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

In this paper, the effects of residual stresses are considered in crack gro wth of cold bent and longitudinally cracked carbon-manganese tubes and pipe s tested under pressure at 360 degreesC, using the fracture mechanics param eters reference stress, sigma (ref), K and C*. Residual stress measurements using the X-Ray diffraction technique and a successive layer removal metho d have been performed through the thickness of the pipe extrados. These dat a have been used in finite element analyses to model effects of the seconda ry stresses acting on the crack tip. Creep crack growth rates versus C* at 360 degreesC, in cold bent tubes, were shown to be faster by a factor of si milar to 50 at constant C* compared to cracking in fracture mechanics CT te st 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 degreesC , there was an increase in incubation time and steady state cracking rate r educed by a factor of 10. Furthermore, it was found that residual stress me asurements performed after a thermal treatment at 360 degreesC for 50 h ind icated a reduction of 40-50%. However, the calculations of C* using a combi nation of primary plus secondary stresses showed only an increase by a fact or of similar to2 which did not fully explain the difference in cracking ra te in the tube specimens. (C) 2001 Elsevier Science Ltd. Ah rights reserved .