Comparison of ring expansion vs flat tensile testing for determining linepipe yield strength

Ring expansion testing has been compared to conventional tensile testing as a means to evaluate the circumferential yield strength of pipe products. It was found that ring expansion testing provides a more accurate determination of hoop yield stress than tensile testing of flattened pipe samples. In addition, ring testing was found to be more sensitive to the metallurgical condition of the steel. Tests were conducted on ERW and Spiral pipes. Different grades and diameter to thickness (D/t) ratios were evaluated. In comparison to tensile testing on flattened specimens, ring expansion tests gave: (a) higher yield strength; and (b) reduced standard deviation in test data. The difference in yield strength measured by ring and tensile tests increased with increasing grade and lower D/t. The higher yield strength measured in the ring test is a result of reducing the Bauschinger effect associated with flattening tensile samples. The reduced standard deviation is due to the elimination of flatness and machining variations that occur on a flattened tensile specimen. It was concluded that ring testing provides a true measure of pipe circumferential yield stress.