Low Cycle Notched Fatigue Behavior and Life Predictions of A723 High StrengthSteels

摘要

Two types of ASTM A723 steels have been investigated for their low cycle fatiguebehavior (less than 104 cycles to failure). Specimens were tested in four-point bending, both with and without notches, and the measured fatigue lives were compared with those predicted by Neuber notch analysis (classic and elastic/plastic remote applied loading), and standard fracture mechanics life prediction techniques. Comparison of measured and predicted lives indicates that the elastic/plastic Neuber analysis underpredicts the measured fatigue life by as much as 6% percent at large strains, and becomes a better predictor of life as the applied strains decrease. The elastic Neuber analysis also underpredicts the measured fatigue lives by 45 percent at large applied strains, but seems to accurately predict lives at reversals-to-failure greater than 100. The fracture mechanics approach assumes elastic stresses at the crack tip, and predicts lives within 30 percent over the full range of strains investigated. The results show that the Neuber notch analysis is not as good an indicator of the low cycle fatigue behavior of A723 steels as is the fracture mechanics life prediction technique. As the life cycles-to-failure decreases, the Neuber analysis predicts lives that are two to three times more conservative than those experimentally measured. Since the fracture mechanics approach and the elastic Neuber approach are fully elastic stress-based, one can conclude that for this class of steels, an elastic-based life predictions technique works even through we are believed to be in a plastically-dominated regime.