Electric-magnetic-force characteristics of rare earth barium copper oxide superconductor high-field coils based on screening effect and strain sensitivity

Rare earth barium copper oxide(REBCO)is the most researched and commercialized second-generation high-temperature superconducting material.Due to the anisotropic structure,strong deformation sensitivity,and central field errors caused by screening current effects,it is still a challenge for commercialization applications.In this study,the transversely isotropic constitutive relationship is selected as the mechanical model based on the structural characteristics of REBCO tapes,and suitable microelements are selected to equate the elastic constants using their average stress-strain relationships.Then,a two-dimensional axisymmetric model for coils wound by single-layer tapes is constructed to analyze the dependence of the electric-magnetic-force distribution in the tape on the strain.Finally,the anisotropic approximation of the homogenized bulk method is used to equate large-turn high-field coils,and the electric-magnetic-force distribution characteristics of the coils with/without screening effects and mechanical strain conditions are investigated,respectively.The results reveal that the mechanical strain has a weakening effect on the electromagnetic field distribution of superconducting tapes,but causes a significant enhancement in the force field distribution.In the presence of 0.5% mechanical strain,the maximum weakening of the peak value of the current density and the critical current density inside the high-field coil can reach about 8% and 13%,respectively,with a nearly 5 times increase in the peak stress.The screening current makes the current field distribution inside the coil improve by about 10 times.The screening current induced magnetic field can reach up to 0.8 T,making the relative error of the high-field coil center up to 7.8%.

Irradiation effect on strain sensitivity coefficient of strain sensing fiber Bragg gratings

The effect of irradiation on the strain sensitivity coefficient of strain sensing fiber Bragg gratings （FBGs） has been investigated through experiments. FBGs were fabricated in single mode fibers with 3 tool% Ge-concentration in the core and with a H2-1oading treatment. In experiments, the FBGs were subjected to y-radiation exposures using a Co6~ source at a dose-rate of 25 Gy/min up to a total dose of 10.5 kGy. The GeO defect in fiber absorbs photons to form a GeE＇ defect; the interaction with H2 is a probable reason for the y-radiation sensitivity of gratings written in hydrogen loaded fibres, The effect mechanism of radiation on the strain sensitivity coefficient is similar to that of radiation on the temperature sensitivity coefficient. Radiation affects the effective index neff, which results in the change of the thermo-optic coefficient and the strain-optic coefficient. Irradiation can change the strain sensitivity coefficient of FBGs by 1.48%-2.71%, as well as changing the Bragg wavelength shift （BWS） by 22 pm-25 pm under a total dose of 10.5 kGy. Our research demonstrates that the effect of irradiation on the strain sensitivity coefficient of FBG is small and that strain sensing FBGs can work well in the radiation environment.

Effect of Strain Rate on Tensile Behavior of Sn-9Zn-xAg-ySb;{(x, y) = (0.2, 0.6), (0.2, 0.8), (0.6, 0.2), (0.8, 0.2)} Lead-Free Solder Alloys

The tensile properties of Sn-9Zn-xAg-ySb;{(x, y) = (0.2, 0.6), (0.2, 0.8), (0.6, 0.2), (0.8, 0.2)} lead-free solders were investigated. All the test samples were annealed at 150°C for 1 hour. The tests are carried out at room temperature at the strain rate of 4.17 × 10-3 s-1, 20.85 × 10-3 s-1, and 208.5 × 10-3 s-1. It is seen that the tensile strength increases and the ductility decrease with increasing the strain rate over the investigated range. From the strain rate change test results, the strain sensitivity values are found in the range of 0.0831 to 0.1455 due to the addition of different alloying elements.

Strain hardening behavior, strain rate sensitivity and hot deformation maps of AISI 321 austenitic stainless steel

Hot compression tests were performed on AISI 321 austenitic stainless steel in the deformation temperature range of 800–1200℃ and constant strain rates of 0.001,0.01,0.1,and 1 s^(−1).Hot flow curves were used to determine the strain hardening exponent and the strain rate sensitivity exponent,and to construct the processing maps.Variations of the strain hardening exponent with strain were used to predict the microstructural evolutions during the hot deformation.Four variations were distinguished reflecting the different microstructural changes.Based on the analysis of the strain hardening exponent versus strain curves,the microstructural evolutions were dynamic recovery,single and multiple peak dynamic recrystallization,and interactions between dynamic recrystallization and precipitation.The strain rate sensitivity variations at an applied strain of 0.8 and strain rate of 0.1 s^(−1) were compared with the microstructural evolutions.The results demonstrate the existence of a reliable correlation between the strain rate sensitivity values and evolved microstructures.Additionally,the power dissipation map at the applied strain of 0.8 was compared with the resultant microstructures at predetermined deformation conditions.The microstructural evolutions strongly correlated to the power dissipation ratio,and dynamic recrystallization occurred completely at lower power dissipation ratios.

STRAIN RATE SENSITIVITY OF ULTRAFINE-GRAINED Cu WITH NANOSIZED TWINS

An ultrafine-grained Cu sample with a high density of growth twins was synthesized by means of pulsed electrodeposition technique. The strain rate sensitivity of the Cu sample was measured by strain rate cycling tests under tension. The effects of grain size as well as twin density on the strength and strain rate sensitivity were discussed.

Strain rate sensitivity and activation volume of electrodeposited nanocrystalline Ni and Ni-Co alloys

Tensile deformation behaviors of the electrodeposited 40 nm grain sized Ni,25 nm Ni-1.7 wt.%Co,and 13 nm Ni-8.6 wt.%Co alloys at various strain rates and room temperature were reviewed with emphasis on strain rate sensitivity and activation volume,respectively.It is found that the strain rate sensitivity and activation volume were strongly grain size dependent.An analytic model based on the bow out of a single dislocation well predicted the relationship between the strain rate sensitivity and the activation volumes for these nanocrystaline metals.

The strain rate sensitive and anisotropic behavior of rare-earth magnesium alloy ZEK100 sheet

To overcome the limitation in formability at room temperature,manufacturers have developed magnesium alloys with remarkable properties by adding rare-earth elements.The rare-earth magnesium alloys behave differently from the conventional alloys,especially with respect to their coupled anisotropic and strain rate sensitive behavior.In the current work,such behavior of the rare-earth Mg alloy ZEK100 sheet at room temperature is investigated with the aid of the elastic viscoplastic self-consistent polycrystal plasticity model.Different strain rate sensitivities(SRSs)for various deformation modes are employed by the model to simulate the strain rate sensitive behaviors under different loading directions and loading rates.Good agreement between the experiments and simulations reveals the importance and necessity of using different SRSs for each deformation mode in hexagonal close-packed metals.Furthermore,the relative activities of each deformation mode and the texture evolution during different loadings are discussed.The anisotropic and strain rate sensitive behavior is ascribed to the various operating deformation modes with different SRSs during loading along different directions.

Effect of strain rate on the compressive deformation behaviors of lotus-type porous copper

Lotus-type porous copper was fabricated by unidirectional solidification, and compressive experiments were subsequently conducted in the strain rate range of 10-3-2400 s-1 with the compressive direction parallel to the pores. A GLEEBLE-1500 thermal-mechanical simulation system and a split Hopkinson pressure bar （SHPB） were used to investigate the effect of strain rate on the compressive deforma-tion behaviors of lotus-type porous copper. The influence mechanism of strain rate was also analyzed by the strain-controlling method and by high-speed photography. The results indicated that the stress-strain curves of lotus-typed porous copper consist of a linear elastic stage, a plateau stage, and a densification stage at various strain rates. At low strain rate （〈1.0 s^-1）, the strain rate had little influence on the stress-strain curves; but when the strain rate exceeded 1.0 s^-1, it was observed to strongly affect the plateau stage, showing obvious strain-rate-hardening characteristics. Strain rate also influenced the densification initial strain. The densification initial strain at high strain rate was less than that at low strain rate. No visible inhomogeneous deformation caused by shockwaves was observed in lotus-type porous copper during high-strain-rate deformation. However, at high strain rate, the bending deformation characteristics of the pore walls obviously differed from those at low strain rate, which was the main mechanism by which the plateau stress exhibited strain-rate sensitivity when the strain rate exceeded a certain value and exhibited less densification initial strain at high strain rate.

Strain-hardening and warm deformation behaviors of extruded Mg-Sn-Yb alloy sheet

Strain-hardening and warm deformation behaviors of extruded Mg-2Sn-0.5Yb alloy(at.%)sheet were investigated in uniaxial tensile test at temperatures of 25-250 ℃ and strain rates of 1×10^(−3) s^(−1)-0.1 s^(−1).The data fit with the Kocks-Mecking type plots were used to show different stages of strain hardening.Besides III-stage and IV-stage,the absence of the II-stage strain hardening at room temperature should be related to the sufficient dynamic recrystallization during extrusion.The decrease of strain hardening ability of the alloy after yielding was attributed to the reduction of dislocation density with increasing testing temperature.Strain rate sensitivity(SRS)was significantly enhanced with increasing temperature,and the corresponding m-value was calculated as 0.07-0.12,which indicated that the deformation mechanism was dominated by the climb-controlled dislocation creep at 200 ℃.Furthermore,the grain boundary sliding(GBS)was activated at 250 ℃,which contributed to the higher SRS.The activation energy was calculated as 213.67 kJ mol^(−1),which was higher than that of lattice diffusion or grain boundary self-diffusion.In addition,the alloy exhibited a quasi superplasticity at 250 ℃ with a strain rate of 1×10^(−3) s^(−1),which was mainly related to the fine microstructure and the presence of the Mg2Sn and Mg2(Sn,Yb)particles.

Annealing and strain rate effects on the mechanical behavior of ultrafine-grained iron produced by SPD

Thermal stability and strain rate sensitivity of ultrafine-grained(UFG)Fe produced by severe plastic deformation(SPD)were investigated.The UFG Fe was processed by equal-channel angular pressing(ECAP)via route Bc.After 6 passes,the grain size of UFG Fe reaches 600 nm, as confirmed by means of electron back scatter diffraction(EBSD).Examination of micro-hardness and grain size of UFG Fe as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization.The critical transition temperature is approximately 500℃,and the material has a bimodal structure after annealing at this temperature.Deformation behaviors of ECAP Fe and ECAP + annealing Fe were studied under both quasi-static and dynamic compressive loadings.The UFG iron shows increased strength and reduced strain rate sensitivity compared with its coarse-grained counterparts.The appropriate post-ECAP annealing can increase strain hardening ability and cancel out thermal softening effect with only a small loss of strength under dynamic loading.

STRAIN RATE SENSITIVTTY OF POLYCRYSTALLINE AND SINGLE CRYSTAL NiAl

The compressive and tensile strain rate sensitivities of polycrystalline and single crystal NiAl have been evaluated at 877 ℃ which is well above DBTT. Samples were prepared to specific sizes by hot press consolidation of appropriate powder sizes and minimizing past consolidation thermal exposures. NiAl single crystals were grown in the [100]orientation using a modified Bridgeman technique. The yield and tensile strength of polycrystalline NiAl as a function of grain size generally follows a Hall-Petch type relationship . A tensile strain rate effect was found at strain rate of 1 to 10-4 sec-1. The strain rate sensitivity coefficient, m,in the equation of σ = Kε is in the range of 0. 10 to 0. 13. The highest strain rate (1 sec-1) resulted in significantly higher strengths and little or no observed ductility. Compressive strain rate testing al strain rate of 10-5to 10-2 sec-1 and at temperature of 1050°to 1250°K was conducted both for polycrystalline and for single crystal NiAl. The calculated stress exponents and the activation energy for creep were compared with the reference data.

Effects of Strain Rate and Texture on the Tensile Behavior of Pre-strained NiCr Microwires

The stress–strain behavior and strain rate sensitivity of pre-strained Ni80Cr20（Ni20Cr） were studied at strain rates from 4.8×10^（–4）s^（–1） to 1.1×10^（–1）s^（–1）. Specimens were prepared through cold drawing with abnormal plastic deformation. The texture of the specimen was characterized using electron backscatter diffraction. Results revealed that the ultimate tensile strength and ductility of the pre-strained Ni20Cr microwires simultaneously increased with increasing strain rate. Twinning-induced negative strain rate sensitivity was discovered. Positive strain rate sensitivity was present in fracture flow stress, whereas negative strain rate sensitivity was detected in flow stress values of σ_（0.5%） and σ_（1%）. Tensile test of the pre-strained Ni20Cr showed that twinning deformation predominated, whereas dislocation slip deformation dominated when twinning deformation reached saturation. The trends observed in the fractions of 2°-5°, 5°-15°, and 15°-180° grain boundaries confirmed that twinning deformation dominated the first stage.

Investigation of Strain-Temperature Cross-Sensitivity of FBG Strain Sensors Embedded Onto Different Substrates

The strain-temperature cross-sensitivity problem easily occurs in the engineering strain monitoring of the self-sensing embedded with fiber Bragg grating(FBG)sensors.In this work,a theoretical investigation of the strain-temperature cross-sensitivity has been performed using the temperature reference grating method.To experimentally observe and theoretically verify the problem,the substrate materials,the preloading technique,and the FBG initial central wavelength were taken as main parameters.And a series of sensitivity coefficients calibration tests and temperature compensation tests have been designed and carried out.It was found that when the FBG sensors were embedded on different substrates,their coefficients of the temperature sensitivity were significantly changed.Besides,the larger the coefficients of thermal expansion(CTE)of substrates were,the higher the temperature sensitivity coefficients would be.On the other hand,the effect of the preloading technique and FBG initial wavelength was negligible on both the strain monitoring and temperature compensation.In the case of similar substrates,we did not observe any difference between temperature sensitivity coefficients of the temperature compensation FBG with one free end or two free ends.The curves of the force along with temperature were almost overlapped with minor differences(less than 1%)gained by FBG sensors and pressure sensors,which verified the accuracy of the temperature compensation method.We suggest that this work can provide efficient solutions to the strain-temperature cross-sensitivity for engineering strain monitoring with the self-sensing element embedded with FBG sensors.

Superplastic behavior of a fine-grained Mg-Gd-Y-Ag alloy processed by equal channel angular pressing

An extruded Mg-6Gd-3Y-1.5Ag(wt%) alloy was processed by 6 passes of equal channel angular pressing(ECAP) at 553 K using route Bc to refine the microstructure. Electron back-scattered diffraction(EBSD) analysis showed a fully recrystallized microstructure for the extruded alloy with a mean grain size of 8.6 μm. The microstructure of the ECAP-processed alloy was uniformly refined through dynamic recrystallization(DRX). This microstructure contained fine grains with an average size of 1.3 μm, a high fraction of high angle grain boundaries(HAGBs), and nano-sized Mg_(5)Gd-type particles at the boundaries of the DRXed grains, detected by transmission electron microscopy(TEM). High-temperature shear punch testing(SPT) was used to evaluate the superplastic behavior of both the extruded and ECAP-processed alloys by measuring the strain rate sensitivity(SRS) index(m-value). While the highest m-value for the extruded alloy was measured to be 0.24 at 673 K, the ECAP-processed alloy exhibited much higher m-values of 0.41 and 0.52 at 598 and 623 K, respectively,delineating the occurrence of superplastic flow. Based on the calculated average activation energy of 118 kJ mol^(-1) and m-values close to 0.5, the deformation mechanism for superplastic flow at the temperatures of 598 and 623 K for the ECAP-processed alloys was recognized to be grain boundary sliding(GBS) assisted by grain boundary diffusion.

Continuous Deformation Monitoring by Polymermatrix Carbon Fiber Sensitive Layer

Composite made of short-cut carbon fiber mat and vinyl ester resin was observed to be an effective sensor for tensile strain up to 6 000με. Based on its strain sensitivity, a skin-like sensitive layer which can continuously cover the structural surface to sense strain in large area was developed. The sensitive layer was applied to continuously monitor the deformation of a simply supported beam. The result indicates that the fractional change in electrical resistance of the sensitive layer reversibly reflects the beam deformation in each section and describes the distribution of the average strain of the beam. The effect of temperature change on the monitoring was studied by monitoring tests conducted at different temperatures ranging from 20 to 80 ℃, which reveals temperature sensitivity in the sensitive layer and the temperature dependence of the piezoresistive behavior when the temperature exceeds 50 ℃. By the application of differential conaection principle, a method for temperature compensation was established and the gauge factor for the monitoring was dramatically increased. This method was verified experimentally.

DETERMINATION OF THE PARAMETERS OF SUPERPLASTIC FORMING FOR LONG RECTANGULAR THIN SHEET TITANIUM ALLOY Ti-6A1-4V

A method for determining the value of the strain rate sensitivity parameter m, of a thin sheet superplastic material, which is based on the results of constant gas pressure tests, has been developed in this paper. Unlike the conventional procedures the method involved provides the test conditions similar to those occurring during an industrial technological process. Such an approach enables one to estimate reliable the superplas- tic properties of the material under study. Theoretical analysis is based upon use of the standard equations of the membrane theory. The experimental investigations have been carried out for Ti-based alloy VT6 (Ti--6Al-4V). Theoretical predictions show satisfactory agreement with experimental data. The results obtained are compared with those measured by means of standard procedures (constant cross-head velocity experiments and load relaxation tests). It is shown that the use of specimens having reduced gage length (which is less than specimen's width) could lead to considerable errors in experimental estimation of the m value which, in its turn, leads to inaccurate calculating the technological parameters.

ANALYTICAL EVALUATION OF PERMANENT DEFLECTION OF A THIN CIRCULAR PLATE STRUCK NORMALLY AT ITS CENTER BY A PROJECTILE

The permanent deflection of a thin circular plate struck normally at its center by a projectile is studied by an approximate theoretical analysis, FEM simulation and experiment. The plate made of rate sensitive and strain-hardening material undergoes serious local deformation but is not perforated during the impact. The theoretical analysis is based on an energy approach, in which the Cowper-Symonds equation is used for the consideration of strain rate sensitive effects and the parameters involved are determined with the aid of experimental data. The maximum permanent deflections predicted by the theoretical model are compared with those of FEM simulation and published papers obtained both by theory and experiment, and good agreement is achieved for a wide range of thickness of the plates and initial impact velocities.

Inverse grain-size-dependent strain rate sensitivity of face-centered cubic high-entropy alloy

It is well documented that the strain rate sensitivity(m)increases at refined grain size for face-centered cubic(FCC)metals and alloys.Through a series of nanoindentation testing,however,we experimentally demonstrated a striking departure from conventional FCC metals that Co Cr Fe Mn Ni high entropy alloy(HEA)with FCC lattice structure exhibits monotonously decreased m as grain size reduced fromμ30.3m to 7.2 nm.Moreover,the apparent activation volume v*,which generally shows an opposite trend of m,exhibited the identical decreasing trend with reduced grain size as that of m.Such an unusual trend of m and its correlation with v*in the FCC HEA alloys can be understood by a distinct deformationmechanism-transitions and unique dislocation morphology evolution that differs from conventional FCC metals.

Unusual He-ion irradiation strengthening and inverse layer thickness-dependent strain rate sensitivity in transformable high-entropy alloy/metal nanolaminates:A comparison of Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu vs Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu

In this work,we prepare transformable HEA/Cu nanolaminates(NLs)with equal individual layer thick-ness(h)by the magnetron sputtering technique,i.e.,Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu and Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu,and comparatively study He-ion irradiation effects on their microstructure and mechanical properties.It ap-pears that the as-deposited HEA/Cu NLs manifest two size h-dependent hardness regimes(i.e.,increased hardness at small h and hardness plateau at large h),while the He-implanted ones exhibit monotonically increased hardness.Contrary to the fashion that smaller h renders less irradiation hardening in bimetal NLs,the Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs manifest the trend that smaller h leads to greater irradiation hard-ening.By contrast,the Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu NLs exhibit the maximum irradiation hardening at a critical h=50 nm.Below this critical size,smaller h results in lower radiation hardening(similar to bimetal NLs),while above this size,smaller h results in greater radiation hardening(similar to Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs).Moreover,these transformable HEA/Cu NLs display inverse h-dependent strain rate sensitivity(SRS m)before and after He-ion irradiation.Nevertheless,compared with as-deposited samples,the irradi-ated Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs display reduced SRS,while the irradiated Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu NLs dis-play enhanced SRS.Such unusual size-dependent irradiation strengthening and inverse h effect on SRS in irradiated samples were rationalized by considering the blocking effects of He bubbles on dislocation nucleation and motion,i.e.,dislocations shearing or bypassing He bubbles.

Positive Strain Rate Sensitivity and Deformation Behavior of a Fe–29Mn–3Al–3Si TWIP Steel

The Fe-29 Mn-3 Al-3 Si twin-induced plasticity(TWIP)steel is used to conduct quasi-static compression and dynamic impact deformation with strain rates ranging from 8.3×10^(-4) to 3800 s^(-1).The microstructures and properties of deformed samples under different strain rates were investigated comparatively.These results show that positive strain rate sensitivity was observed with the increase in strain rates and that there was a significant difference in strain rate sensitivity factor(m)between quasi-static compression(m=0.029)and dynamic impact deformation(m=0.190).Compared to the quasi-static compression,the dynamic impact deformation exhibited higher yield strength.Microstructural examination reveals that the primary twins were frequently found during the quasi-static compression process,and the secondary twins were rarely observed.However,the secondary and multi-fold deformation twins were florescent in the dynamic impact samples.At the initial stage of dynamic impact deformation,partial dislocations and staking faults on multiple conjugate{111}planes were simultaneously activated and produced a large number of Lomer-Cottrell dislocations,resulting in a large increase in yield strength during dynamic impact.