Secondary Creep Stage Research Articles

Secondary creep stage behavior of copper-clad aluminum thin wires submitted to a moderate temperature level

This work focuses on the role of the microstructure on the creep behavior of thin copper-clad aluminum (CCA) wires. Creep tests were performed at 150°C on hard drawn and annealed CCA and on pure metals. It is shown that the Cu-Al interface of CCA ensures a mechanical resistance leading to lower creep velocities than for metals. Creep mechanisms are driven by aluminum at lower stresses and copper for higher stresses, independently of the physical nature of the interface.

Low-temperature creep of austenitic stainless steels

Plastic deformation under constant load (creep) in austenitic stainless steels has been measured at temperatures ranging from 4 K to room temperature. Low-temperature creep data taken from past and unreported austenitic stainless steel studies are analyzed and reviewed. Creep at cryogenic temperatures of common austenitic steels, such as AISI 304, 310 316, and nitrogen-strengthened steels, such as 304HN and 3116LN, are included. Analyses suggests that logarithmic creep (creep strain dependent on the log of test time) best describe austenitic stainless steel behavior in the secondary creep stage and that the slope of creep strain versus log time is dependent on the applied stress/yield strength ratio. The role of cold work, strain-induced martensitic transformations, and stacking fault energy on low-temperature creep behavior is discussed. The engineering significance of creep on cryogenic structures is discussed in terms of the total creep strain under constant load over their operational lifetime at allowable stress levels.

Multi-step triaxial compressive creep behaviour and induced gas permeability change of clay-rich rock

Clay-rich rocks are potential geological barriers for geological repositories of radioactive waste. The long-term safety of a repository requires a full characterisation of the time-dependent thermal hydro-mechanical behaviour of the clay-rich rocks. This paper investigates the creep deformation and induced permeability change of the Callovo-Oxfordian (COx) claystone by a series of multi-step triaxial compressive creep tests. The effects of confining pressure and deviatoric stress are both studied. Three values of effective confining pressure are 2 MPa, 6 MPa and 12 MPa, according to the in situ stress estimation at the Bure underground research laboratory in France. In each test, the deviatoric stress is progressively loaded in five steps so that the primary, secondary and tertiary stages of creep can be evaluated. In the last loading step, the tested sample may exhibit a creep failure process. During the creep tests, the gas permeability of the partially saturated COx claystone is measured by a transient pulse decay method in concurrence with the material creep deformation. The results obtained show that both the confining pressure and deviatoric stress have important effects on the time-dependent deformation and permeability evolution of the claystone. Analysis of creep strain rates during the last creep step indicates that there exists a threshold for the creep-induced failure of the COx claystone. The permeability evolution in the triaxial creep process is closely related to the microstructural evolution of rock, including the closure of pre-existing micro-cracks, the compaction voids and the creation, propagation and coalescence of micro-cracks.

Indenter load effects on creep deformation behavior for Ti-10V-2Fe-3Al alloy at room temperature

Abstract Indentation creep behavior at room temperature and its mechanism of Ti-10V-2Fe-3Al alloy with dual phase structure were investigated. Micro-indentation creep tests were performed under the maximum indenter load ranging from 1500 mN to 4500 mN and the holding time of 300 s. The effect of indenter load on creep behavior and creep parameters, like creep rate, creep strain rate, indentation stress and creep stress exponent, were analyzed at the steady-state creep. The results revealed that creep parameters exhibited significantly indentation depth dependent. At the secondary stage of creep, creep strain rate and creep rate increased with the increase of maximum indenter load, while creep stress and creep stress exponent exhibited an opposite trend. Especially, creep stress exponent of 7.65 ± 1.25 in power-law creep behavior of Ti-10V-2Fe-3Al alloy, which was consistent with dislocation process, indicated that the secondary stage of creep may be dominated by dislocation climb. Furthermore, experimental data and figures were used to evaluate and comprehend likely creep mechanisms during a micro-indentation scheme of Ti-10V-2Fe-3Al alloy.

Secondary creep of porous metal supports for solid oxide fuel cells by a CDM approach

The creep behaviour of porous iron-chromium alloy used in solid oxide fuel cells (SOFCs) becomes relevant under SOFC operating temperatures. In this paper, the secondary creep stage of infiltrated and non-infiltrated porous metal supports (MS) was investigated and theoretically modelled by a continuum damage mechanics (CDM) approach. The behaviour of the porous metal support, in the range from 1 to 17MPa and temperatures between 650 and 700°C, was combined and compared with data from literature of Crofer® 22 APU, taken as zero porosity reference material. The variation of the elastic modulus as function of temperature, determined by the high temperature impulse excitation technique, was directly used to account for the porosity and the related effective stress acting during the creep tests. The proposed creep rate formulation was used to extend the Crofer® 22 APU Monkman-Grant diagram in the viscous creep regime. The influence of oxide scale formation on creep behaviour of the porous MS was assessed by comparing the creep data of pre-oxidised samples tested in reducing atmosphere.

Effect of Nitrogen on Primary and Steady State Creep Deformation Behaviour of 316LN Austenitic Stainless Steel

Effect of nitrogen on primary and steady state creep deformation behaviour of 316LN austenitic stainless steel has been studied. Nitrogen content was varied between 0.07 and 0.22 wt%. Creep tests have been carried out on the steels over the stress range of 120–225 MPa at 923 K. The creep behaviour of the steels has been assessed based on Garofalo relationship, \(\varepsilon = \varepsilon_{o} + \varepsilon_{T} \left[ {1 - exp\left( { - rt} \right)} \right] + \dot{\varepsilon }_{s} t\). The rate of exhaustion of primary creep rate ‘r’ and steady state creep rate ‘\(\dot{\varepsilon }_{s}\)’ decrease and time to onset the secondary creep stage increases with nitrogen content. The relationships between the initial creep rate ‘\(\dot{\varepsilon }_{i}\)’and rate of exhaustion of primary creep rate ‘r’ with steady creep rate ‘\(\dot{\varepsilon }_{s}\)’ reveal that the same deformation mechanism prevails during the transient and steady state creep deformations. Attempt has been made to generate a master transient creep curve for the steels having different nitrogen contents using the above parameters. Even though for a steel of specific nitrogen content, master curve has been generated for different stress levels, a master curve for all the steel could not be established. Different deformation characteristics of the steel having different nitrogen content which effect the stacking fault energy as well as precipitation behaviour is considered for the disagreement.

Прогибы предварительно натянутой гибкой весомой нити, закрепленной в двух произвольных точках

In this paper, it is obtained the differential equation of sagging of flexible heavy rope stretched at an arbitrary angle to the horizon and its solution in the case of small deflections. It is shown that the maximum deflection is always achieved in the middle of rope projection, and its magnitude does not depend on the position of supports. For the first time in the formulation of the problem, it is assumed that the rope is stretched under the influence of two independent factors: deformation of the prestress and deformation due to gravity, and each of the factors may not be taken into account. It is considered the case of rope deformation in a homogeneous temperature field, and it is established that the presented model becomes incorrect in the case of small pretension in comparison with thermal deformation. For cases of absence of pretension and the equality of temperature deformation and pretension, solutions to the problem are represented for the linearly elastic and rheologically active materials. It is noted methodical features of solving the problem for different stages of creep using the Kelvin-Voigt equation. Solutions are given for the combined use of the Kelvin-Voigt model and linear hereditary viscoelasticity for the primary and secondary creep stages, respectively. The problems of modeling drying and reduction of biological filament are considered.

Evolution of superdislocation structures during tertiary creep of a nickel-based single-crystal superalloy at high temperature and low stress

A second-generation nickel-based single crystal superalloys DD6 were creep tested in the [001] direction (within 15°) at 1100 °C/140 MPa. The specimen tested until rupture was investigated using scanning and transmission electron microscopy to determine the evolution of the dislocation behaviors during tertiary creep. It was found that the tertiary creep deformation was highly localized and inhomogeneous along the gauge length, and the types of superdislocations in γ′ rafts varied with the distances from the rupture surface, including individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. It can be concluded that γ′ rafts shearing events occur in the following sequence with the evolution of tertiary creep: individual screw dislocations, antiphase boundary-coupled dislocation pairs, and superlattice intrinsic stacking faults. The origin of these transformations of superdislocation types and its influence on tertiary creep rate are discussed. It is proposed that at the microscopic level, a more reasonable explanation for the strain softening mechanism during the tertiary creep of nickel-based superalloys at high temperatures and low stresses is the emergence of new superdislocation types with higher mobility rather than the density rise of a single type of superdislocation produced during the later secondary creep stage.

Positive Effects of High-Temperature Steel Creep Behavior on Continuous Casting Slab

Mechanical properties and creep behavior of continuous casting slab subjected to uniaxial tensile tests at 1100 and 1150 °C were considered in this paper. All of specimens was given a typical in-situ melting thermal history and interrupted cooling cycle before testing. An increase in load and temperature accelerates the creep rate thereby decreasing the steady state range. A new creep equation whose parameters are calculated by inverse-estimation using the regression analysis was proposed based on Norton-Bailey creep law. Primary and secondary creep stages could be described accurately by the new equation. Positive creep effects on straightening technology in continuous casting slab were presented in this paper. Involute continuous straightening technology could give full play to the role of steel creep deformation at high temperature. This research is helpful for the design of the new casting machine and improvement of old casting machine in using depended on high temperature creep behavior.

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.

Microstructural Evolution in a 9%Cr-3%Co-3%W-VNb Steel during Creep

Microstructural evolution in a 9Cr-3Co-3W-0.2V-0.06Nb-0.05N-0.005B steel crept at T=650°C under an applied stress of 140 MPa up to strains of 1, 3, 5.75 and 12%, which represent primary, secondary and tertiary creep stages and rupture, respectively, was studied. The steel was initially normalized from 1050°C, and finally tempered at 750°C for 3h. After tempering the boundaries of tempered martensite lath structure (TMLS) were decorated by M23C6 carbides, M6C carbides and Laves phase particles. The 3% W additives provide the narrow size distribution of the boundary particles excepting M6C carbides. The depletion of thermodynamically none-equilibrium content of W from the solid solution during creep leads to following events. (i) Continuous precipitation of small Laves phase particles occurs during all creep stages and results in the formation of bimodal size distribution. As a result, the average size of Laves phase particles remains unchanged during creep. (ii) Coarsening of M23C6 carbides starts to occur only at the transition to tertiary creep. (iii) Transformation of laths to subgrains followed by their growth is observed during all stages of creep. The density of particle located at lath/subgrain boundaries decreases from 5.6 to 2.6 μm-1 during creep up to rupture. However, no full transformation of TMLS into subgrain structure has been revealed.

Anisotropic creep damage and fracture mechanism of nickel-base single crystal superalloy under multiaxial stress

The influence of orientation on the creep rupture properties of a single crystal superalloy DD6 under multiaxial stress was carried out at 980 °C and 400 MPa. A circumferential V-type notched specimen had been designed to investigate the effect of multiaxial stress state on the creep behavior. It was found that the creep lifetimes of the [111] oriented notched specimen were slightly longer than that of the [011] orientation and 1.36 times longer than that of the [001] orientation. The computational results showed that the strain and damage distribution revealed fourfold symmetry, double symmetry and threefold symmetry for [001], [011] and [111] orientation respectively, which was confirmed by experimental observation. At high temperature the creep anisotropy in three different orientations exhibited mainly in primary and secondary creep stages. Through the study of notched specimens by SEM, the morphology evolution of γ′-phase proved that the directional coarsening was strongly dependent on the local effective stress and the direction of the local max principal stress with respect to loading axis. Fracture morphology displayed uneven cleavage configuration with multi-level feature, and the cleavage planes parallels (001), (011) and (111) crystal plane for [001], [011] and [111] orientation, respectively. The cleavage plane, which is attributed to the cracks propagated along the interfaces of γ/γ′phases, displayed square-like, rhombus-like and hexagon-like feature for [001], [011] and [111] orientation, respectively. Due to the higher density in dislocations of {111} planes, it is more easily as the secondary crack when the crack reach the {111} planes, which is thought to be the main reason of forming different feature on primary cleavage plane.

Voiding mechanisms in semi-crystalline polyamide 6 during creep tests assessed by damage based constitutive relationships and finite elements calculations

Notched round bars made of a semi-crystalline polymer, polyamide 6, were submitted to creep tests. In order to study the microstructural evolution of the material during creep deformation, these tests were stopped at two characteristic creep times: i) the end of the secondary creep stage, ii) the onset of the failure during the tertiary creep stage, allowing 3D through thickness Synchrotron Radiation Tomography (SRT)-inspections. This SRT-technique allowed features of the damage to be assessed at a micrometre resolution within the notched region, such as spatial distributions of void volume fraction and void heights and diameters. These deformation mechanisms have already been observed and studied during steady strain rate loading and a multi-mechanism model coupled with damage formulation proposed to simulate notched specimens submitted to tensile loads. This model, based on porous visco-plasticity, was used here to simulate the creep behaviour of this PA6 material. The optimization procedure has led to a set of material coefficients capable of reproducing both macroscopic behaviour (creep curves) and damage micro-mechanisms. The level of maximum damage within the material and the spatial distributions of void volume fraction obtained numerically were compared successfully to experiments and the Cauchy stress tensor has been proved to be related to the void deformation mechanisms.

A fractional order creep constitutive model of warm frozen silt

A series of triaxial creep tests were conducted on warm frozen silts extracted from Qinghai–Tibet Plateau at temperature of −1.5 °C under confining pressures of 0.5, 1.0, and 2.0 MPa, respectively. The applied test stress levels were 30, 50, 60, and 70% of triaxial shear strength, respectively. The test results indicate that the creep strain increases with the increase in applied stress level and there is a stress threshold, based on which the test results can be classified into two types of creep strain curves. The creep strain curve only includes primary and secondary creep stages when the stress level is less than the threshold value. When the stress level exceeds the threshold value, the creep strain velocity gradually increases and the specimen quickly fails in tertiary creep stage. Based on the creep test results, a fractional order rheological element model is established for warm frozen silt, which is also generalized from uniaxial stress state to the three-dimensional stress state. From the analysis on the features of the stress threshold, a creep strength criterion is also proposed simultaneously. Comparing the calculated results of the warm frozen silt with the tested ones, it is found that the predicted results of the proposed model are in good agreement with the test results. In the proposed fractional order model, the relationship between the damage factor and time is established to describe the damage degree of the specimen. Compared with the existing creep constitutive model of frozen soil, the proposed fractional order model has advantages of fewer model parameters, higher simulation precision and wider applicability in analyzing the mechanical properties of warm frozen silt.

Creep behaviors of methane hydrate coexisting with ice

A series of creep tests were carried out in order to clarify the creep behaviors of methane hydrate coexisting with ice, which may exist in the ice-bearing permafrost. Measurements on synthetic methane hydrate-ice specimens indicate that (1) higher external load will increase the initial strain and the axial strain during the whole creep test. And the creep strain rate will increase with the increase of external load; (2) the initial strain, axial strains and creep strain rate of the specimens under certain external load increase with the decrease of confining pressure; (3) temperature drop leads to the decrease of initial strain and axial strains of the specimens under certain external load. And the creep strain rate firstly increases and then decreases with the increase of temperature, due to the refreezing of dissociated water from methane hydrate; (4) the specimens run to the secondary creep stage earlier when they endure lower external load, higher confining pressure and lower temperature; (5) the specimens under higher confining pressure and lower temperature show higher failure strength.

Effect of thermal exposure on the residual stress relaxation in a hardened cylindrical sample under creep conditions

This paper describes the effect of thermal exposure (high-temperature exposure) (T = 675◦C) on the residual creep stress relaxation in a surface hardened solid cylindrical sample made of ZhS6UVI alloy. The analysis is carried out with the use of experimental data for residual stresses after micro-shot peening and exposures to temperatures equal to T = 675◦C during 50, 150, and 300 h. The paper presents the technique for solving the boundary-value creep problem for the hardened cylindrical sample with the initial stress–strain state under the condition of thermal exposure. The uniaxial experimental creep curves obtained under constant stresses of 500, 530, 570, and 600 MPa are used to construct the models describing the primary and secondary stages of creep. The calculated and experimental data for the longitudinal (axial) tensor components of residual stresses are compared, and their satisfactory agreement is determined.

Experimental investigation of tension and compression creep-ageing behaviour of AA2050 with different initial tempers

Creep-ageing behaviour of aluminium alloy 2050 with different initial tempers (T34, T84 and as-quenched) has been experimentally investigated under both tension and compression creep-ageing conditions, with different stress levels at 155°C for 18h. Corresponding strengthening phenomena have been studied by interrupted creep-ageing tests and subsequent tensile tests. Moreover, the microstructures of some selected specimens after creep-ageing tests have been examined by transmission electron microscopy (TEM) and the precipitation process has been analysed. It has been found that creep strains under tensile stresses are much larger than those under compressive stresses during the tests. A new “double primary creep feature” has been observed in both the as-quenched alloys and the pre-stretched/natural-aged (T34) alloys, in which an intermediate inverse creep stage with an increasing creep strain rate locates between the initial primary+transient steady-state creep stages and the second primary+second steady-state creep stages. While for the alloy with peak-aged initial temper (T84), typical primary and steady-state secondary creep stages are observed during tension creep-ageing tests and little creep strain occurs under compressive stresses of 150 and 175MPa. The mechanisms for these phenomena are discussed in terms of microstructural interactions among the changing dislocations, solute-matrix bonding and precipitates, and their effects on the creep resistance of the alloy during creep-ageing tests are analysed.

Creep Rupture Properties of Indigenously Developed 304HCu Austenitic Stainless Steel

The 304HCu austenitic stainless steel tubes are being produced for the first time indigenously in India. Creep tests have been carried out at 923 and 973 K on two heats with different solution annealing conditions. Stress levels ranged from 180 to 300 MPa at 923 K and 100 to 170 MPa at 973 K. The creep curves showed a small instantaneous strain followed by primary, secondary and tertiary creep stages. The rupture lives varied in the range of 50–8700 hours. Higher rupture life was observed in the heat with higher solutionising temperature. The variation of creep rupture life with applied stress at different temperatures is compared with internationally reported data and it is found that the indigenous 304HCu SS tube material has creep rupture strength comparable with internationally reported data. The variation of steady state creep rate with stress showed a power law relationship. Rupture ductility was generally found to decrease with increase in rupture life. SEM fractography revealed mixed mode failure with predominance of intergranular creep cavitation.

Characterization of creep behavior of TiAl alloy with high Nb content at elevated temperatures

Creep experiments of the TiAl alloy with high Nb content were conducted to consider both the temperature dependence and stress dependence of its creep behavior. The creep curves were characterized by Theta constitutive model, and Newton–Gauss method was applied to obtain the material parameters. Based on the rupture strain, the rupture life of the TiAl alloy was able to be predicted by Theta constitutive model. The results show that the creep curves of the TiAl alloy contain primary creep, secondary creep and tertiary creep stages, especially for the lower stress. The rupture life of the TiAl alloy decreases with the increase in either applied stress or temperature. Theta constitutive model is able to describe the creep deformation of the TiAl alloy accurately, and the predicted life agrees well with the experimental result.

Experimental study and statistical theory of creep behavior of warm frozen silt

To investigate the soil parameters and stochastic mechanical characteristics of warm frozen silt, a series of triaxial compression tests were conducted on frozen silt at the temperature of -1.5°C under confining pressures of 0.5, 1.0, and 2.0 MPa. The results indicate that the creep properties of warm frozen silt are affected considerably by stress levels. The creep show present primary and secondary creep stages under low stress level and indicate that the specimen is destroyed quickly under high stress levels. Based on phenomenological theory, a unified creep constitutive model of warm frozen silt was proposed. The results predicted by the proposed model agree well with corresponding experimental data. Investigation of the random distribution has revealed that the parameters of the proposed creep model could be better described by a normal distribution. Based on the dynamic theory of frozen soil strength, the long-term strength of warm frozen silt was obtained, and was found to exhibit nonlinear behavior with increasing confining pressure. The Weibull function can represent the random distribution well, and the reliabilities of long-term strength were given under different confining pressures.