| Accurate creep crack growth(CCG)life assessment is the core issue of integrity assessment,life design,operation and maintance for high temperature components.Due to the lack of crack-tip constraint incorporation in life assessment method based on the single-parameter creep fracture parameter C*,excessive conservative or non-conservative assessment results are obtained.In order to improve the accuracy of creep life assessment for high temperature components containing cracks,it is necessary to develop a method and a technique of CCG life prediction and assessment incorporating constraint effect for high temperature components.In this paper,aiming at estbalishment of CCG life assessment method incorporating crack-tip constraint effect for pressurized pipes,based on the creep constraint parameter R*,the creep crack-tip constraint characterization of three-dimensional specimens with different geometries,material constraint-dependent CCG rate,creep constraint analysis and constraint parameter solutions for inner axial and circumferential semi-elliptical surface cracks in pressurized pipes,effect of material creep properties on constraint parameter R*and CCG life prediction and assessment method incorporating crack-tip constraint effect based on two paramters C*-R*for pressurized pipes have been systematicly investigated by the combination of finite element simulation,creep test data and theretical analysis.The main work and conclusions are listed as follows:(1)The creep crack-tip constraint along three-dimensional crack front of various test specimens with different specimen geometries,crack depths and loading configuration has been calculated and characterized by finite element method.The constraint-dependent CCG rate equations of 316H steel have been established by combining constraint parameter R*and CCG test data of specimens with different constraint levels.(2)The creep crack-tip constraint of inner axial and circumferential semi-elliptical surface cracks in pressurized pipes with different crack sizes and inner raidus-thickness ratios under high temperature air environment has been characterized by three-dimensional finite element analysis.The results show that the constraint level of semi-elliptical surface cracks increases with the increase of crack depth and length.Empirical quations of constraint parameter R*were fitted and their accuracy and load-independence in pressurized pipes were validated.(3)The constraint-dependent CCG rate of pressurized pipes with cracks was analysed based on the constraint parameter R*solutions of axial and circumferential semi-elliptical surface cracks in pressurized pipes and constraint-dependent CCG rate equations of 316H steel.The result shows that the constraint level and CCG rate of pressurized pipes with cracks are lower than that of C(T)specimen.The CCG rate of pressurized pipes with shallower and shorter cracks is 0.33-0.58 and 0.26-0.44 times that of standard C(T)specimen for axial and circumferential semi-elliptical cracks,respectively.The constraint level of circumferential cracks is lower than that of axial cracks for the same crack size.(4)The effect of material creep properties on constraint parameter R*was systermatically investigated and the engineering estimation method of constraint parameter R*was proposed through extensive finite element analyses.Results show that the pramater R*increases with increasing creep exponent n and is independent on creep coefficient A.The R*is more sensitive to creep exponent n for specimens with lower constraint.(5)The creep crack-tip constraints for three specimen geometries(C(T),SEN(T)and M(T))with various crack depths and thickness-to-width ratios have been analyzed by three-dimensional finite element method and quantified by using three constraint parameters R*,h and Tz respectively.Results show that the constraint parameters R*and h could mainly quantify the in-plane creep constraint and partially incorporate out-of-plane constraint.The constraint parameter Tz is adequate to incorporate and quantifiy the out-of-plane creep constraint and has a better universality.The specimen geometries with higher constraint strengthen the out-of-plane constraint effect.For all sepcimens considered,the effect of crack depth on constraint parameters R' and h is smaller than that of specimen thickness.(6)The CCG life prediction and assessment have been conducted for pressurized pipes with inner axial cracks under high temperature air environment and the assessment method has been given based on the two-paramter C*-R*creep fracture mechanics incorporating crack-tip constraint effect.Results show that the crack growth length and area calculated based on the two-paramter C*-R*is smaller than that calculated based on the single-parameter C*method for a specific intial crack size.More accurate crack growth profile can be obtained by the two-paramter C*-R*approach.Over-conservative CCG life assessment was obtained based on the traditional single-parameter C*and additional conservatism was produced by the calculation of parameter C*by reference stress method.Therefore,in orde to reduce the excessive conservatism in CCG life assessment,it is necessary to incorporate the constraint effect and adopt finite element method to calculate the parameter C*value. |
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