Testing of Brazilian Disk Specimens with a Delamination between a Transversely Isotropic and a Tetragonal Composite Ply

Fracture toughness tests using the Brazilian disk specimen (BD) were carried out to determine the toughness properties of a delamination between two fiber reinforced composite plies. The upper ply is a transversely isotropic UD fabric with fibers oriented in the 0-direction and the lower ply is a tetragonal plain balanced weave with fibers oriented in the +45°/-45°-directions. The composite is manufactured as a wet-layup. With the BD specimen, mixed mode combinations are achieved by changing the loading angle ω, between the load line and the delamination. Eight specimens were tested at four loading angles, two for each angle. Based on the test results, finite element (FE) analyses were carried out in conjunction with two methods, the three-dimensional conservative M-integral and displacement extrapolation (DE) to obtain the stress intensity factors along the delamination front for each specimen resulting from mechanical loads, as well as residual stresses. The two methods were used in order to validate the results. Both methods were extended for this specific interface and made use of the first term of the asymptotic solution of the displacement field. The stress intensity factors were superposed, and the critical interface energy release rates and phase angles were calculated and plotted.