The traditional approach to high temperature design of process equipment has been directed towards prevention of creep rupture in nominally defect free structures. However, experience has shown that many failures are by the growth of cracks that may have been introduced during fabrication, repairs, or operation. This growth is via a localized creep mechanism termed creep crack growth. Both load-controlled, primary stress and deformation-controlled (e.g. thermal), secondary stress can contribute to this crack growth. Simplified methods were used to predict creep crack growth due to primary and secondary stress. This required development of a solution for crack growth due to secondary stress that considered simultaneous crack growth and load relaxation. Closed-form solutions were developed based on the C* integral that yielded crack advance as a function of time and initial load. The predictions compared very well with the results of a deformation-controlled compact tension creep crack growth experiment. Parametric evaluations showed that only very shallow cracklike defects can be tolerated in highly stressed areas. Cracks can advance very quickly due to both high primary or high secondary stresses. With primary stresses at the ASME Boiler and Pressure Vessel Code Section VIII, Div. 1 limits, cracks as shallow as 1.3 mm (0.05 in.) can reduce average life to less than 100,000 hr. Where cyclic secondary stresses exceed shakedown limits, life may be limited to a relatively small number of start-up/shutdown cycles.

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