静的および衝撃三点曲げ試験によるアルミナの破壊じん性評価

Abstract

The static and impact three point bending tests were carried out to evaluate the fracture toughness of alumina. A precrack was introduced into the specimen by BI method. In order to eliminate the influence of residual stress caused by the roughness of the crack surface, a notch was made on the crack with diamond cutter, leaving the crack tip.The static fracture toughness was evaluated by SEPB method. Because of rising R-curve behavior of alumina, the fracture toughness calculated from the maximum load would be greater than the inherent one. In this experiment, the fracture toughness was calculated from the load at the onset of crack extension, which was detected by the output of a strain gage glued near the crack tip. This value was almost the same as the fracture toughness obtained by IF method. In addition, the relation between load and strain near the crack tip observed in the static three point bending test was explained by using a simple model.In the impact three point bending test, the impact forces were loaded on the specimen by dropping a cylindrical bar from various heights. It was found that the fracture toughness had a tendency to increase with an increase of impact velocity. Furthermore, the time variation of dynamic stress intensity factor measured in this test was compared with the calculated value by a simple evaluating method which was proposed in the previous paper. As a result, both values showed good agreement until the crack began to extend.