Error identified as hysteresis in flexure testing of advanced ceramics

Abstract

Flexure testing has been widely used to determine the strength, fracture toughness, fatigue and creep behavior of brittle ceramic materials at both ambient and elevated temperatures. The sources of error have been identified in detail by several investigators. A previous study by the present authors showed that friction between the support-rollers of a four-point flexure fixture and the specimen surface resulted in a well-defined hysteresis in the load (stress) versus strain cycle when the rollers were fixed. Based on the previous finding that hysteresis is a measure of stress error, a particular attempt was made in this study to establish a quantitative correlation between hysteresis and stress error for the four-point, fixed-roller fixture system. For this purpose, friction between support rollers and specimen surface was arbitrarily changed by using five different mediums at the contact points: air, distilled water, silicon oil, solid lubricant, and Teflon tape. The stress error thus obtained was correlated in terms of a hysteresis index that quantifies the degree of hysteresis. The frictional effect of various contact mediums was determined by loading a strain gaged silicon nitride specimen in a SiC, four-point flexure fixture with SiC rollers that were fixed in V-grooves.