Damage evaluation of refractories under cyclic loading-unloading processes using ultrasonic method

Accumulation of damage by cyclic loading-unloading under uni-axial compressive stress was evaluated using ultrasonic method for two kinds of refractories: alumina/graphite (AG) and isotropic graphite (IG) refractories. Change of relative degradation in apparent sonic velocity V_RD was used for a barometer on damage evaluation of IG and AG refractories. Measurement of change in V_RD actualizes in-situ evaluation of damage during loading-unloading processes. Difference of change in V_RD during loading-unloading processes was observed between both specimens, which suggested the damage mechanism is different in IG and AG. Accumulation of damage for AG specimens by cyclic loading-unloading was very little, while it is a little larger for IG specimens. An increase in applied stress ratio R contributed to an increase in degree of damage accumulation. To explain the damage mechanism for both AG and IG specimens, we consider damage models, respectively. In these models, loading-unloading processes were classified into 4 steps based on the behavior of change in V_RD, and damage of the two kinds of specimens was explained by relating to crack nucleation and crack opening-closing mechanism in each step.