Effect of Pipe Bend Shape Imperfections on Creep-fatigue Crack Growth

Abstract

This study is concerned with assessing the structural integrity of power plant pipes which operate under creep and fatigue conditions. In the manufacturing process of pipe bends it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross section of the bend also becomes non circular due to bending process; this tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of pipe bend on stress-strain redistributions and creep-fatigue crack growth rate was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bend was performed for different stage of lifetime under considering loading conditions. As a result it was found that the effect of ovality and thinning cross sections interaction is significant and is a function of creep time. The tensile tests were performed for determination the main mechanical properties of pipe bend's material after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross sections as well as operating time. The creep-fatigue crack growth rate tests were performed on special designed program test-cycle. Comparison between new and old materials shows tensile properties of 12Cr1MoV steel have degraded by long-term high-temperature service; while comparison between the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature is used in the form of creep stress intensity factor.