardized (ASTM E1820) and extensively used to quantify the fracture toughness of homogeneous nonlinear materials, its validity to unidirectional FRPs remains uncertain. This work addresses this issue by comparing different methods to quantify the energy dissipation during fracture of glass fiber-reinforced polymers with a nonlinear polyurethane matrix. To this end, the J-integral under mode I transverse intralaminar crack growth is evaluated for the first time by directly computing J versus load-line displacement curves using strain maps obtained around the crack tip with digital image correlation. This direct approach is then compared with 2 established data reduction methods and data obtained from virtual tests to determine the suitability of standardized procedures to quantify the fracture energy of such heterogeneous materials. The results show that the best agreement among the values obtained with the different experimental methods is reached when the initial crack length accounts for 40 to 50% of the specimen's width. Accounting for the experimental uncertainties, the procedure described in the standard ASTM E1820 is suitable to quantify the fracture energy of unidirectional laminates displaying nonlinear mechanical response.