Isothermal and thermal-mechanical fatigue of alumina

Isothermal four-point-bending tests and uniaxial tension-compression tests with different mean stresses at 25 degrees C and 900 degrees C as well as thermal mechanical fatigue (TMF) test with T-min = 400 degrees C and T-max = 900 degrees C were carried out in laboratory air on an alumina ceramic with a small amount of intergranular glass phases resulting from sintering aids (MgO, SiO and CaO). From four-point-bending tests characteristic strength date were determined. The fatigue tests were performed at different mean stresses. From the resulting lifetime data Haigh diagrams were evaluated. The fatigue strength for mean stress below about 180 MPa at T = 900 degrees C is higher than at T = 25 degrees C because of stress relaxation processes in the failure critical sites like triple points, grain boundaries and micro-pores. The thermal-mechanical fatigue (TMF) behaviour was investigated using the "out-of-phase" test procedure with a triangle shape temperature time curve and total mechanical strain control. Under TMF loading conditions the number of cycles to failure decreases with increasing mechanical total strain amplitude. The maximal stress induced in the particular cycles within a particular test increases with increasing number of cycles. This is the more pronounced the higher the total mechanical strain amplitude is. On average, failure of the specimen take place when the induced maximal stress reaches 150 MPa.