Fracture behaviour of ceramic cylinders due to thermal stresses

Abstract

Recently developed domestic stoves are equipped with ceramic lining that surround the fire. During use these linings are exposed to thermal stresses arising due to the temperature gradients across the walls of the linings and restraint of expansion resulting from the presence of different phases with different coefficients of thermal expansion in a multiphase ceramic system. These stresses cause the ceramic lining to fracture after a considerably short service life. A numerical model is designed in order to study the thermal and stress characteristics of the lining. The effects of geometric parameters,thermal conductivity, specific heat capacity, density and heat transfer conditions have been studied. Experimental temperature and strain analyses were employed on hollow cylinders produced from Nyeri clay to establish the validity of the numerical results Thermal fracture behaviour was investigated using quenching method of quenching of cylinderical specimens from a certain preselected higher temperature. Some specimens were produced from clay and sand mixture from Maragua and others from clay from Nyeri Chemical canposi tion of the specirrens was determined and thermal and physical properties were measured and/or estimated. Thermal shock behaviour was monitored by the use of low frequency (40 kHz) longitudinal ultrasonic waves and four points flexural loading. The minimum thennal shock severity required to initiate cracks and damage caused after cracking was determined. Thennal fatigue behaviour was monitored by visually observing the fonnation and propagation of cracks on the specimen surfaces. It was possible to construct the 'thermal shock severity - probability - tirre' (T-SPI') diagram and hence use the result to predict the thermal fatigue life. Use of ultrasonic technique was extended so as to monitor thermal fatigue and decrease in the speed of propagation of ultrasonic waves with number of thennal cycles (shocks)was obtained. It was found that after several thermal shocks the Nyeri clay suffered more damage (loss of strength) than the ~ Maragua clay. The Maragua clay was found to have a longer fatigue life than the Nyeri clay. Use of ultrasonic wave to monitor fatigUe was found to be applicable to the Nyeri clay whereas there was an insignificant interaction effect between the measured speed of ultrasonic pressure waves through the Maragua samples and the numbers of thermal shocks they were Subjected to. Based on these conclusions, a number of suggestions have been made as to the future work on this project.