AN ASSESSMENT OF THE ROLE OF NEAR-THRESHOLD CRACK-GROWTH IN HIGH-CYCLE-FATIGUE LIFE PREDICTION OF AEROSPACE TITANIUM-ALLOYS UNDER TURBINE-ENGINE SPECTRA

Increasingly accurate life prediction models are required to utilize t he full capability of current and future advanced materials in gas tur bine engines. Of particular recent interest are predictions of the lif etimes of engine airfoil materials that experience significant interva ls of high-frequency, high-cycle fatigue (HCF). Conventional life mana gement practices for HCF in the turbine engine industry have been base d principally on a total-life approach. There is a growing need to dev elop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), l ow cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine com ponents, a review is presented of the extensive results of an Air Forc e research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil ma terial are used to examine the applicability of linear elastic fractur e mechanics methods for prediction of service lifetimes under load spe ctra that include high cycle fatigue. The roles of fatigue crack initi ation and growth are examined for materials that are nominally-defect- free, as well for materials that have experienced significant prior st ructural damage. An assessment is presented of the potential utility o f the conventional threshold stress intensity factor range, Delta K-th , defined by testing specimens containing large cracks. Although the g eneral utility of a large-crack-Delta K-th approach is questionable du e to the potentially rapid growth of small fatigue cracks, the low all owable stresses involved in turbine engine high cycle fatigue appear t o limit and simplify the small-crack problem. An examination is also p resented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.