Andreas Börger

The ball on three balls test for strength testing of brittle discs: stress distribution in the disc

Biaxial strength testing of brittle materials is claimed to have some benefits compared to uniaxial testing, e.g. the much simpler specimen preparation, the avoiding of tensile loaded edges, the similarity of the stress state to those from typical loading (e.g. during a thermal shock loading) and the fact, that biaxial stress states are more revealing of defects than uniaxial stress states. The experience of the past showed, that biaxial strength testing has its own problems, to avoid these led to the development of several variants. One of these variants, the ball on three balls test, seems to be extremely simple: a disc is supported by three balls and then axially loaded from the opposite side via a fourth ball. In this system small deviations from the requested geometry, especially some out of flatness of the disc, are mentioned to be tolerable, but the threefold bending symmetry makes an exact analytical assessment of the stress state in the loaded disc extremely difficult. A numerical approach has yet not been performed. In this paper a FE analysis of the stress state in a ball on three balls tested disc is performed. The stress field scales with the maximum principle stress, which occurs in the centre of the tensile surface. For this stress an analytical approximation (which has been fitted to the numerical results) is given, which accounts for the influence of all relevant geometrical and material parameters. The investigated range of parameters considers the values typical for testing of brittle materials.

Biaxial Strength Testing on Mini Specimens

Biaxial strength testing of brittle materials using the ball on three balls (B3B) test is a new method for strength testing of disc or plate specimens [1] (see Fig. 1). The analysis of the stress fields and the calculation of the effective volumes and surfaces for several types of specimens and testing geometries has been performed recently. The ball on three balls testing method has several advantages compared to the common three- or four point bending tests: the results are very stable against small geometrical inaccuracies of the specimen or the test assembly, edge defects are not relevant and there exists only a very low influence of friction [2]. This makes this type of experiment ideal for testing very small specimens [3].

Application of the Brazilian Disc Test for Strength Measurements on Ceramic Green Bodies

Some ceramic parts (e.g. electro-ceramics) are produced in high numbers and they need to be transported in the green state. During the handling before firing the ceramic green body has to withstand mechanical loading. Therefore a minimum strength is required. A lot of effort has been made in recent years to establish methods for characterising ceramic green bodies, particularily for measuring the green strength. For this investigation ceramic green bodies were pressed uniaxially and their strength was measured. Data acquired from cylindrical samples using the Brazilian disc test were compared with bending test results on large specimens. In the Brazilian disc test the influence of sample size and loading rate on the measured strength values was investigated. Test results can also be used to analyse the influence of binder type and content on the green strength.