A study of variables in the mechanical testing of beryllia

Abstract

Hot-pressed beryllia of 2.9 to 3.0 g/cm 3 density and 15–20 microns grain size was studied to determine the effects of specimen size, shape, surface finish, strain rate, prior mechanical treatment, and loading geometry on the modulus of rupture measured in four point bending. The modulus of rupture was found to be insensitive to variations in volume of the test piece, prestraining, and loading geometry over the range studied; when considered as a normally distributed sample, the variations in modulus of rupture at the extremes of volume were not significant at the 95 per cent confidence level. The standard deviation of results for a normal population was about 1900 psi. The results were analysed according to the Weibull “weakest link” theory. Values of the Weibull “constant” varied from 10 to 35 with a mean of 16; the variations in modulus of rupture with volume under stress, predicted from this model were therefore small but probably significant at the 95 per cent confidence level for a normal distribution. Specimens prepared with various surface finishes (20 micron to 150 micron abrasives) showed no significant variation in modulus of rupture. An increase in strain rate from 0.06 per cent per minute (extreme fibre strain) to 1.2 per cent per minute showed no significant increase in strength, but further increase in strain rate to 48 per cent per minute resulted in 25 per cent increase in the modulus of rupture.