Fatigue life of elbow pipe for in-plane loading (Part II: carbon steel)

It has been pointed out that the fatigue life of elbow pipes made of carbon steel subjected to cyclic in-plane bending was shorter than that predicted for uniaxial cyclic loading. This study aimed to investigate the fatigue life of elbow pipes and to quantify the reduction in fatigue life for component design. The fatigue test was conducted at room temperature using eight elbow specimens made of carbon steel. To suppress the ratcheting strain, only slight internal pressure was applied to detect crack penetration. Uniaxial and plate bending fatigue tests were also conducted using specimens taken from an elbow pipe of the same heat. The equivalent strain range at the crack initiation point, which was the inner surface of the crown, was successfully estimated from the measured strain using finite element analysis results. It was shown that the fatigue life obtained by the elbow test was shorter than that obtained by the axial and plate bending fatigue tests for the same equivalent strain range, and the reduction in fatigue life became more significant as the strain range increased. It was deduced that the ratcheting strain was not a major factor that caused the fatigue life reduction of elbow pipes. Then, the degree of the fatigue life reduction of the elbow specimens was quantified. The ratio of the fatigue life of the elbow specimen to that for uniaxial fatigue loading did not exceed 4 when the fatigue life of the elbow pipe was more than 120 cycles.