Magnetic Interactions with Strain Gradient in Ultrathin Pr_(0.67)Sr_(0.33)MnO_(3) Films

Strain gradient is a normal phenomenon around a heterostructural interface in ultrathin film,and it is important to determine its effect on magnetic interactions to understand interfacial coupling.In this work,ultrathin Pr_(0.67)Sr_(0.33)MnO_(3)(PSMO)films on different substrates are studied.For PSMO film under different in-plane strain conditions,the saturated magnetization and Curie temperature can be qualitatively explained by double-exchange interaction and the Jahn-Teller distortion.However,the difference in the saturated magnetization with zero field cooling and 5 T field cooling is proportional to the strain gradient.Strain-gradient-induced structural disorder is proposed to enhance phonon-electron antiferromagnetic interactions and the corresponding antiferromagnetic-to-ferromagnetic phase transition via a strong magnetic field during the field cooling process.A non-monotonous structural transition of the MnO_(6) octahedral rotation can enlarge the strain gradient in PSMO film on a SrTiO_(3) substrate.This work demonstrates the existence of the flexomagnetic effect in ultrathin manganite film,which should be applicable to other complex oxide systems.

Spalling characteristics of high-temperature treated granitic rock at different strain rates

The dynamic spalling characteristics of rock are important for stability analysis in rock engineering.This paper presented an experimental investigation on the dynamic spalling characteristics of granite with different temperatures and strain rates.A series of dynamic spalling tests with different impact velocities were conducted on thermally treated granite at different temperatures.The dynamic spalling strengths of granite with different temperatures and strain rates were determined.A model was proposed to correlate the dynamic spalling strength of granite,high temperature and strain rate.The results show that the spalling strength of granite decreases with increasing temperature.Moreover,the spalling strength of granite with a higher strain rate is larger than that with a lower strain rate.The proposed model can describe the relationship among dynamic spalling strength of granite,high temperature and strain rate.

Experimental and numerical study on dynamic mechanical behaviors of shale under true triaxial compression at high strain rate

High-energy gas fracturing of shale is a novel,high efficacy and eco-friendly mining technique,which is a typical dynamic perturbing behavior.To effectively extract shale gas,it is important to understand the dynamic mechanical properties of shale.Dynamic experiments on shale subjected to true triaxial compression at different strain rates are first conducted in this research.The dynamic stress-strain curves,peak strain,peak stress and failure modes of shale are investigated.The results of the study indicate that the intermediate principal stress and the minor principal stress have the significant influence on the dynamic mechanical behaviors,although this effect decreases as the strain rate increases.The characteristics of compression-shear failure primarily occur in shale subjected to triaxial compression at high strain rates,which distinguishes it from the fragmentation characteristics observed in shale under dynamic uniaxial compression.Additionally,a numerical three-dimensional Split Hopkinson Pressure Bar(3D-SHPB),which is established by coupling PFC3D and FLAC3D methods,is validated to replicate the laboratory characteristics of shale.The dynamic mechanical characteristics of shale subjected to different confining stresses are systematically investigated by the coupling PFC3D and FLAC3D method.The numerical results are in good agreement with the experimental data.

Accurate Measurements of High Strain Gradients near Notches Using a Feature-Based DIC Algorithm

In this work, the efficiency of a feature-based DIC （digital image correlation） algorithm for measuring high strain gradients was investigated by means of numerical and actual experiments. The so-called SIFT-Meshless method consisted of a novel formulation involving the SIFT （scale-invariant feature transform） feature detector with a self-adaptive meshless formulation. Whereas the numerical experiments aimed to evaluate the accuracy and the spatial resolution, the actual experiments aimed to demonstrate in practice the above findings. A stereoscopic system and a micro-stereoscopic system were used to perform high strain gradient measurements in notched specimens of different materials and notch sizes. This paper concludes that the feature-based algorithm is able to provide accurate strain measurements at high strain gradient regions, even under conditions of plasticity. Moreover, the algorithm showed its efficiency to capture the peak strain near the notch boundary. Lastly, a spatial resolution study proposes a link between the desired accuracy and the pixel resolution required to perform accurate measurements of high strain gradients.

Flow behavior and microstructure of ZK60 magnesium alloy compressed at high strain rate

Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dynamic recrystallization （DRX） developed mainly at grain boundaries at lower strain rate （0.1-1 s^-1）, while in the case of higher strain rate （10-50 s^-1）, DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 ℃, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10-50 s^-1 and in the temperature range of 250-350 ℃ was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.

HIGH STRAIN RATE SU PERPLASTICITY IN SiCp REINFORCED AZ31 MAGNESIUM MATRIX COMPOSITE

Superplasticity of AZ 31 magnesium matrix composites reinforced with 10 vol% SiC（2 μm） particulate i s investigated at temperature range from 365℃ to 565℃ and strain rate from 2.0 8×10-3 to 5.21×10-1 s-1. The maximum total elongation of 228 % is obtained at a strain rate of 2.08×10-1 s-1. The strain rate se nsitivity exponent （m） higher than 0.3, is observed when the strain rate is high er than 10-1 s-1 at 525℃. Increasing the test temperature to 540℃, the maximum total elongation exceeding 195% is achieved at a higher strain rate of 5.21×10-1 s-1 than that at 525℃. SiC in AZ31/SiCp composite ca n fine the matrix grain size. Filament is observed on the fracture surface of th e specimens showing superplasticity.

Strain rate-dependent high temperature compressive deformation characteristics of ultrafine-grained pure aluminum produced by ECAP

To explore the effect of strain rate ε on the high temperature deformation characteristics of ultrafine-grained materials, the deformation and damage features as well as microstructures of ECAP-treated pure Al at different temperatures T and strain ratesε were systematically studied through compression tests and microscopic observations. The increase in ε eliminates strain softening at T≤473 K, and largely enhances the yield strength and flow stress at 473?573 K. The shear deformation dominates the plastic deformation of ECAP-treated Al. Many cracks along shear bands (SBs) are formed at T≥473 K and secondary SBs basically disappear at 1×10?3 s?1; however, at 1×10?2 s?1, cracks are only observed at temperature below 473 K, and secondary SBs become clearer at T≥473 K. The microstructures of ECAP-treated Al mainly consist of sub-grains (SGs). The increase in ε inhibits the SG growth, thus leading to the increases both in yield strength and flow stress at high temperatures.

Highly-Strained InGaAs/GaAs Single-Quantum-Well Lasers Grown by Molecular Beam Epitaxy

Highly stained InGaAs/GaAs Quantum Wells (QW) are grown by using molecular beam epitaxy.The room-temperature photoluminescence (PL) peak wavelength as long as 1160nm is obtained from QW with the In composition of 38% and the well width of 6 8nm.The full-width at half-maximum of the PL peak is 22meV,indicating a good quality.InGaAs/GaAs QW ridge-waveguide lasers with emission wavelength of 1120nm are demonstrated.For 100-μm-wide ridge-waveguide lasers with a cavity length of 800μm,the kink-free output power up to 200mW is achieved with the slope efficiency of 0 84mW/mA under the continue-wave operation.For 10μm-wide ridge-waveguide lasers,the lowest threshold current density of 450A/cm2 and the characteristic temperature of 90K are obtained.

Isolation of High-virulence Strains and Antagonistic Bacteria of the Pathogen of Ginger Blast (Ralstonia solanacearum)

[ Objective] The paper was to screen Bacillus with strong antagonistic effect. [ Method] The diseased ginger and the surrounding soils were collected from Laiwu of Shandong Province, and the high-virulence strains of the pathogen of ginger blast （Ralstonia solanacearum） were isolated, Bacillus was used to carry out antagonistic test. [Result] Three strains LW-4, LW-7 and LW-32 had strong antagonistic effect against R. solanacearum, the area of their inhibition zone was larger than other strains. [ Conclusion] The study provided theoretical basis for the control of ginger blast.

Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.

An analytical study of vibration in functionally graded piezoelectric nanoplates: nonlocal strain gradient theory

In this paper,we analytically study vibration of functionally graded piezoelectric(FGP)nanoplates based on the nonlocal strain gradient theory.The top and bottom surfaces of the nanoplate are made of PZT-5H and PZT-4,respectively.We employ Hamilton’s principle and derive the governing differential equations.Then,we use Navier’s solution to obtain the natural frequencies of the FGP nanoplate.In the first step,we compare our results with the obtained results for the piezoelectric nanoplates in the previous studies.In the second step,we neglect the piezoelectric effect and compare our results with those obtained for the functionally graded(FG)nanoplates.Finally,the effects of the FG power index,the nonlocal parameter,the aspect ratio,and the lengthto-thickness ratio,and the nanoplate shape on natural frequencies are investigated.

Small scale effects on buckling and postbuckling behaviors of axially loaded FGM nanoshells based on nonlocal strain gradient elasticity theory

By means of a comprehensive theory of elasticity, namely, a nonlocal strain gradient continuum theory, size-dependent nonlinear axial instability characteristics of cylindrical nanoshells made of functionally graded material(FGM) are examined. To take small scale effects into consideration in a more accurate way, a nonlocal stress field parameter and an internal length scale parameter are incorporated simultaneously into an exponential shear deformation shell theory. The variation of material properties associated with FGM nanoshells is supposed along the shell thickness, and it is modeled based on the Mori-Tanaka homogenization scheme. With a boundary layer theory of shell buckling and a perturbation-based solving process, the nonlocal strain gradient load-deflection and load-shortening stability paths are derived explicitly. It is observed that the strain gradient size effect causes to the increases of both the critical axial buckling load and the width of snap-through phenomenon related to the postbuckling regime, while the nonlocal size dependency leads to the decreases of them. Moreover, the influence of the nonlocal type of small scale effect on the axial instability characteristics of FGM nanoshells is more than that of the strain gradient one.

Solidification microstructure of directionally solidified superalloy under high temperature gradient

The effect of solidification rate on the microstructure development of nickel-based superalloy under the temperature gradient of 500 K·cm-1 was studied. The results show that, with the increase of directional solidification rate from 50 to 800 μm·s-1, both the primary and the secondary dendrite arm spacings of the alloy decrease gradually, and the dendrite morphologies transform from coarse dendrite to superfine dendrite. The sizes of all precipitates in the superalloy decrease gradually. The morphology of γ' precipitate changes from cube to sphere shape and distributes uniformly in both dendrite core and interdendritic regions. MC carbide morphology changes from coarse block to fine-strip and then to Chinese-script and mainly consists of Ta, W, and Hf elements. The γ-γ' eutectic fraction increases firstly and then decreases, and similar regularity is also found for the variation of segregation ratio of elements.

A nonlocal strain gradient shell model incorporating surface effects for vibration analysis of functionally graded cylindrical nanoshells

In this pap er, a novel size-dep endent functionally graded (FG) cylindrical shell model is develop ed based on the nonlocal strain gradient theory in conjunction with the Gurtin-Murdoch surface elasticity theory . The new model containing a nonlocal parameter, a material length scale parameter, and several surface elastic constants can capture three typical typ es of size e ects simultaneously , which are the nonlocal stress ef- fect, the strain gradient e ect, and the surface energy e ects. With the help of Hamilton’s principle and rst-order shear deformation theory , the non-classical governing equations and related b oundary conditions are derived. By using the prop osed model, the free vibra- tion problem of FG cylindrical nanoshells with material prop erties varying continuously through the thickness according to a p ower-law distribution is analytically solved, and the closed-form solutions for natural frequencies under various b oundary conditions are obtained. After verifying the reliability of the prop osed model and analytical method by comparing the degenerated results with those available in the literature, the in uences of nonlocal parameter, material length scale parameter, p ower-law index, radius-to-thickness ratio, length-to-radius ratio, and surface e ects on the vibration characteristic of func- tionally graded cylindrical nanoshells are examined in detail.

Gradient nanoporous phenolics as substrates for high-flux nanofiltration membranes by layer-by-layer assembly of polyelectrolytes

Thin film composite(TFC) membranes represent a highly promising platform for efficient nanofiltration(NF)processes. However, the improvement in permeance is impeded by the substrates with low permeances. Herein,highly permeable gradient phenolic membranes with tight selectivity are used as substrates to prepare TFC membranes with high permeances by the layer-by-layer assembly method. The negatively charged phenolic substrates are alternately assembled with polycation polyethylenimine(PEI) and polyanion poly(acrylic acid)(PAA)as a result of electrostatic interactions, forming thin and compact PEI/PAA layers tightly attached to the substrate surface. Benefiting from the high permeances and tight surface pores of the gradient nanoporous structures of the substrates, the produced PEI/PAA membranes exhibit a permeance up to 506 L? m-2?h-1?MPa-1, which is ~2–10 times higher than that of other membranes with similar rejections. The PEI/PAA membranes are capable of retaining N 96.1% of negatively charged dyes following the mechanism of electrostatic repulsion. We demonstrate that the membranes can also separate positively and neutrally charged dyes from water via other mechanisms.This work opens a new avenue for the design and preparation of high-flux NF membranes, which is also applicable to enhance the permeance of other TFC membranes.

Generalizing J_2 flow theory: Fundamental issues in strain gradient plasticity

It has not been a simple matter to obtain a sound extension of the classical J2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent behaviour of metals at the micron scale. Two classes of basic extensions of classical J2 theory have been proposed： one with increments in higher order stresses related to increments of strain gradients and the other characterized by the higher order stresses themselves expressed in terms of increments of strain gradients. The theories proposed by Muhlhans and Aifantis in 1991 and Fleck and Hutchinson in 2001 are in the first class, and, as formulated, these do not always satisfy thermodynamic requirements on plastic dissipation. On the other hand, theories of the second class proposed by Gudmundson in 2004 and Gurtin and Anand in 2009 have the physical deficiency that the higher order stress quantities can change discontinuously for bodies subject to arbitrarily small load changes. The present paper lays out this background to the quest for a sound phenomenological extension of the rateindependent J2 flow theory of plasticity to include a de- pendence on gradients of plastic strain. A modification of the Fleck-Hutchinson formulation that ensures its thermo- dynamic integrity is presented and contrasted with a comparable formulation of the second class where in the higher or- der stresses are expressed in terms of the plastic strain rate. Both versions are constructed to reduce to the classical J2 flow theory of plasticity when the gradients can be neglected and to coincide with the simpler and more readily formulated J2 deformation theory of gradient plasticity for deformation histories characterized by proportional straining.

High strain rate superplasticity of rolled AZ91 magnesium alloy

The high strain rate superplastic deformation properties and characteristics of a rolled AZ91 magnesium alloy at temperatures ranging from 623 to 698 K（0.67Tm-0.76Tm） and high strain rates ranging from 10^-3 to 1 s^-1 were investigated.The rolled AZ91 magnesium alloy possesses excellent superplasticity with the maximum elongation of 455% at 623 K and a strain rate of 10-3 s-1,and its strain rate sensitivity m is high up to 0.64.The dominant deformation mechanism responsible for the high strain rate superplasticity is still grain boundary sliding（GBS）,and the dislocation creep mechanism is considered as the main accommodation mechanism.

High strain rate superplasticity of SiC whisker reinforced pure aluminum composites

A β SiC whisker reinforced pure aluminum composites expected to exhibit high strain rate superplasticity has been successfully fabricated by a new processing route consisting of pressure infiltration, extrusion with a low extrusion ratio and rolling. The composites exhibite a total elongation of 220%～380% in the initial strain rates within 1.0×10 -2 ～1.0×10 -1 s -1 and at 893～903 K. According to differential thermal analysis(DTA) and microstructure observation, it is concluded that an appropriately small amount of liquid phase is necessary to cause a good high strain rate superplasticity in aluminum matrix composites in addition to fine and uniform microstructure.

CRACK TIP FIELD AND J-INTEGRAL WITH STRAIN GRADIENT EFFECT

The mode I plane strain crack tip field with strain gradient effects is presented in this paper based on a simplified strain gradient theory within the framework proposed by Acharya and Bassani.The theory retains the essential structure of the incremental version of the conventional J_2 deformation theory.No higher-order stress is introduced and no extra boundary value conditions beyond the conventional ones are required.The strain gradient effects are considered in the constitutive relation only through the instantaneous tangent modulus.The strain gradient measures are included into the tangent modulus as internal parameters.Therefore the boundary value problem is the same as that in the conventional theory.Two typical crack problems are studied:(a)the crack tip field under the small scale yielding condition induced by a linear elastic mode-I K-field and(b)the complete field for a compact tension specimen.The calculated results clearly show that the stress level near the crack tip with strain gradient effects is considerable higher than that in the classical theory.The singularity of the strain field near the crack tip is nearly equal to the square-root singularity and the singularity of the stress field is slightly greater than it.Consequently,the J-integral is no longer path independent and increases monotonically as the radius of the calculated circular contour decreases.

Influences of Stress Wave Propagation upon Studying Dynamic Response of Materials at High Strain Rates

How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SHPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.