Creep Strain Data Research Articles

Creep strain prediction in power plant material via ANN modelling of nonlinear ultrasonic test results

ABSTRACT Reliable and accurate prediction of the creep life of power plant components is crucial for both economic and safety reasons. Existing prediction models, based on creep test data, can be complex and time-consuming. Nonlinear ultrasonic (NLU) is a widely-accepted non-destructive testing (NDT) technique for evaluating damage progression in crept specimens. The information from NLU measurements alone is insufficient to forecast the life of any component. In real-time applications, intelligent NDT protocols are needed to enable fast and accurate life prediction of such components. A methodology for creep life prediction using artificial neural networks (ANN) has been introduced based on NLU test results of crept P92 steel specimens. The technique involved creep tests of P92 specimens exposed to a temperature of 625⁰C with applied stress ranging from 120MPa to 160MPa, NLU measurements at each step load, and prediction of creep life of the material with a ANN trained with creep strain and NLU test data. The technique involves prediction from previously generated historical data, thus saving both cost and time of conducting continuous experiments. This approach for ANN modeling of NLU data can be considered a reliable, time-saving, and effective technique for assessing creep damage progression in power plant components.

Basic Analytical Modeling of Creep Strain Curves.

Creep strain versus time curves (creep curves) have traditionally been described with the help of empirical models where a number of adjustable parameters are involved. These models are simple to use, but they cannot be applied for prediction. For understanding the general behavior of primary and tertiary creep, they are still useful. In fact, the phi model can represent primary creep, and the Omega model tertiary creep for a number of materials. However, in recent years, basic analytical models have been formulated that can predict and describe creep strain data without using fitting parameters. In the paper, a review of these models is given. A number of applications of the models are also given. It is demonstrated that the basic models can quantitatively predict observations. They also provide derivations of some empirical findings.

Modelling and extrapolation of creep strain data from a P-NID rupture model and a characteristic strain relationship

This article demonstrates a model for creep strain that requires only a Parametric Numerical Isothermal Datum (P-NID) model for the stress dependence of rupture life and a Characteristic Strain model established from creep strain data. The combination of these models is shown to replicate strain and strain rate data for a modified 9Cr1Mo steel satisfactorily and compares favourably to a recently published alternative. Creep strain models are discussed in the context of their relevance to the design of high-temperature plant.

Creep life estimation of a service‐exposed P91 cross‐weld joint based on a primary‐secondary creep deformation model

AbstractCreep deformation and rupture behavior of welded joints from service‐exposed P91 steel were investigated by creep tests and microstructure examination. A series of miniature tensile creep tests were conducted at 569°C under applied stresses ranging from 182.5 to 286 MPa for the heat‐affected zone (HAZ), base metal, and weld metal. Creep deformation behavior was modeled using the experimentally measured creep strain data as the primary creep‐secondary creep (PC‐SC) model and fitting closely with the measured creep curves of each metal. Power‐law type creep equations were employed to describe the primary and secondary creep behavior. The creep rupture life of the cross‐weld specimen was predicted using the PC‐SC model with HAZ properties. An accurate prediction was possible if the stress in estimation was increased to 1.14 times of the actual applied stress. The creep test results with the cross‐weld specimens were used for verification. Applying the Monkman–Grant relationship and the creep damage tolerance factors to the cross‐weld joint life prediction are also discussed.

Determination of Creep Behavior of Concrete Beams Made with Rice Husk Ash

Background: Creep in concrete is a long-term deformation under sustained loading. It is influenced by many factors, including constituent materials, environmental conditions, among others. Whenever there is an alteration in the convectional concrete preparation process, the creep characteristics need to be realistically assessed. In the present construction, rice husk ash has been used for partial replacement of cement in concrete production. This is because its properties of both tensile and compressive strength in concrete have been tested and found comparable with plain concrete. However, durability characteristics such as creep, which take place in the long run, have not been realistically assessed. Therefore, it is important to study the creep of rice husk ash concrete, which will further help in the development of a creep prediction model for such concrete for use by design engineers. Objectives: Rice husk ash was used as supplementary cementitious material in concrete, and the creep behavior was studied with the aim of producing a creep prediction model for this concrete. Methods: The cement was replaced with 10% of rice husk ash in concrete with a design strength of 30MPA. Reinforced concrete beams were cast and loaded for flexural creep 35 days after casting. The loading level was 25% of the beam’s strength at the time of loading. The creep observation was done for 60 days. The rice husk used was obtained locally from Mwea irrigation scheme in Kenya. The experiments were carried out in our school laboratory at Jomo Kenyatta university of Agriculture and Technology. Results: The creep strain data of rice husk ash concrete beams was obtained with the highest value of 620 micro strain for 60 days. The results were used to develop a creep prediction model for this concrete. Conclusion: A creep prediction model for rice husk ash concrete has been developed, which can be adopted by engineers for class 30 of concrete containing rice husk ash at a 10% replacement level.

Characterization of heterogeneous creep deformation in vanadium-modified 2.25Cr1Mo steel weldments by digital image correlation

Low alloy ferritic steels are widely used to manufacture critical components that operating at elevated temperatures in the process industry, but a better understanding of the creep responses of their weldments are still needed to provide guidance for the design of high-temperature welded structures. In the present paper, the heterogeneous creep behavior of vanadium-modified 2.25Cr1Mo ferritic steel weldments was investigated by using the digital image correlation (DIC) technique. Three-dimensional plot of creep strain across the weld joint was constructed to intuitively represent the spatio-temporal evolution of creep strain for the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). Combining the analysis of strain contour evolution during creep with the microstructural examination, it is found that creep strain was primarily concentrated in the HAZ, resulting in the creep cavitation at grain boundaries of the coarse-grain bainitic HAZ. In addition, BM exhibited a faster deformation rate than WM during creep. In order to quantitatively describe the heterogeneous creep deformation, a sub-region extensometer method based on the construction of virtual extensometers was proposed to obtain the representative creep strain data of individual regions of weldments. Furthermore, a parameter identification procedure combining a global optimization method (i.e. genetic algorithm) with an initial value determination scheme, was developed to determine the model parameters of BM, WM and HAZ for the hyperbolic-sine creep constitutive model. The results show that the heterogenous creep behavior of the V-mod 2.25Cr1Mo steel weldment can be well characterized by combining the sub-region extensometer method, the hyperbolic-sine creep constitutive model and its genetic algorithm-based parameter identification method.

On the Interrupted Creep Test Under Low Stress Levels for the Power Law Parameter Extraction

Abstract Low stress interrupted creep test, as an interim compromise, can provide essential data for creep deformation design. However, there are no clear guidelines on the characterization of the terminating time for interrupted low-stress creep test. To obtain a suitable terminating time in terms of economy and effectiveness, long-term creep strain data of 9%Cr steels are collected from literatures and their creep deformation characterization is analyzed. First, the variations of normalized time and strain of each creep stage with the stress level are discussed. Then, the effect of the terminating time on final fitted results of Norton–Bailey equation is estimated. Third, the relationship between demarcation points at different creep stages and minimum/steady-state creep rate is analyzed. The results indicate that when the creep rupture life is considered as an important factor for creep design, the tertiary creep stage is of greatest significance due to the largest life fraction and creep strain fraction at low stress level. However, the primary and secondary creep stages are of great significance for design due to their larger contribution to 1% limited creep strain. And the long-term secondary creep data could be extrapolated by combining the primary creep strain data obtained from interrupted creep tests with the time to onset of tertiary creep derived from a similar Monkman–Grant relationship.

Research on creep constitutive model of steel cables

Creep and stress relaxation of steel cables caused by long-term loads will affect the mechanical behavior of tensile structures. A creep experiment of small-diameter steel cables was conducted at normal temperatures to determine the creep constitutive model of the cables using the time–creep strain data for different prestress levels. The viscoelastic Kelvin chain model was used to fit the data, and the creep constitutive equation of the cable was determined. An iterative expression suitable for ANSYS finite element analysis was derived, and the constitutive model was validated with MATLAB programs. The creep constitutive equation was used for the time-dependent analysis of a cable net structure. The results showed that creep caused stress relaxation of the cable net and caused structural element displacements.

Prediction of the non-isothermal creep strain of a glassy polymer on the basis of dynamic analysis results

In the present work, the non-isothermal creep response of a glassy polymer was experimentally studied and analyzed theoretically by a viscoelastic model, introduced in previous works. The main concept of this research is the prediction of the non-isothermal creep strain data on the basis of independent experimental results, namely dynamic mechanical analysis and stress–strain results at various temperatures. Two different approaches regarding the non-isothermal constitutive equation were studied, i.e., one established in the literature and the second one based on the assumption that the temperature dependence can be taken into account by utilizing the concept of the reduced time. A quite successful prediction of the non-isothermal creep strain has been achieved.

Experimental Investigation of the Creep Behaviour of Coal by Monitoring Changes of Acoustic Properties

Time-dependent deformation of coal is a major concern for deep coal mining. When experiencing high stress conditions, the enhanced time-dependent deformation of coal will become a very possible factor to trigger the violent failure of underground openings. For this study, the inner structural variation of an extremely inhomogeneous coal subjected to incremental creep stress has been experimentally investigated by monitoring both the macroscopic deformation and acoustic property. Results show the following: (1) The pronounced inhomogeneity of coal induces considerable irregularities in creep strain data collected during the increase of stress level. These flawed data highlight the shortcomings of conventional strain-time relationship experiments for deciphering the mechanisms that control creep; (2) The mechanical response of samples under creep compression is the result of competition between compaction and cracking. Cracking deteriorates the structure and increases the ultrasonic attenuation, whilst the compaction causes the densification along the axial axis and reduces attenuation; (3) Long-term creep involves multiple structural deformations that alter the internal structure of the coal. This results in fluctuations of the ultrasonic velocities and their dynamic moduli; (4) During uploading phase of short term creep, vertical cracks develop and transform the sample into a structure consisting of separated columns. Further deterioration of the coal increases the possibility of tensile and shear failure of individual columns, which can trigger the violent collapse of an already fragile structure.

Prediction of creep curve of HP40Nb steel using artificial neural network

Simulation of creep curves using data obtained from a limited number of short-time creep tests is helpful for predicting the long-time creep life of materials by extrapolation techniques. The present paper demonstrates the application of artificial neural network (ANN) for the prediction of creep curves for HP40Nb micro-alloyed steel. The network consists of stress, temperature and time as the input parameters and the creep strain as the output parameter. The data used are taken from accelerated creep tests carried out at constant temperatures in the range 650–1050 °C and constant stresses 47–120 MPa. The network was trained using a three-layer feed-forward back-propagation network, having a 15-neuron hidden layer, using the Levenberg–Marquardt optimization algorithm. After successful network training, the model was subjected to several tests to demonstrate consistent prediction capability. 98% of the creep strain data could be predicted within an error of ±10% deviation from the experimental values. An additional experiment carried out to check the prediction capability of the model confirms very good prediction capability, with a correlation coefficient of 0.994, by ANN modeling.

Defining a negligible creep temperature curve for Gr. 91 steel

Time independent rules for design can be applied if the design temperature of a component is restricted below the negligible creep temperature (TNEC). The TNEC curve describes the time and temperature limits below which accumulated creep strain and damage is insignificant at a specified reference or design stress. For the ferritic/martensitic steel Gr. 91 (X10CrMoVNb9-1) there is currently no TNEC curve available in the nuclear design code RCC-MRx. In this paper the TNEC curve for Gr.91 is defined using a new methodology developed in the European FP 7 project MATTER. The TNEC curve is based on time to 0.2% creep strain data. Supporting test data from low temperature (375–500 °C) creep and tensile tests are presented and assessed. The TNEC curve can alternatively be constructed by using time factor corrected creep rupture data. These time factors can be used for evaluating the safety against significant creep at both reference and design stress. The presented methodology is will be used for determination of TNEC curves for the next revision of the EN-13445.

The Pin-Loaded Small One-Bar Specimen in Use to Determine Uniaxial and Multiaxial Creep Data

AbstractTwo novel small specimen creep testing techniques are presented in this paper. The pin-loaded small one-bar specimen (OBS) and the small notched specimen with four loading pins (SNS4) are designed to determine the remaining lifetime for the high-temperature components. The small OBS is suitable for use in obtaining both uniaxial creep strain and creep rupture life data and the SNS4 is designed to obtain the multiaxial behaviour using small material samples. The specimens can be made from small material samples removed from the component surface or from the heat-affected zone. The specimens can be loaded through pin connections for testing. A conversion relationship and conversion factor have been obtained and used to convert the OBS creep data to the corresponding uniaxial data. For validation two materials have been used, P92 and P91 steels at 650°C. The advantages of these testing techniques are highlighted; the recommendations for future research are also given.

Creep Behavior, Deformation Mechanisms, and Creep Life of Mod.9Cr-1Mo Steel

The creep behavior, deformation mechanisms, and the correlation between creep deformation parameters and creep life have been investigated for Mod.9Cr-1Mo steel (Gr.91, 9Cr-1Mo-VNb) by analyzing creep strain data at 723 K to 998 K (450 °C to 725 °C), 40 to 450 MPa, and t r = 11.4 to 68,755 hours in NIMS Creep Data Sheet. The time to rupture t r is reasonably correlated with the minimum creep rate $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ and the acceleration of creep rate by strain in the acceleration region dln $$ {\dot{\varepsilon }} $$ /d e, as t r = 1.5/[ $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ (dln $$ {\dot{\varepsilon }} $$ /d e)], where $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ and dln $$ {\dot{\varepsilon }} $$ /d e reflect the creep behavior in the transient and acceleration regions, respectively. The $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ is inversely proportional to the time to minimum creep rate t m, while it is proportional to the strain to minimum creep rate e m, as $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ = 0.54 (e m/t m). The e m decreases with decreasing stress, suggesting that the creep deformation in the transient region becomes localized in the vicinity of prior austenite grain boundaries with decreasing stress. The duration of acceleration region is proportional to the duration of transient region, while the dln $$ {\dot{\varepsilon }} $$ /d e is inversely proportional to the e m. The t r is also correlated with the t m, as t r = g t m, where g is a constant. The present creep life equations reasonably predict the degradation in creep rupture strength at long times. The downward deviation takes place in the t r vs $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ curves (Monkman–Grant plot). At the same $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ , both the e m and t m change upon the condition of t m ∝ e m. The decrease in e m with decreasing stress, corresponding to decreasing $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ , causes a decrease in t m, indicating the downward deviation of the t r vs $$ {\dot{\varepsilon }}_{ \hbox{min} } $$ curves.

3D CFD/FEM analysis of thermomechanical long-term behaviour in SOFCs: Furnace operation with different fuel gases

Full commercialisation of the solid oxide fuel cell (SOFC) technology is limited by technological challenges, in particular due to the thermomechanical reliability of the components. The long-term thermomechanical behaviour is especially affected by the operation environment. Most of the research and development studies, as well as laboratory studies are performed within furnaces rather than operating in a real system environment. However, the thermomechanical response of fuel cells operating in the furnace, especially operating with different fuel gas compositions is not fully understood, yet. The present study investigates the thermomechanical comparison of H2, 77% CH4 and 38% CH4 operation. A three dimensional coupled CFD/FEM sub-model, derived from a whole stack model and experimentally determined data is used. The analyses account both for the thermo fluid flow and the nonlinear elastoplastic, creep strain material behaviour. The thermomechanical behavior of the most favourable operation condition has been analysed for 1200 h operation time. The results are validated qualitatively and quantitatively using post-mortem images and creep strain data, respectively.

Creep behavior of starch-based nanocomposite films with cellulose nanofibrils.

Nanocomposite films were successfully prepared by incorporating cellulose nanofibrils (CNFs) from sugar beet pulp into plasticized starch (PS) at CNFs concentration of 5-20%. The storage (G') and loss (G″) moduli, creep and creep-recovery behavior of these films were studied. The creep behavior of these films at long time frame was studied using time-temperature superposition (TTS). The CNFs were uniformly distributed within these films up to 15% of CNFs. The PS-only and the PS/CNFs nanocomposite films exhibited dominant elastic behavior. The incorporation of CNFs increased both the G' and G″. The CNFs improved the creep resistance and reduced the creep recovery rate of the PS/CNFs nanocomposite films. TTS method was successfully used to predict the creep behavior of these films at longer time frame. Power law and Burgers model were capable (R(2)>0.98) of fitting experimental G' versus angular frequency and creep strain versus time data, respectively.

The potential for major extrapolation of creep rupture and creep strain data

A set of creep rupture data, for which two independent investigators have previously published models, is re-examined using the recently proposed parametric numerical isothermal datum (P-NID) approach to modelling. It is shown that neither of the existing models can be presumed reliable in extrapolation. In contrast, P-NID models are found to represent the data with much greater accuracy and to satisfy appropriate criteria for reliability in extrapolation. According to the principles of this approach, the close correspondence between the P-NID models and the data justifies extrapolation in rupture time at mid-range and lower test temperatures in excess of 15-fold. This expectation is confirmed by excluding from the analysis all data at more than 7% of the longest known rupture life, then comparing the resultant extrapolation to the excluded data. The success of the procedure suggests a potential for greatly reducing the extent of rupture testing necessary to produce reliable extrapolation to times of practical interest. This is demonstrated for a set of data including simulated results at intermediate temperatures. The study shows a potential reduction to less than one-tenth of the aggregate test time conventionally considered necessary. The analysis of further sets of rupture data shows how a P-NID model may reliably be extrapolated even when the material behaviour necessitates a model partitioned according to temperature range, or exhibits instability. Limited investigation indicates that, for a given material, creep strain data are consistent with parametric models for rupture data and may be extrapolated over similar intervals.

Effect of creep constitutive equation on simulated stress mitigation behavior of alloy steel pipe during post-weld heat treatment

Post-weld heat treatment (PWHT) is usually conducted to reduce the residual stress and to improve the mechanical properties of the welded region. The thermal elasto-plastic creep analysis is used to estimate the mitigation of the residual stress in the welded region analytically. The stress mitigation is caused by effect of the creep relaxation behavior during PWHT. Thus, creep constitutive equation is used to estimate the residual stress mitigation. The material properties of Norton's law and the Norton-Bailey law for alloy steel pipe JIS STPA23 (equivalent to ASME SA335P11 seamless steel tubes) were experimentally investigated. Creep tests were conducted at 400, 500, 600, and 700 °C. Specimens were subjected to certain stresses, and creep strain was measured. The material properties of Norton's law and the Norton-Bailey law were calculated from the measured creep strain data. Five bead-on curved plate specimens were fabricated in order to verify the thermal elasto-plastic creep analysis done with these creep constitutive equations. The curved plate was made from a pipe cut in the hoop direction. The specimens were welded without filler metal by gas tungsten arc weld (GTAW) on both surfaces in the center of the curved plate. The residual stress in one specimen was measured without PWHT. The other specimens were measured under different PWHT conditions. The results of analysis with the two creep constitutive equations agreed well with the experimentally measured results. In addition, no differences in the creep constitutive equations were observed at high PWHT temperature. Accordingly, these analyses conducted with both types of creep constitutive equation were verified to be effective. The effect of creep constitutive equation on simulated stress mitigation behavior during PWHT has been clarified for analytically estimating residual stress in the welded region.

Analysis and Design of a Small, Two-Bar Creep Test Specimen

In this paper, a new small-sized (two-bar) specimen type, which is suitable for use in obtaining both uniaxial creep strain and creep rupture life data, is described. The specimen has a simple geometry and can be conveniently machined and loaded (through pin-connections) for testing. Conversion relationships between the applied load and the corresponding uniaxial stress, and between the measured load-line deformations and the corresponding uniaxial minimum creep strain rate, have been obtained, based on the reference stress method (RSM), in conjunction with finite element analyses. Using finite element analyses the effects of the specimen dimensions on reference stress parameters have been investigated. On this basis, specimen dimension ratio ranges are recommended. The effects of friction, between the loading pins and the specimen surfaces, on the specimen failure time, are also investigated. Test results obtained from two-bar specimen tests and from corresponding uniaxial specimen tests, for a P91 steel at 600 °C, are used to validate the test method. These results demonstrated that the specimen type is capable of producing full uniaxial creep strain curves. The advantages of this new, small, creep test specimen, for determining uniaxial creep data, are discussed and recommendations for future research are given.

3D thermomechanical behaviour of solid oxide fuel cells operating in different environments

Hermetic sealing and long-term structural reliability of fuel cell stacks depend strongly on the thermomechanically induced stress–strain behaviour. These are especially affected by the environment; the fuel cell is operating in. Most of the research and development studies, as well as laboratory studies are conducted within electrically heated furnaces rather than operating in an insulated system environment. The thermomechanical comparison of them is not fully understood, yet. The present study utilises a previously developed full scale three dimensional planar type 6-cell SOFC short stack model to shed light on the thermomechanical response of high temperature fuel cells operating in system and furnace environments. The physically resolved coupled computational fluid dynamics and computational structural mechanics model has been improved, accounting for the rate dependent creep strain, as well as including the furnace domain and thermal radiation to fully describe the thermal and deformation behaviour of the stack. The non-linear elastoplastic behaviour of the metal components as a function of temperature is considered. The results are validated using creep strain data from the literature and in-house post-mortem images. The study gives an insight about the critical regions prone to failure due to creep strain operating in different environments and the long-term fuel cell behaviour. Moreover, the critical locations appear to be prone to high creep strain after 1000 h operation time.