Using Torsion Bar Testing to Determine Fracture Toughness of Ceramic Materials

A new method, designated as Spiral Notch Torsion Test (SNoTT), is introduced for determining fracture toughness K_IC of materials ranging from metallic alloys to brittle ceramics and their composites. A round-rod specimen having a V-grooved spiral line with a 45° pitch is subjected to pure torsion. This loading configuration creates a uniform tensile-stress crack-opening mode, Mode-I, with a transverse plane-strain state along the grooved line. This technique is analogous to the conventional test method using a compact-type specimen with a thickness equivalent to the full length of the spiral line. K_IC values are determined from the fracture load and crack length with the aid of an in-house developed 3-D finite element program (TOR3D-KIC). A mixed mode (modes I and III) fracture toughness value can be determined by varying the pitch of the spiral line or varying the ratio of axial to torsion loads. Since the key information needed for determining K_IC values is manifested within a small region near the crack tip, the specimen can be significantly miniaturized without the loss of generality. Limited results obtained for various materials are compared with published K_IC values, showing differences of less than 2% in general and 6% maximum in one case. The experimental technique and theoretical basis of the proposed method are presented in detail.