Creep Life Assessment Of Structures Containing Crack Incorporating Constraint Effect

Crack-tip constraint can affect creep crack growth (CCG) rate. The use of CCG data measured from standard compact tension (CT) specimens with deep crack and high constraint will produce over-conservative or non-conservative assessment results for the creep life of practical cracked structures with different constraints. In order to improve the accuracy of creep life assessment of high temperature components, there is a strong incentive to establish life assessment method incorporating crack-tip constraint effects. However, the studies in this area have been very limited. In this dissertation, with a comprehensive approach of combining testing, finite element numerical simulation and theoretical interpretation, macro and micro-analysis, and solid mechanics and materials science, the constraint effect and mechanism on CCG rate of the Cr-Mo-V steel, the characterization and correlation of creep crack-tip constraint between axially cracked pipelines and test specimens, the establishment and application of constraint-dependent CCG rate equations of the Cr-Mo-V steel were systematically studied in order to provide a theoretical basis for creep life assessment incorporating crack-tip constraint effects. The main work and conclusions are summarized as follows:1) The CCG rate in a wide range of C*was measured by using CT specimens with different thicknesses. It was found that the effect of out-of-plane constraint induced by specimen thickness on the CCG rate was related to C*levels. In the low C*region related to the service loads of practical components, constraint significantly influences CCG rate. The mechanism of the constraint effect on CCG rates was explored through micro and meco-scale observation. It provided a physical basis for creep life prediction models and methods incorporating constraint effects.2) A modification of the new developed constraint parameter R was proposed, and a load-independent creep constraint parameter R*was defined. The load independence was validated by using extensive calculation results of constraint parameter R of various specimens and pipes under different loading levels. The influence factors of the parameter R*were deeply analyzed, including specimen/structure geometry, crack size, distance from the crack tip and creep time. A simplified definition and calculation method was proposed for the engineering application of R*. It provided a creep fracture mechanics basis using two parameter (C*-R*) for creep life assessment of structures containing crack incorporating constraint effects.3) The crack-tip constraints of five kinds of specimens with various geometries and loading modes and pipes with axial surface cracks were investigated by using three-dimensional (3-D) finite element method. The characterization method of the R*was determined for3-D cracked components. The effects of geometries, crack sizes and loading levels on crack-tip constraint of specimens and pipes were analyzed. The load independence of the constraint R*was validated for3-D cracked pipes and test specimens. It is found that the single-edge notched tension specimen provides a closely matched creep crack-tip constraint with the axially cracked pipes. The correlation between them was analyzed using the average constraint R*avg over the3-D crack front.4) Based on the parameter R*and CCG rates of out-of-plane constraint experiments, the constraint-dependent CCG rate equation was obtained for the Cr-Mo-V steel. The availability and portability of the equation were validated by the CCG rates tested from specimens with various loading modes. In addition, the creep crack initiation time and creep fracture toughness were measured for the specimens with different constraints of the Cr-Mo-V steel. Based on the constraint parameter R*, the constraint-dependent creep crack initiation time and creep fracture toughness equations were established for the Cr-Mo-V steel.