Comparison of biaxial strength measured with the Ball-on-Three-Balls- and the Ring-on-Ring-test
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The Ball-on-Three-Balls-test (B3B) and the Ring-on-Ring-test (RoR) were conducted on alumina discs and compared based on Weibull-Theory. The influence of various intermediate layers was evaluated. To support these findings, Finite-Element-Analysis was conducted to analyze the effects of deviations from ideal loading conditions. The influence of friction between sample and fixture and the effects of an inhomogeneous load distribution on the maximum stress were investigated. The experiments demonstrated that measuring corresponding strength values with both testing methods is possible. To properly asses the strength using the RoR-test, intermediate layers must be used. Teflon-foils or adhesive tapes are considered suitable. If no intermediate layer is used, the materials strength will be underestimated. Finite-Element-calculations show that this effect cannot be explained by the influence of friction and is rather caused by a non-homogeneous load distribution along the load-ring. Fractography supports these findings.