Conventional Creep Tests Research Articles

Theoretical and experimental analysis on the small punch creep deformation of Incoloy800H

Abstract The small punch creep deformation behavior estimation of Incoloy800H was performed theoretically and experimentally in this study. The results indicate that three creep stages of the small punch test are similar to that of the conventional creep tests. Considering the deformation characteristics and geometry form, an obvious initial elastic-plastic deformation is presented and the primary creep stage is proven to be nonnegligible in the small punch test. Both the three-point bending model and the modified geometric model were proposed. It indicates that creep strain is proportional to small punch deflection in the primary and steady creep stages. Meanwhile, membrane stretching model was modified and a constant stress phase during deformation was found. The results indicate that small punch character parameters are capable to replace traditional creep data in the creep curve analysis of Incoloy800H. Thus, the creep curves with three deformation phases were analyzed and predicted by both of the modified Norton-Bailey model and modified theat model. Finally, the primary, steady and tertiary creep stages were described, and the creep life prediction of Incoloy800H was realized.

Nano-dynamic mechanical analysis (nano-DMA) of creep behavior of shales: Bakken case study

Understanding the time-dependent mechanical behavior of rocks is important from various aspects and different scales such as predicting reservoir subsidence due to depletion or proppant embedment. Instead of using the conventional creep tests, nano-dynamic mechanical analysis (nano-DMA) was applied in this study to quantify the displacement and mechanical changes in shale samples over its creep time at a very fine scale. The results showed that the minerals with various mechanical properties exhibit different creep behavior. It was found that under the same constant load and time conditions, the creep displacement of hard minerals would be smaller than those that are softer. On the contrary, the changes in mechanical properties (storage modulus, loss modulus, complex modulus and hardness) of hard minerals are larger than soft minerals. The results from curve fitting showed that the changes in creep displacement, storage modulus, complex modulus and hardness over creep time follow a logarithmic function. We further analyzed the mechanical changes in every single phase during the creep time based on the deconvolution method to realize each phase’s response independently. Two distinct mechanical phases can be derived from the deconvolution histograms. As the creep time increases, the volume percentage of the hard mechanical phase decreases, while this shows an increase for soft phases. The results suggest that nano-DMA can be a strong advocate to study the creep behavior of rocks with complex mineralogy.

Long-term response and design of two geosynthetics: effect of field installation damage

This paper contributes to understanding how installation damage of geosynthetics influences their long-term response and design. A geotextile and a geogrid were exhumed after installation under real conditions; their long-term tensile behaviour was investigated using conventional creep and creep rupture tests. Reduction factors for installation damage, creep and their combined effect were computed. The main aim of the paper was to assess how the long-term response of the geosynthetics was influenced by installation damage and whether it would be necessary to update current design approaches. The installation damage affected the mechanical response of both geosynthetics; important strength reductions were observed, particularly for the most severe installation conditions. The results indicate that, contrary to what has been reported in the literature, the creep rupture response of the geotextile changed after installation damage. When tested under similar creep loads (fraction of the sample tensile strength), both geosynthetics exhibited reduced potential for creep rupture and smaller strains at the end of primary creep. Changes in stiffness were less important than those for strength, for both short- and long-term response. There was synergy between installation damage and creep; the traditional approach to design was unsafe for the geotextile and slightly conservative for the geogrid.

A triaxial creep model for frozen soil based on hypoplasticity

This paper presents a triaxial constitutive model for the creep behaviour of frozen soil within the framework of hypoplasticity. The model is obtained by introducing a time-dependent function and breaking the stress-homogeneity and rate independence of the basic hypoplastic constitutive model for sand. Parameters in the model are classified into two groups, which can be determined from conventional triaxial test and creep test. The performance of the model is demonstrated by simulating some creep tests at different stress levels and temperatures. The simulated results suggest that the proposed model is capable of describing the three-stage creep process in a unified way, and of handling the cases with or without creep failure. Besides, the radial strain, radial strain rate and volumetric strain during the creep process are also presented in the paper.

A Variable-Parameter Creep Damage Model Incorporating the Effects of Loading Frequency for Rock Salt and Its Application in a Bedded Storage Cavern

Laboratory tests were conducted to assess the effects of the loading frequency on the time-dependent behavior and damage properties of rock salt under confining stress states. Axial two-stage irreversible deformation based on the loci of the minimum load of each cycle was observed, and this observation was similar to the result of conventional creep tests under static loads. The unloading modulus decreased exponentially with respect to time, and the damage variable was represented in terms of the decay of the material stiffness. To account for the effects of the loading frequency on the time-dependent degradation of rock salt, a unified damage evolution equation was formulated based on the experimental results. A creep damage model of rock salt was proposed by introducing non-stationary modular components into the Burgers viscoelastic model. Numerical simulation was performed using the newly developed model to evaluate the stability and serviceability of a storage cavern in a bedded salt formation under various loading scenarios. The simulated results indicate that a lower injection–withdrawal frequency results in a greater volume convergence rate and a wider dilatancy region of the storage cavern. Additionally, the stress concentration and dilatancy of the surrounding rock mass extend much deeper into the mudstone interbeds than into other regions of the cavern.

Comparison of Results of Accelerated and Conventional Creep Tests of Dissimilar Weld Joint of Steels FB2 and F

The recently developed simulative accelerated creep testing (ACT) on a Gleeble thermal mechanical simulator allows the microstructural transformation of creep-resisting materials in a relatively short time (less than 100 hours) to a state resembling that of multiyear application under creep conditions. This contribution deals with the comparison of long-term conventional creep testing (CCT) with ACT on a dissimilar weld joint prepared from steels COST FB2 (X13CrMoCoVNbN 9-1-1) and COST F (X14CrMoVNbN 10-1). Creep tests to rupture were carried out at temperatures from 550 °C to 650 °C in a stress range from 70 MPa to 220 MPa. Creep rupture strength was evaluated using the Larson-Miller parameter. Evolution of the microstructure, changes of precipitates and dislocation of the substructure in different zones of the weld joint (Fig. 1) were correlated with the position of fracture and the creep strength. The fine grained part of the heat affected zone of steel COST F was found to be a critical zone of creep damage. ACT of samples machined from various positions in the weldment was performed at 600°C under 100 MPa. Changes in the hardness and the microstructures of the samples, which underwent both types of creep tests, were compared. The results of both creep testing methods are compared from a point of view of the potential for utilization of ACT in industry. It seems that microstructural processes taking place during creep exposures are accelerated by ACT, however, further experiments have to be performed to improve the accuracy of fracture prediction and verification if long-term CCT is to be replaced by ACT.

Use of master curves based on time-temperature superposition to predict creep failure of aluminium-glass adhesive joints

Advancements in materials technology and the use of innovative designs have led to extensive application of adhesive bonding techniques in the electric appliance industry. While the static strength of such joints is sufficient for the intended applications, long term durability remains a major concern, mainly due to creep effects. Conventional creep testing can be performed at the service temperature but it is a long test that can take decades, although it can be accelerated using high temperatures. In this work, glass-aluminium joints were studied under static and creep loads. Glass-aluminium specimens were subjected to creep testing at various temperatures. Using the time temperature superposition principle, the results of these individual creep tests were combined in a master curve that approximates the creep behaviour of the adhesive joint in a long time period. These master curves were used to guarantee a minimum service life of the joint.

A fractal derivative constitutive model for three stages in granite creep

In this paper, by replacing the Newtonian dashpot with the fractal dashpot and considering damage effect, a new constitutive model is proposed in terms of time fractal derivative to describe the full creep regions of granite. The analytic solutions of the fractal derivative creep constitutive equation are derived via scaling transform. The conventional triaxial compression creep tests are performed on MTS 815 rock mechanics test system to verify the efficiency of the new model. The granite specimen is taken from Beishan site, the most potential area for the China’s high-level radioactive waste repository. It is shown that the proposed fractal model can characterize the creep behavior of granite especially in accelerating stage which the classical models cannot predict. The parametric sensitivity analysis is also conducted to investigate the effects of model parameters on the creep strain of granite.

Creep life prediction of small punch creep testing specimens for service-exposed Cr5Mo using the theta-projection method

Abstract In this paper, small punch creep tests under different loads and conventional creep tests with various stresses at 550 °C were conducted on service-exposed Cr5Mo material. The theta-projection and modified theta-projection methods were used to analyze the creep curve and the minimum creep rate. The actual fracture time in the small punch creep test was 377 h, and the theta-projection method predicted a time of 339 h, which represents an error of 10.08%. The predicted time from the modified theta-projection method was 316.25 h, which represents an error of 16.11%. The curves predicted by the two models are similar. The theta-projection method is suitable for applications in small punch creep life analysis.

Determination of creep behaviour of Ti60 alloy by small punch creep test

The small punch (SP) creep test has distinct advantages in the creep property assessment of materials at elevated temperatures. However, there are few creep properties of Ti alloys obtained by the SP creep test in the current literature. In this paper, the SP creep behaviour of Ti60 alloy has been evaluated under various loads in the range 550–800 N over a temperature range 873–973 K. The SP creep curves obviously indicated the primary, secondary and tertiary stages of creep. The test results have been compared with those of conventional creep tests. The European Code of Practice (CoP) for Small Punch Testing, Dorn equation and Monkman–Grant relationship have also been used to analyse the results of the SP creep tests. The ratio of load of the SP creep tests to equivalent stress of conventional creep tests, the load exponent value of steady deflection rate and activation energy for creep deformation were estimated from the SP creep tests. In conclusion, it was found that dislocation creep may be the main mechanism that dominates the SP creep deformation of Ti60 alloy in the range of load and temperature.

Numerical modeling of creep degradation of natural soft clays under one-dimensional condition

Creep degradation is a common phenomenon of natural soft clays. This paper focuses on developing a one-dimensional constitutive model considering the influence of bond degradation on the creep behavior for natural soft clays. First, conventional oedometer creep tests are studied and a creep based structure indicator ϖ denoting the difference between the creep coefficient of the reconstituted samples and that of intact samples is proposed. Then, the creep coefficient of intact clay is formulated by this indicator and the intrinsic creep coefficient corresponding to reconstituted clay. This formula is then incorporated into a one-dimensional creep model to describe the creep degradation behavior induced by bond degradation. The model parameters can be determined in a straightforward way from oedometer tests which leads to an easy application of the model for practice. Furthermore, coupled with the consolidation theory, the model is used to simulate oedometer tests at constant strain rate as well as long-term creep tests on sensitive Batiscan clay. The destructuration effects on the evolution of creep coefficient are analyzed. The comparisons between experimental and numerical results show that the proposed model can precisely describe the creep degradation behavior induced by destructuration of natural soft clay under one-dimensional loading.

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.

Effect of lattice misfit on the evolution of the dislocation structure in Ni-based single crystal superalloys during thermal exposure

In this study, we investigate the movement of dislocations in Ni-based superalloys under the influence of lattice misfit stresses between the ordered γ'-cuboids and the disordered γ-matrix during thermal exposure in the absence of an applied load. This study focuses on a different condition than the conventional creep testing, and thus offers a unique opportunity to study the intrinsic behavior of the alloy. The dislocation density increases substantially with time during thermal exposure, leading to the formation of various configurations of dislocation networks on the {100}γ/γ′ interfaces, including <110> diamond-shaped, <110>/<100> mixed polygon-shaped and <100> square-shaped networks. During thermal exposure, the b=12<110> native dislocations first move and evolve into 60° mixed dislocations along the <110> directions on the {100}γ/γ′ interfaces, forming the <110> diamond-shaped dislocation networks. In the case of longer thermal exposures, the dislocations further evolve into pure edge dislocations along the <100> on the {100}γ/γ′ interface, leading to the evolution of the <110> diamond-shaped dislocation networks into <100> square-shaped networks, with the mixed <110>/<100> dislocation networks as an intermediate stage during the transition. These movements occur by sweep glide in the {111} planes and diagonal climb on the {100} planes for the edge and mixed dislocations and by cross-slip for the screw dislocations. The driving force for all of these movements is the interaction between the normal misfit stresses and the edge components of the Burgers vectors of the dislocations to relax the misfit stresses. An analysis based on the elastic strain energy considerations is presented to explain the driving forces for the dislocation movements.

Evaluation of creep rupture strength of high nitrogen ferritic heat-resistant steels using small punch creep testing technique

The development of next generation ultra-heat-resistant ferritic steels for the future advanced thermal power plants which operate in excess of 700°C are currently in progress in Japan. The small punch (SP) creep testing technique which uses miniature specimens has been applied to evaluate creep rupture strength of newly developed high nitrogen ferritic steels. The SP creep tests have been carried out on different grades of high nitrogen ferritic steels in the temperature range of 600–800°C, under different loads in the range of 70–400N, using specimens of dimension ∅8mm x 0.5mm. The SP creep rupture results of high nitrogen ferritic steels have been compared with that of conventional steel (Gr.91). The high nitrogen steels exhibited higher creep rupture strength when compared to the Gr.91. The creep rupture results obtained from SP creep tests were correlated to the uniaxial creep rupture results obtained from conventional creep tests using a stress conversion coefficient of 2.05. The paper investigates the applicability of SP creep testing technique for the evaluation of creep rupture strength of high nitrogen ferritic heat-resistant steels.

Fracture-based correlation of uniaxial and small punch creep data

A critical problem of small punch test is determining the relation between this test and the conventional uniaxial test. In high-temperature creep conditions studied by constant-force variant of small punch test, this problem is often approached empirically by comparing force in small punch test and the applied stress in conventional creep test of the same duration, i.e. of the same time to fracture. A different approach is used in the evaluation of constant-rate small punch test: the force is normalized by the square of specimen thickness or, recently, by the product of the specimen thickness and a relevant deflection. Modifications of this advanced approach in the creep area are examined in the present paper. The deflection observed at fracture of specimen in small punch creep test is taken as a starting normalization quantity. The suggested correlations are illustrated by means of data obtained by creep testing of steels usually employed in power plants (P91, P92) or proposed for heavy-duty service (Eurofer’97).

Creep rates of heat-affected zone of grade 91 pipe welds as determined by stress-relaxation test

Cross-weld creep testing conducted at 650 °C under 69 MPa stress has reproduced either the Type I or Type IV failure mode in Grade 91 welds, depending on the post-weld heat treatment procedures. Welds post-weld heat treated below the AC1 temperature have ruptured in the Type VI failure mode, while welds heat-treated above the AC1 temperature of the alloy have ruptured in the Type I failure mode. Heat-treatments at lower temperatures and shorter durations have produced a reduced creep rate. The accelerated short term stress-relaxation test has been conducted to obtain the input creep rates for a finite element model for the cross-weld creep testing. The model predicted secondary creep rates are in good agreement with the results from the conventional cross-weld creep tests. From the finite element model, the creep damage by cavitation is believed to start at regions where the first principal stress and stress triaxiality concentrate.

Room temperature creep behavior of Ti–Nb–Ta–Zr–O alloy

The room temperature creep behavior and deformation mechanisms of a Ti–Nb–Ta–Zr–O alloy, which is also called “gum metal”, were investigated with the nanoindentation creep and conventional creep tests. The microstructure was observed with electron backscattered diffraction analysis (EBSD) and transmission electron microscopy (TEM). The results show that the creep stress exponent of the alloy is sensitive to cold deformation history of the alloy. The alloy which was cold swaged by 85% shows high creep resistance and the stress exponent is approximately equal to 1. Microstructural observation shows that creep process of the alloy without cold deformation is controlled by dislocation mechanism. The stress-induced α" martensitic phase transformation also occurs. The EBSD results show that the grain orientation changes after the creep tests, and thus, the creep of the cold-worked alloy is dominated by the shear deformation of giant faults without direct assistance from dislocations.

Evolution of a modern mechanical testing and design standard for high temperature materials

The basis for high-temperature testing and analysis was established a century ago using test equipment that was available at the time. This fostered a tradition in research and engineering applications that has not changed fundamentally. The limitations of this approach are described, with emphasis on problems associated with predicting the remaining life of operating components. A new testing method and analysis philosophy is proposed based on measurement of current performance, with end of life unambiguously identified with limiting acceptable values of these current performance metrics. Instead of a single test to measure creep strength and fracture resistance, the new approach uses separate tests. For creep strength, the stress vs. creep rate curve determined at nearly constant state and covering five decades in creep rate in a one-day test is obtained from a high-precision stress relaxation test (SRT). The results for many alloys and materials often show a sigmoidal curve. Conventional long-time creep tests of steels have shown a similar effect. However, rather than recognising this as a fundamental rate-dependent phenomenon, it has been typically explained in terms of microstructural coarsening. For fracture resistance, a constant displacement rate test across a notch is used at the temperature of greatest sensitivity to high-temperature fracture. This latter test has been very successful in detecting embrittlement in superalloys. For more ductile alloys, a non-destructive measure of ductility based on the strain rate sensitivity determined from the SRT data is shown to have great potential. This introduces the concept of intrinsic ductility that can be determined without ambiguity over a wide range of stress and temperature for both new and serviced components.

Creep Deformation and Rupture Behavior of Single- and Dual-Pass 316LN Stainless-Steel-Activated TIG Weld Joints

Creep deformation and rupture behavior of single-pass and dual-pass 316LN stainless steel (SS) weld joints fabricated by an autogenous activated tungsten inert gas welding process have been assessed by performing metallography, hardness, and conventional and impression creep tests. The fusion zone of the single-pass joint consisted of columnar zones adjacent to base metals with a central equiaxed zone, which have been modified extensively by the thermal cycle of the second pass in the dual-pass joint. The equiaxed zone in the single-pass joint, as well as in the second pass of the dual-pass joint, displayed the lowest hardness in the joints. In the dual-pass joint, the equiaxed zone of the first pass had hardness comparable to the columnar zone. The hardness variations in the joints influenced the creep deformation. The equiaxed and columnar zone in the first pass of the dual-pass joint was more creep resistant than that of the second pass. Both joints possessed lower creep rupture life than the base metal. However, the creep rupture life of the dual-pass joint was about twofolds more than that of the single-pass joint. Creep failure in the single-pass joint occurred in the central equiaxed fusion zone, whereas creep cavitation that originated in the second pass was blocked at the weld pass interface. The additional interface and strength variation between two passes in the dual-pass joint provides more restraint to creep deformation and crack propagation in the fusion zone, resulting in an increase in the creep rupture life of the dual-pass joint over the single-pass joint. Furthermore, the differences in content, morphology, and distribution of delta ferrite in the fusion zone of the joints favors more creep cavitation resistance in the dual-pass joint over the single-pass joint with the enhancement of creep rupture life.

The Application of Stress-Relaxation Test to Life Assessment of T911/T22 Weld Metal

A dissimilar weld metal was obtained through submerged arc welding of a T911 steel to a T22 steel, and its creep property was explored by stress-relaxation test assisted by some conventional creep tests. The creep rate information of the stress-relaxation test was compared to the minimum and the average creep rates of the conventional creep test. Log-log graph showed that the creep rate of the stress-relaxation test was in a linear relationship with the minimum creep rate of the conventional creep test. Thus, the creep rate of stress-relaxation test could be used in the Monkman-Grant relation to calculate the rupture life. The creep rate of the stress-relaxation test was similar to the average creep rate, and thereby the rupture life could be evaluated by a method of “time to rupture strain.” The results also showed that rupture life which was assessed by the Monkman-Grant relation was more accurate than that obtained through the method of “time to rupture strain.”