Creep-fatigue Crack Growth Rates Research Articles

A Phenomenological Model for Creep and Creep-Fatigue Crack Growth Rate Behavior in Ferritic Steels

A model to rationalize the effects of test temperature and microstructural variables on the creep crack growth (CCG) and creep-fatigue crack growth (CFCG) rates in ferritic steels is described. The model predicts that as the average spacing between grain boundary particles that initiate creep cavities decrease, the CCG and CFCG rates increase. Further, the CCG data at several temperatures collapse into a single trend when a temperature compensated CCG rate derived from the model is used. The CCG and CFCG behavior measured at different temperatures is used to assess the effects of variables such as the differences between the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions. The model is demonstrated for Grade 22 and Grade 91 steels using data from literature. It is shown that differences between the CCG behavior of the Grade 22 steel in new and ex-service conditions are negligible in the BM and WM regions but not in the HAZ region. The CCG behavior of the Grade 91 steels can be separated into creep-ductile and creep-brittle regions. The creep-brittle tendency is linked to the presence of excess trace element concentrations in the material chemistry. Significant differences found in the CCG rates between the BM, WM, and HAZ regions of the Grade 91 steel are explained.

A new creep-fatigue crack growth model and a correlation of the creep-fatigue crack growth rate with unified constraint parameter

Creep-fatigue and constraint are important factors affecting the fracture behavior and service life of components in high-temperature service. Some researchers have attempted to describe creep-fatigue crack growth (CFCG) behavior using constraint parameters. However, there is still no consensus regarding the effect of the constraint on the CFCG rate, and a correlation between the constraint and the CFCG rate must be established. Thus, in this study, a new CFCG model was developed and verified by finite element simulations and experiments. Then, the CFCG behavior for single edge-notched tensile (SENT), centre-cracked tension (CCT), and compact tension (CT) specimens under different constraint conditions was studied using the new model to reveal the effect of constraint on the CFCG rate. The relationship between the dwell time and the CFCG rate was also studied. Moreover, a linear correlation between the CFCG rate and the constraint was established for the first time using the unified constraint parameter Ap, and it is shown that the CFCG rate has a strong correlation with the area surrounded by the equivalent plastic strain (εp) contour at the crack tip. This indicates that the constraint effect on the creep-fatigue could be further revealed by Ap. Furthermore, Ap was extended from monotonic loading to creep-fatigue loading, and a method is provided for using CFCG test data to evaluate high-temperature components with defects.

Results of a Pilot Study in Support of a Round-Robin on Creep-Fatigue Crack Growth Testing of a Creep-Brittle Material for ASTM E2760

Abstract This paper describes pilot test results from a round-robin conducted in support of the standard ASTM E2760, Standard Test Method for Creep-Fatigue Crack Growth Testing, on creep-fatigue crack growth testing. Results from the pilot study are recommended to participants to develop their individual experimental testing. This round-robin is an international effort with 10 participants from five countries from Asia, North America, and Europe. All participants are testing compact type (C(T)) specimens according to the procedure described in the standard. A few participants are evaluating the Kb-bar (surface crack) specimen configuration. The test material is Inconel-718 (IN718) nickel-based alloy, which is a creep-brittle alloy. Pilot creep-fatigue crack growth tests were conducted at 600°C using C(T) specimens under constant load amplitude conditions. The tests were conducted at hold times of 0, 60, and 600 s. Creep-fatigue crack growth rates were analyzed using the methods proposed and included in the test standard. It was found that the procedures followed throughout the pilot test program were sufficient in generating the desired outcomes. As a result, it is proposed that the various participants follow the procedures and test matrix as close as possible and document any deviations. This will allow data from the various participants to be assessed and determine the levels of precision and bias in creep-fatigue crack growth data obtained using the ASTM standard.

Crack growth rate prediction based on damage accumulation functions for creep-fatigue interaction

The present study is concerned with formulation of a model for the creep–fatigue crack growth rate prediction on the base of fracture damage zone concepts. It is supposed that crack growth rate can be determined by integration of damage accumulation rate equations into the fracture process zone for low-cycle fatigue and creep loading independently. In the case of low-cycle fatigue loading the damage accumulation function proposed by Ye and Wang was used as well as a classical Kachanov-Rabotnov power law was employment for the creep damage accumulation characterization. Fracture process zone size is calculated on the base of the nonlinear stress intensity factors concept proposed by Shlyannikov. The background for the proposed general model of crack growth rate under creep and fatigue interaction is given in order to comparison with the experimental data. Experimental study of crack growth rate under creep and fatigue interaction is performed for compact tension specimen made from 20CrMoV5. Crack growth rate carried out at the elevated temperature of 550°C according to ASTM E2760 standard. The predictions of the crack growth rate were compared with the experimental data for the 20CrMoV5 steel obtained at an elevated temperature, and the agreement was found to be satisfactory.

Fatigue and creep-fatigue crack growth in alloy 709 at elevated temperatures

ABSTRACTFatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) in austenitic stainless-steel Fe-25Ni-20Cr (Alloy 709) were measured experimentally and simulated using the finite element method. Temperature conditions investigated were 550°C, 600°C and 700°C, with load hold times of 0 s, 60 s and 600 s. Fracture surface was investigated using scanning electron microscopy and optical imaging. Experiments indicated that crack growth exhibits minimal sensitivity to the various loading conditions evaluated. At 600°C, crack growth rates were independent of hold time or loading frequency. At 700°C, there was a small increase in crack growth rate as a function of hold time, with a 600s hold time causing a factor of 2 increase in crack growth rate over FCG. Finite element simulations were performed to compute plasticity-induced crack closure in the presence of creep deformations at the crack tip. The simulations produced FCG and CFCG rates similar to the experimental results.

Crack growth behavior, fracture mechanism, and microstructural evolution of G115 steel under creep–fatigue loading conditions

Creep–fatigue crack growth (CFCG) behavior, fracture mechanism, and microstructural evolution of G115 steel under different hold times and initial stress intensity factor ranges (ΔKin) were investigated. The crack propagation durations increased significantly with decreasing ΔKin and increasing dwell time. In da/dN vs. ΔK plots, a “hook” was formed, and it entered the fast crack growth zone at approximately ΔK=36MPam, which corresponds to σref = σ0.2 under plane stress. In (da/dt)avg vs. (Ct)avg plots, data for a specific hold time in a wide load range exhibited a unique relationship. It appears that the parameter D0 in this relationship is affected by hold time and constraint, while φ is independent of these factors. Scanning electron microscopy analyses revealed that with increasing hold time (60–600 s), the fracture mode changed from transgranular-dominated to intergranular-dominated. Meanwhile, the fracture mode changed from intergranular-dominated to predominantly transgranular with high ductility when ΔKin increased (17–29 MPam). According to electron backscatter diffraction observations, many substructured and recrystallized grains formed after CFCG and the deformed grains further decreased as ΔKin increased. The increased crack resistance with decreased deformation grains may combine with the ductility fracture effect to decrease the CFCG rates under high load levels. Additionally, the local misorientation angle reduced significantly after CFCG, and the differences among various load levels were limited.

Creep–fatigue crack growth rate prediction based on fracture damage zone models

Based on a new fracture damage zone formulation and nonlinear elastic–plastic and creep crack tip fields, the prediction of creep–fatigue crack growth rate is studied. A unified crack growth rate equation is derived using the equivalent nonlinear stress intensity factor, monotonic low-cycle fatigue, and creep material properties at an elevated temperature. The crack growth rate model includes the size of the fracture process zone and damage parameters. The predicted creep–fatigue crack growth rates are compared with the experimental data of the 12Cr1MoV power plant steel and good agreement is observed.

The influence of plasticity-induced crack closure on creep-fatigue crack growth in two heat-resistant steels

Finite element simulations are performed to predict plasticity-induced crack closure and creep-fatigue crack growth rates in two heat-resistant steels, the austenitic 20Cr-25Ni (Alloy 709) and martensitic 9Cr-1Mo (modified 1Cr-9Mo) steels. In a creep-fatigue cycle, total crack growth rate is usually computed by the addition of the fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions, with the consideration of elastic-plastic and creep deformations of the material at the crack tip. The simulations quantify the effect of hold time on crack opening load induced by the combined action of plasticity-induced crack closure and creep-induced stress relaxation at the crack tip. It is shown that increasing hold time during a creep-fatigue cycle results in a decrease of crack-tip opening load, thus increasing crack growth rate during the next cyclic loading. Experimental results of creep-fatigue crack growth rates in Alloy 709 are presented. The finite element simulations predicted crack growth rates under creep-fatigue loading in agreement with experimental values for both Alloy 709 and modified 9Cr-1Mo steels.

Creep-fatigue crack growth rate assessment using ductility damage model

In this study, we analyzed the behavior of creep damage and fracture resistance parameters in a 3D finite thickness compact specimen by employing damage-evolution equations. A damage model for the fracture process zone is represented using a ductility-based formulation. Following the ductility model, a new multi-axial failure strain condition is derived. Both damage-free and defective creeping solids are analyzed. The variations in the creep damage contour and crack tip governing parameters in terms of the creep In-integral with respect to time, as well as the crack size, are considered using full-field finite element model solutions. Creep-fatigue crack growth rate tests are performed on a specially designed program test cycle. The application of the previously introduced creep stress intensity factor, as a creep-damage-sensitive parameter for creep-fatigue interaction, is discussed.

Predicting creep-fatigue crack growth rates in Alloy 709 using finite element simulations of plasticity and creep-induced crack closure

This paper reports on a computational study and experimental validation of creep-fatigue crack growth rates at high temperature in two structural materials. The objectives are to develop a methodology to predict creep-fatigue crack growth rates using plasticity-induced crack closure under creep-fatigue loading conditions by characterizing the effect of hold time on crack growth rates during cyclic loading. In this study, the computation of fatigue crack growth rates is based on the crack closure phenomenon. The total crack growth rate during creep-fatigue loading is based on the addition of fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. The study identifies the effects of frequency and shape of loading cycle on crack-tip opening stresses induced by the combined action of the plasticity-induced crack closure and creep relaxation at the crack tip. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions. Elastic-plastic-creep material behavior is considered in these simulations. Closure levels are computed for high temperature structural materials such as 9Cr-1Mo steel and Alloy 709. The numerical predictions provide satisfactory agreement with experimental data of creep-fatigue crack growth rates in modified 9Cr-1Mo and Alloy 709 steels at high temperatures.

Modeling of creep-fatigue interaction effects on crack growth at elevated temperatures

The well-known load frequency effect on creep-fatigue crack growth is explained by the interactions between fatigue and creep loading and is quantified using the concept of plasticity-induced crack closure. It is shown that the hold time during creep loading affects crack growth rates during subsequent fatigue cycles. Longer hold times lead to lower crack-tip opening stresses and faster crack growth rates during fatigue loading. To model the impact of hold time on crack opening stresses during fatigue loading, a strip-yield model was developed for creep-fatigue crack growth. The strip-yield model computes crack-tip opening stresses, which determine the effective stress intensity factor range and crack growth rate during the fatigue portion of each loading cycle. Maximum stress intensity factor is used to compute the crack growth rate during the creep portion of each cycle. The proposed strip-yield model is used to compute creep-fatigue crack growth rates for several structural materials, i.e., an Astroloy, aluminium alloy 2650 and 316 stainless steel. The model predictions of crack growth rates compare well with published experimental data for these alloys. This model achieves reliable predictions of crack growth rates and life prediction on components subjected to creep-fatigue loading at elevated temperatures by considering loading interaction effects.

Creep crack growth assessment in terms of continuum damage mechanics

The stress, strain rate, and process zone with respect to the creep-crack growth in compacttension C(T) specimen is analyzed by employing damage-evolution equations. The damage model for the fracture of the process zone is represented using a stress based formulation. Both damage free and defective creeping solids have been studied. The variations in the creep stress/strain and crack-tip governing parameter in terms of the creep stress intensity factors with respect to time and the evolution of creep damage are analyzed using an FE model for C(T) specimen. The creep-fatigue crack growth rate tests were performed on special designed program test-cycle. The interpretation of the experimental creep crack growth rate data was given in terms of introduced creep stress intensity factors based on undamaged and damaged stress/strain fields.

Creep-fatigue crack growth behavior in GH4169 superalloy

This study aims to examine the crack growth behavior of turbine disc GH4169 superalloy under creep-fatigue loading. Crack growth experiments were performed on compact tension specimens using trapezoidal waveform with dwell time at the maximum load at 650 °C. The crack growth rate of GH4169 superalloy significantly increased with dwell time. The grain boundaries oxidize during the dwell process, thereby inducing an intergranular creep-fatigue fracture mode. In addition, testing data under the same dwell time showed scattering at the crack growth rate. Consequently, a modified model based on the Saxena equation was proposed by introducing a distribution factor for the crack growth rate. Microstructural observation confirmed that the small grain size and high volume fraction of the δ phase led to a fast creep-fatigue crack growth rate at 650 °C, thus indicating that two factors, namely, fine grain and presence of the δ phase at the grain boundary, increased the amount of weakened interface at high temperature, in which intergranular cracks may form and propagate.

Loading history effect on creep-fatigue crack growth in pipe bend

This study is concerned with assessing the structural integrity of power plant pipes that operate under creep and fatigue conditions. In the process of manufacturing pipe bends, it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross-section of the bend also becomes non-circular due to the bending process, which tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of a pipe bend on stress-strain redistributions and creep-fatigue crack growth rates was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bends was performed for different stages of lifetime while considering various loading conditions. As a result, it was determined that the combined effect of ovality and thinning cross-sections is significant and is a function of creep time. The tensile and creep tests were performed for determination of the main mechanical properties of the material of the pipe bends after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross-sections, as well as operating time. The creep-fatigue crack growth rate tests were performed on a specially designed program test-cycle. A comparison demonstrates that the tensile properties of 12Cr1MoV steel were degraded by long-term high-temperature service, whereas a comparison of the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature was used in the form of a creep stress intensity factor.

Crack Growth Resistance Characteristics Determination Algorithm for Creep-fatigue Interaction

A creep-fatigue crack growth rate interpretation method and algorithm and test results are described. The algorithm allows to obtain a crack growth rate diagram in terms of creep stress intensity factor. The proposed algorithm is realized on steel compact tension specimens (12Х1MF).Crack growth rate was determined on standard compact specimens under temperature 550°C. The waveforms for the loading and unloading portions were trapezoidal, and the loading/unloading times were held constant – 5 s. A hold time of predetermined duration, 60 s, was superimposed on the trapezoidal waveforms at maximum load. The potential drop and the unloading compliance methods were used to monitor crack length during the creep–fatigue tests.For the experimental crack paths in tested specimens the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral was calculated by finite element analysis along crack front. The governing parameter of the stress-strain fields in form of In-integral was used as the foundation of the creep stress intensity factor. The creep stress intensity factor approach was applied to the fatigue crack growth. The numerical and experimental results demonstrated that Kcr is the most effective crack tip parameter in correlating the creep–fatigue crack growth rates in power plant materials and can be used reliably for practical purposes.

Creep-Fatigue Crack Growth in Power Plant Steels

An important consideration in the characterisation of creep-fatigue crack growth rates and the defect assessment of high temperature components is the size of pre-existing or service-induced flaws relative to the dimensions of associated cyclic plastic zones. In particular, long cracks are those whose size exceeds r p and short cracks are those whose size is less than r p, and the way in which their propagation is analytically represented can differ depending on the form of loading and the respective contributions of creep and fatigue damage accumulation. Examples are given for a number of power plant steels. There are now published procedures for the assessment of structural integrity in high temperature components subject to creep-fatigue loading, and guidance on how to generate the necessary properties for their implementation, and these are reviewed.

A creep stress intensity factor approach to creep–fatigue crack growth

A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature is introduced in the form of creep stress intensity factor. On this base a crack growth rate approach is developed for the case of creep and fatigue interaction. A numerical method is proposed for estimating the governing parameter of the creep crack tip field in the form of an In-integral along the through-the-thickness straight and curved crack fronts. The application of the introduced parameters through experimental study of the creep–fatigue crack growth rate confirms effectiveness of the proposed approach.

Effect of Pipe Bend Shape Imperfections on Creep-fatigue Crack Growth

This study is concerned with assessing the structural integrity of power plant pipes which operate under creep and fatigue conditions. In the manufacturing process of pipe bends it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross section of the bend also becomes non circular due to bending process; this tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of pipe bend on stress-strain redistributions and creep-fatigue crack growth rate was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bend was performed for different stage of lifetime under considering loading conditions. As a result it was found that the effect of ovality and thinning cross sections interaction is significant and is a function of creep time. The tensile tests were performed for determination the main mechanical properties of pipe bend's material after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross sections as well as operating time. The creep-fatigue crack growth rate tests were performed on special designed program test-cycle. Comparison between new and old materials shows tensile properties of 12Cr1MoV steel have degraded by long-term high-temperature service; while comparison between the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature is used in the form of creep stress intensity factor.

Low Cycle Fatigue Crack Growth Behavior of Modified Rotor Steel under High-Temperature

Many components in power plants are subjected to low cycle loading under high-temperature during continuing service. Therefore, creep-fatigue damage has been become a significant concern in the design and operation of these components. This damage mechanism initiates cracks at critical locations and subsequently creates a potential risk to safe and reliable operation. Creep-fatigue crack growth rate data are necessary inputs for assessing the structural integrity of high temperature components in power plants. Creep-fatigue crack growth tests were conducted for a modified X12Cr rotor steel at 600°C under trapezoidal wave shape load amplitude conditions with various hold times using C(T) specimens. A direct current electrical potential-drop was used to monitor crack growth length during the tests.It is shown that crack growth behavior under creep-fatigue conditions is characterized better by the (Ct)avg parameter, and the formula of assessment low cycle fatigue crack growth under high-temperature was fitted by use of ΔK and (Ct)avg parameter.

Round robin on creep fatigue crack growth testing for verification of ASTM standard 2760-10

This paper presents early results from an ongoing round robin study to verify the newly developed ASTM standard E-2760-10 on creep fatigue crack growth testing. This round robin is an international effort with 18 participants from 11 countries from Asia, North America and Europe. All participants are testing compact type, C(T) specimens according to the procedure described in the standard. The test material is P91 taken from a section of retired steam pipe that was heat treated to rejuvenate the microstructure. All background materials data such as tensile, creep deformation and creep fatigue crack formation data are already available on the test material. Creep fatigue crack growth testing was conducted as part of a pilot program at 625°C using C(T) specimens under constant load amplitude conditions at two load levels. The tests are conducted at hold times of 0, 60 and 600 s. Creep fatigue crack growth rates are analysed using the two methods included as part of the test standard. The data from the various participants will be assessed to determine the expected levels of precision and bias in creep fatigue crack growth data obtained using the ASTM standard. Modifications to the current version of the standard will be proposed and balloted as needed from the round robin results.