Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

The mechanical properties and reliability aspects of advanced transparent fineand coarse-grained MgAl2O4 spinel have been characterized at ambient and high temperature. The studies were based on a combination of micro- and macro-mechanical methods to assess Young’s modulus, hardness, fracture toughness, strength and crack growth kinetics. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics. Strength was analyzed using two- and three-parameter Weibull statistics. The experimental limits of Young’s modulus determination using standard ringon- ring testing are highlighted, and an approach is outlined to correct the measured apparent values. Experimentally obtained strength data as a function of loading rate are used to assess the potential effect of subcritical crack growth, yielding the failure time under static loading, via a strength/probability/time plot for a lifetime prediction. Furthermore, the Brinell indentation showed potential for local strength measurement and the data supported a strength-loaded area relationship that was based on the ring-on-ring test data. Novel experimental methods were implemented for fracture determination and strain detection. Particular attention was directed to the effect of defects and associated local strain fields, as assessed using polarized light in photoelastic measurements. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure initiating defect size and fracture stress.