A Method for Dynamic Fracture Initiation Testing of Ceramics

An experimental method is described whereby the dynamic fracture initiation toughness of ceramics and ceramic composites can be measured in pure tension or pure torsion at stress intensity factor rates of 10⁵ to 10⁶ MPa$m$/s. In this procedure, circumferentially notched cylindrical rods are subjected to uniaxial cyclic compression at room temperature to introduce a self-arresting, concentric Mode I fatigue pre-crack, following the technique presented by Suresh et al. (1987) and Suresh and Tschegg (1987). Subsequently, dynamic fracture initiation is effected by stress wave loading with a sharp-fronted pulse which subjects the specimen to a dynamic load inducing either Mode I or Mode III fracture. Instrumentation appropriate to the loading mode provides a record of average stress at the fracture site as a function of time. The capability of this method to yield highly reproducible dynamic fracture initiation toughness values for ceramics is demonstrated with the aid of experiments conducted on a polycrystalline aluminum oxide. The dynamic fracture toughness values are compared with the results obtained for quasi-static Mode I and Mode III fracture in the ceramic material at stress intensity factor rates of 10⁻¹ to 1 MPa$m$/s. Guidelines for the dynamic fracture initiation testing of ceramics and ceramic composites are discussed.