Torsional Creep Tests Research Articles

Creep strength of Mod.9Cr1Mo steel under biaxial stress states

This paper describes the development of tension, torsion, and combined tension-torsion creep testing machine with hydro static thrust bearing, criterion on creep rupture strength under biaxial stress states was examined. Tension, torsion, and combined tension-torsion creep tests were carried out at 873 K in air employing the developed creep testing machine and hollow cylindrical specimen of Mod.9Cr1Mo steel. Ratio of rupture time tR/tR* increased with the decreasing principal stress ratio λ at constant Mises' equivalent stress, where tR, and tR* denote rupture times for biaxial and tension creep tests, and λ expresses ratio of minimum principal stress to maximum principal stress. Hayhurst's stress successfully correlated with tension, torsion, and combined tension-torsion loading creep lives but Mises' type equivalent stress and maximum principal stress were inadequate to predict creep lives of Mod.9Cr1Mo steel. Oxide film and micro cracks on specimen surfaces were observed using SEM. Micro cracks and oxide film cracks initiated to the perpendicular direction to maximum principal stress axis for -1≦λ≦0. Main crack grew in direction perpendicular to specimen axis independent on the maximum stress ratio λ. The residual lives under biaxial stress states were estimated using Omega method. The Omega method was able to estimate the residual lives within a factor of 4 under biaxial stress states.

Measurement of Very Low Creep Strains: A Review

Abstract Various experimental techniques using mechanical springs for high strain sensitivity creep testing are described. A theory of stress and strain using these techniques is briefly summarized. Problems and advantages of the helicoid spring specimen technique are also analyzed. The helicoid spring specimen technique provides high efficiency for low-stress creep experiments in the self-loaded mode, while the external load mode allows extraordinary strain sensitivity. A new machine for conducting high-sensitivity torsion creep tests is presented together with some examples of creep curves obtained by the technique. The machine was designed to enable high sensitivity creep tests of inherently brittle materials such as intermetallics and ceramics.

Influence of chromium and vanadium in the mechanical resistance of steels

Steel exposed to high temperatures and stress in a corrosive environment suffers detriment in the mechanical properties and finally embrittlement. The objective of this work is to study the mechanical properties of steels of different chemical compositions under hydrogen attack (HA). The influence of chromium and vanadium in the mechanical resistance was analyzed. The 1.25Cr1Mo0.25V and 2.25Cr1Mo ferritic steels previously hydrogenated are studied at different temperatures and loads. The temperature, electric charge density and electrolyte concentration were modified during the electrochemical charge in order to find the optimum hydrogen income to the material. The metal test conditions were characterized by scanning electron microscopy. The material remains in stationary state for a period of time and the residual damage was evaluated. The steels were subject to torsion creep tests while the temperature and the load were maintained constant along the experience. We studied the relationship between the strain rate at the secondary stage and both the stress and temperature. The stress exponent and the activation energy were also determined. A comparison with the same steel samples without hydrogen charge was also performed. The microstructural mechanics during the tests and the embrittlement degree were also discussed.

Grain-Boundary Viscosity of Preoxidized and Nitrogen-Annealed Silicon Carbides

Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO2 glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2-added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials.

Superplastic Deformation Behavior of the Y-TZP Tested in Torsion

Constant torque torsional creep tests for 3Y-TZP have been conducted in an ambient atmosphere at 1623K to 1773K in the torque range of 0.53 to 1.51Nm. While there are no traces of primary and tertiary creeps, the creep-rate decreases with the increase in strain due to concurrent grain growth. In the shear stress versus shear strain-rate relations obtained in the present experimental range, there exist two regions; one in which m-value is 0.43 to 0.45 and the other in which m-value is 0.21 to 0.22 above and below the shear stress of around 14MPa, respectively. In the former, i.e., high m region, grain size exponent, p ≃ 2 and activation energy, Q=443KJ/mol, whereas in the latter, i.e., low m region, p ≃ 3 and Q=490KJ/mol. The former region corresponds to the superplastic region II. Regarding the latter region, it seems to be relating to the existence of threshold stress.

Proton irradiation creep of Inconel 718 at 300°C

Torsional creep tests were conducted on Inconel 718 in the precipitation hardened condition under 17 MeV proton irradiation at 300°C upto a maximum dose of 0.35 dpa. The stress dependence of the irradiation creep rate was linear for the applied shear stresses which ranged from 150 to 450 MPa. The results are discussed in relation to the operating conditions of an ITER-like machine, where Inconel 718 bolts are used to mechanically attach the shielding blanket to the backplate. The irradiation creep induced stress relaxation amounted to about 30% after a dose of 0.35 dpa.

Irradiation creep of advanced silicon carbide fibers

The bend stress relaxation (BSR) method was applied to study irradiation enhanced creep (IEC) of small diameter silicon carbide (SiC) fibers after 10 MeV proton irradiation. A first series of tests was conducted on Sylramic™ fibers irradiated at 600°C with average bending stresses of 400 and 667 MPa and for irradiation doses smaller than 0.04 dpa. The BSR results are compared to previously obtained torsional creep test results for the Textron SCS-6™ type SiC fibers by calculating the tensile equivalents for both testing methods. For the Sylramic fibers, the creep constant κ=4.7×10−6 Mpa−1 dpa−1, was a factor of 6 smaller than the κ-value determined for SCS-6 fibers at 600°C. In contrast, for T<900°C the κ-value determined by R.J. Price [Nucl. Technol. 35 (1977) 320] for high purity monolithic β-Si after 7.7 dpa neutron irradiation was only 0.4×10−6 MPa−1 dpa−1.

Irradiation creep induced stress relaxation of Inconel 718

Torsional creep tests were conducted on Inconel 718 specimens under 17 MeV proton irradiation at 300°C for doses up to ∼0.08 dpa. The stress dependence of the irradiation creep rate was measured by increasing the maximum applied shear stress from 150 up to 450 MPa. The experimental results show that the stress dependence of the irradiation creep rate is linear in the whole stress range applied. The irradiation creep constant κ=1.2×10 −11 (dpa −1 Pa −1) at 300°C. The calculated creep induced stress relaxation amounts to ∼10% after 0.1 dpa. The results are discussed in relation to the operating condition of an ITER-like machine, where Inconel 718 bolts are used to mechanically attach the shielding blanket to the back plate.

522 多軸応力状態におけるオーステナイト鋼SUS310Sのクリープボイドの発生・成長挙動に関する検討(OS 耐熱材料)

Creep fracture characteristics consist of the initiation and growth behavior of creep voids , creep fracture mode, and rupture life. However, initiation and growth behavior of creep voids in multi-axial stress states have not been made clear. In this study, using ductile austenitic steel SUS310S. tensile creep, torsional creep and tensile-torsional creep tests with tubular specimens are conducted at 700℃. Creep voids in test-stopped specimens are examined in detail by observation on the scanning electron micrographs. The initiation and growth behaviors of creep voids under multi-axial stress conditions are discussed.

608 ねじり応力下における3Y-TZPの超塑性変形挙動(オーガナイズドセッション : 超成形・超塑性と材料)

Constant torque torsional creep tests for 3Y-TZP have been conducted in an ambient atmosphere at 1623K to 1773K in the torque range of 0.53 to 1.51Nm. While there are no traces of primary and tertiary creeps, the creep-rate decreases with the increase in strain due to concurrent grain growth. In the shear stress versus shear strain-rate relations obtained in the present experimental range, there exist two regions; one in which m-value is 0.43 to 0.45 and the other in which m-value is 0.21 to 0.22 above and below the shear stress of around 14MPa, respectively. In the former, i.e., high m region, grain size exponent, p &sime; 2 and activation energy, Q=443KJ/mol, whereas in the latter, i.e., low m region, p &sime; 3 and Q=490KJ/mol. The former region corresponds to the superplastic region II . Regarding the latter region, it seems to be relating to the existence of threshold stress.

Light ion irradiation creep of SiC fibers in torsion

Creep tests were conducted in torsion on TEXTRON type SCS-6™ silicon carbide (SiC) fibers during irradiation with 14 MeV deuterons for 450°C, 600°C and 800°C. The fibers, produced by chemical vapor deposition (CVD), should be representative of the chemical vapor infiltrated (CVI) matrix of a SiC/SiC composite. SiC is known to undergo irradiation induced swelling which occurs without an incubation dose for temperatures below about 1000°C [R.J. Price, J. Nucl. Mater. 33 (1969) 17]. Such swelling in SiC may mask the irradiation creep strain in a tensile experiment, but plays a minor role in torsional creep tests. The torsional irradiation creep curves are characterized by long lasting strain transients during which the creep rate slows down before reaching approximately constant values. The steady state torsional creep rate γ ̇ s exhibited a linear dependence on stress and particle flux and it decreased when the temperature was increased. The temperature dependence of γ ̇ s in the range 450–800°C is similar to that of swelling for neutron irradiated SiC.

Creep-Fatigue Life Evaluation of Solder Based on Creep Characteristics.

Torsional fatigue tests and creep tests were carried out on a 60Sn-40Pb solder. In both tests three temperatures (303∼333K) were used, and in the fatigue test five cycling frequencies (0.001∼5.0 Hz) were used. An evaluation method for the solder fatigue life under relatively high temperature or low cycling frequency was proposed from the test results, noting that the characteristics of fatigue under the above mentioned conditions were similar to the creep characteristics. Furthermore, this evaluation method was applied to the estimation of the crack initiation life of through-hole-type joints. By comparing the estimated values with the experimental ones, it was clarified that this method was valid for the assessment of the crack initiation life of solder joints.

Acoustic Emission in Single Crystals of Ice

The dislocation dynamics during the creep deformation of single crystals of ice Ih was studied using acoustic emission (AE) measurements. The AE activity was recorded during uniaxial compression and torsion creep tests. The results were interpreted in terms of dislocation dynamics with the help of an AE source model relating the amplitude of an acoustic event to the number of dislocations involved in the event and to their velocity. This model was first validated by a comparison between the global AE activity and the global strain rate. Then, it was possible to evaluate the density of moving dislocations during creep deformation. Two regimes were revealed. Without significant polygonization, the density of mobile dislocations, deduced from AE, was proportional to the stress, but increased much faster after polygonization, in agreement with theoretical arguments. Finally, the power law distributions observed for AE amplitudes, the slow driving process, the very large number of interacting dislocations invo...

High-temperature triaxial torsional creep tests of concrete at various hygral conditions

Results of an initial series of six-day creep tests under a program aimed at establishing fundamental deformation properties of concrete at high temperatures under triaxial loading with shear and various hygral conditions, both constant and transient, are reported. The tests are conducted in a novel large triaxial torsional testing machine with hygrothermal control. Tested are cylinders of six inch diameter, sealed or unsealed, loaded by compressive axial force, chamber pressure and torque. Some specimens are sealed wet, some are sealed after drying in an oven, and some are let to dry during the test. Significant differences in creep at various hygro-thermal conditions are observed. Particularly interesting is the fact that hygro-thermal changes affect not only normal creep strains but also shear creep strains. The results are of interest for the formulation of constitutive relations needed for the analysis of nuclear reactor accidents, radioactive waste disposal, and fire resistance.

The creep behaviour of alloy 800H under tension and torsion

Torsional creep test results for Alloy 800H at 800°C are presented for the mean shear stress range 26–52 MPa. Comparisons with the secondary creep rate and rupture life under tension are made on the basis of the octahedral or von Mises definitions of equivalent stress and strain rate. It is shown that the creep rates and rupture lives from the two tests are not equivalent when such definitions are employed. However, an improvement in the comparison between creep lives is found when those for torsion are redefined at the onset of buckling. The effect of section size on tensile creep life (and ductility) appears to be significant when making a comparison of this kind. Further tensile rupture data in the temperature range 650–1000°C correlate satisfactorily with a time-temperature parameter either of the Larson-Miller type or which effectively translates rupture curves rigidly on to a single reference curve. The use of these parameters in the extrapolation to longer lives within the given temperature range is more reliable at high temperatures when the effect of γ′ precipitation on strength and ductility over prolonged periods of creep is known to be much less significant than it is for lower temperatures.

Simultaneous Stress Relaxation in Tension and Creep in Torsion of 2618 Aluminum at Elevated Temperature

Experiments are reported for stress relaxation and simultaneous stress relaxation and creep with proportional and nonproportional loading and unloading. Results were compared with predictions of a viscous-viscoelastic theory, and modifications, and strain hardening. Predictions were calculated from results of combined constant-stress tension and torsion creep and recovery tests only, which were reported previously. Results showed that a modified viscous-viscoelastic theory predicted all the observed features and predicted the creep and relaxation rates reasonably well.

Locked in stresses, creep and dilatancy of rocks, and constitutive equations : Rock Mech, V13, N1, Aug 1980, P5–22

Locked in Stresses, Creep and Dilatancy of Rocks, and Constitutive Equations A micro rheological analysis is presented of the deformation characteristics of rocks based on the multi mineral and polycrystalline structure of igneous metamorphic and some sedimentary rocks; in addition the polygranular structure of sandstone-type rocks is also considered. Due account is taken with the history of rock formation and tectonics. Torsional creep test results on Ichang sandstone are presented and analysed. A new method of testing is introduced whereby the sample is subjected to a step wise loading function and the deformation measured as a function of the time. In this manner the creep as a function of stress and time can be obtained very easily from tests on only one sample. A hypothesis is presented on the origin and formation of “locked in” stresses the release of internal strain energy is studied at the hand of some typical test results. The practical importance of these stress pockets is discussed and it is stressed that creep and “locked in” stresses are fundamental factors in the behaviour of rocks which must be carefully studied in practice. On the basis of experimental results and physical reasoning constitutive equations are set up, which are three dimensional generalisations of the experimental Griggs equation:γ =a +b logt +ct, whereby the material parameters are scalar functions of the stress invariants.

Locked in stresses, creep and dilatancy of rocks, and constitutive equations

Locked in Stresses, Creep and Dilatancy of Rocks, and Constitutive Equations A micro rheological analysis is presented of the deformation characteristics of rocks based on the multi mineral and polycrystalline structure of igneous metamorphic and some sedimentary rocks; in addition the polygranular structure of sandstone-type rocks is also considered. Due account is taken with the history of rock formation and tectonics. Torsional creep test results on Ichang sandstone are presented and analysed. A new method of testing is introduced whereby the sample is subjected to a step wise loading function and the deformation measured as a function of the time. In this manner the creep as a function of stress and time can be obtained very easily from tests on only one sample. A hypothesis is presented on the origin and formation of “locked in” stresses the release of internal strain energy is studied at the hand of some typical test results. The practical importance of these stress pockets is discussed and it is stressed that creep and “locked in” stresses are fundamental factors in the behaviour of rocks which must be carefully studied in practice. On the basis of experimental results and physical reasoning constitutive equations are set up, which are three dimensional generalisations of the experimental Griggs equation:γ =a +b logt +ct, whereby the material parameters are scalar functions of the stress invariants.

Physical aging of margarine

Margarine samples were quenched from 30°C to measuring temperatures of −10, 0, 5 and +10°C. At each measuring temperature a sequence of short small-strain torsional creep tests was performed giving the effect of the time elapsed after the quench on the creep properties. Aging effects were found at +5°C and lower, and surprisingly, the creep and aging behaviour of margarine strongly resembles that of synthetic polymers.

High Temperature Torsional Creep Test for Refractories

Measurement of hot strength is becoming increasingly significant for refractories. At high temperatures, the bricks of furnace linings are subjected, mainly, to three types of stresses, namely tensile, transverse or compressive stress, and undergo stretching, bending or compression. For the above purpose, measurements of high temperature tensile, torsional, transverse and compressive strengths are adopted. Due to appearance of liquid phases at high temperature, the strength measurements are complicated and practically no standardisation in the apparatus nor in the method of evaluation has been done. In the present investigation attempt has been made to standardise the high temperature torsional-creep test by taking into consideration the factors which cause variation in the result. This method will enable to measure the apparent viscosity of refractories at high temperatures, utilizing a torsion testing apparatus.