Low-cycle fatigue/high-cycle fatigue interactions in notched Ti-6Al-4V

Combined low-cycle fatigue/high-cycle fatigue (LCF/HCF) loadings were inves tigated for smooth and circumferentially V-notched cylindrical Ti-6Al-4V fa tigue specimens. Smooth specimens were first cycled under LCF loading condi tions for a fraction of the previously established fatigue life. The HCF 10 (7) cycle fatigue limit stress after LCF cycling was established using a st ep loading technique. Specimens with two notch sizes, both having elastic s tress concentration factors of K-t = 2.7, were cycled under LCF loading con ditions at a nominal stress ratio of R = 0.1. The subsequent 10(6) cycle HC F fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. :after LCF cycling and prior to HC F cycling, the notched specimens were heat tinted, and final fracture surfa ces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life f or the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity-induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.