THE EFFECT OF TEMPERATURE ON THE BEHAVIOR OF SHORT FATIGUE CRACKS IN WASPALOY USING AN IN-SITU SEM FATIGUE APPARATUS

Short fatigue crack growth behaviour was studied in Waspaloy at 25-deg rees-C, 500-degrees-C and 700-degrees-C under cyclic loading condition s at a stress ratio of 0.1. Specimens were tested within an elevated t emperature fatigue apparatus coupled to a scanning electron microscope which allowed in situ observations to be made of the fatigue process. Crack formation was dominated by slip band cracking at 25-degrees-C a nd 500-degrees-C, while at 700-degrees-C cracks formed along slip band s and twin boundaries. Crack propagation proceeded by means of slip ba nd cracking at 25-degrees-C and 500-degrees-C, while at 700-degrees-C crack growth was primarily stage 11-mode I type growth. Short fatigue crack growth rates were faster at 500-degrees-C than at 25-degrees-C f or an equivalent stress intensity range, DELTAK(I), owing to a change in slip character. Short crack growth rates tended to be lower at 700- degrees-C than at 500-degrees-C, owing to changes in the material lead ing to precipitate coarsening. As the temperature was increased, micro structural contributions to crack growth decreased, resulting in a red uced microstructural short crack effect.