Comparison of hydraulic-burst and ball-on-ring tests for measuring biaxial strength

The statistics of failure of the hydraulic-burst (HB) test were compared with those of the ball-on-ring (BOR) test. Polycrystalline Al₂O₃ tape-cast specimens, both square and circular, in two different sizes, were tested. Both the mean strengths and the Weibull moduli from the BOR tests were approximately twice the values from the HB tests. The area (volume) under stress is much larger for the HB test than the BOR test; therefore, the HB data can be considered as a low-probability-of-failure, low-strength tail of the BOR curve that has a lower Weibull modulus than the high-stress portion. Thus, BOR tests give a misleading picture of improvements in mechanical strength, because of changes in the fabrication and handling of substrates. However, previous observations that the incidence of edge and support failures was very high in the HB test were confirmed. Also, the apparent strength of the HB specimens was affected more strongly by size and shape than was that of the BOR specimens.