An Investigation on Formulation of Cyclic Local Strain Range and Estimation of Fatigue Life in Notched Plate (2nd Report)

Abstract

In order to evaluate the fatigue life in the stress-concentrated region of structures using the results of strain-controlled fatigue tests, it is necessary to know the cyclic local strain range rather than the monotonous strain behavior. In the present study, a simple method to estimate the local strain range under cyclic loading was investigated.
Namely, the elastic-plastic finite element analyses under cyclic tensile load were conducted on the highly stress-concentrated models besides the results in the first report. Then an investigation was made into the effects of applied cyclic stress condition, elastic stress concentration factor and material properties on the shakedown and the behaviors of local stress, strain ranges and their concentration factors.
As a result, the formulae to estimate the shakedown limit and the cyclic local strain range were derived. Therefore, when the elastic stress concentration factor and the material properties such as cyclic yield stress, workhardening modulus and parameter of the Bauschinger effect are given, the cyclic local strain range can be estimated for a certain applied cyclic stress range. Then, by substituting the estimated strain range into a relationship between the strain range and the failure life obtained by the strain-controlled fatigue test, the fatigue evaluation can be done reasonably for the structural members.
The stress-controlled fatigue test on notched plates and the strain-controlled low cycle fatigue test on small smooth specimens were conducted on 50 kgf/mm² class steel. Then the estimated local strain range and fatigue life were compared to the experimental results. As a result, the estimated values agreed well with the measured values. In addition, the comparisons were made between the estimated fatigue lives on notched plates and the results so far published. Then it was found that the method proposed in this study can be applied to evaluate well the fatigue lives in the wide range from 5 × 10² to 5 × 10⁶ cycles in the notched plates of 41 to 80 kgf/mm² strength levels.