Sub-Critical Crack Growth in Alumina – a Comparison of Different Measurement and Evaluation Methods

Abstract

Every brittle failure of a ceramic component is preceded by a certain amount of sub-critical crack growth (SCCG). In order to incorporate lifetime predictions into design considerations, material data on this phenomenon have to be known. Such material data are only available for a few materials. This is primarily due to the fact that the necessary experiments are time consuming and the evaluation of the data may be complicated. In this work two common measurement methods, static bend tests and constant stress-rate tests were used to determine the parameters for sub-critical crack growth, n and B of a commercial alumina ceramic. Several evaluation procedures were applied to the constant stress-rate data. The data evaluation turned out to be straightforward because a clear separation of the inert strength plateau from the SCCG influenced strength could be observed. A large range of stress-rates spanning five orders of magnitude could be used for the evaluation. All evaluation procedures lead to comparable values for n and B, constant stress-rate tests provide more accurate values for the SCCG parameters than the evaluation of the lifetime distribution. While n could be determined with reasonable scatter, the variety in B was much bigger.