Creep crack initiation and growth in thick section steel pipes under internal pressure

In this study, creep crack growth in pre-cracked straight and bent pipes of a 9% Cr-steel, containing multiple cracks and tested at 625 degreesC under static and slow cyclic pressure loading, is investigated. The results have been interpreted in terms of the creep fracture mechanics parameter C* and compared with data obtained on standard compact tension (CT) specimens of the same material and batch. In making the assessments, reference stress methods have been used to determine C* for the pipes. Several formulae can be employed for calculating reference stress depending on whether it is based on a 'global' or a 'local' collapse mechanism. When using this approach, it is shown that the values obtained for C* are sensitive to the material properties, geometric dimensions and crack lengths chosen in the analysis. However, it has been found that, the most satisfactory comparison of crack growth rates with standard CT specimen data is obtained when the global reference stress solution is used in conjunction with the nominal thickness of a pipe and the mean parent uniaxial creep properties. Also, no difference has been observed between the crack growth rates measured in the straight and bent pipes. The main effect of the slow pressure cycling was to cause an acceleration in the early stages of cracking. (C) 2003 Elsevier Ltd. All rights reserved.