Studies on Estimation of Cyclic Strain Amplitude at Stress Concentrated Region

Abstract

In order to estimate crack initiation life of structural members, it is necessary to know relation between nominal stress, stress concentrated factors and local strain at the place where fatigue crack will initiate. For this purpose, step tests were carried out for notched specimens of several types and cyclic nominal stress-local strain curves were obtained. To calculate local strain at notched part, Neuber's equation and modified Stowell's equation, etc. have been employed, but the calculated strain does not always agree with measured strain accurately. Using Neuber's equation, relation between K_ε/K_t and S_a・K_t/σ_y^' is expressed by the equation K_ε/K_t=f[(S_a・K_ι)/σ'_y] But this equation does not give accurate results as mentioned above, the following modified equation was proposed K_ε/K_t=f[(S_a・K_t)/(σ'_y)-((ε'^^_y)/(ε'_y)-1)] where K_ε ; strain concentration factor (at stress concentrated region) K_t ; elastic stress concentration factor (at stress concentrated region) S_a ; nominal stress amplitude σ_y^', ε_y^' ; cyclic yield stress and cyclic yield strain at uniform strain field <ε_y^'>^^^- ; cyclic yield strain at non-uniform strain field The above modified Neuber's equation was applied to two kind of materials of SM41B and A5083 Al Alloy, and calculated local cyclic strain amplitude at stress concentrated part agrees to the measured one with good accuracy.