Stress-strain assessment of dents in wall of high pressure gas pipeline

The aim of this work was experimental study of the dents effect on structural integrity of a high-pressure gas pipeline with nominal diameter 300 mm after long-terme operation. Stress-strain behaviour of two dents was evaluated by strain gauge chains oriented in hoop direction. Dent profiles were measured by digital slide caliper in circumferential and longitudinal direction in points distant 10 mm from each other. Residual deformations were detected after internal pressure loading by water to the operating level. Negative and positive values of residual deformation were found wuth the help of strain gauges. Fatigue resistance was tested by 10 000 pressures cycles with load asymmetry R = 0 and maximum internal pressure pmax = 6,3 MPa (nominal operating level). Internal pressure cyclic loading caused an increase of residual deformation values. By means of subsequent pressure burst test, the limit state of this type defect was found. Two dwells were realized in 66% and 100% yield stress. Negative values of strain were observed at strain gauges placed on edge of dents. Positive values of strain were found at strain gauges placed inside the dents. The increase of internal pressure led to the changes of dent shape. Dent depth decreased and the original circular tube profile was reached again. The pipe fracture occurred at the internal pressure of 19 MPa. Tensile properties and Charpy absorbed energy were measured on standartized specimens of the pipe material.