EFFECT OF THREAD FORM ON THE STRENGTH OF CERAMIC BOLTS

The strength of various designs of screw thread in partially stabilize d zirconia (PSZ) bolts has been measured. It was found that the thread geometry usually used for metal fasteners and connections is not the optimum for a brittle ceramic material such as PSZ. An increase in the flank angle of the thread from the normal 60 degrees to 90 degrees ca n produce a 40 per cent increase in strength. The results are discusse d in terms of the stress concentration factor acting at the notch root of the thread and the tensile strength of PSZ. This stress concentrat ion or notch embrittlement factor reduces the strength of the threaded connection in a brittle material, compared to that of a metal with th e same tensile strength. In the present experiments a stress concentra tion factor of between 2 and 4 was observed. This factor is combined w ith Weibull statistics to obtain safe working stresses for various des igns of screw thread in PSZ. This information should be of direct use to designers interested in using this new engineering material.