Evaluation of Elevated-Temperature Crack Growth in Ceramics under Static and Cyclic Loads.

Abstract

An experimental investigation was conducted to study the crack growth characteristics of alumina, AD90, silicon nitride, EC-141, and magnesia, MgO, at elevated temperatures in air under static and cyclic loads. The test temperatures were 750, 900 and 1050°C for AD90, and 1000, 1100 and 1200°C for EC-141 and MgO. Crack growth behavior was similar to that at room temperature below specific temperatures, depending on the materials. Above these temperatures, the crack growth mechanism depended on the loading conditions and the materials. For AD90 and EC-141, subcritical or slow crack growth (SCG) was observed on the fracture surfaces. The SCG behavior occurred in a narrower range of the stress intensity factor under cyclic load than under static load. The SCG rate tended to be lower under cyclic load than under static load in AD90, while it was higher under cyclic load in EC-141. For MgO, crack growth resistance was dependent on the loading condition even if the SCG region was not observed on the fracture surface. Based on the experimental results, the importance of the threshold stress intensity factor for SCG, KISCG, at elevated temperatures was discussed, and methods for evaluating KISCG were proposed.