A compact Mode II fracture specimen together with its loading frame is analyzed and tested. This system is a further development of an earlier circular Mode II fracture specimen presented by the authors. For fatigue precracking in two different loading configurations, formulas relating the Mode I stress intensity factor, applied load, and specimen geometry are developed. In order to determine K_IIc from the fracture load, a relationship between K_II, the applied load, and specimen/loading frame geometry is obtained. All of these relations are calculated by means of the finite element method.

These formulas are then employed to perform a series of Mode II fracture toughness tests with Perspex (also called Plexiglas or polymethyl methacrylate [PMMA]). The average measured K_IIc value was 0.94 MPa√m. The K_Ic tests were performed with the compact tension specimen, yielding K_Ic = 1.06 MPa√m; thus K_Ic/K_IIc was found to be 1.127. In addition, crack propagation angles were found to be between 63 and 70°. These results are compared with the maximum tangential stress and maximum tangential principal stress criteria.