Experimental Study on Stability of Yield Strength of Line Pipe

Aimed at the instability problems of yield strength tested by cold-pressed method according to API Spec 5L-2009 “Specification for line pipe” for line pipes, four tensile test methods such as round bar specimens with double-shoulder, flat specimens with flat-spread ratio and flat specimens with pre-tension strain to elasticity limits of Rp0.02 and Rp0.01 were adopted to test the yield strength of line pipe specimens with different sampling directions, and test results of these four methods were compared and analyzed. The results show that since round bar specimens with double-shoulder are not full-thickness specimens, so yield strength gained by this method can not truly reflected the yield strength of the line pipes. At the same time, processing of round bar specimens with double-shoulder is more complex than that of rectangle cross-section specimens. Because the flat specimens with flat-spread ratio only calculate one specimen’s flat-spread ratio at the same angle, the test results are dispersed, and meanwhile using hysteresis loop method to process test dates requires more calculation. Flat specimens with pre-tension strain to elasticity limits method can conveniently accurately calculate the yield strength of line pipes, but percentage non-proportion extension to elasticity limits Rp0.02 is too large. Therefore it is suggested using flat specimens with pre-tension strain to elasticity limits Rp0.01 method to test the yield strength of line pipes.