J II fracture testing of a plastically deforming material

A compact model II fracture specimen was previously analyzed and employed to determine the mode II fracture toughness K_IIc, of perspex. In employing this specimen for a more ductile material such as aluminium, it was observed that the load vs. crack sliding displacement record becomes nonlinear for small loads. Thus, concepts of linear elastic fracture mechanics cannot be employed. To this end, the specimen was calibrated for J-integral testing, so that J_IIc mesurements can be performed.In this study, mode I and II tests are carried out on an aircraft aluminium alloy, AI 7075-T7351. First, standard K_Ic tests are performed leading to a value of 27.9 15-1 which would be equivalent to a J_Ic of 10.7 kN/m. Then standard J_Ic tests are carried out on this material with specimen thicknesses, of 5, 7.5 and 9.9 mm, leading to an average J_Ic value of 10.5 kN/m. Methods for J_II testing are proposed; a series of specimens of six thicknesses between 5 and 16 mm are employed for testing. An average J_IIc value was found to be 40.2 kN/m which yields a K_IIc value of 54.1 15-2. Thus, K_IIc is seen to be approximately twice that of K_Ic for this material.