Effects of residual stress in creep crack growth analysis of cold bent tubes under internal pressure

Abstract

In this paper, the effects of residual stresses are considered in crack growth of cold bent and longitudinally cracked carbon–manganese tubes and pipes tested under pressure at 360°C, using the fracture mechanics parameters reference stress, σ ref, K and C ∗. Residual stress measurements using the X-Ray diffraction technique and a successive layer removal method have been performed through the thickness of the pipe extrados. These data have been used in finite element analyses to model effects of the secondary stresses acting on the crack tip. Creep crack growth rates versus C ∗ at 360°C, in cold bent tubes, were shown to be faster by a factor of ∼50 at constant C ∗ compared to cracking in fracture mechanics CT test specimens. This difference is due to geometry as well as the method of C ∗ analysis. In cold-bend tubes after 1000 h thermal soaking at 650°C, there was an increase in incubation time and steady state cracking rate reduced by a factor of 10. Furthermore, it was found that residual stress measurements performed after a thermal treatment at 360°C for 50 h indicated a reduction of 40–50%. However, the calculations of C ∗ using a combination of primary plus secondary stresses showed only an increase by a factor of ∼2 which did not fully explain the difference in cracking rate in the tube specimens.