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

The statistics of failure of the hydraulic-burst (HB) test mere compared wi th those of the ball-on-ring (BOR) test. Polycrystalline Al2O3 tape-cast sp ecimens, both square and circular, in two different sizes, were tested. Bot h the mean strengths and the Weibull moduli from the BOR tests were approxi mately twice the values from the HE tests. The area (volume) under stress i s much larger for the HE test than the BOR test; therefore, the HE data can be considered as a low-probability-of-failure, low-strength tail of the BO R curve that has a lower Weibull modulus than the high-stress portion. Thus , BOR tests give a misleading picture of improvements in mechanical strengt h, because of changes in the fabrication and handling of substrates. Howeve r, previous observations that the incidence of edge and support failures wa s very high in the HE test were confirmed. Also, the apparent strength of t he HE specimens was affected more strongly by size and shape than was that of the BOR specimens.