Uniaxial Model Research Articles

Nonlinear Analysis of Reinforced Concrete Core Walls Based on Three-Vertical-Line-Element Model

Three-vertical-line-element model and uniaxial spring model were adopted to simulate walls and beams. Based on the existing experiment data, numerical simulation is used to analysis the nonlinear behavior of reinforced concrete core walls. The analysis also considered the influence of relevant parameters on the elasto-plastic in core walls structure. Results from the simulation anlysis match well with those from the tests. It is shown that the higher axial compression ratio enhance the bearing capacity; the ratio of height to width has effect on the bearing capacity, deformation performance and the failure mode; the decrease of span-depth ratio of coupling beam reduce ductility and deformation ability of the core walls; the imposed horizontal angle has important effect on the overall space behavior of reinforced concrete core wall.

Elasto-Plastic History Analysis of Reinforced Concrete Frame Structure Based on Fiber Beam-Column Model

The three parameters of the earthquake ground motions(peak amplitude, spectrum and duration) and dynamic elasto-plastic properties of the building structure can be considered in elasto-plastic history analysis method, therefore it is significant for seismic performance assessment, nonlinear development progress and failure mechanic analysis of building structures under different level earthquake. The accuracy and availability for constitutive model of concrete and reinforcement subjected to cyclic loading have an important influence on reliability of the elasto-plastic history analysis of building structure based on material model. However, due to the complexity of concrete material, there is no unified constitutive model can be used to analyze nonlinear mechanical behavior of concrete until now. In this paper, a practical uniaxial hysteretic constitutive model of concrete and reinforcement is proposed, a computer program of material model is developed and the model is inserted into ABAQUS software with the user subroutine, using fiber beam element model and explicit time integration method, a elasto-plastic history analysis for shaking table test model of 12-story reinforced concrete frame is carried out. The comparison between computation analysis and experimental results shows that the model has a higher accuracy, efficiency and better availability, it is suitable for the constitutive model of concrete and reinforcement used in elasto-plastic history analysis of building structure.

Direct observation of dendritic domain growth in perpendicular magnetic anisotropy CoFe/Pt multilayers

We present the experimental results on thermally activated magnetization reversal for [Co 0.9Fe 0.1(5.0 Å)/Pt(20 Å)] 4 multilayer. Direct domain observations show that magnetization reversal is initiated with rare nucleation and followed by dendritic growth of domain walls. Based on macroscopic magnetic parameters from experimental data, the dendritic domain growth mode is qualitatively interpreted by Monte Carlo simulations in terms of a simple uniaxial magnetic anisotropy model. Moreover, both time evolution of domain growth observation and magnetic relaxation measurements reveal that CoFe/Pt multilayer has a relatively large activation volume compared with Co/Pt multilayers.

Nonlinear response of masonry wall structures subjected to cyclic and dynamic loading

The assessment of the dynamic or seismic performance of complex structures often requires the integration in the time domain of the structural equation of motion in the frame of a nonlinear analysis. Although sophisticated methods have been developed for the nonlinear analysis of masonry wall structures, including the macro- and micro-modeling approaches, these require large computational effort still limiting the extent and complexity of the structures analyzed. This paper presents an alternative method based on the Generalized Matrix Formulation for masonry skeletal structures and load bearing wall systems, which has been proved as an efficient formulation for the analysis of the strength capacity of these kinds of structures (Roca et al. (2005) [17]). The basic formulation has been complemented with a uniaxial cyclic constitutive model for masonry and a time integration scheme. The ability of the resulting approach to predict the nonlinear dynamic response of masonry structures is shown through its application to the time domain analysis of an experimental scale masonry building with available experimental results on its dynamic response.

Experimental and constitutive model study of structural steel under cyclic loading

In order to study extremely low cycle fatigue performance of structural steel and find a suitable constitutive relationship under cyclic loading, a total of fifty Q235B and Q345B steel extremely low cycle fatigue experiments have been carried out. The mechanical behavior of Q235B and Q345B structural steel including monotonic loading behavior, hysteresis loading behavior and hysteresis criterion are discussed. With fully recognized cyclic behavior of structural steel, a uniaxial and simplified constitutive relationship of structural steel under cyclic loading is proposed. Then the uniaxial steel constitutive relationship is developed as user-defined material based on the user subroutine interfaces UMAT provided by Finite Element Software ABAQUS. By introducing the fiber beam element method, the steel uniaxial constitutive model can be used for steel structural analysis. With comparison of Q235B and Q345B tests data under various loading systems, the model proposed in this paper is proved correct and can be applied in nonlinear time history analysis of steel frame. Both tests and analysis results show that the response of Q235B and Q345B steel under cyclic loading and monotonic loading are quite different, and the skeleton curve under cycle loading is much higher than monotonic loading after steel yielding. Both cyclic loops and amplitudes seriously affect the fracture ductility of steel material. While the structural steel bearing cyclic loading, the necking and the fracture behavior will occur ahead of time, it means that the cumulative damage makes the ductility of steel reduced.

Engine Crankshaft Remanufacturing Life Prediction and Experimental Verification

In this paper, the engine crankshaft remanufacturing life was estimated before and after. As long as the known material properties, loading, local stress, and then select the appropriate fatigue damage model can predict the fatigue life estimation. Material parameters of the experiment and the approximate method are obtained; load is on the basis of dynamic simulation; local stress obtained by finite element analysis; injury models were selected: the uniaxial damage model S-N method and multi-axis model of damage is strain method, SWT-Bannantine method, shear strain method, Fatemi-Socie method. New crankshaft and 500 hours, 1000 hours and 3000 hours after the crankshaft bench fatigue tests were conducted, and the experimental results were compared to verify the correctness of the theoretical calculation, and explain the crankshaft to remanufacture the fatigue life is sufficient.

(Invited) Periodic Buckling as Predominant Deformation Mechanism in Carbon Nanotube Foams as Revealed by In situ Uniaxial Compression Experiments and Modeling

not Available.

Cyclic behaviour of short glass fibre reinforced polyamide: Experimental study and constitutive equations

Polymer matrix composites are widely used in the automotive industry and undergo fatigue loadings. The investigation of the nonlinear cyclic behaviour of such materials is a required preliminary work for a confident fatigue design, but has not involved many publications in the literature. This paper presents an extensive experimental study conducted on a polyamide 66 reinforced with 35 wt% of short glass fibres (PA66 GF35), at room temperature. The material was tested in two conditions: dry-as-moulded (DAM) and at the equilibrium with air containing 50% of relative humidity (RH50). An exhaustive experimental campaign in tensile mode has been carried out, including various strain or stress rates, complex mechanical histories and local thermo-mechanical recordings. Such an extended database allowed us to highlight several complex physical phenomena: viscoelastic effects at different time scales, irrecoverable mechanisms, non-linear kinematic hardening, non-linear viscous flow rule, cyclic softening. Taking into account this advanced analysis, a constitutive model describing the cyclic behaviour is proposed. As the experimental database only includes uniaxial tensile tests, the general 3D anisotropic frame is reduced to an uniaxial model valid for a specific orientation distribution. The robust identification process is based on tests which enable the uncoupling between the underlined mechanical features. This strategy leads to a model which accurately predicts the cyclic behaviour of conditioned as well as dry materials under complex tensile loadings.

Phonon States and Dispersive Spectra of Polar Optical Phonons in Quasi-One-Dimensional Nanowires of Wurtzite ZnO and Zinc-Blend MgO Semiconductors

Within the framework of the macroscopic dielectric continuum model and Loudon's uniaxial crystal model, the phonon modes of a wurtzite/zinc-blende one-dimensional (1D) cylindrical nanowire (NW) are derived and studied. The analytical phonon states of phonon modes are given. It is found that there exist two types of polar phonon modes, i.e. interface optical (IO) phonon modes and the quasi-confined (QC) phonon modes existing in 1D wurtzite/zinc-blende NWs. Via the standard procedure of field quantization, the Fröhlich electron-phonon interaction Hamiltonians are obtained. Numerical calculations of dispersive behavior of these phonon modes on a wurtzite/zinc-blende ZnO/MgO NW are performed. The frequency ranges of the IO and QC phonon modes of the ZnO/MgO NWs are analyzed and discussed. It is found that the IO modes only exist in one frequency range, while QC modes may appear in three frequency ranges. The dispersive properties of the IO and QC modes on the free wave-number kz and the azimuthal quantum number m are discussed. The analytical Hamiltonians of electron-phonon interaction obtained here are quite useful for further investigating phonon influence on optoelectronics properties of wurtzite/zinc-blende 1D NW structures.

Compact modeling of CMOS transistors under variable uniaxial stress

This paper presents a novel implementation of variable uniaxial mechanical stress model to be used with DC circuit simulation, e.g. using BSIM3v3 transistor model. Based on transistor measurements under various uniaxial stress conditions two stress-dependent parameters are identified, namely the carriers mobility and to a lesser extend the carrier saturation velocity. The effect of the parasitic source/drain resistance on the piezoresistive coefficient determination is addressed in detail. Using the fundamental piezoresistive coefficients, the model has implemented a general relation to calculate the coefficients for arbitrary directions of current and stress in the (0 0 1) silicon (Si) plane. The extended transistor model allows for simulating the effect of uniaxial stress on any MOSFET geometry, under different operation conditions and for any combination of the drain current and stress orientations in the (0 0 1) silicon (Si) plane. The method proposed in this paper is validated by static and dynamic stress-dependent simulations and comparison with experimental data. The method is simulator-independent and can be adapted to other bulk CMOS technologies including SOI processes.

Uniaxial Modelling of Behavior of the Concrete in Fast Dynamics: Approach to Seismic Behavior

The advantages of the computer enable us to simulate of complex structures subjected to dynamic loading. To come up to the necessity to know the real behavior of such material, we exploit these advantages basing on experimental data available in the literature. Since the response of the material depends on stress velocity, so it is essential to provide a computational code with dynamic behavior. To perform such simulations, we have elaborated a behavior law governed by loading velocity effect on concrete and their attitudes cyclic non elastic, for an approach of seismic behavior. This paper shows the processes we have followed to formulate this viscous damage law whose aim is behavior prediction for concrete under dynamic stresses. Then, the model is validated with experimental results and simulations of some available tests on Hopkinson’s bar.

Biaxial elastic–viscoplastic behavior of Nafion membranes

Durability is a major limitation of current proton exchange membrane fuel cells. Mechanical stress due to hygro-thermal cycling is one failure mechanism of the polymer electrolyte membrane (PEM). In a fuel cell the PEM is highly constrained in the membrane plane and relatively unconstrained in the through-thickness direction, leading to primarily biaxial loading upon hygro-thermal cycling. The rate, temperature, and hydration dependent elastic–viscoplastic mechanical behavior of Nafion, the benchmark PEM, has been extensively investigated in uniaxial tension in prior work which also served as a data basis for a three dimensional constitutive model. Here, the important effects of the biaxiality of the loading conditions on the elastic–viscoplastic Nafion stress–strain behavior are investigated for the first time via experiments and simulation. Biaxial stress–strain behaviors were shown to exhibit similar features to uniaxial behavior including linear elasticity followed by a highly non-linear transition to yield followed by post-yield strain hardening with highly non-linear unloading and reloading; these features were each quantitatively dependent on the biaxiality of the loading conditions. The constitutive model was found to successfully quantitatively predict the loading behavior and its dependence on biaxiality. The constitutive model was also found to predict the magnitude of the yield shoulder during unloading and reloading, but to underestimate the gradual nature of both the forward and reverse plastic deformation processes as well as the strain recovery at zero load. These errors are consistent with those seen in the uniaxial model indicating that the framework used to incorporate the uniaxial behavior into a three dimensional model is capable of predicting the biaxial deformation response of the membrane.

Uniaxial Stress-Strain Model for Concrete Confined by Rectangular Steel Tubes

This paper presents a new model for uniaxial stress-strain relationship of concrete confined by rectangular steel tubes. The difference between concrete confinement effect provided by broad faces and that provided by narrow faces of steel tube is considered in the proposed model. The failure criteria for concrete subjected to triaxial compression is applied to estimate the ultimate strength of concrete core. The parameters of the model are determined based on the test results and the calculation of complete load-stress relationship curves is conducted for axially loaded rectangular CFT specimens using the model proposed in the paper. The concrete core strength and stress-strain behavior of rectangular CFT columns is found to exhibit good agreement with test results.

Simplified uni-axial hysteretic damage model for panel zone of structural steel under earthquake loads

In structural systems, it is a very important issue to predict damage levels of structural components subjected to earthquakes. To appreciate ultimate behaviors and fracture mechanism on the panel zone of structural steels under earthquake loads, it is essential to comprehend damage levels quantitatively. Thus, simple damage models are needed to predict structural components behaviors for random earthquakes effectively. This study proposes a simplified uni-axial hysteretic damage model for panel zone, which is applicable to multi-axial cyclic loading. To confirm the validity of the model, experimental tests are performed under the cyclic shear loading with constant or variable axial loading. The results from the tests are compared with those of the proposed hysteretic model considering the effect of damage. Though limitations are given to the application of this model, this model can predict well the hysteretic behavior of general panel zone applied to multi-axial cyclic loading.

Numerical simulation on seismic retrofitting performance of reinforced concrete columns strengthened with fibre reinforced polymer sheets

This paper evaluates the seismic performance of reinforced concrete columns retrofitted with fibre reinforced polymer (FRP) sheets through numerical simulations of the load–deformation response using two-dimensional finite element analysis (2D-FEA). The relatively rational mesh configuration is verified through comparison of analysis results obtained from the different mesh configurations. The seismic performance of three reinforced concrete (RC) columns strengthened with FRP sheets is assessed through a series of parametric studies, and the applicability of existing crack models and constitutive relationships on crack discontinuity and concrete compressive behaviour are validated. Comparisons of analysis results with tests shows that an equivalent uniaxial strain model and a failure criterion can be used to accurately simulate nonlinear behaviour and the failure of concrete under a biaxial stress state, respectively. Moreover, it is shown that a modified confinement model can be simply adopted to evaluate confined effects from hoop steel and FRP on concrete, which generally operate in three-dimensional confinement. Lastly, the seismic retrofitting performance of RC columns wrapped with FRP sheets is verified by analysing load–deformation responses and the progression of stress–strain at inflection points and bottoms of the columns.

Coverage dependence of magnetic domain structure and magnetic anisotropy in supported Fe nanoparticles on Al2O3/NiAl(100)

Studies of magnetic domain and magnetic anisotropy in collected nanoparticles are crucial for both understanding interparticle interaction and engineering in applications. In order to characterize the microscopic surface morphology and the nanoscale magnetic domain structure of Fe nanoparticles, a scanning tunneling microscope and a scanning electron microscope with polarization analysis (SEMPA) were used in our experiment. For the coverage of 9–13 monolayers (MLs) Fe deposited on Al2O3/NiAl(100), circular and well-separated nanoparticles were grown. As the coverage increased up to 23–33 ML, these Fe nanoparticles started to coalesce and form elongated islands. Therefore a transition from isotropic to anisotropic in-plane magnetism was observed. Our proposed uniaxial magnetic anisotropy models effectively explain the azimuthal angle dependent two-step hysteresis loops. Moreover, the in situ measured SEMPA images clearly show the coverage dependent evolution of magnetic domain structure. Variations in interparticle interaction and magnetic correlation length with increasing Fe coverage are also reported.

In situ investigation of CuPc thin films grown on vicinal Si(111)

In order to characterize the growth process of copper phthalocyanine (CuPc) thin films on vicinal Si(111) substrates, in situ spectroscopic ellipsometry (SE) and reflection anisotropy spectroscopy (RAS) were combined. The analysis of the in situ SE data implies a structural change occurring with increasing film thickness during growth. The ex situ SE data are fitted using a uniaxial model, and different out-of-plane molecular orientations are found: lying molecules on Si(111)-6° and standing molecules on Si(111)-0.35°. The average tilt angles of molecules relative to the substrate surface are calculated to be 41.5°±1.0° and 81.1°±3.5°, respectively. The in situ RAS spectra show that the optical anisotropy of CuPc/Si(111)-6° is induced by the substrate anisotropy, and the strength of the RAS signal of CuPc films is proportional to the film thickness.

Characterization of inclined GaSb nanopillars by Mueller matrix ellipsometry

Inclined GaSb nanopillars prepared by low energy ion sputtering with oblique ion beam incidence have been characterized by two different Mueller matrix ellipsometric tools. The optical properties of the nanopillars were found to be well described by a uniaxial anisotropic graded effective medium model. The pillar height and inclination angle were determined by fitting the parameters of the effective medium model to spectroscopic (1.44–2.88 eV) Mueller matrix measurements at multiple azimuth sample orientations. A set of different samples with various average pillar height and inclination angle was studied; results from the optical characterization correspond well with those from scanning electron microscopy analysis. For samples with nanopillars inclined by 45° or less, the height could be determined from a single Mueller matrix measurement at only one azimuth orientation, allowing real-time in situ observation of the formation. The nanopillars were also studied using a single wavelength angle resolved Mueller polarimeter, which also can be used to determine height and inclination of the pillars, in addition to validating the optical model over a wide range of incident and azimuth angles.

Modeling and simulation of uniaxial strain effects in armchair graphene nanoribbon tunneling field effect transistors

In this paper, we perform a modeling and simulation study on strained armchair graphene nanoribbon (AGNR). Two uniaxial strain models based on a tight binding method are compared with results from first-principles calculation. Tunneling field effect transistors (TFETs) with channels made of strained AGNR of different widths are modeled and simulated by a ballistic quantum transport model based on nonequilibrium Green’s function and nonparabolic effective mass approximation. Compared with TFETs with narrow AGNR, those with strained wide AGNR can achieve better device performance.

Closure to “Nonlinear Uniaxial Material Model for Reinforcing Steel Bars” by Sashi K. Kunnath, YeongAe Heo, and Jon F. Mohle

Closure to “Nonlinear Uniaxial Material Model for Reinforcing Steel Bars” by Sashi K. Kunnath, YeongAe Heo, and Jon F. Mohle