Motohisa Hirose

Evaluation of Elevated-Temperature Crack Growth in Ceramics under Static and Cyclic Loads

Since advanced ceramics such as alumina and silicon nitride are considered for use in a variety of structural applications at elevated-temperature, it is very important to evaluate the strength and fracture resistance at elevated-temperature. The results reported so far revealed that the viscous glass phase which exists at the grain boundaries significantly influences the creation of intergranular cavities and the subcriticai crack growth (SCG) above a specific temperature depending on materials1~9. Above that temperature, it has been observed that the fracture toughness and R-curve depended on loading rate7, and that the fracture toughness increased abruptly with increasing temperature8. The fatigue strength under cyclic loading above the temperature depended on loading frequency, and was higher than that under static loading9.

The Effect of Low Temperature Quenching and Quench Ageing on the Fatigue Strength of Notched Specimens of Low Carbon Steels

The fatigue limits, σw1 and σw2, which are critical stresses for initiation and propagation of stage 2 cracks respectively, were compared among annealed, quenched and quench aged steels. To examine the effect of heat treatment on the fatigue life, crack propagation rate, d(2a)/dN, was measured at a stress above σw2. The contribution of quenching and quench ageing to the increase in the fatigue limits is greater for σw2 than for σw1, and the relationship, ασw1sσw0, is kept independent of the heat treatments, where σw0 is the fatigue limit of unnotched specimens. Therefore, the notch sensitivity factor, ( σw0/σw2-1)/(α-1), is reduced by both treatments. The strengthening effect is more remarkable with quench aged steel than with quenched steel. At higher ΔK level, d(2 )/dN is independent of the heat treatment, and is correlated with the streas intensity factor ΔK. The increase in the fatigue life by quenching and quench ageing is due to the decrease in d(2a)/dN at lower ΔK level.