Total Tensile Strain Research Articles

Enhanced critical axial tensile strain limit of CORC® wires: FEM and analytical modeling

Conductor on Round Core (CORC®) cables and wires are composed of spiraled high-temperature superconducting (HTS) rare-earth barium copper oxide (REBCO) tapes, wound in multiple layers, and can carry very high currents in background magnetic fields of more than 20 T. They combine isotropic flexibility and high resilience to electromagnetic and thermal loads. The brittle nature of HTS tapes limits the maximum allowable axial tensile strain in superconducting cables. An intrinsic tensile strain above about 0.45% will introduce cracks in the REBCO layer of straight HTS tapes resulting in irreversible damage. The helical fashion at which the REBCO tapes are wound around the central core allows tapes to experience only a fraction of the total axial tensile strain applied to the CORC® wire. As a result, the critical strain limit of CORC® wires can be increased by a factor of more than 10 that of REBCO tapes. Finite element (FE) and analytical models are developed to predict the performance of CORC® wires under axial tensile strain. A parametric analysis is carried out by varying the winding angle, the Poisson’s ratio of the CORC® wire core, the core diameter, and the tape width. The results show that a small variation in winding angle can have a significant impact on the cable’s axial tensile strain tolerance. While the radial contraction of the helically wound tapes in a CORC® wire under axial tensile strain depends on its winding angle, it is mostly driven by the Poisson’s ratio of the central core, affecting the tape strain state and thus its performance. Contact pressure from multiple layers within the CORC® wire also affects the CORC® wire performance. The FE model can be used to optimize the cable design for specific application conditions, resulting in an irreversible strain limit of CORC® cables and wires as high as 7%.

On the necessity of determining the dynamics of development of deformation during mechanical tests under conditions of cable production

The analysis of results of comparison of mechanical properties of polyolefin filled with fire retardants is carried out. Experimental data for evaluating the dynamics of the development of uniaxial tensile deformation during standard tests of cable polymer compositions under production conditions are presented. The organization of technological control of ensuring the mechanical strength of structural elements made of halogen-free plastics in a specific production environment is one of the urgent tasks in the manufacture of fire-safe cables. To analyze the dynamics of the development of deformation of halogen-free cable plastics and to determine a practically important quantitative characteristic, it is necessary to study the initial section of the relationship between deformation and time, at which the deformation of the polymer is limited by elastic and viscoelastic deformation. The characteristic dependences of the elongation deformation of the samples on the time of uniaxial tension at a constant rate of separation of the clamps are presented in the process of the standard procedure for determining the mechanical characteristics for halogen-free cable plastics. For all samples, an initial (up to 10 – 15 seconds) linear section is observed with a different growth rate of the total tensile strain.

Cyclic tensile behavior of high strain hardening UHPC analyzed by acoustic emission techniques

Understanding the cyclic tensile behavior of high strain hardening ultra high performance concrete (UHPC) is important in structural design and for predicting its damage state. In this study, acoustic emission (AE) analysis was applied to monitor the damage evolution process of high strain hardening UHPC under direct tensile cyclic loading at different objective tensile strain levels. The test results show that when the objective strain is in the elastic state, the tensile cyclic stress–strain curves show marginal change, whereas when it is in the strain hardening state, the residual strain and the stiffness of UHPC are reduced by increasing the number of cycles and finally tends toward stability, the maximum stress remains stable. The AE analysis results can help to explain the evolution of the mechanical properties of high strain hardening UHPC under cyclic tensile loading and proves the existence of the Kaiser effect, which can be used to quantify the deformation and the degree of damage of UHPC. The stiffness of UHPC under cyclic tension is controlled by the nominal debonding length of the steel fibers. Finally, the relationship between the total tensile strain and the loading stiffness degradation ratio under cyclic tension was proposed and could be employed in evaluate of stiffness of the high strain hardening UHPC under cyclic tension in some extent.

A modified Johnson-Cook model of AA6061-O aluminum alloy with quasi-static pre-strain at high strain rates

In this paper, specimens were first pre-stretched to different pre-strain coefficients (0, 0.4 and 0.8) under quasi-static tensile for AA6061-O aluminum alloy. Then, the specimens with different pre-strain coefficients were stretched by a high-speed tensile machine (HTM) at different strain rates (200 s− 1, 400 s− 1 and 600 s− 1). Digital image correlation (DIC) technique was employed to measure the strains. The results showed that the yield ratio increased significantly with the pre-strain increased. As the pre-strain coefficient increased, high-speed tensile strain limitation decreased and the total tensile strain limitation increased. A modified Johnson-Cook (JC) model considering pre-strain coefficient was proposed. Numerical simulations were performed using LS-DYNA with the modified JC model. The strain field of the specimen taken by the camera agreed well with the simulated strain field.

In situ measurement of elastic and total strains during ambient and high temperature deformation of a polygranular graphite

In situ neutron diffraction and synchrotron X-ray diffraction, combined with image correlation analysis of 2D optical and 3D X-ray tomography datasets, have been used to investigate the relationship between elastic lattice strain and total strain during deformation of Gilsocarbon (IM1-24) polygranular nuclear grade graphite. The specimens were flat-end Brazilian discs under diametral loading, such that a compressive-tensile biaxial stress state was developed in the central region. The X-ray study was at ambient temperature, and the neutron diffraction was conducted at temperatures from ambient to 850 °C. When under compression, there is a temperature-insensitive linear relationship between the total strain and the lattice strain that is measured perpendicular to the graphite basal planes. However, when under tensile stress, the total strain and elastic strain relationship is temperature sensitive: below 600 °C, the lattice tensile strain saturates with increasing total tensile strain; above 600 °C, significantly higher tensile lattice strains are sustained. The saturation in tensile lattice strain is attributed to microcracking in the graphite microstructure. Improved resistance to microcracking and damage tolerance at elevated temperature explains the increase in tensile strength of polygranular graphite.

A New 3D Creep-Fatigue-Elasticity Damage Interaction Diagram Based on the Total Tensile Strain Energy Density Model

Fatigue damage, creep damage, and their interactions are the critical factors in degrading the integrity of most high-temperature engineering structures. A reliable creep-fatigue damage interaction diagram is a crucial issue for the design and assessment of high-temperature components used in power plants. In this paper, a new three-dimensional creep-fatigue-elasticity damage interaction diagram was constructed based on a developed life prediction model for both high-temperature fatigue and creep fatigue. The total tensile strain energy density concept is adopted as a damage parameter for life prediction by using the elastic strain energy density and mean stress concepts. The model was validated by a great deal of data such as P91 steel at 550 °C, Haynes 230 at 850 °C, Alloy 617 at 850 and 950 °C, and Inconel 625 at 815 °C. The estimation values have very high accuracy since nearly all the test data fell into the scatter band of 2.0.

Microstructure Evolution of Extruded Mg–6Gd Alloy Under 175 °C and 150 MPa

The tensile creep behavior of extruded Mg–6Gd alloy, having the tensile yield strength of ~ 110 MPa at 175 °C, has been investigated under 175 °C and 150 MPa. In this study, the extruded Mg–6Gd sample exhibits the total tensile strain of ~ 10.5% after the creep time of 1100 h, and the fast plastic strain of ~ 4.6% at the beginning of the creep test. The microstructure result suggests that the dislocation deformation is the main deformation mode during creep, and the grains with orientation close to 〈0001〉 || ED disappear after creep. The creep process containing a low creep strain has no effective promotion for the precipitation compared with the aging process without strain. The origination of creep crack is related to the formation of precipitate-free zone during creep. The work offers an important implication to research the microstructure evolution under an applied stress in a weak aging response Mg alloy.

Elasto-plastic behaviour of a columnar structure of nanocrystalline iron with sharp 〈011〉 fibre texture

We explore, using molecular dynamics (MD), the strength and mechanical stability at 298 K of pure bcc Fe with an oriented columnar nanostructure of long grains randomly distributed about 〈011〉 and average size ranging from 20 to 5 nm in the cross-section. According to the simulations, the tensile flow stress of such aligned nanostructure spans from 4 GPa to 6 GPa as grain size decreases from 20 nm to 5 nm. The axially oriented tilt boundaries are very stable to axial plastic elongations as large as 0.5 true total (elastoplastic) tensile strain. The structure shows an important Bauschinger effect in compression after tensile elongation, but remains stable for relatively large compressive deformations. However, moderate plastic transverse shear deformations, which promote shear-coupled tilt boundary migrations, destabilize it, by provoking grain growth and softening. Our simulations connect with real world materials: very fine wires obtained by large-strain drawing bcc metals or alloys develop a similar columnar structure of 〈110〉 axially oriented grains of nanometric cross section and extraordinary mechanical strength.

Shape-memory characterization of NiTi microtubes fabricated through interdiffusion of Ti-Coated Ni wires

Near-equiatomic NiTi microtubes were fabricated using an additive alloying method consisting of two steps: (i) depositing a Ti-rich coating onto ductile, pure Ni wires (50 μm in diameter) via pack cementation, resulting in a Ni core coated with concentric NiTi2, NiTi and Ni3Ti shells, and (ii) homogenizing the coated wires to near-equiatomic NiTi composition via interdiffusion between core and shells, while concomitantly creating Kirkendall pores. Because of the spatial confinement and radial symmetry of the interdiffusing core/shell structure, the Kirkendall pores coalesce near the center of the wire and form a continuous longitudinal channel, thus creating a microtube. Both the mechanical and thermal response of the NiTi microtubes were characterized in this study using a combination of dynamic mechanical analysis and differential scanning calorimetry, respectively, in conjunction with conventional metallography and X-ray tomographic microscopy. Due to slight compositional variations, both shape-memory and superelastic behaviors were observed within the same microtube, which achieved a total tensile strain of ∼8% before failure: the largest contribution to the strain recovery was the thermal shape memory effect showing near complete strain recovery occurring during multiple cycles. A second microtube exhibited only superelastic behavior, achieving a maximum, recoverable strain of 2.5% at 110 MPa, likely limited by the presence of a remaining Ni3Ti core as a result of under-titanization. Finite-element analysis of elastic stresses in a wire segment modeled from actual tomography data illustrates the extent of stress concentrations due to inner and outer tube surface roughness. The stress concentrations are responsible for a 65% increase in the top 1% average von Mises stress, which may further affect the shape-memory behavior of the tubes.

Effects of solution treatment on microstructure and superelasticity of FeNiCoAlTaB alloy

The effects of solution treatment on the microstructure, macrotexture and superelastic properties of FeNiCoAlTaB alloy were investigated in this study. The results showed that rolling texture of (011) [2-11] (Brass-type) developed by cold-rolling with reduction of 98.5% transferred to (023) [100] and (032) [100] texture after solution treatment with the temperature above 1200 °C, forming a strong 〈100〉 fiber recrystallization texture. Solution treatments at the temperature above 1250 °C facilitated the formation of a single austenite phase which was in favor of the precipitation of γ′ phase during aging treatment and martensitic transformation in cyclic tension. In addition, microcracks appeared readily in the alloy for solution treatment with the temperature above 1300 °C, which negatively affected the superelastic performance. Under the experimental conditions of this study, the 98.5% cold-rolled FeNiCoAlTaB alloy acquired a preferable superelasticity after solution treatment at 1250 °C for 1 h and aging treatment at 600 °C for 72 h, with the total tensile strain of 3.8%, the complete cyclic superelastic strain of 1.6% and the tensile strength of 885 MPa.

Influence of technological defects and neutron irradiation on the mechanical properties of single-crystal molybdenum pipes

The state of single-crystal molybdenum pipes irradiated in an IVV-2M reactor at the peak temperature of 1400°C for 1700 and 3200 h by the fast neutron fluence of 3 × 1020 and 5 × 1020 n cm–2, respectively, is investigated. The aim is to analyze the shape and surface state of pipes, the metallography of the structure, the short-time mechanical properties, and the character of destruction of the ring pipe specimen under testing. Radiation strengthening of molybdenum single-crystal is not registered. The strength parameters of the irradiated material are the level of initial values. The plastic properties of samples approach zero under room temperature tests. When the temperature is above 250°C, the total tensile strain of the sample exceeds 15% and ductile fracture with neck thinning to zero takes place. The perfect structure of an irradiated single crystal is preserved. Technological defects such as microcracks on the inner surface of pipes may act as stress concentrators.

Deformation microstructural evolution and strain hardening of differently oriented grains in twinning-induced plasticity β titanium alloy

The {332}<113> twinning structure evolution in nearly [1¯22] and [001] oriented grains was quantitatively examined in a polycrystalline Ti-15Mo alloy at various tensile strains. Twinning with a single variant, which obeyed Schmid’s law, was induced in [1¯22] grain after yielding. The area fraction of twins rapidly increased from 3% to 69% with strain from 0.02 to 0.15, and changed gradually to 81% at strains of up to 0.25. In [001] grain, twin formation violating Schmid’s law with three variants was confirmed after the strain reached 0.01. Twins with an area fraction of 0.7% showed no significant change with further deformation. The contribution of deformation modes to the total tensile strain in [1¯22] grain was dominated by twinning at strains of up to 0.15, and became dislocation slip with further deformation. In [001] grain, dislocation slip mainly contributed to the plastic deformation over the entire strain range. Dynamic microstructure refinement arising from twinning, namely the dynamic Hall–Petch effect, was the main strain hardening mechanism in [1¯22] grain at strains of up to 0.15. However, strain hardening caused by twinning was negligible in [001] grain. The effects of local stress concentration and geometric constraint between neighboring grains on the deformation microstructural evolution and strain hardening behavior should also be considered.

Enhancement of mechanical and electrochemical properties of Al0.25CrCoFe1.25Ni1.25 high-entropy alloys by coating Ni–P amorphous films

The Al0.25CrCoFe1.25Ni1.25 high-entropy alloys (HEAs) are coated with Ni–P amorphous films with a thickness of 35μm. The as-cast HEAs have a yielding strength of 52MPa. In contrast, the yielding strength of coated HEAs exhibits over 130MPa, with a 160% improvement. A considerable total tensile strain as high as about 50% is detected in the as cast and coated HEAs. In addition, Ni–P coatings have a stable passive characteristic and form a thick passive film, suggesting that Ni–P coatings provide an effective barrier against active dissolution process. The current Ni–P coated HEAs combine the excellent mechanical and electrochemical properties.

Quantitative Measurements of Grain Boundary Sliding in an Ultrafine-Grained Al Alloy by Atomic Force Microscopy

In the current study, quantitative measurements for grain boundary sliding (GBS) in ultrafine-grained (UFG) 5083 Al by atomic force microscopy (AFM) were performed. An ion beam polishing and etching technique was used to reveal grain boundaries in the alloy for AFM characterization. A comparison between the average grain sizes measured from AFM images and those estimated from transmission electron microscopy micrographs and electron backscatter diffraction (EBSD) maps showed excellent agreement. The vertical offset of GBS was measured by comparing predeformation and postdeformation AFM images. By analyzing these measurements, the contribution of GBS to the total tensile strain in 5083 Al was estimated as 25 pct at a strain rate of 10−4 seconds−1 and a temperature of 473 K (200 °C). It was demonstrated that the relatively low value of the contribution of GBS to the total strain is most likely the result of testing UFG 5083 Al under experimental conditions that favor the dominance of region I (low-stress region) of the sigmoidal behavior characterizing high-strain-rate superplasticity, which was reported previously for the alloy.

Ductility enhancement in ultrafine-grained Fe–Ni–Mn martensitic steel by stress-induced reverse transformation

The effect of large strain deformation on the mechanical properties was investigated in a martensitic Fe–Ni–Mn alloy. After a combined deformation route (cold rolling plus wire drawing), the mechanical properties improved. The tensile strength of alloy in this study reaches to 2540 MPa and total tensile strain to 7% which are much higher than our previous study which after cold rolling and aging tensile strength was 1840 MPa with total tensile strain of 2.8%, although in both cases the improvement are significant for this brittle alloy. The enhancement of ductility after aging might be attributed to the effective role of grain refining down to nanoscale and the formation of austenite during deformation process and stabilization of that austenite in the aged wire. The austenite transforms to ɛ-martensite during tensile testing because of its low stacking fault energy (SFE) and increases the total measured tensile strain more. X-ray diffraction (XRD) analyses and transmission electron microscopic (TEM) images clarified the suggested reason for this ductility enhancement.

The effect of axial fuel rod power profile on fuel temperature and cladding strain

The most limiting design criteria for nuclear reactor normal operating conditions (ANS Condition I) are known to be rod internal pressure and cladding oxidation, while those for nuclear reactor transient operating conditions (ANS Conditon II) to be fuel centerline temperature and transient cladding total tensile strain. However, the design margins against fuel temperature and transient cladding tensile strain become smaller since power uprating is being or will be utilized for the most of nuclear power reactors to enhance the economics of nuclear power. In order to secure sufficient design margins against fuel temperature and cladding total tensile strain even for power uprating, the current axial rod power profiles used in the reactor transient analysis were optimized to reduce over-conservatism, considering that 118% overpower of a steady-state peak rod average power was not exceeded during the reactor transients. The comparison of the current axial rod power profiles and the optimized ones indicates that the latter reduces the fuel centerline temperature and cladding total tensile strain by 26°C and 0.02%, respectively.

Analysis of macroscopic crack branching patterns in chemically strengthened glass

Residual stress profiles were introduced in sodium aluminosilicate glass disks using an ion-exchange process. They were fractured in two loading conditions: indentation and biaxial flexure. The fractal dimension of the macroscopic crack branching pattern called the crack branching coefficient (CBC), as well as the number of fragments (NOF) were used to quantify the crack patterns. The fracture surfaces were analyzed to determine the stresses responsible for the crack branching patterns. The total strain energy in the body was calculated. The CBC was a good measure of the NOF. They are directly related to the tensile strain energy due to the residual stress profile for fractures due to indentation loading. However, in general for materials with residual stresses, CBC (or NOF) is not related to the strength or the stress at fracture, or even to the total stored tensile strain energy. Instead, the CBC appears to be related, in a complex manner, to the distribution of stresses in the body. Therefore, in general, the characterization of the CBC of fractured materials cannot be used to ascertain the prior stress distribution.

Computer simulation of the tension of noncrystalline silicon nanoparticles

Vitreous and amorphous silicon nanoparticles consisting of 500 atoms are investigated using the molecular dynamics method after a series of uniform tensions with a total tensile strain Δl/l ≈ 0.10 and subsequent relaxation. The disappearance of the second peak in the radial distribution function for an amorphous nanoparticle subjected to a uniform tension with a tensile strain Δl/l ≈ 0.06 indicates the destruction of the tetrahedral packing. In nanoparticles of both types, the energetically most favorable atomic packing is retained in middle layers for all the strains under investigation. The distribution of Si-Si bond lengths and the larger mean number of bonds per atom suggest that the structure of the vitreous nanoparticle is characterized by a higher statistical resistance to tension. This nanoparticle also has a higher kinetic resistance to uniform tension. Unlike the amorphous nanoparticle, the radial component of the coefficient of mobility of atoms in the vitreous nanoparticle neither dominates over its tangential component nor increases regularly in going from the center of mass of the nanoparticle toward the nanoparticle surface. As the number of tensions increases, the mean length of Si-Si bonds decreases in the vitreous nanoparticle and, by contrast, increases in the amorphous nanoparticle.

An Analysis of Microband Orientation in Hot Deformed Aluminium Alloy AA5052 Using Forward and Reverse Torsion

The effects of strain path reversal on the macroscopic orientation of microbands in AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. In both cases microbands were found in the majority of grains examined with many having more than one set. The microbands appear to cluster at specific angles to the macroscopic deformation. For the forward/forward condition microbands clustered around -20° and +45° to the maximum principle stress direction and at ± 30-35° to the principal strain direction. For the forward/reverse condition significantly more spread in microband angle was observed though peaks were visible at ±35° with respect to principal stress direction and at -40° and +30° with respect to the principal strain direction of the reverse torsion. This suggests the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path.

An analysis of microband orientation in a commercial purity aluminium alloy subjected to forward and reverse torsion using Electron Backscatter Diffraction (EBSD)

High-resolution electron backscatter diffraction has been used to study the effects of strain reversal on the evolution of microbands in commercial purity aluminium alloy AA1200. Deformation was carried out using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300 degrees C and strain rate of 1 s(-1) to a total equivalent tensile strain of 0.5. In both cases, microbands were found in the majority of grains examined with many having microband walls with more than one orientation. For the forward/forward condition, the microband clusters were centred around -20 degrees and +45 degrees to the equivalent tensile stress axis, whereas for material subjected to a strain reversal, the clusters were at -65 degrees and -45 degrees . There was no evidence of microbands that were formed in the forward deformation step in the reversed material, which would suggest that a strain of 0.25 is sufficient to dissolve any microstructure history generated by the first step. Furthermore, the microbands within the strain-reversed material had a reduction in misorientation compared with the lineally strained material, suggesting that these microbands only formed at the onset of the second deformation step. This confirms that microband formation is complex and sensitive to strain path; however, it is still unclear to what extent microband formation is dependent on strain path history compared with the instantaneous deformation mode.