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 VRD was used for a barometer on damage evaluation of IG and AG refractories. Measurement of change in VRD actualizes in-situ evaluation of damage during loading– unloading processes. Difference of change in VRD 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 VRD, and damage of the two kinds of specimens was explained by relating to crack nucleation and crack opening-closing mechanism in each step