The creep and failure of engineering ceramics under multiaxial states of stress.

The effort dedicated to developing the material properties of engineering ceramics has not been accompanied by a similar effort in developing design methods that would allow engineers to make full use of these materials. In particular the high temperature creep behaviour of engineering ceramics has received little attention. In this thesis two parallel approaches, one theoretical and one practical, have been taken towards the final aim of constructing design codes for the creep of ceramic materials. In the theoretical work the principles developed for modelling creep and failure in metals were employed and adapted where necessary to provide new models that describe the behaviour of ceramics under multiaxial stresses. Important changes were made to account for differences in microstructure between these two classes of materials. In the practical work equipment was developed to provide suitable multiaxial creep test data with which to verify and further construct models. This involved the construction of a tension/torsion creep testing machine featuring a radio-frequency heating furnace, cooled grip heads, extensometry equipment, biaxial loading system and a temperature measurement and control system. The machine was capable of operating for at least 300 hours at a temperature of at least 1400 °C. Nine creep tests were conducted on reaction bonded silicon nitride specimens including two unique tests under pure torsion and combined tension/torsion. Four tests were conducted on aluminium oxide specimens including a unique test under combined tension/torsion. Tensile test results showed good agreement with previously published data for both materials confirming the equipment accuracy. Results from the multiaxial tests indicated that reaction bonded silicon nitride fails in response to the value of the effective stress. In addition reasonable agreement was obtained between the test data and predictions from the new models.