Mixed mode fracture toughness: An empirical formulation for GI/GII determination in asymmetric DCB specimens

Asymmetric DCB specimens (ADCB), in which the crack plane is out of the laminate midplane, are simple and useful test configurations to produce a mixed mode load state at the crack tip of the samples. This test configuration is as simple as pure mode I tests. However, the analytical procedures developed in the ASTM D5528-01 standard to calculate GI are no longer valid to analyze the ADCB specimen. In this test configuration, the position of the crack plane controls the mode I and mode II load levels at the crack tip.In this work an empirical formulation, based on Finite Element Modelling (FEM), has been developed in order to calculate GI/G and GII/G (where G=GI+GII) in ADCB specimens. On the other hand, the Modified Beam Theory (MBT) has been adapted by means of experimental calibration to allow the calculation of GG for this test configuration.These formulations have been validated by means of FE modelling for different crack plane positions.Finally, the formulations developed in this paper were used to calculate GI and GII in experimental results on glass fibre epoxy laminates and the obtained results were compared with formulations developed by other authors. A good agreement was found among the different analytical, empirical, and numerical methods studied in this paper.