Creep Crack Growth Life Research Articles

A Creep Constitutive Model, Based on Deformation Mechanisms and Its Application to Creep Crack Growth

In this paper, a constitutive model, based on the creep deformation mechanism in P91 steel, under a wide range of stress levels, was established and embedded into finite element software. The accuracy and reliability of the model was verified by comparing the simulation of uniaxial creep tensile test results and the experimental data under different stress levels for P91 steel at 600 °C. The creep crack growth behavior of P91 steel, under a wide range of stress levels was simulated using a ductility-exhaustion-based damage model, combined with the stress-dependent creep ductility model, and the predicted creep crack growth (CCG) rates were compared with the experimental data. Finally, the established model was used to predict the CCG behavior for the pressurized pipes with axial surface cracks. The results show that the constitutive model, established on the basis of the creep deformation mechanism, agrees better with the experimental data than other constitutive models. The CCG rate varies at different direction angles θ for the axial surface cracks. The direction angle θ corresponding to the maximum creep crack length is about 33°, when the internal pressure exceeds 10 MPa. The initial crack shape (a0/c0) = 1, and it does not change with different initial crack depth ratios (a0/t). The established constitutive model can be well used in CCG life analyses and designs of high-temperature structures.

Unified creep constraint parameter solutions for surface cracks in plates under tensile and bending loads

AbstractIn order to accurately assess creep crack initiation (CCI) and growth (CCG) life for elevated temperature cracked structures, both out‐of‐plane and in‐plane constraints need to be considered, and the unified creep constraint parameter solutions should be investigated. In this work, the new unified creep constraint parameter Ac values for plate structures with various surface crack sizes subjected to tensile and bending loads have been investigated and obtained using three‐dimensional (3D) finite element (FE) analysis. The effects of material creep properties, crack sizes, loading modes, and plate thickness on the parameter Ac have also been analyzed. The estimation equation of Ac for materials with different creep exponent n has been fitted. The parameter Ac solutions can be used in CCI and CCG life evaluation of the C*‐Ac two‐parameter approach for cracked plates subjected to tensile and bending loads.

A comparison between two parameter and ductility exhaustion approaches for creep life assessment

To improve the accuracy of creep life assessment for cracked components, the two-parameter fracture mechanics approaches with considering the constraint effect have been investigated and developed over the past decade. To examine the accuracy and applicability of different approaches, in this work, the creep life (including both creep crack initiation and growth life) is comparatively assessed using the two-parameter C*-R*, two-parameter C*-Ac and ductility exhaustion based approaches for pressurized pipes with different initial axial crack sizes. The results show that the life assessment results of the C*-R* approach are sensitive to initial crack sizes. For the shallower initial cracks, the C*-R* approach can give reasonable life prediction results, while for the deeper and longer cracks, it produces conservative results due to the sensitivity of the in-plane constraint parameter R* to initial crack sizes. The ductility exhaustion based approach verifies that the two-parameter C*-Ac approach has better life prediction accuracy for a wide range of initial crack sizes due to that the parameter Ac can characterize both in-plane and out-of-plane constraints and it is not very sensitive to initial crack sizes. The C*-Ac approach also can accurately predict the creep crack growth morphology which is mainly dependent on the initial crack aspect ratio.

A stress triaxiality based modified Liu–Murakami creep damage model for creep crack growth life prediction in different specimens

Liu–Murakami creep damage model is improved to predict the creep life of various cracked specimens. The modified creep damage law is implemented in the framework of extended finite element method (XFEM) for performing elasto-plastic creep crack growth simulations. Experiments show that the crack tip constraints vary from component to component which leads to variation in crack growth rates. A stress triaxiality function is introduced in the modified Liu–Murakami damage model to address the variation in crack growth rates. Moreover, a new definition of stress triaxiality (ratio of a linear combination of maximum principal stress and hydrostatic stress to von Mises stress) is proposed based on Leckie and Hayhurst failure criterion. The new definition of stress triaxiality is a key parameter in the prediction of time to failure. The modified Liu–Murakami creep damage model is used for the creep crack growth (CCG) simulations of several specimens under different loading conditions. Parametric studies are also performed to study the influence of various parameters on the CCG. Moreover, a combined framework of continuum damage mechanics and XFEM is used to predict the CCG life of a turbine blade. This work establishes that the modified Liu–Murakami creep damage law accurately predicts the creep life of cracked components under different constraint conditions.

A creep crack growth life assessment method for pressurized pipes based on a two-parameter approach

The unified creep constraint parameter Ac based on crack tip equivalent creep strain can incorporate both in-plane and out-of-plane constraints in specimens and components. To improve the accuracy of creep crack growth (CCG) life assessment for cracked components, it needs to develop new life assessment procedures to consider the unified constraint. In this work, the methodology of the CCG life assessment based on two-parameter C*-Ac has been studied and given for pressurized pipes with a wide range of initial crack sizes. The assessment results of CCG life and crack size/shape evolution based on the two-parameter C*-Ac approach are quantitatively analyzed and compared with those based on the conventional single-parameter C* approach. The effects of calculation methods of C* on the CCG life assessments are also analyzed. The results show that the application of the two-parameter approach can reduce conservatism and predict reasonable crack growth size and shape. The conventional CCG life assessment based on the single-parameter C* and the reference stress C* calculation for shallower and shorter surface cracks with lower constraint in cracked pipes may produce very high overall conservatism degree. Therefore, in the CCG life assessments for these cracks, it is highly suggested that the constraint effect is considered and the finite element solution of C* is used.

Estimation of Ct for weld cracks including HAZ softening region

Ct is an important parameter in predicting creep crack growth life from the small-scale creep stage to the extensive steady state creep stage. In this paper, weld interface and non-interface creep cracks for compact tension (CT) specimens with heat-affected zone (HAZ) were studied by finite element method (FEM) to systematically research the estimation method of Ct for weld creep cracks. Simulation results showed that Ct of weld interface cracks could be estimated by Yoon’s method. For weld non-interface cracks, Ct under the small-scale and transition creep stages could be exactly estimated by a proposed method. The accuracy of the proposed method was not affected by HAZ, but Yoon’s method was slightly affected by HAZ.

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.

In-plane and out-of-plane unified constraint-dependent creep crack growth rate of 316H steel

The unified characterization parameter Ac of in-plane and out-of-plane creep constraints along three-dimensional crack front in different specimen geometries has been analyzed. Based on the parameter Ac and the experimental creep crack growth rate data of different specimen geometries in the literature, the in-plane and out-of-plane unified constraint-dependent creep crack growth rate equation of 316H steel has been obtained. The predicted creep crack growth rate by using the equation for each specimen agrees well with the experimental data. Based on the two-parameter C∗−Ac concept, the equation may be used in creep crack growth life assessments incorporating in-plane and out-of-plane constraint effects.

Creep crack growth prediction and assessment incorporating constraint effect for pressurized pipes with axial surface cracks

Creep crack growth life is firstly predicted and assessed by two-parameter concept to incorporate constraint effect for pressurized pipes with inner axial surface cracks. The prediction and assessment results are compared with those based on conventional single-parameter C∗ and finite element calculations. For shallower and shorter cracks, the life assessments based on single-parameter C∗ are excessively conservative. To reduce excess conservativity, it is strongly recommended to incorporate constraint effect. With decreasing initial crack sizes, the benefits gained from two-parameter assessments increase. The crack growth profile may also be correctly predicted by two-parameter calculations. The C∗ calculation of reference stress method leads to extra conservativity.

Effect of stress dependent creep ductility on creep crack growth behaviour of steels for wide range of C*

In this work, the effect of stress dependent creep ductility on the creep crack growth (CCG) behaviour of steels has been investigated by finite element simulations based on ductility exhaustion damage model. The relationship between the transition region of creep ductility and the transition behaviour of CCG rate on da/dt-C* curves has been examined and the CCG life assessments of components and CCG resistance of materials for a wide range of C* were discussed. The results show that with increasing the transition region size of creep ductility, the transition C* region size on da/dt-C* curves increases. With moving transition region position of creep ductility to high stress region (increasing transition stress levels), the transition C* region on the da/dt-C* curves also moves to high C* region. Decreasing transition stress levels and transition region sizes of creep ductility and increasing the lower shelf and upper shelf creep ductility values can improve the CCG resistance of materials. If the extrapolation CCG rate data from the high C* region or from the transition C* region are used in life assessments of the components at low C* region, the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data should be obtained for a wide range of C* by long term laboratory tests or numerical predictions using the stress dependent creep ductility and model.

The influence of stress-regime dependent creep model and ductility in the prediction of creep crack growth rate in Cr–Mo–V steel

In this paper, the stress-regime dependent creep model and ductility have been implemented in a ductility exhaustion based damage model, and their influence on creep crack growth (CCG) behavior of materials have been analyzed. By using the stress-regime dependent creep model and ductility, the CCG rate in a Cr–Mo–V steel over a wide range of C* has been predicted by finite element analyses. The predicted CCG rates agree with the available experimental data in the literature. The analysis results show that with increasing C*, the creep model and ductility for determining crack-tip creep damage accumulation change from the low-stress regime model and ductility through a combination of low- and high-stress regime model and ductility to high-stress regime model and ductility. These changes lead to the line segments of the da/dt−C* curves. In CCG life analyses and designs of high temperature components, the stress-regime dependent creep model and ductility need to be used.

A review of a time-dependent fracture life law (or model) based on a proposed multi-scale analysis

To solve problem of structural safety under time-dependent conditions, an algorithm of multi-scale hy- brid mechanics from nano-scale to macro-scales is described. The paper presents an algorithm that links physics of materials with life prediction determined by creep crack growth and addresses practical application which enables predic- tion of fracture to be developed from the science of clarification of mechanism on basis of algorithm. In particular, in order to predict life of real components, importance of link between void mechanics caused by vacancy diffusion (micro mechanical factor), stress tri-axiality, TF (a structural mechanical factor), and material structure (a meso-mechanical factor) is shown in proposed formulation for activation energy of thermally activated process of creep crack growth. A detailed discussion on these issues as well as on algorithm relevant to establishment of law of creep crack growth life based on multi-scale analysis is included ranging from scale of micro damage to structural brittleness.

143 高温クリープ・疲労条件下におけるIN100合金のき裂成長寿命に及ぼす応力多軸効果と巨視力学と空孔拡散凝集挙動による損傷形成挙動の解明(材料力学II,一般講演)

143 高温クリープ・疲労条件下におけるIN100合金のき裂成長寿命に及ぼす応力多軸効果と巨視力学と空孔拡散凝集挙動による損傷形成挙動の解明(材料力学II,一般講演)

Characterizations of Creep-fatigue Crack Initiation and Growth Life for P92 using Circular Notched Round Bar Specimen

Abstract A testing method under creep-fatigue interaction conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimens have been recommended as a testing specimen. Additionally, creep-fatigue crack initiation and growth life are usually defined as the attainment of a specific rate of decrease in the maximum tensile stress or the modulus of elasticity ratios. However, these criterions of crack initiation and growth life on ASTM standard are not characterized in terms of physical crack size. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens. An attempt is made to measure the crack length during the tests by the direct current potential drop method, which results in the establishment of a high accuracy crack length measurement. Additionally, the life of creep-fatigue crack growth was characterized by linear cumulative damage law combined with the Q*concept which has been proposed as fracture parameter to describe the life of creep crack growth.

G030022 W添加9Cr鋼溶接材のクリープき裂発 生・成長挙動([G03002]材料力学部門一般セッション(2):クリープ損傷)

Most of boiler component structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. However, the technique of predicting the life of creep crack initiation and growth have not been clearly developed for weldments of W-added 9 %Cr ferritic heat-resistant material (ASME Code Case 2179, ASTM A335 P92) due to many factors such as the variation in micro-structure and the residual stress caused by welding and thermal cycles. In the present paper, high temperature creep crack growth tests were conducted using C(T) specimens and circular notched specimens with the cracks in the heat affected zone of weldments for P92. To derive the algorithm of predicting the life of creep crack growth for weldments, the creep crack growth rate were estimated by C*, d&dt and Q* parameters.

Behavior of Branch Cracking and the Microstructural Strengthening Mechanism of Polycrystalline Ni-Base Superalloy, IN100 under Creep Condition

Recently, the development of the high-efficiency technology for gas turbine and jet engine is required to minimize carbon dioxide and nitrogen oxide emission. It is effective way to increase the operational temperature to develop the high-efficiency technology for high temperature instruments. To increase the operating temperature, advanced Ni-base superalloys have been developed as a turbine blade material.Even though a Ni-base superalloy is used for a structural component, creep damages and creep cracks may be caused due to the external tensile load under high temperature conditions. Therefore, a predictive law of creep crack growth life is necessary to maintain operational safety.This study is aimed to clarify the branch cracking behavior due to the microstructural strengthening mechanism of polycrystalline Ni-base superalloy IN100 under the creep condition. The creep crack growth tests were conducted at a temperature of 900°C. The creep crack growth behavior and creep damage formulation were observed by in-situ observational system and FE-SEM/EBSD. Additionally, two-dimensional elastic-plastic creep finite element analyses were conducted for the model which describes the experimental results. The creep crack growth behavior and the creep damage progression were found to be affected by the distribution behaviors of grains and grain boundaries around the notch tip. By comparison of experimental results with mechanical analysis using FEM analyses, mechanisms of the creep crack growth and the creep damage formulation were clarified.

Evaluation of Creep Crack Growth Rate of P92 Welds Using Fracture Mechanics Parameters

High Cr ferritic heat resisting steels have been widely used for boiler components in ultrasupercritical thermal power plants operated at about 600°C. In the welded joint of these steels, type-IV crack initates in the fine-grained heat affected zone during long-term use at high temperatures and their creep strength decreases. In this paper, creep properties and creep crack growth (CCG) properties of P92 welds are presented. The CCG tests are carried out using cross-welded compact tension C(T) specimens at several temperatures. The crack front was located within the fine-grained HAZ region to simulate type-IV cracking. Finite element analysis was conducted to simulate multiaxiality in welded joints and to compare experimental results. The constitutive behavior for these materials is described by a power-law creep model. C∗ and Q∗ parameters are used to evaluate CCG rate of P92 welds for comparison. C∗ parameters can characterize approximately 20% of the total life of CCG in P92 welds, and Q∗ parameters can characterize approximately 80% of the total life. Q∗ parameter is one of the useful parameters to predict CCG life in P92 welds.

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.

Creep crack initiation and growth characteristics in weldment for P92 based on C*, dδ/dt and Q* parameters

Most boiler component structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. However, the technique of predicting the life of creep crack initiation and growth have not been clearly developed for weldments of W-added 9%Cr ferritic heat-resistant material (ASME Code Case 2179, ASTM A335 P92) due to many factors such as the variation in micro-structure and the residual stresses caused by welding and thermal cycles. In the present paper, high temperature creep crack growth tests were conducted using C(T) specimens with the cracks in the heat affected zone of weldments for P92. To derive the algorithm of predicting the life of creep crack growth, the creep crack growth rate were estimated by C∗, dδ/dt and Q∗ parameters. Additionally, the definition and estimation method of creep crack initiation and growth life for weldments were discussed on the basis of the concept of structural brittleness.

高温クリープ条件下における IN100 合金のき裂分岐挙動と材料組織強靭化機構

Recently, the development of the high-efficiency technology for gas turbine and jet engine is required to minimize carbon dioxide and nitrogen oxide emission. It is effective way to increase the operational temperature to develop the high-efficiency technology for high temperature instruments. To increase the operating temperature, advanced nickel based superalloys have been developed as a turbine blade material. Even though a nickel based superalloy is used for a structural component, creep damages and creep cracks may be caused due to the external tensile load under high temperature conditions. Therefore, a predictive law of creep crack growth life is necessary to maintain operational safety. This study is aimed to clarify the branch cracking behavior due to the microstructural strengthening mechanism of polycrystalline nickel based superalloy IN100 under the creep condition. The creep crack growth tests were conducted at a temperature of 900°C. The creep crack growth behavior and creep damage formulation were observed by in-situ observational system and SEM/EBSD. Additionally, two dimensional elastic-plastic creep finite element analyses were conducted for the model, which describes the experimental results. The creep crack growth behavior and the creep damage progression were found to be affected by the distribution behaviors of grains and grain boundaries around the notch tip. By comparison of experimental results with mechanical analysis using FEM analyses, mechanisms of the creep crack growth and the creep damage formulation were clarified.