Fatigue Properties of Socket-Weld-Jointed Pipes. Analysis of the Fatigue Life Using Fracture Mechanics.

Abstract

The fatigue properties of socket-weld-jointed pipes have not yet been investigated in detail. The fatigue limit and failure mode of socket-weld-jointed pipe specimens were revealed in a previous study. To extrapolate the very long fatigue lives which cannot be obtained experimentally, we analyzed the fatigue lives of socket-weld-jointed pipes using fracture mechanics. The results obtained are summarized as follows. (1) In most weld joints, the slit was formed between pipe and socket, and behaved like a crack. The stress intensity factors (K) of these slit like cracks were analyzed using the FEM. This crack is subjected to mixed mode I-II loading. Kθ had a maximum value in a direction at an angle of about 65 degrees to the axial direction. (2) About half of the weld joints had weld defects 0.5∼1.5mm high. This crack is subjected almost to mode I loading. (3) The propagation path angles were predicted to be 65∼85 degrees using the maximum principal stress criterion. This value is in good agreement with the experimental results. (4) The fatigue crack propagation lives were calculated using the da/dN-ΔK relation. The S-N diagram theoretically was compared with the test results. They were in good agreement. (5) For pipe maintenance, a map that could be used to evaluate the fatigue life from the stress and weld defect size was constructed.