MECHANISTICALLY BASED MODELS FOR THE PREDICTION OF FATIGUE DAMAGE IN A BETA-TITANIUM ALLOY

This paper presents the results of recent studies of the micromechanis ms of room temperature fatigue damage elucidated in a metastable beta Ti-15V-3Cr-3Al-3Sn alloy. The fatigue damage mechanisms observed inclu de: grain boundary sliding, crack initiation/propagation, and crack co alescence prior to the onset of catastrophic failure. Mechanistically based models are presented for the prediction of fatigue damage (plast icity and cracking). The models are based on fracture mechanics ideali zations of the complex damage modes observed during fatigue experiment s, in which acoustic emission signals were collected from deformed sec tions. Following appropriate noise filtration and careful analysis of the detected acoustic emission signals, the number of counts due to cr acking is shown to represent a scalar measure of damage. A modified po wer law expression (modified Paris law) is also proposed to describe t he relationship between the cracking count rate and the effective stre ss intensity factor. Estimates of the fatigue lives are obtained by in tegrating between appropriate limits, after the separation of variable s in the modified Paris law expression. The measured and predicted fat igue lives were generally in good agreement.