Creep-fatigue Conditions Research Articles

Characterization crack growth behavior in creep-fatigue loading conditions through different specimen geometries

Creep-fatigue interaction would accelerate crack growth behavior and change crack growth mode, which is different from that presenting in pure creep or fatigue regimes. In addition, the constraint ahead of crack tip affects the relationship between crack growth rate and fracture mechanics parameter and thus influences the accuracy of the life prediction for defected high-temperature components. For the aim of revealing the role of various specimen geometries in the creep-fatigue regimes, crack growth behaviors were investigated using five different types of cracked specimens (including C-ring in tension CST, compact tension CT, single notch tension SENT, single notch bend SENB, middle tension MT). The crack growth and damage evolution were simulated using finite element method coupled with a non-linear creep-fatigue interaction damage model. Moreover, the solutions of (Ct)avg for different specimen geometries were given to expand the range of testing specimens. By analysing crack growth behavior with various specimen geometries under creep-fatigue conditions, CT and CST showed the highest crack growth rates, which were eight times as the lowest crack growth rates in MT. This revealed that distinctions in specimen geometry influenced the in-plane constraint level ahead of crack tip. Furthermore, a load-independent constraint parameter Q* was introduced to correlate the crack growth rate. The sequence of crack growth rate at a given value of (Ct)avg was same to the reduction of Q*. A normalized constraint level was employed to correlate with the normalized crack growth rate for the aim of transferring lab results into engineering issues.

Ratcheting testing of polytetrafluoroethylene (PTFE) under multiple-step compression

Abstract Uniaxial stress-controlled ratcheting experiments of PTFE (polytetrafluoroethylene) under cyclic compressive loads with multiple load steps were performed. The influence of stress rate, temperature, stress amplitude, loading sequence and peak holding on the compressive ratcheting behavior have been discussed systematically. Results indicate that ratcheting deformation increases significantly when enhancing temperature and stress amplitude and reducing stress rate. Only a slight, accumulated deformation occurs during the first 20 or so cycles at room temperature (RT), but it always turns to shakedown. Moreover, ratcheting strain is clearly influenced by the loading sequence. When greater compressive deformation was obtained during the prior load step due to a greater stress level (such as higher temperature or a lesser stress rate), increased compressive strain hardening and deformation resistance could be produced, which in turn restricts or even reduces ratcheting deformation in the pursuing load step. It is of great interest that no initial cyclic stress strain curve was observed under cyclic compression at peak holding time. This indicates that creep recovery due to anelastic relaxation decreases the ratcheting rate of PTFE material under compressive creep-fatigue conditions. This research provides an important testing approach and data for PTFE material subjected to repeated compressive loads influenced by temperature modulation.

Numerical implementation of a phase mixture model for rate-dependent inelasticity of tempered martensitic steels

Tempered martensitic steels with a high chromium content are used for power plant components at elevated temperatures under creep–fatigue conditions. In order to model the complex mechanical behavior of the alloy X20CrMoV12-1, a phase mixture model is used. This model is based on the distinction of hard and soft constituents, which are connected via an iso-strain approach. Nonlinear kinematic hardening and softening effects are taken into account by introducing a backstress and a softening variable. This paper focuses on the numerical implementation of the phase mixture model, i.e., the stress update algorithm and the consistent tangent operator. For implicit time integration of the governing equations, the backward Euler method in combination with Newton–Raphson iterations is applied. The model is implemented as user material subroutine into the commercial finite element code ABAQUS. For verification, several benchmarks considering uniaxial and multi-axial stress states are presented and analyzed. Furthermore, a thermo-mechanical fatigue test based on a typical sequence of start-ups and shutdowns of power plants is simulated.

Creep-Fatigue Life Prediction of 316H Stainless Steel by Utilizing Non-Unified Constitutive Model

True stress and strain are necessary to estimate the rupture life under creep-fatigue conditions. Finite element analysis (FEA) is one of the most reliable method to calculate true stress and strain, but the accuracy of the obtained result is often greatly dependent on the constitutive model used. Non-unified constitutive model has been proposed, where inelastic strain is decomposed into creep strain and visco-plastic strain. In this paper, a cyclic hardening effect and a plastic strain range dependency were introduced into the non-unified constitutive model to predict the creep-fatigue damage of 316H stainless steel. This model was implemented in a finite element program and FEA were conducted to develop a life assessment method and calculate the creep-fatigue damage by modified ductile exhaustion method. As a consequence, it was revealed that the predicted creep-fatigue lives showed high correspondence with the experimental results by the modified ductile exhaustion method utilizing the non-unified constitutive model.

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 the cyclic softening and lifetime of ferritic-martensitic steels under creep-fatigue loading

Ferritic-martensitic (F/M) steels are a common choice for high-pressure, high-temperature boilers and steam piping in power plants, but cyclic softening without saturation is a major degradation mechanism of F/M steels. This degradation mechanism is more pronounced and complex under creep-fatigue loadings. For reliable, but not overly conservative design a better understanding of this phenomenon is necessary. The intent of this work is to characterize and model the softening and lifetime behavior under creep-fatigue conditions at high temperature. Therefore, low-cycle fatigue (LCF) tests with hold time were performed at 550 °C.A distinct dependence of the softening behavior on hold time was found. Additionally, detrimental effect of compressive hold time on lifetime was also observed in this study. All effects of hold time were more pronounced for small strain amplitudes. Based on these experimental observations, a unified viscoplastic deformation and damage model is further developed to describe the high-temperature cyclic softening and lifetime behavior of F/M steels under creep-fatigue loading. An optimized parameter set for the extended model is determined based on experimental results. Good agreement of experimental results and model descriptions is achieved both under pure fatigue as well as under creep-fatigue conditions. The detrimental effect of compressive hold times on lifetime of ferritic-martensitic steels is well represented.

Engineering models for softening and relaxation of Gr. 91 steel in creep-fatigue conditions

Purpose There are a number of different approaches for calculating creep-fatigue (CF) damage for design, such as the French nuclear code RCC-MRx, the American ASME III NH and the British R5 assessment code. To acquire estimates for the CF damage, that are not overly conservative, both the cyclic material softening/hardening and the potential changes in relaxation behavior have to be considered. The data presented here and models are an initial glimpse of the ongoing European FP7 project MATISSE effort to model the softening and relaxation behavior of Grade 91 steel under CF loading. The resulting models are used for calculating the relaxed stress at arbitrary location in the material cyclic softening curve. The initial test results show that softening of the material is not always detrimental. The initial model development and the pre-assessment of the MATISSE data show that the relaxed stress can be robustly predicted with hold time, strain range and the cyclic life fraction as the main input parameters. The paper aims to discuss these issues. Design/methodology/approach Engineering models have been developed for predicting cyclic softening and relaxation for Gr. 91 steel at 550 and 600°C. Findings A simple engineering model can adequately predict the low cycle fatigue (LCF) and CF softening rates of Gr. 91 steel. Also a simple relaxation model was successfully defined for predicting relaxed stress of both virgin and cyclically softened material. Research limitations/implications The data are not yet complete and the models will be updated when the complete set of data in the MATISSE project is available. Practical implications The models described can be used for predicting P91 material softening in an arbitrary location (n/Nf0) of the LCF and CF cyclic life. Also the relaxed stress in the softened material can be estimated. Originality/value The models are simple in nature but are able to estimate both material softening and relaxation in arbitrary location of the softening curve. This is the first time the Wilshire methodology has been applied on cyclic relaxation data.

Characterizing crack growth behavior and damage evolution in P92 steel under creep-fatigue conditions

The present paper characterized crack growth behavior and damage evolution in P92 steel under creep-fatigue interaction conditions. Creep-fatigue crack growth tests were conducted at a constant load amplitude with various hold times. To reveal the role of constraint in creep-fatigue regime, specimens with different crack depths and thicknesses were used. Combination with a non-linear creep-fatigue interaction damage constitutive model, the crack growth and damage evolution behaviors were simulated using finite element method. Under creep-fatigue condition, time dependent crack growth rate increased as the duration period was reduced. This was attributed to the role of enhanced fatigue damage with the decreasing of the dwell time. Moreover, the deviation of crack growth rate with different dwell times became small as crack grew. With the crack length increasing, the creep damage level was improved regardless the dwell time, which may be due to the fact that the creep damage dominated crack growth in this stage. Furthermore, the crack growth rate increased as the crack depth became deep and the specimen thickness became large, as a result of the increased constraint level. A load-independent constraint parameter Q* was introduced to correlate the crack growth rate, which provided a good prediction for specimens under different constraint conditions.

Experimental investigation of hot-work tool steels performances under the creep-fatigue regime

In the present research, an innovative testing method, specifically developed to characterize the tool steels under creep-fatigue conditions, was carried out on a TQ1 (X36CrMoV5-2) hot-work tool steel in cooperation with Constellium R&D center. The experimental campaign consisted of different testing conditions, and most of the specimens were nitrided to account for the specific surface state of the tools. Tests were performed on a 100 kN MTS fatigue machine equipped with a heating furnace. A creep-fatigue loading type was applied to the specimens, i.e., a cyclic load with a dwell time, in order to properly reproduce the conditions acting on extrusion tools and dies. Under a constant temperature of 520 °C, the effects of four different load levels and two different values of dwell times were evaluated. In addition, selected test conditions were replicated with specimens not nitrided with the aim to evaluate and quantify the influence of the superficial treatment. Final results were presented in terms of fatigue diagram of the TQ1 steel and compared to the performances of the H11 tool steel tested in a previous research by the same authors.

Effect of tensile dwell on high-temperature low-cycle fatigue and fracture behaviour of cast superalloy MAR-M247

Hot isostatically-pressed, nickel-based superalloy MAR-M247 was tested under fatigue and creep-fatigue conditions. Continuous cycling and cycling with 10min tensile dwells in each cycle were applied in symmetrical strain control cycle with constant total strain amplitude at 900°C in air. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained. The introduction of tensile dwells resulted in lower fatigue life in total strain representation. Surface relief, specimen sections and fracture surface observations revealed crack initiation sites, crack density and crack paths, and help to clarify the effect of dwells on fatigue behaviour.

Notch Behavior of Components Under the Stress-Controlled Creep–Fatigue Condition: Weakening or Strengthening?

Notch-related weakening and strengthening behavior under creep–fatigue conditions was studied in terms of the elastic–viscoplasticity finite-element method (FEM). A coupled damage analysis, i.e., the skeletal point method for creep damage evaluation coupled with the equivalent strain range method for fatigue damage, was employed in the notch effect evaluation. The results revealed that, under the short holding time condition, a weakening behavior was observed for the notch, while a strengthening effect was detected with the increase of holding time. The difference could be ascribed to the creep damage contribution in the holding stage. The influence of stress concentration factor (SCF), stress ratio, and the maximum stress was strongly dependent on the competition of creep and fatigue mechanism.

Influence of coarsened and rafted microstructures on the thermomechanical fatigue of a Ni-base superalloy

The aging of the microstructure of Ni-base superalloys during service is primarily characterized by coarsening and rafting of the γ′ precipitates. The influence of these different aged microstructures on thermomechanical fatigue (TMF) under either continuously cycled (CC) and creep-fatigue (CF) was investigated. Three different aged microstructures, generated through accelerated aging and pre-creep treatments, were studied: stress-free coarsened γ′, rafted with orientation perpendicular to loading direction (N-raft), and rafted with orientation parallel to loading direction (P-raft). Under most conditions, the aged microstructures were less resistant to TMF than the virgin microstructure; however, there were exceptions. Both stress-free coarsened and N-raft microstructures resulted in a reduction in TMF life under both CC and CF conditions in comparison to the virgin material. P-raft microstructure also resulted in reduction in TMF life under CC conditions; however, an increase in life over that of the virgin material was observed under CF conditions. These differences are discussed and hypothesized to be related to the interactions of the dislocations in the γ channels with γ′ precipitates.

Analysis of damage behavior based on EBSD method and the law of fracture life under creep–fatigue conditions for the polycrystalline nickel-base superalloy

Analysis of damage behavior based on EBSD method and the law of fracture life under creep–fatigue conditions for the polycrystalline nickel-base superalloy

Characterisation of crack growth behaviour for W-added 12%Cr ferritic heat resistant steel under the creep-fatigue interaction conditions

The W-added 12%Cr ferritic heat resistant steel has been developed as a turbine rotor material which is used under high temperature creep-fatigue conditions. In this study, crack growth tests using C(T) specimen under the creep-fatigue interaction conditions were conducted under various conditions of stress holding time, applied stress and temperatures. As a result, the effect of load frequency on crack growth behaviour and its life for this steel was characterised by the three-dimensional curved surface based on the concept of non-equilibrium science. Furthermore, a predictive law of crack growth life under the creep-fatigue interaction conditions was derived based on the concept of Q* with the transition function of crack growth life from fatigue to creep.

Non-destructive prediction method of creep damage and remnant life related to those under creep–fatigue interactive conditions for nickel base superalloys

In order to clarify the microdamage formation behaviour for high temperature brittle IN100 alloy, a polycrystalline nickel base superalloy, in situ observational test was carried out to observe creep damage formation behaviour of a notched IN100 specimen. Furthermore, the experimental findings of creep damage formation were evaluated by an original elastic–plastic creep finite element analysis. As a result, microcracks and damage forming areas were found to develop in the creep damage areas suggested by this analysis. This has resulted in the development of a non-destructive prediction method for creep damage accumulation and material’s remnant life. The application of this method to the cases under creep–fatigue conditions was addressed.

Small crack propagation behaviour during thermo-mechanical creep-fatigue loading of Ni based superalloy specimen under non-uniformly distributed temperature condition

A new testing system has been developed to get a basic understanding on the thermo-mechanical creep-fatigue (TMCF) small crack propagation behaviour around the cooling holes in high temperature Ni base superalloy used for advanced gas turbine components. By means of the test system the TMCF tests under creep-fatigue condition were carried out for a directionally solidified Ni base superalloy hollow specimen with nine cooling holes which were machined in a 3×3 square array within the specimen gauge length, simulating the cooling system in air cooled gas turbine blades. Special attention was paid to the early growth of creep-fatigue crack(s) around the holes. Some fracture mechanics discussions are made to interpret the experimental results and the effect of cooling system on the TMCF failure, taking account of the roles of stress concentration and the local thermal stress induced under a significant temperature gradient.

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.

J0310202 環状切欠き試験片を用いた応力およびひずみ制御クリープ・疲労条件下での切り欠き近傍における微視損傷解析

J0310202 環状切欠き試験片を用いた応力およびひずみ制御クリープ・疲労条件下での切り欠き近傍における微視損傷解析

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.

Creep–fatigue crack growth behaviour based on EBSD observations for notched specimen made of directionally solidified Ni-base superalloy

In order to characterise the creep–fatigue crack growth behaviour of a directionally solidified Ni-base superalloy, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method using notched specimens has been carried out. The average misorientation in the vicinity of notches increases linearly up to the initiation of cracks with the increasing creep strains under creep or creep–fatigue conditions, whereas under fatigue conditions fatigue cracks grew without an increase in misorientation. The stress holding time clearly influenced the growth behaviour of creep–fatigue cracks and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation and the relative notch opening displacement was independent of test conditions such as temperatures, stresses and stress wave forms.It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiation of creep–fatigue cracks and their growth behaviours.