Conventional Uniaxial Creep Tests Research Articles

Experimental study on uniaxial creep characteristics of sandstone with pre-peak unloading damage

Deep resource extraction has been affected by the complex geological environment of "three highs and one disturbance" for a long time. The surrounding rocks experience strong unloading stress disturbance during the underground resource extraction. The creep characteristics of the perimeter rocks are of great theoretical and practical value. Firstly, the triaxial pre-peak loading and unloading tests on the prepared samples are conducted on the intact rock samples using the TAW-200 rock mechanics test system. Then the rock samples with different degrees of pre-peak unloading damage were prepared under the condition of pre-peak yield. Finally, the uniaxial creep tests were carried out to study the uniaxial creep mechanical properties. The results show that the wave velocities of the damaged rock samples are reduced to different degrees compared with those of the intact rock samples, the creep of the pre-peak unloading damaged rock has a time-dependent damage effect, and the study results are similar to those of the conventional uniaxial creep test of the white sandstone. The instantaneous strain at all stress levels of the damaged rock samples increased gradually with the stress level, which is consistent with the non-linear functional relationship; Based on the time-dependent damage effect, the two-parameter Weibull distribution function was introduced into the West Plains viscoelasticity model, and a creep damage model for white sandstone was proposed. The improved Nishihara model can simulate the uniaxial creep characteristics of each damaged rock sample at various stress levels.

An initial study on the impression creep behaviour of stir-cast GNP reinforced AA6061 composite (AMMC)

Impression creep testing technique is used to screen out creep behaviour of materials as it is relatively faster method, requires small volume of the specimen, and is relatively material non-invasive technique when compared to conventional uniaxial tensile creep test method. This technique also possesses the benefit of maintaining constant stress throughout the experiment because a flat cylindrical indenter is used to apply load on the specimen. Owing to commercial unavailability of the materials under development such as metal matrix composites (MMC), the impression creep testing approach for such materials becomes a best method. In present work, creep behaviour of stir-cast graphene nanoplatelets (GNP) reinforced AA6061 composite (AMMC) is studied by using impression creep tests. The AMMC was prepared by adding GNPs in AA6061 through stir casting process by melting the alloy up to 1073 K. The impression creep experimentation included stress levels of 140 MPa, 170 MPa, and 200 MPa at a temperature of 573 K, 603 K and 633 K. The tests were conducted at constant stress by varying the temperature and vice-versa in vacuum. Two distinct regions i.e., primary and secondary/steady-state were observed and minimum creep strain rate was obtained in steady-state region. Tertiary region was absent, since the load/stress applied was compressive. During the experiment, depth of indentation of 0.03 mm to 0.14 mm was produced in the specimen. The microstructure of the composite has been analyzed using optical microscope and scanning electron microscope (SEM) to correlate with impression creep behaviour of the studied materials.

Assessment of Creep Properties Using Small Punch Test for a 9%Cr-Mo-Co-B Power Plant Steel

The present study provides a feasible method to evaluate creep properties for a 9%Cr-Mo-Co-B power plant steel by comparing two sets of data obtained from small punch tests and conventional uniaxial creep tests. The method includes three steps: firstly, conduct a series of small punch tests and conventional creep tests in different load and temperature conditions; secondly, convert the load and central deflection data obtained from the small punch test to stress and strain data; thirdly, determinate the best fit correlation factor by comparing the two sets of data in selected creep models. It is found that two sets of data show a similar trend in stress–rupture time relation, stress–minimum strain rate relation and LMP–stress relation. The correlation factor, ksp, can effectively bridge the gap between the load in small punch test and the stress in conventional creep test. For a high-Cr martensitic heat-resistant steel named as CB2, the ksp value 1.4 can make a good prediction for rupture time, while for minimum creep rate and the Larson–Miller parameter, the ksp value 1.4 will lead a conservative prediction in the low-stress range.

The creep strength of aberrant grade 91 steel

ABSTRACT The high-temperature steel grade 91 is intended to operate in service with a fully martensitic microstructure, achieved by a well-defined heat treatment. Unfortunately, lack of control, either during the initial material manufacturing or later during post-weld heat treatment, can lead to components entering service in a substandard condition. In the worst cases, this can be a fully ferritic microstructure with a complete absence of martensite. In this aberrant condition, the steel can have a creep strength significantly lower than that of normal martensitic grade 91 with an associated risk of early failure in plant life. This paper reports on investigations into the creep strength of aberrant grade 91,,using both conventional uniaxial rupture testing and small-scale impression creep testing. The creep strength has been found to fall into the range mean-35% to mean-50% in terms of stress relative to the most recent (2019) ECCC rupture life assessment of grade 91.

New flat-punch indentation creep testing approach for characterizing the local creep properties at high temperatures

An indentation creep testing approach has been developed which allows measuring creep properties at high temperatures. In contrast to existing indentation or impression creep experiments, the approach described here allows to achieve a quite high spatial resolution, as flat punch indenters with a diameter of only 20μm are used. First indentation creep tests have been performed on single crystalline nickel and nickel binary solid solution alloys with Re, Ta or W as alloying elements, respectively. The indentation creep tests have been carried out at a temperature of 650°C and stress levels in the range of 85 to 400MPa. Using crystal plasticity finite element modeling, the indentation creep response is converted into equivalent uniaxial creep properties. It is shown that the conversion parameters, evaluated for differently oriented single crystals, can be chosen independently of the creep rate exponent in the power law creep regime. It is found that the indentation creep results agree well with conventional uniaxial creep tests. Furthermore, the results show that Ta is the most effective solid solution strengthener of all tested solid-solution strengtheners at 650°C because of the large atomic size mismatch, followed by W and Re.

Methodology of obtaining intrinsic creep property of concrete by flexural deflection test

The shrinkage of concrete when exposed to external drying influences the accurate quantification of concrete creep. To reduce the effect of shrinkage on the measured creep, particularly for concrete at early ages when the shrinkage is pronounced, a flexural creep test set-up was designed in this study to investigate the creep property of concrete beams with w/c ratios of 0.3 and 0.4 at the age of 7 days. The concrete beams were exposed to both sealed and drying conditions. A sequential coupled hydromechanical finite element analysis was conducted to back-calculate the creep parameters in the original microprestress solidification theory-based creep model from the measured creep-induced deflection. The results show that the flexural creep test is capable of obtaining the intrinsic creep property of concrete without incorporating the shrinkage effect as long as the relative humidity distribution in the concrete beams is symmetric about the neutral plane. The intrinsic creep property depends only on the mixture proportions of concrete, i.e., it is independent of the loading and drying conditions, which can be used to predict the deflection development of concrete beam under other complex drying conditions. The findings in this study provide a new methodology of obtaining concrete creep, which solves the problem of the conventional uniaxial creep testing method that both creep strain and shrinkage strain need to be measured during the creep test.

A two-material miniature specimen test method and the associated inverse approach for high temperature applications

A miniature two-material testing method has been developed for material property characterization. Elastic-plastic and creep damage models have been applied for a semi-analytical model and a finite element (FE) model to simulate the miniature specimen tensile and creep tests on a two-material system. An inverse optimization algorithm has been developed to extract the creep properties of the unknown material in the two-material system from the experimental results. Single-material miniature specimen tensile tests have been performed for an aluminum alloy at room temperature and 400 °C and full stage creep tests have been performed for a P91 steel at 650 °C. The miniature specimen testing results are comparable to the results from the corresponding conventional uniaxial tensile and creep tests on the same materials and under the same test conditions. This miniature specimen testing technique and the developed inverse method have the potential to become a new approach for determining the elastic-plastic and the full-stage creep properties until rupture for a two-material system at high temperature, e.g. a coating-substrate system.

The impression creep Monkman Grant relationship

Impression creep testing is an established small-scale testing technique in which the indentation rate of a rectangular indenter can be converted into equivalent creep strain rate. It is a versatile technique in that, once a stable indentation rate is established, either the stress, temperature, or both, can be varied to provide data under multiple test conditions on the same specimen. It does not however produce a specimen failure. In order to overcome this limitation, use can be made of an empirical relationship between the creep strain rate obtained in the impression test and the rupture life obtained in a conventional uniaxial creep test at the same stress and temperature. This relationship, termed the Impression Monkman Grant relationship, has been applied successfully to grade 91 steel where it has been shown that rupture life predicted from impression testing is in good agreement with actual rupture life obtained by conventional uniaxial testing. The relationship has proved particularly useful for plant application in situations where mis-heat treated grade 91 pipework with lower than expected creep strength has been encountered, requiring an estimate of creep strength to justify continued operation in service.

Correlation Factor Study of Small Punch Creep Test and Its Life Prediction.

A small punch test is one of the innovative methods that can be used to evaluate the properties of a material without destructive harm to the in-service component. Conventionally identifying material properties by a uniaxial test is widely applied to engineering. How the properties obtained from a small punch test can be applied with the same utility has been a goal. In 2006, European Code of Practice (CoP) of small punch tests was first released, in which the correlation factor, ksp, was introduced to bridge the gap between the above methods. The author investigates the relationship between a uniaxial creep text and a small punch creep test by exploring the correlation factor ksp. Various sets of experiments and a comparative study of the conventional uniaxial creep test and small punch creep test were carried out. Methods including Norton, Larson-Miller and Time versus Stress relation were employed to identify the value of ksp. Different ksp values were found in different materials, which indicate that ksp values of materials need to be identified separately. In addition, the life prediction of a small punch creep test was carried out and the results of the life prediction predict a reasonable accuracy, which indicates that the small punch creep test is a reliable method for life prediction.

予クリープ変形を受けたフェライト系ステンレス鋼のスモールパンチクリープ試験による累積クリープ損傷評価

Small Punch (SP) Creep test has been recognized as a practical procedure to examine creep lives only sampling a very small volume. The SP creep test enables assessment of local points that conventional uniaxial creep test cannot evaluate. In order to assess the applicability of cumulative damage rule using the time fraction rule, a series of SP creep tests were carried out in air at 700°C employing ferritic stainless steel. Experimental test results show that the time fraction rule could be applied for the residual life of slighter pre-damaged specimens. However it would overestimate the severely damaged specimens.

GS0410 JIS規格外の微小試験片によるクリープ破断特性

Some destructive techniques are required to evaluate the degradation of components for thermal power plants. However, conventional uniaxial creep specimens were sometimes too large to collect a sample from local area of target components. Miniature Creep (MC) test that employs smaller size of the specimen can be collected from the local site is well known as a better semi-destructive testing technique. Although the MC specimen of round bar needed a sample of 4 mm in thickness, that of thin plate shape can be collected under 2 mm in thickness. In this study, MC tests using thin plate and round bar specimens were performed in Argon gas and in vacuum to evaluate their creep properties. Test results were compared with uniaxial ones employing 2.25Cr-1Mo steel. It was shown that creep rupture lives of the MC plate specimens in vacuum were a similar trend to those of uniaxial creep, and that the MC test using the thin plate is able to substitute for the conventional uniaxial creep test.

Creep life analysis by an energy model of small punch creep test

Currently most creep life prediction methods by small punch test are focused on the steady creep deformation phase which is similar to conventional uniaxial creep tests. This paper presents a novel creep life extrapolation approach by small punch creep test. An energy model believed to have advantages in modeling creep deformation process of small punch tests is introduced. Linear expression between general strain and creep deflection was recommended as the deformation criterion in the analytical prediction. Meanwhile, the combination of ball radius and hole size of the lower die was analyzed to accommodate the difference of experimental devices. Based on the small punch creep data with different materials and creep environments, the creep life in the multiaxial form of large deformation was analyzed. Finally, by selecting different creep displacements from initial creep phase, the results were in agreement with the expected trend. The creep life extrapolation method was verified.

Creep Properties Assessment of P92 Steel by Small Punch Creep Tests

Small punch creep (SPC) tests have the potential as an alternative to conventional uniaxial creep tests when limited amount of material is available. This paper investigates the applicability of SPC to evaluate the creep deformation behavior of P92 steel. SPC results have been compared with uniaxial creep test results in order to establish the correlation between the two tests. Small punch tests on P92 steel have been performed at 600 °C in the load range of 1000–400 N. The load dependence of minimum deflection rate was found to obey Norton’s power law but with a two slope behavior exhibiting distinct values of load exponents in the low and high load regimes. Transition in fracture mode of SPC specimens from ductile transgranular at high loads to brittle intergranular fracture at low loads was in accordance with the large variation in load exponent values. Monkman–Grant equation interrelating minimum deflection rate with rupture time has been found to be valid for SPC data. Correlations between SPC load and minimum deflection rate with uniaxial creep stress and minimum creep rate respectively, established based on empirical relations suggest the relevance of SPC for evaluating creep properties in relatively short time.

10805 微小丸棒および微小平板試験片によるクリープ破断特性

Some destructive techniques are required to evaluate the degradation of components under elevated temperatures. However, conventional uniaxial creep specimens were sometimes too large to collect them from local area of target components. Miniature Creep (MC) test has been recognized as a semi-destructive technique to examine the damage condition of components. They employ small size specimens that can reduce the conventional uniaxial creep specimen in similarity. In this study, MC tests using round bar and thin plate specimens were performed in Argon gas to measure creep properties, and compared with uniaxial creep test results employing 2.25Cr-1Mo steel. It was shown that MC rupture lives were a similar trend those of uniaxial creep, and the MC test is able to substitute for conventional uniaxial creep test.

Assessment of creep constitutive properties from three-point bending creep test with miniaturized specimens

Three-point bending creep tests using miniaturized specimen are proposed to assess creep constitutive properties of materials. Based on Norton’s creep law, the analytical model for the three-point bending specimen is given. Through the analytical model, creep parameters can be obtained by regressing the bending test data. Maximum stress approach and reference stress approach are used to convert bending test data to equivalent uniaxial tensile test data. In order to verify these approaches, both three-point bending and uniaxial creep tests of 23CrMoNiWV88 steel are performed. The results show that the derived creep parameters from the bending test data are in reasonable agreement with those obtained from conventional uniaxial creep tests. The equivalent uniaxial creep data converted using reference stress approach are more reasonable than that using maximum stress approach. It has also been found that the large deformation may have a significant influence on the conversion relationship. The results are only accurate at relatively low load levels during testing.

On the Determination of Material Creep Constants Using Miniature Creep Test Specimens

Full size creep test specimens, i.e., conventional uniaxial creep test specimen and Bridgman notch specimens are usually used to determine the full set of material constants for any creep model. However, in many situations, sufficient material is not available for theses specimens to be manufactured from it. Therefore, small creep test specimens have been introduced and used to determine (i) creep constants and (ii) the remaining life time for engineering components. Two commonly used small creep specimen types, i.e., the impression and the small ring creep tests, are used in this paper to determine the steady state creep constants. However, these specimen types are limited for use in determining the secondary creep properties, i.e., they are unable to determine the full set of material creep constants for creep damage models. In this paper the recently developed small two-bar creep test specimen and the newly developed small notched specimen test are described and used to determine a full set of material constants for Kachanov and Liu-Murakami creep damage models. The small notched specimen manufacturing, loading and testing procedures are described in this paper. P91 steel at 600 °C and (Bar-257) P91 steel at 650 °C have been used to compare the material constants obtained from the small two-bar and the small notched creep test specimens with those obtained from the conventional uniaxial creep test specimens and Bridgman notch specimens. The results show remarkably good agreement between the two sets of results.

Application of Impression Creep Technique for Development of Creep Resistant Austenitic Stainless Steel

The present paper highlights the usefulness of impression creep technique as applicable to alloy development studies of a creep resistant austenitic stainless steel which has been used as structural material for Fast Breeder Reactors. The effect of nitrogen on creep deformation behavior of type 316LN stainless steel has been studied using impression creep testing technique. Impression creep tests have been carried out at 923 K on 316LN stainless steel containing 0.07, 0.11, 0.14, and 0.22 wt. % nitrogen, in the punching stress range of 472-760 MPa. The impression creep curves were characterized by a loading strain, a primary creep stage and a secondary creep stage similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep curves was absent. The steady state impression velocity was found to increase with increasing punching stress. It was observed that impression velocity was sensitive to the variation in nitrogen content in the steel. Steady state impression velocity was found to decrease with increasing nitrogen content as was observed in conventional uniaxial creep tests. A good correlation between the impression creep test and the conventional creep test results was observed which demonstrates the applicability of impression creep testing technique for alloy development programme.

Impression Creep Behavior of 316LN Stainless Steel

Impression creep tests have been carried out at 923 K on 316LN SS containing 0.07, 0.14, and 0.22 wt.% nitrogen, under different applied stress levels. It was observed that the impression creep depth versus time curves were similar to the creep curves obtained from conventional uniaxial creep tests. The impression creep curves were characterized by a loading strain and primary and secondary creep stages similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep curves was absent. The steady-state impression velocity was found to increase with increasing applied stress. The equivalent steady-state creep rates calculated from impression velocities were found to be in good agreement with the steady-state creep rates obtained from conventional uniaxial creep tests. Equivalence between applied stress and steady-state impression velocity with uniaxial creep stress and steady-state creep rate, respectively, has been established based on the laws of mechanics for time-dependent plasticity. It was found that impression velocity was sensitive to the variation in nitrogen content in the steel; impression velocity decreased with increasing nitrogen content, and the results obtained in this study were in agreement with those obtained from uniaxial creep tests.

Evaluation of the effect of nitrogen on creep properties of 316LN stainless steel from impression creep tests

Impression creep testing technique is an innovative tool to study creep deformation behavior of materials mainly because it requires relatively short test time and small volume of material for evaluating the creep properties when compared with conventional uniaxial creep testing which is both material and time consuming. The effect of nitrogen on creep deformation behavior of type 316LN stainless steel has been studied using impression creep testing technique. Impression creep tests have been carried out at 923K up to 1000h on 316LN stainless steel containing 0.07, 0.11, 0.14, and 0.22wt.% nitrogen, in the punching stress range of 400–800MPa. The impression creep curves were characterized by a loading strain, a primary creep stage, and a secondary creep stage similar to uniaxial creep curves. The tertiary stage observed in uniaxial creep tests was absent. The steady state impression velocity was found to increase with increasing punching stress. The calculated stress exponent values varied between 3.3 and 8.2 with increasing in nitrogen content. It was observed that impression velocity was sensitive to the variation in nitrogen content in the steel. The steady state impression velocity was found to decrease with increasing nitrogen content. Correlation between the impression creep parameters and conventional uniaxial creep parameters has been established based on the laws of mechanics for time dependent plasticity.

Creep characterization of power-law materials through pseudo-steady indentation tests and numerical simulations

A constant-indentation creep rate test (CICRT) has been carried out for an Al-Mg solid-solution alloy using a microindenter in the temperature range of 636–773 K. When a conical indenter is pressed into the specimen surface under a load condition of F=F0 exp(γt) (F the indentation load, F0 the initial load, γ the loading rate parameter, t the loading time), the indentation pressure and indentation creep rate approach constant values of ps and ϵ̇in(s), respectively. The representative points of the deformation in the underlying material are defined on a contour line of the equivalent stress σr = C1ps, where C1 is the so-called constraint coefficient of 1/3 reported by Tabor. The finite element simulation of a power-law material subjected to the CICRT shows that the relationship between the equivalent plastic strain rate ϵ̇r at these points and ϵ̇in(s) is ϵ̇r=C2ϵ̇in(s) and that C2 ≊ 1/3.6 in the case of a creep stress exponent of 3.0. The constitutive equation of ϵ̇r versus σr obtained from experimental data and the computed value of C2 is in good agreement with that evaluated from conventional uniaxial creep tests.