A mixed-mode fracture specimen: analysis and testing

A mixed-mode fracture specimen which is employed to measure K_Iand K_IIfailure values for 0≤K_I/K_II≤0.28 is analyzed. In order to perform experiments, specimen calibration formulas relating K_Iand K_IIto the applied load and specimen geometric parameters must be obtained. To this end, a mixed-mode weight function method is applied to the specimen, requiring the stress intensity factors and displacement fields induced by two different loading cases. These quantities are determined by means of a finite element analysis which makes use of a singular crack tip element. For this range of K ratio, the stress field in the central region of the specimen before crack introduction is nearly uniform; hence, expressions for the desired mixed-mode stress intensity factors may be written as the product of two factors; one depending upon crack length and one depending upon loading angle. Once the K calibration formulas are obtained as a function of applied load and specimen geometry, testing may be carried out. Here, perspex specimens are tested; results for K_I, K_IIand crack propagation angle at fracture compare reasonably well with failure curves determined from an extended maximum tangential stress criterion.