The ball on three balls test for strength testing of brittle discs:: Part II:: analysis of possible errors in the strength determination

Biaxial strength testing of brittle materials using the ball on three balls test (11313 test) is a useful tool for direct strength testing of disc specimens offering some advantages compared with the testing of bended beams: there are less problems with the alignment of the specimens in the fixture, in many cases the production of the specimens is easier, the area of maximum tensile stress amplitudes is far from the edges of the specimen (were often machining defects exist) and the testing procedure can easier be miniaturised. In a preceding paper, a proposal for a suitable testing set up was made, the stress fields in the disc specimens were analysed and first testing results were reported. A simple solution for the maximum tensile stress amplitude in the discs was given for a range of parameters concerning the geometry of the specimen, the supporting situation during the test and the material properties. In this paper, possible sources for deviations of this stress from the reported idealised solution (which would result in measuring errors) are discussed. For example, the not accounted influence of friction forces between the disc and the support balls, the influence of a possible buckling of the disc and the influence of geometric inaccuracies are analysed. It is shown, that the thickness of the disc is the most sensitive parameter determining the maximum tensile stress. Therefore it has to be determined with high accuracy. (C) 2003 Elsevier Ltd. All rights reserved.