HIGH-TEMPERATURE CREEP AND RESULTANT ANISOTROPY IN ULTRASONIC VELOCITY IN ISOTROPIC GRAPHITE

Plastic deformation of isotropic graphite in the vicinity of 2500-degr ees-C was studied using an Instron-type testing machine. The load-defl ection curve was found to show a proportional limit at high temperatur e. Creep curves of graphite were also measured and were simulated by a n empirical equation based on parabolic creep. Following the creep tes ts, ultrasonic velocity in the elongated graphite was measured. The ve locity showed 5%-20% decrease after the creep. Below 2500-degrees-C, t he rate of velocity decrease along the elongation axis was always larg er than that across the elongation axis. Above 2500-degrees-C, this be haviour was reversed with the rate of velocity decrease across the elo ngation axis being the larger. The rate of decrease appeared to depend firstly on temperature and secondly on stress.