Loading history effect on creep-fatigue crack growth in pipe bend

Abstract

This study is concerned with assessing the structural integrity of power plant pipes that operate under creep and fatigue conditions. In the process of manufacturing 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 the bending process, which 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 a pipe bend on stress-strain redistributions and creep-fatigue crack growth rates was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bends was performed for different stages of lifetime while considering various loading conditions. As a result, it was determined that the combined effect of ovality and thinning cross-sections is significant and is a function of creep time. The tensile and creep tests were performed for determination of the main mechanical properties of the material of the pipe bends 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 a specially designed program test-cycle. A comparison demonstrates that the tensile properties of 12Cr1MoV steel were degraded by long-term high-temperature service, whereas a comparison of 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 was used in the form of a creep stress intensity factor.