A simple strength test for brittle discs

The Ball-on-three-balls test (B3B) is a simple method for strength tests of brittle discs. A disc shaped sample is supported on three points at the top surface and centrally loaded from the opposite site. Both the support and the application of load is realised with spherical parts. This test assembly shows a threefold symmetry. In this work the B3B test assembly is used for tests on samples made of alumina. The size of the samples, the ratio of disc thickness to disc diameter and the support situation is varied systematically. It was possible to confirm the numerical results for the maximum stress in the disc with fracture experiments. Therefore the Weibull behaviour of the ceramic has to be taken into account. Advantages of the ball-on-three-balls test are especially the great tolerance for small deviations of the real from an idealised geometry, the ability to test slightly bended discs, i.e. as sintered samples, the small influence of frictional effects and the great potential for testing very small samples. A disadvantage is the more complicated test evaluation compared to the common bending test.