J _II fracture testing of a plastically deforming material

Abstract

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.