Creep Crack-tip Constraints Research Articles

Effect of constraint on creep fracture toughness of a Cr-Mo-V steel

The creep fracture toughness of materials is required for the assessment of defective components at high-temperatures. Existing evidence indicates that creep crack-tip constraint state has a great influence on the fracture behavior of materials. In this work, the effect of constraint on creep fracture toughness was explored by conducting a series of creep crack growth tests on specimens with various thicknesses and loading modes. Results showed that the curves of creep fracture toughness with creep crack initiation time on the logarithmic scale for different groups of specimens were almost parallel with one another. Through the characterization of crack-tip constraint level for different groups of specimens in terms of a load-independent creep fracture constraint parameter, R*, the creep fracture toughness was found to increase with the decrease in the crack-tip constraint level. Moreover, evident effect of specimen geometry on creep fracture toughness was observed. Of all types of specimens, the CCT2 and CT10-SG exhibited the highest and lowest values of Kcmat at the same ti, respectively. In addition, the Kcmat was found to increase following a specimen group order of CT10-SG, CT10, CT5, SENT5, CT2, SENT2 and CCT2 at a given ti. Finally, a constraint-dependent creep fracture toughness equation was established for the Cr-Mo-V steel, which is expected for practical applications to improve accuracy of life assessment methods. Follow-up work on life assessment methods incorporating the constraint parameter R* is still necessary.

A more Accurate Constraint Parameter to Characterize the Creep Constraint Effect at Crack Tip of Cr-Mo-V Steel

A suitable constraint parameter is a key to quantify the creep crack tip constraint levels. In this study, a new more accurate creep constraint parameter As of brazed joint was proposed based on the existing constraint parameters R* and Ac. The CCG rate equation of Cr-Mo-V steel related to the constraint parameter As is obtained. The results show that the constraint parameter As is basically unchanged with the increase of creep time. The crack growth data predicted by the constraint dependent CCG rate equation are in good agreement with the experimental data. The established constraint dependent CCG rate equation can predict the CCG rate at other constraint levels very well.

Creep constraint analysis for test specimens with a wide range of dimensions and comparison with constraint of cracked pipes

Based on extensive three-dimensional finite element analyses and the unified creep constraint parameter Ac, systematic studies have been carried out to examine effects of a wide range of dimensions of test specimens (C(T), SEN(T) and M(T)) on creep crack-tip constraint. The parameter Ac solutions for the test specimens also have been obtained and are tabulated. It has been shown that the thickness B/W of the test specimens has the most significant effect on creep constraint, and the second is effect of the specimen width W. With increasing B/W and W, the constraint levels of specimens increase. The effect of crack depth a/W on creep constraint is not obvious. The comparison and matching of creep constraint between the standard test specimens recommended in ASTM-E1457 and non-standard specimens and pipes with a wide range of axial and circumferential crack sizes have been investigated. The results obtained are suitable for wider materials and may be used to choose constraint designed test specimens in creep life assessments of pipes with different crack size ranges, and to avoid non-conservative assessments for deeper and longer pipe cracks.

Characterization of creep crack-tip constraint levels for pressurized pipelines with axial surface cracks

Through extensive 3D finite element analyzes, the creep crack-tip constraint levels were characterized using a load-independent creep constraint parameter Q* for pipelines with axial surface cracks of different geometrical sizes, which involved in various crack depths and crack shapes. The Q* distribution along the crack front for axial internal surface cracks and axial external surface cracks exhibited the same distribution tendency. However, the constraint level of internal surface cracks was higher than external surface cracks. In addition, the constraint level was improved as the crack depth became deep; in contrast, the constraint level showed a reduction tendency as the crack shape ratio a/c increased from 0.2 to 1.0. The highest constraint levels for the axial surfaces cracks in pipelines approached to that of single-edge notched tension specimen. Finally, two empirical equations for estimating the constraint level were established as a function of crack depth ratio and crack shape ratio for axial surface cracks in pipelines.

Effects of creep properties of materials on creep crack-tip constraint parameter R*

Extensive finite element analyses have been conducted to investigate the effect of creep properties of materials on the creep crack-tip constraint parameter R*. The results show that the parameter R* increases with increasing power law creep stress exponent n of materials, and it is sensitive to lower n values and lower in-plane and out-of-plane constraints. In the engineering estimations of the creep constraint parameter R*, only the creep exponent effect needs to be considered for cracked components with lower exponent n and lower constraint, and the Norton’s coefficient A of power law creep materials can be ignored due to its insensitivity to the R*. The R*-n relation formulae have been established for single-edge-notched tension specimen (it has similar constraint level to pressurised pipes) with different in-plane and out-of-plane constraints, and they may be used to estimate the creep crack-tip constraint levels for cracked pipes with lower exponent n and lower constraint.

Unified correlation of in-plane and out-of-plane creep constraints with creep crack growth rate

In this paper, the equivalent creep strain distributions ahead of crack tips in SEN(B), SEN(T) and M(T) specimens with different in-plane and out-of-plane constraints were calculated by extensive finite element analyses, and the creep crack growth (CCG) rates of these specimens were simulated over a wide range of C* by using stress dependent creep ductility and strain rate model in a ductility exhaustion based damage model. The capability and applicability of the constraint parameter Ac based on crack-tip equivalent creep strain for characterizing both in-plane and out-of-plane creep crack-tip constraints and establishing a unified correlation with CCG rate of a steel were investigated. The results show that with increasing constraints, the CCG rate increases, and the effect of out-of-plane constraint on CCG rate is more obvious than that of in-plane constraint. The CCG rate of high constraint specimen geometry (such as SEN(B)) is more sensitive to the out-of-plane and in-plane constraints than that of low constraint specimen geometry (such as M(T)). The parameter Ac is a unified characterization parameter of in-plane and out-of-plane creep constraints for the different specimen geometries. Base on the parameter Ac, the unified correlation formulas of in-plane and out-of-plane constraints with CCG rate of the steel have been obtained, and they are independent on the choices of the standard specimen. The unified correlation formulas may be used in constraint-dependent CCG life assessments of high-temperature components (such as pressure vessels and pipes) with any in-plane and out-of-plane constraint levels.

Creep constraint analysis and constraint parameter solutions for circumferential surface cracks in pressurized pipes

Creep crack-tip constraints for inner circumferential surface cracks in pressurized pipes have been analyzed by three-dimensional finite element method and the constraint parameter R∗ solutions have been obtained for different pipe geometries and crack sizes. It has been shown that the constraint level of circumferential cracks is lower than that of axial cracks, and the constraint effects need to be considered in creep life assessments of pressurized pipes. The benefits from incorporating constraint effects are greater for the circumferential cracks than for the axial cracks and for the shallower and shorter cracks than for the deeper and longer cracks.

Characterization and correlation of 3-D creep constraint between axially cracked pipelines and test specimens

Three-dimensional creep crack-tip constraints of axially cracked pipes and conventional test specimens have been characterized and analyzed by a load-independent creep constraint parameter R∗ proposed by authors. An average measure of constraint over the crack front is utilized to correlate the creep constraint between the 3-D axially cracked pipelines and test specimens. It was found that single-edge notched tension specimens provide a closely matched creep crack-tip constraint with axially cracked pipes. The investigation results provide a strong support for use of constraint-designed specimens in creep life assessment of pressurized pipes.

Unified Correlation of In-Plane and Out-of-Plane Creep Constraints with Creep Crack Growth Rate

In this paper, the equivalent creep strain distributions ahead of crack tips in different specimens were calculated by extensive finite element analyses, and the creep crack growth (CCG) rates of these specimens were simulated over a wide range of C*. The capability and applicability of the constraint parameter Ac for characterizing both in-plane and out-of-plane creep crack-tip constraints and establishing a unified correlation with CCG rate of a steel were investigated. Base on the parameter Ac, the unified correlation formulas of in-plane and out-of-plane constraints with CCG rate of a steel have been obtained.

Three-dimensional finite element analyses of in-plane and out-of-plane creep crack-tip constraints for different specimen geometries

Based on extensive three-dimensional finite element analyses, the creep crack-tip constraints for three specimen geometries (C(T), SEN(T) and M(T)) were quantified by using three constraint parameters (namely R*, h and Tz) for 316H steel at 550°C and steady-state power law creep. The three parameters were comparatively analyzed with respect to their capabilities for characterizing in-plane and out-of-plane creep constraints as well as the interaction between them. The results show that the specimen geometries with higher constraint strengthen the out-of-plane creep constraint effect. The parameters R* and h can quantify in-plane creep constraint, but they are inadequate to characterize higher out-of-plane creep constraint. The Tz can adequately incorporate out-of-plane creep constraint effect.

Creep constraint analysis and constraint parameter solutions for axial semi-elliptical surface cracks in pressurized pipes

Creep crack-tip constraints for axially cracked pipes with different geometries and sizes of semi-elliptical surface cracks have been analyzed by three-dimensional finite element method and the constraint parameter solutions were obtained. It has been shown that the constraint levels of cracks in the pipes increase with increasing crack depths and lengths. The constraint levels and creep crack growth rates of all cracks in the pipes are lower than those of the standard compact tensile specimens. Without considering the creep constraint effects, the excessive conservatism may be produced in creep life assessments of the axially cracked pipes.

Three-dimensional numerical analysis of out-of-plane creep crack-tip constraint in compact tension specimens

In this paper, the C (t) and C∗ integrals, stress redistribution time tred and creep crack-tip stress distributions in the compact tension (CT) specimens with various thicknesses have been calculated by the three-dimensional (3D) finite element method, and the out-of-plane creep crack-tip constraint induced by specimen thickness is quantitatively characterized and analyzed in detail. The results show that the out-of-plane creep crack-tip constraint in the CT specimens could be characterized and analyzed by the new constraint parameter R. The 3D creep crack-tip constraint increases with increasing specimen thickness, and the 3D effect becomes more pronounced with increasing the parameter C∗. There is a central region with higher creep constraint along the crack front, the size of the region increases with increasing specimen thickness, and decreases with increasing creep time and parameter C∗. The factors of influencing the out-of-plane creep crack-tip constraint are analyzed.

Correlation of creep crack-tip constraint between axially cracked pipelines and test specimens

In this paper, based on the creep constraint parameter R, the creep crack-tip constraints in axially cracked pipelines and various test specimens have been analyzed under conditions of plane strain and steady-state creep, and correlation of the constraint between the cracked pipes and test specimens has been investigated. The creep crack-tip constraint of the single-edge notched tension (SENT) specimen is close to that of axially cracked pipes. The pin-loaded SENT specimen (SENTp) with a/W = 0.35 and 0.05 W loading point offset provides a closely matched creep crack-tip constraint with the axially cracked pipes. It will be more accurate if the creep crack growth rate and creep fracture toughness data measured by the SENTp specimen are used in integrity assessment of axially cracked pipes. The results provide a strong support for use of constraint-designed specimens in creep failure assessments of pressurized pipes at high temperature.

Numerical investigation on the creep crack-tip constraint induced by loading configuration of specimens

In this paper, the creep crack-tip constraint induced by loading configuration of specimens is quantitatively investigated by the finite element method. The results show the creep constraint induced by the loading configuration could be characterized and analyzed by the constraint parameter R. The specimen order in terms of the creep crack-tip constraint from high to low is compact tension, single edge-notched bend, single edge-notched tensile and middle-cracked tension specimen. With increasing C ∗ the constraint decreases in the specimens with different loading configurations. The effect of distances from the crack tips on the creep constraint parameter R is also analyzed.