Mechanical Properties of Cold Bend Pipes

Cold bends are frequently required in energy pipelines in order to change the vertical and horizontal orientation of the pipeline route. They are produced by plastically bending a pipe joint in a cold bending machine, by creating a series of uniformly spaced kinks. This procedure acts to reduce the moment capacity and buckling strain of the pipe, and studying the changes in pipe properties caused by cold bending is valuable in assessing the level of this strength reduction. Accordingly, the initial imperfections and material transformations of five full-scale cold bend pipes were assessed in this research program. The imperfections were measured at several locations around the circumference of the specimens, along the entire bend length. It was determined that the distribution of imperfections was similar in shape to a sine function, and their amplitude ranged from 0.3 to 1.0 mm. Tension coupon tests were conducted on material from the intrados, extrados, and virgin material of the specimens. It was revealed that the extrados material exhibited an increase in yield strength due to work hardening, and that the intrados material demonstrated a reduction in yield strength due to the Bauschinger Effect. It was established that the imperfections, and material transformations in the specimens were predominantly unaffected by the kink magnitude or spacing that was employed during the cold bending procedure.