Effect by Average Tensile Stress on Fatigue Life of Linepipe

Studied are the methods for estimating the fatigue behavior and fatigue life under the average tensile stress conditions.A more universal fatigue life formula suitable for non-zero average stress is derived by means of normalizing the stress amplitude.The S-N curve equation of X65Q linepipe is fitted via the small sample fatigue test with average stress of 0 MPa,and the fracture morphology is analyzed.As a result,it is identified that the fatigue life calculated by the average stress equivalent formula as established by the Gerber is in full agreement with the fatigue life value from the actual test.Therefore it is suggested to estimate the fatigue life of the said linepipe under the condition of average tensile stress by means of the S-N curve with the fitted average stress of 0,and the Gerber-established average stress equivalent formula.