Digital Repository, ECF14, Cracow 2002

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Fracture of Brittle Discs under Biaxial Loading

Last modified: 2013-02-10


In industrial mass production ceramic parts are often disc shaped, especiallywhen producing electroceramics. To be able to assure a constant quality or to tracechanges in the production cycle a quick, cheap and meaningful mechanical testing ofsamples of different production batches has to be performed. Often used approaches for thedirect testing of disc shaped specimens are either the Brazilian disc test or different discflexure tests. A very common disc flexure test is the ring-on-ring test. This test as well as amodification of it, the ring-of-balls on ring-of-balls test, is quite easy to perform and toevaluate but the gained results of these test assemblies are very vulnerable to geometricimperfections of the tested disc and the support rings.In this paper a special variant of a disc flexure test will be discussed, the so called ball onthree balls test. In this test configuration a disc is supported by three balls and then axiallyloaded from the opposite side via a fourth ball. Through to the loading on three welldefined points this test is quite tolerant to a slight out of flatness of the disc.Caused by the three support balls the stress state in the disc is not axisymmetric but showsa threefold symmetry making an exact analytical assessment of the stress state in the loadeddisc rather difficult. Therefore former estimations of the stress amplitude are quiteinaccurate. In this paper a FE analysis of the stress field was performed and differentequivalent stress models were investigated and compared with the results of experiments ofuniaxial bent bar and biaxial bent disc Al2O3 specimens. Furthermore fractography wasused to localise the fracture origin on the tested Al2O3 discs and bars.

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