A Practical Use of Simplified Fatigue Life Prediction Method at Stress Concentration of Metals

Yoichi TAMIYA and Takuma WADA

Abstract:Fatigue crack initiation life (FCI life) of stress concentration can be predicted according to compare between elastic-plastic strain at there and total strain-fatigue life curve (Δε_t-N_f curve) of smooth specimen. Elastic-plastic strain is estimated by use of Neuber's rule etc., and Δε_t-N_f  curve is estimated by use of Manson's universal slope method (US method) etc. At this point, cyclic stress-strain curve (CSS curve) is needed in order to estimate the elastic-plastic strain using Neuber's rule etc. However, CSS curve formulated by Ramberg-Osgood relation can be transform from Δε_t-N_f  curve formulated by Coffin-Manson & Basquin relation. Consequently, simplified fatigue life prediction methods can be constructed by only mechanical properties (tensile strength, true fracture ductility, Young modulus, etc.), according to combine these estimation methods (Neuber's rule, US method, etc.). In this report, we compared actual fatigue fracture life (FF life) of some stress concentration specimens with predicted FCI life of them obtained by use of simplified methods, and then evaluated about a practical use of simplified methods. The conclusions are obtained as follows:
(1) About the precision of predicted fatigue life, modified universal slope method (MUS method) was better for steels, and Median method was better for aluminum alloys and titanium alloys.
(2) For steels, the predicted fatigue life using ESED rule was more effective than using Neuber's rule.
(3) In case of using elastic stress concentration factor K_t, the ratio of predicted FCI life N_p and actual FF life N_f (N_p/N_f) was conspicuously decreasing with increasing of K_t.
(4) Taking account of decrease of N_p/N_f, simplified method can be employed for preliminary fatigue design.