OXIDE WEDGING EFFECTS ON NEAR THRESHOLD FATIGUE CRACK GROWTH AT ELEVATED TEMPERATURE

ABSTRACT RECENTLY linear fracture mechanics has been successfully applied to the evaluation of the fatigue crack growth in the elastic creep range at elevated temperature. Many studies have been reported on the fatigue crack growth at elevated temperature mainly at intermediate stress intensity range. However, very few data have been obtained on the fatigue crack growth at near threshold stress intensity because of the difficulties of the fatigue crack growth tests at near threshold at elevated temperature. To evaluate the fatigue life of structural components based on fracture mechanics, the threshold for fatigue crack growth at elevated temperature is of practical importance. At elevated temperature, the oxidation is known to influence fatigue crack growth. Especially at near threshold stress intensity, the influence of the oxidation seems to be very remarkable because the specimen is exposed to elevated temperature for very long time. To investigate oxide effects on near threshold fatigue crack growth, a set of AK decreasing fatigue tests were carried out in air for various stress ratios R at elevated temperature up to 650°C., by using a center cracked plate specimen of solution annealed type 304 stainless steel. Fatigue crack growth rate and the crack closure behaviour at near threshold stress intensity have been investigated at elevated temperature in comparison with those at room temperature. The fatigue crack growth rate at near threshold stress intensity at elevated temperature is found to be lower and the threshold value? K th higher than those at room temperature, although at intermediate stress intensity range the fatigue crack growth rate is known to be higher than that at room temperature. This tendency is more remarkable at low stress ratio and at high temperature. The reason for the increase of? K th value at elevated temperature can be considered that the oxide scale formed on the crack surface has blocked the fatigue crack growth and that the oxide scale has reduced the stress intensity range effective to fatigue crack growth. This was confirmed by the measurement of crack closure behaviour and the observation of the oxide scale on the fractured surface. This oxide wedging effects on the fatigue crack growth was analyzed quantitatively by using a new crack model with continuous springs inside. Numerical results of this model can explain the effects of the oxide thickness on the crack closure. Thus at elevated temperature the oxidation is found to be important. It is interesting that the oxidation accelerates the fatigue crack growth at intermediate stress intensity range and decelerates it at near threshold stress intensity.