Dependence of the Threshold Stress of Microcrack Propagation on the Stress Ratio

抄録

Plane bending and cyclic torsion tests, and their combination are first carried out at a stress ratio R=0 using annealed cracked specimens made of carbon steel. The threshold stress of small fatigue crack propagation under a mixed mode is determined using the maximum principal stress criterion since there is not crack closure effect in this case. Next, plane bending test are performed for various values of the stress ratio R. Assuming that the crack opening displacement on the threshold level is constant regardless of R, and empirical relation between R and the threshold stress is developed. The suggested expression is found to be in good agreeement with the test results and can be applied to both the positive and the negative ranges of the stress ratio.