Creep Test Specimens Research Articles

Analysis of shrinkage and creep effect for bridge widening based on TB-FEM method and experimental research

Based on TB-FEM method and laboratory test, the influence of shrinkage and creep on the structure of widened bridge is studied. The shrinkage test uses C50 concrete and C50 double-mixture (steel polyester fiber) concrete to prepare the standard prism specimen, and the creep test uses the 15m hollow slab which exactly has the same material, size, prestress and construction method of a real bridge as the creep test specimen. The strain-time curve obtained by shrinkage and creep is used to define the shrinkage and creep characteristics of the material, and Trost-Bazant is combined with finite element method to analyze the mechanical response of the new and old bridge splicing structure caused by shrinkage and creep. The result shows that under the condition of rigid splicing, the new bridge should be supported by the simply supported structure, and it is not suitable to use the same simply supported and continuous hollow slab as the original bridge. Theoretical analysis and example study show that combination of laboratory test and TB-FEM method can reasonably analyze the effect of shrinkage and creep on bridge widening, and provide a theoretical basis for the type selection of widening structure.

Notched Creep Damage Assessment Based on EBSD Observation for Austenitic Stainless Steel

Creep damage processes for smooth and notched specimen of austenitic stainless steel through interrupted creep tests using multiple specimens. The material used was 18-8 stainless steel for boiler tube use. The mid-sections of interrupted creep test specimens were observed through SEM(Scanning Electron Microscope) instrumented with EBSD(Electron BackScatter Diffraction patter) equipment. IPF(Inverse Pole Figure) maps, Phase maps and GOS(Grain Orientation Spread) maps were used for investigating creep damage process. For smooth specimen, the relationship between macroscopic creep time fraction and GOS averaged for all pixels showed linearity, while the relationship between creep strain and the averaged GOS showed non-linearity regressed by Green function successfully. For notched specimen, the EBSD maps became noisy possibly due to extensive phase transformation under highly concentrated notch stress. Obtained GOS data for gamma phase only showed non-monotonic change with time and nominal strain. The evaluated local strains in the vicinity of the notch showed relatively small amount, which might cause the very long creep life compared with smooth specimen under the same nominal stress condition.

Small ring testing of high temperature materials

In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under high temperature in industry.

Effect of stress regime‐dependent creep behaviour on measurement of creep strain rate based on small specimen techniques

AbstractSmall specimen creep testing technique has become a hot topic of research as bulk materials are not available in many occasions. The stress distributions in the small specimens such as small punch and 3‐point bending specimens are essentially nonuniform. As it is known, the creep deformation/damage accumulation mechanisms exhibited at a high stress regime are not the same as the ones at a lower stress regime for many engineering alloys. The potential measurement errors because of stress regime‐dependent creep behaviour, however, has not been considered in the determination of the creep parameters based on small specimen testing in the previous studies. In this paper, 2 kinds of materials that show different Norton's parameters at the corresponding stress regimes are examined. A simple case of 2‐bar structure is firstly adopted to illustrate the measurement error of creep strain rate because of stress regime‐dependent creep behaviour. Furthermore, clamped beam bending testing and small punch testing are investigated to demonstrate the significance of measurement error using the same materials. It is shown that an error of more than 8 times may occur near the transition point of creep deformation mechanism depending on specimen types and materials. Attention should thus be paid to the selection of stress level in the small specimen testing to avoid significant measurement errors.

Correlation and capability of using site inspection data and small specimen creep testing for a service-exposed CrMoV pipe section

ABSTRACTThis paper presents for the first time an investigation on the creep damage evolution of an ex-service CrMoV pipe section through impression creep test (ICT) and metallurgical inspection data. The study emphasises the importance of correlating the operating conditions (temperature and stress) of power plant components with the results from metallurgical examinations and small specimen creep tests. The paper seeks for a correlation among micro- and macro-hardness measurements, surface replicas data and minimum creep strain rates (obtained by ICTs) of the parent material of the pipe section. Also, optical and scanning electron microscope (SEM) micrographs have been used to assess possible metallurgical differences through the thickness of the pipe section. This investigation shows how miniature creep test specimen data could be practically used in a holistic approach for the evaluation of life consumption of power plant components and concludes that the studied parent material could have been retired from service too early.

An F82H steel pressurized tube creep capsule for irradiation in HFIR

A novel capsule design has been developed for measurement of irradiation creep in pressurized tubes and is being used to irradiate reduced activation ferritic/martensitic F82H steel creep test specimens. These tests are being conducted in the flux trap of the High Flux Isotope Reactor at the Oak Ridge National Laboratory. The capsule design uses a tight-fitting corrugated aluminum foil placed in the center of a vanadium alloy holder to conduct heat from the centrally located test specimen. The foil acts as a compressible thermal interface between the pressurized tube and holder, maintaining a constant thermal resistance (and thus a constant tube temperature gradient) during irradiation, regardless of differential thermal expansion, creep, and swelling in the test specimen. Mechanical interference with creep deformation of the specimen tube is minimized by using a thin (0.05 mm) foil with sufficient room to crush. Finite element analysis of the contact pressure between the specimen and foil, combined with thermal creep in the foil, showed little interference with specimen stress conditions. Specimens were designed to experience hoop stresses of 380, 300, 150, and 0 MPa at a temperature of 300 °C while being irradiated to a dose of 3.7 dpa. Passive SiC thermometry is located within the pressurized tube and the holder material for confirmation of experiment irradiation target temperatures. This work discusses aspects of the capsule's fabrication and design, including thermal models of the capsule during irradiation.

Influence of specimen thickness on the creep behavior of a directional solidification nickel-based superalloy

The effects of thickness and surface signature on the creep life of a directionally solidified nickel based superalloy were investigated. The creep tests of ground specimens in the thickness of 0.61 mm and 0.89 mm, as well as polished ones with 0.66 mm and 0.87 mm in thickness were performed at 980 °C/160 MPa. Based on the experimental results, the thin ones had inferior creep life compared with thick specimens. XRD exhibited that all the specimens were oxidized during the creep tests, however, the non-loading bear zones which caused by oxidation was insufficient to explain the creep life of specimens under different wall thicknesses. On the other hand, the morphology of fracture sections indicated that the interaction of cracks and creep damage could reveal this phenomena. It worth noting that the cracks played a more important role in the creeping of thin specimens, but creep damage influenced the thick specimens more effectly. In addition, the polished specimens had a better behavior during creep tests than the ground ones.

Impression creep test of a P91 steel: a round robin programme

The process of standardisation of small specimen creep testing techniques, specifically the impression creep test requires the repeatability of the test method. In this study it is accomplished through a round robin programme involving four different labs which have slightly different test set-ups adhering to predefined recommendations stated in previous work. The labs all conducted the same stepped stress test on a reference heat of grade 91 power plant steel and the displacement traces of the tests are analysed to outline the effects of different test set-ups and their efficacies. Main differences are in temperature control and loading application and control.

The role of small specimen creep testing within a life assessment framework for high temperature power plant

ABSTRACTThe safe operation of ageing materials and structures operating at high temperature and pressure is challenging. This paper, for the first time, clearly addresses the role of small specimen creep testing methods within a practical life assessment framework, with a case study used to illustrate the main principles and applications. To enhance the current practice for assessing the condition of creep ageing components, more proactive use of small specimen testing methods is proposed for the in-service condition assessment of power plant materials, earlier in the plant lifecycle and within a holistic life assessment framework. The paper describes how small specimen creep testing methods and other complementary tools can be used together in a new structured approach to life management. There is a clear need to provide the plant owner with more reliable and effective life prediction tools, based on earlier and more rigorous assessment of in-service life consumption. A novel, holistic, life assessment methodology including the use of small specimen creep testing has been developed by the authors.

Experimental and Numerical Analysis of Initial Plasticity in P91 Steel Small Punch Creep Samples

To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret. This paper presents a novel numerical model able to generate results that match the experimental findings. For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 ∘C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen. Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data. The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model. Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected. The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a ’state of the art’ creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture. The new modelling method will potentially lead to improved capability for SPCT data interpretation.

Monitoring creep damage at a weld using a potential drop technique

Welds of high-temperature pressurised components act as preferential sites for creep damage accumulation and will frequently be the life limiting feature. Type IV failures, where cracking occurs between the heat affected zone (HAZ) and parent material, are of particular concern in power station components. Typically damage will be characterised by locally high strain rates in the HAZ and local grain boundary separation leading to crack initiation and growth. A potential drop creep monitoring technique is suggested and demonstrated on cross-weld creep test specimens to be sensitive to both strain and cracking. The technique can be implemented as an array, discretizing a larger area into local measurements, providing contextual information and enabling the identification of local increases in damage. The influence of strain and cracking can be effectively separated to indicate crack initiation. The proposed sensor has very simple and robust high-temperature hardware lending itself to in-situ measurements in the field.

Determination of Creep Damage Properties from a Miniature Three Point Bending Specimen Using an Inverse Approach

In this work, a novel method for determining the creep damage properties which can be used to represent the full life until failure from a miniature bending creep test specimen is developed based on a mathematical analysis and an inverse approach. Using the Kachanov-Rabotnov creep damage model, a mathematical expression for the deflection of a simply supported, rectangular miniature thin beam creep test specimen, under three-point bending (TPB), is derived. The outputs of these equations are iterated numerically using a MATLAB program. The time-dependent deflection curves are computed as the virtue TPB tests at various loads. The accuracy of the mathematical solutions is evaluated by the corresponding results obtained from finite element analysis. On this basis, an inverse method is then developed to obtain the creep and damage constants using a MATLAB optimisation scheme, where the primary creep is neglected. The results obtained for a power plant Grade 91 Cr steel is used for demonstration. The inverse approach developed has potential applications for assessing the high temperature material strength as part of a NDT procedure and for deriving the full life creep damage constitutive properties from a small volume of material, in particular, for various microstructure regions within a heat-affected zone of weldments, e.g. of power plant pipelines and aero-engine components.

Small Ring Testing of High Temperature Materials

In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under high temperature in industry.

Effects of carnallite content on mechanical properties of Maha Sarakham rock salt

Mechanical characterization tests are performed to determine the effects of carnallite contents on the strength, elasticity, and time-dependent parameters of rock salt specimens obtained from the Lower Member of the Maha Sarakham formation. The specimens are prepared with carnallite content (C %) varying from 0% (pure halite) to 100% (pure carnallite). The compressive and tensile strengths and elastic moduli of the specimens exponentially decrease with increasing C %. Specimens with higher C % tend to dilate more than those with lower C %, as evidenced by the increasing of the Poisson’s ratio. The strength reduction due to the carnallite content decreases as the confining pressures increase. The elastic, visco-elastic, and visco-plastic parameters of the creep test specimens are defined as a function of C %. They rapidly decrease with increasing C %. Pure halite tends to behave as the Burgers material while pure carnallite behaves as the Maxwell material. The different creep rates and deformation mechanisms between halite and carnallite explain the occurrences and structures of potash ore deposit at the shelves and flanks of the salt domes and anticlines in the salt basin.

Influence of Hold Time and Stress Ratio on Cyclic Creep Properties Under Controlled Tension Loading Cycles of Grade 91 Steel

Abstract Influences of hold time and stress ratio on cyclic creep properties of Grade 91 steel were systemically investigated using a wide range of cyclic creep tests, which were performed with hold times (HTs) of 1 minute, 3 minutes, 5 minutes, 10 minutes, 20 minutes, and 30 minutes and stress ratios (R) of 0.5, 0.8, 0.85, 0.90, and 0.95 under tension loading cycles at 600°C. Under the influence of HT, the rupture time increased to HT = 5 minutes at R = 0.90 and R = 0.95, but there was no influence at R = 0.50, 0.80, and 0.85. The creep rate was constant regardless of an increase in the HT, except for the case of HT = 5 minutes at R = 0.90 and R = 0.95. Under the influence of stress ratio, the rupture time increased with an increase in the stress ratio, but the creep rate decreased. The cyclic creep led to a reduction in the rupture time and an acceleration in the creep rate compared with the case of monotonic creep. Cyclic creep was found to depend dominantly on the stress ratio rather than on the HT. Fracture surfaces displayed transgranular fractures resulting from microvoid coalescence, and the amount of microvoids increased with an increase in the stress ratio. Enhanced coarsening of the precipitates in the cyclic creep test specimens was found under all conditions.

Creep Evaluation of Traditional and Nuclear Power Plant High-Temperature Components, Using Small Pin-Loaded One-Bar and Two-Bar Specimens

The high-temperature components in thermal power plants are subject to creep deformation as a result of operating at elevated temperature and high steam pressure. Creep is nonlinear deformation leading to rupture and component failure; therefore, it has to be monitored closely, especially when the high-temperature components approach the last stage of their designed lifetime. This paper presents the design and application of two small specimen types that can be used to assess the severity of creep damage in these components as they age. These specimens can be used to assess the creep strength and remaining lifetime of in-service components. Small material samples can be removed safely from operating component surfaces and then be used to manufacture these small specimens. These specimens can be manufactured and tested easily using pin connection. This paper places emphasis on specimen design and loading for creep testing. Two high-temperature materials (P91 and P92 steels) are used to validate the accuracy of the new testing technique. The creep results obtained from these small creep test specimens are compared with results obtained from corresponding uniaxial creep tests. Very good correlation is found between the two sets of results.

Determination of the creep crack initiation properties using pre-cracked small punch tests

The use of pre-cracked small punch creep test specimens (p-SPCT) is analyzed in this paper as an alternative method to obtain the Creep Crack Initiation (CCI) time in those cases where there is not enough material to use CT specimens. The material selected was an AZ31B magnesium alloy. The geometry of the specimens used was 10×10mm square with a thickness of 0.5mm. An initial crack-like notch was created in the SPT specimens by means of a laser micro-cutting technique. Conventional compact specimens (CT-20) were tested at T=150°C as a reference for CCI determination. The definition of an equivalent load for p-SPCT was seen to be a good methodology, and a λ parameter was defined to convert the p-SPCT to an equivalent C(T) load that produces the same CCI time. The influence of temperature and material on the λ parameter still remains to be investigated.

Small Two-Bar Specimen Creep Testing of Grade P91 Steel at 650°C

AbstractCommonly used small creep specimen types, such as ring and impression creep specimens, are capable of providing minimum creep strain rate data from small volumes of material. However, these test types are unable to provide the creep rupture data. In this paper the recently developed two-bar specimen type, which can be used to obtain minimum creep strain rate and creep rupture creep data from small volumes of material, is described. Conversion relationships are used to convert (i) the applied load to the equivalent uniaxial stress, and (ii) the load line deformation rate to the equivalent uniaxial creep strain rate. The effects of the specimen dimension ratios on the conversion factors are also discussed in this paper. This paper also shows comparisons between two-bar specimen creep test data and the corresponding uniaxial creep test data, for grade P91 steel at 650°C.

Ni基合金におけるスモールパンチクリープ試験の荷重/応力換算係数に及ぼす延性の影響

Remaining-life assessment of high temperature components using the small punch (SP) creep testing technique necessitates the evaluation of SP load (F)/uniaxial stress (σ) conversion coefficient, F/σ, obtained by comparing the SP and uniaxial creep test results. In the present study, the SP creep tests were carried out at 850°C on various Ni-base alloys having different rupture elongations in the range of 0.06-0.63 to investigate the influence of ductility on the value of F/σ. The F/σ value was determined for each alloy by correlating SP creep rupture data with corresponding uniaxial creep ones. The experimental results revealed that the F/σ value increased with increasing rupture elongation up to 0.45 and remained almost constant at around 2 with further increase of rupture elongation. This result indicated that the SP creep rupture test result could be converted to that of uniaxial one if the ductility on a given material was available. Moreover, the correlation between the F/σ and the equivalent fracture strain in SP creep test specimen were also analyzed to establish a procedure for estimating the stress equivalent to the SP load from the SP creep test result alone. As the result, the variation of F/σ with the equivalent fracture strain was similar to its variation with the rupture elongation.

Gamma Prime Precipitation, Dislocation Densities, and TiN in Creep-Exposed Inconel 617 Alloy

Inconel 617 is a solid-solution-strengthened Ni-based superalloy with a small amount of gamma prime (γ′) present. Here, samples are examined in the as-received condition and after creep exposure at 923 K (650 °C) for 574 hours and 45,000 hours and at 973 K (700 °C) for 4000 hours. The stress levels are intermediate (estimated, respectively, as of the order of 350, 275, and 200 MPa) and at levels of interest for the future operation of power plant. The hardness of the specimens has been measured in the gage length and the head. TEM thin foils have been obtained to quantify dislocation densities (3.5 × 1013 for the as-received, 5.0 × 1014, 5.9 × 1014, and 3.5 × 1014 lines/m2 for the creep-exposed specimens, respectively). There are no previous data in the literature for dislocation densities in this alloy after creep exposure. There is some evidence from the dislocation densities that for the creep-exposed samples, the higher hardness in the gage length in comparison with the creep test specimen head is due to work hardening rather than any other effect. Carbon replicas have been used to extract gamma prime precipitates. The morphology of γ′ precipitates in the ‘as-received’ condition was spheroidal with an average diameter of 18 nm. The morphology of these particles does not change with creep exposure but the size increases to 30 nm after 574 hours at 923 K (650 °C) but with little coarsening in 45,000 hours. At 973 K (700 °C) 4000 hours, the average gamma prime size is 32 nm. In the TEM images of the replicas, the particles overlap, and therefore, a methodology has been developed to estimate the volume fraction of gamma prime in the alloy given the carbon replica film thickness. The results are 5.8 vol pct in the as-received and then 2.9, 3.2, and 3.4 vol pct, respectively, for the creep-exposed specimens. The results are compared with predictions from thermodynamic analysis given the alloy compositions. Thermodynamic prediction shows that nitrogen content is important in determining the gamma prime volume fraction. This has not previously been identified in the literature. The higher the nitrogen content, the lower the gamma prime volume fraction. This may explain inconsistencies between previous experimental estimates of gamma prime volume fraction in the literature and the results here. The observed decrease in the γ′ volume fraction with creep exposure would correspond to an increase in TiN. At present, there are insufficient experimental data to prove that this predicted relationship occurs in practice. However, it is observed that there is a higher volume fraction of TiN precipitates in the gage length of a creep sample than in the head. This suggests that secondary TiN particles are precipitating at the expense of existing γ′ due to the ingress of N from the atmosphere, possibly via creep cracks penetrating in from the surface of the gage length. This effect is not expected to be observed in real components which are much larger and operate in different atmospheres. However, this highlights the need to be conscious of this possibility when carrying out creep testing.