جستجوی مقالات مرتبط با کلیدواژه « پیش کرنش » در نشریات گروه « مکانیک »

Extraction of mechanical properties, including plastic and fracture properties, of sheet metal under quasi-static and dynamic states is one of the fundamental issues in crashworthiness studies. According to standards, samples with various geometries can be employed to extract dynamic and quasi-static mechanical properties. Given the dependence of the yield behavior results and fracture strain on the geometry of the test specimen, it is essential for a specimen used under dynamic conditions to have maximum compatibility with quasi-static uniaxial tension test specimens and to be usable in both conditions. Therefore, in the present study, various geometries were initially designed and evaluated to determine plastic behavior. The effect of geometric parameters on the results of uniaxial tension tests was evaluated, and a suitable specimen for extracting dynamic and quasi-static mechanical properties was presented. Additionally, based on the results of quasi-static and dynamic uniaxial tension tests on the selected specimen at strain rates of 0.01/s to 30/s, the sensitivity of the plastic behavior to strain rate was determined. Furthermore, through the calibration of the coefficients of the Johnson-Cook constitutive model, strain rate-dependent plastic behavior was determined within the desired range and strain rates beyond that.

Fiber metal laminates have garnered attention from the aviation and automotive industries due to their favorable mechanical properties. These laminates consist of thin metal layers and a fiber-reinforced epoxy layer. This study focuses on investigating and analyzing the impact of temperature and strain rate on GLARE 2/1 under bending conditions. GLARE 2/1 are composed of one layer of epoxy reinforced with glass fibers sandwiched between two layers of aluminum 2024. The samples were subjected to bending with loading rates of 0.03 and 0.3 1/s at temperatures of 25, 60, and 100 degrees Celsius.The force-displacement curves were extracted for three-point bending at different temperatures and strain rates. The deformation of the layers was observed using digital image correlation technology. The results indicate that, in GLARE 2/1 and at a constant loading speed, with increasing temperature, the maximum bending force decreases, this value decreased by 51% at a loading rate of 0.031/s and at 100 degrees Celsius compared to 25 degrees Celsius and this value decreased 30% at a loading rate of 0.3 1/s. Increasing the loading speed in GLARE 2/1 samples leads to a greater increase in the maximum bending force. The bending elastic coefficient at a loading rate of 0.3 1/s is higher than at a loading rate of 0.03 1/s at all temperatures. By utilizing the deformations obtained under different temperature and loading conditions, it is possible to control the deformation of GLARE 2/1 in forming processes.

Incremental forming of metal sheets is one of the new methods of forming, in which the local exertion of forming forces and the absence of a matrix enhances the forming limit of the sheet and extends the flexibility of process in producing of complex geometries. In this research, the forming limit of aluminum/copper bilayer sheets in the single-point incremental forming of different geometries was studied. Considering the instability mechanism of the sheet, the Xue-Wierzbicki damage criterion was used in the form of the VUMAT subroutine of Abaqus in the numerical prediction of the growth and damage initiation of bilayer sheets. Experimental tests showed that the type of geometry has influences on the forming height limit due to the different induced stress and strain states on the sheet. The prediction of the numerical model of the forming height limit on average for different geometries with difference of 8% compared to the experimental tests, which indicates the validity of the numerical model. Accordingly, using the numerical model, the effect of changes in equivalent plastic strain and triaxial stress as crucial variables on the distribution of surface strains and damage was analyzed. Also, cyclic and nonlinear loading in this process was shown by plotting the strain path for different geometries.

Nowadays, the utilization of smart materials is experiencing rapid growth across a wide range of industrial systems. Magnetorheological elastomers are a class of smart materials and possess two unique features, i.e. adjustable hardness and damping capabilities. These characteristics make them widely used in various industrial applications. Hence, understanding their behavior in different systems is necessary. The focus of this work is to study the dynamic behavior of magnetorheological elastomers under different static pre-strains in tension-compression mode. In this study, three isotropic samples were fabricated and the force-deflection features of them were acquired under harmonic excitation with various strain amplitudes, static pre-strains, frequencies, and magnetic flux densities. Assess the effect of static pre-strain on the dynamic response of the magnetorheological elastomers, studying the effects of other parameters like strains, frequencies, and magnetic flux densities on the dynamic modulus of magnetorheological elastomers, and proposing a novel phenomenological-based model to predict the viscoelastic behavior of magnetorheological elastomers are the innovative aspects of this study. The results showed that the dynamic modulus of magnetorheological elastomers will increase by superimposing the static pre-strain. Furthermore, the relative MR effect decreases when the static pre-strain is applied. The maximum relative MR effect of 288.3158% have been achieved at a strain of 4%, a frequency of 7 Hz, and a static pre-strain of 0%.

Polymers are used in a wide range of industries. In this research, the mechanical behavior of polymers used in the glass industry has been studied. The investigated polymers included thermoplastic polyurethane (TPU), polyvinyl butyral (PVB) and sentry glas (SG). These polymers were subjected to tension and compression tests at different strain rates from 0.001 to 0.25 s-1. Also, the mechanical dynamic properties of the polymers were extracted using the mechanical dynamic analysis test at a constant frequency. The tensile test results showed that the mechanical behavior of polyurethane is not dependent on strain rate, but SG is highly sensitive to strain rate. Also, with increasing strain rate, the fracture stress of SG decreased drastically. The pressure test results showed that TPU can withstand more stress. The glass transition temperature of TPU was lower than the other two polymers. Overall, it can be concluded that among the polymers studied in this research, TPU had better mechanical behavior.

Numerical simulation of turbulent flames with laminar flamelet models is not easily possible under high turbulence intensity conditions. Experimental results and direct numerical simulations show that introducing strain effects in the production of flamelet tables can significantly increase the accuracy of modeling. In this work, implementing the strain effects in the model results in 30 mm increase in the flame height relative to the unstrained model. In this work, a strained flamlet model has been implemented and evaluated in the simulation of turbulent premixed flames. The premixed counterflow flame has been used to produce the flamelet tables. These tables are used in the computational fluid dynamics solver using two reaction progress variables and the presumed probability density function method. The model obtained in this research has been used in Reynolds-Averaged simulation of a turbulent piloted premixed flame in a bunsen burner. This burner utilized a novel approach to highly increase the input turbulence intensity. The results show that the strained flamelet model predicts the flame propagation speed and consequently the flame length, significantly better compared to the unstrained flamelet model.
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Split Hopkinson pressure bar apparatus (SHPB) is used commonly for determining high strain rate material properties. The validity of the SHPB test results depend on many parameters. In this paper, the effect of specimen dimension (aspect ratio) as well as friction on test results is studied both numerically and experimentally. ABAQUS/Explicit is used for numerical study. To this end, the signals at strain gage positions (on input and out bars) are extracted and stress-strain curve of the specimen is determined using wave propagation analysis. By comparing these stress-strain curves for different states of friction and specimen dimensions, the validity of the experimental and numerical results are checked. By increasing friction, the stress increase and strain decrease in the output stress-strain curve and the best result is for no friction condition. Its is observed that by increasing the aspect ratio, the discrepancy of the results for different friction conditions decreases. Specifically, in simulation results, the difference between maximum stresses of with and without lubrication for aspect ratios 0.25, 0.5, 1.0 and 1.5 are 58%, 18%, 9% and 5%, respectively which is in agreement with experimental trends. In other words, different stress-strain curves are obtained for different aspect ratios in the presence of friction which is mainly due to the friction effect rather than strain rate effect. As the friction increases, the error will reduce the overall strain of the sample and increase the yield stress.

In the current research, the effect of strain path by two processes of conventional asymmetric rolling and asymmetric cross rolling, as well as natural aging on the microstructure and hardness of AA7075 aluminum alloy was investigated. The microstructure was examined by light microscopy and the hardness by macro-Vickers hardness tester. The results showed that the rolled sample (initial sample) had elongated grains due to rolling and the average width of the grains in this sample was 13.4 μm. By applying conventional asymmetric rolling up to 60%, the grains became more elongated and the average grain width reached 2.6 μm. By performing asymmetric cross rolling up to 40%, the average grain width reached 3.7 μm. The distribution of particles did not change significantly with rolling deformation. Shear bands were also formed in the sample after 40% and 60% conventional asymmetric rolling, as well as after 40% asymmetric cross rolling. At zero aging time, the hardness of the 60% conventionally rolled sample was higher than the 40% cross rolled sample. With increasing the aging time, the hardness of all samples increased due to natural aging. As the thickness reduction percentage increased (increasing the strain), the hardness increase percentage due to natural aging decreased. The increase in hardness due to natural aging was more noticeable in the cross-rolling process than in the conventional rolling process. After 7 days of natural aging, the hardness of the material reached its saturation limit.

In this study, vibration and stability analysis of micro-pipes conveying fluid under magnetic, electric, and thermal fields using classical, modified coupled stress, and modified strain gradient theories are presented. The Euler-Bernoulli beam theory with clamped-pinned, clamped-clamped, and pinned-pinned boundary conditions is used for modeling the pipe. The differential equations governing the vibration of conveying fluid micro-pipe are derived through extended Hamilton’s method. Additionally, the extended Galerkin’s method is used to convert the governing partial differential equations into ordinary differential equations. The effects of size, boundary conditions, magnetic field, electric field, and thermal field on eigenvalues and critical velocity are investigated. The results indicated that the strain gradient theory predicts the highest natural frequencies and critical fluid velocities among the other two theories. The effects of magnetic, electric, and thermal fields along with different boundary conditions on eigenvalues and critical fluid velocity have been studied. It has also been concluded that the impact of these fields on the stability regions is different for different boundary conditions. Furthermore, the results showed that the stability of the micro-pipes increases with the increase of the magnetic field coefficient, but decreases with the increase of the coefficient of electric and thermal fields.

According to the declining trend of fossil fuel resources and the need to use renewable energies, appropriate research should be conducted for technical and functional studies in this regard. Therefore, in this research, a tubular PEM fuel  cell as a suitable energy source with three-dimensional geometry has been numerically simulated and investigated. For a comprehensive study, the equations of continuity, momentum, energy, stress-strain, and fluid-solid-heat interaction at steady state are defined, coupled together, and then solved by a finite element numerical code. Assuming the cell voltage changes from 0.95 to 0.4 volts, the passage of compressible fuel and air through the channels and porous media of the electrode and catalyst, and also about 6 degrees increase in the average cell temperature, causes approximately 35 nm displacement in different parts. These displacements, due to fluid-solid-heat interactions, cause thermal and mechanical stresses. The maximum stress is about 3500 kN/m2  in the electrolyte due to its displacement limit (average displacement 12.8 nm). Then the relation of voltage variation with current density, stress, fuel flow rate, displacement and fuel cell temperature was shown. Also the results showed that the assumption of fluid-solid-heat interaction reduces the fuel cell power density by about 3%. Finally, the effect of different parameters such as fuel and air channel radius, electronic and ionic conductivity were investigated. For example, at a voltage of 0.4 volt, 20 percent reduction in the radius of air or fuel channels, or 100 percent increase in the electron or ionic conductivity, increases the electrical current density by about 2.17, 0.05, 3.69, and 40 percent, respectively.

In this paper, the effects of size on spherical cell membrane under hydrostatic pressure and heat field are investigated using strain gradient theory. Two most important parameters related to the cellular niche are mechanical forces and thermal fields. Thermal and mechanical fields affecting the cell are used in the food industry to inactivate microorganisms. The governing equations are obtained using the minimum potential energy principle and finally the governing equations are solved numerically. The effect of size parameters, heat field and hydrostatic pressure were investigated. The results indicate that the effects of small-sale cannot be ignored and predict the hardening properties compared to classical theory. In addition, increasing the heat field and hydrostatic pressure increases the displacement. The results of this research can be used for food industry and tissue engineering.In addition, increasing the heat field and hydrostatic pressure increases the displacement. The results of this research can be used for food industry and tissue engineering.

Due to the impossibility of performing destructive tests, the need for new and Non-Destructive Tests (NDT) in this area is strongly felt. In the present study, using the non-destructive method of Automated Ball Indentation (ABI) test method, some mechanical properties of 4340 annealed steel and 7075 raw aluminum and prepared from aged aircraft are determined. To perform calculations related to ABI method, the explicit relations routine was coded in FORTRAN and verified. Then samples of uniaxial tensile test of 4340 annealed steel and 7075 raw aluminum and prepared from aged aircraft are prepared and their stress-strain curves are obtained. Also, ABI tests are performed on 4340 annealed steel samples and 7075 raw aluminum and prepared from aged aircraft and the results of the two methods were compared. The comparison of results showed that the calculated error of yield strength and ultimate strength was less than 9 and 14%, respectively. With increasing force, this value in some areas reached less than 0.7%.

In this study, plastic deformation of the metallic bipolar plate with serpentine flow field was investigated during stamping process. Strain path and thickness distribution in 304 stainless steel bipolar plate with the thickness of 0.1 mm were determined. To this aim, the process was simulated by the commercial finite element code. The validity of the result was evaluated by comparing the experimental and numerical thickness distribution and force-displacement curve which represent 4.76 and 3.85% prediction error, respectively. According to the results, flow of the material has significant effect on the thickness distribution of the central and lateral channels, and the thickness reduction percentage of the central channel in longitudinal, diagonal and transverse direction is much more than that of lateral one. Maximum thickness reduction (critical area) in central channels is placed in longitudinal direction (33% at channel side) while the diagonal direction is considered as critical direction for lateral channels. Due to the existence of the equibiaxial tension strain path in diagonal direction, significant thickness reduction is observed in both the side and the rib zone of the channels.

In the present paper, a predictive strain-rate-dependent model of localized necking is developed by using a modified Vertex theory. A novel ductile damage -based criterion is proposed to control the necking parameters including on stress triaxiality, strain hardening exponent and Lode parameters. As a characterization parameter, elastic modulus is eventually chosen to measure the ductile damage during process of plastic deforming. Furthermore, a vectorized user - defined material subroutine is developed to finite element simulation by ABAQUS software, according to original formulations, in order to create linkage between related essential models. A typical strain rate-dependent metal is selected to validate the modified Vertex theory. To examine the accuracy of the results from present simulated study, the applicability is considered to compare with the experimental results. Tests of forming are also performed for St 13 and St14 sheets to measure forming limit diagram (FLD). It should be noted that the simulated FLDs are in good agreement with the experimental data. However, this correlation at low strain rates is better than high strain rates. However, this increase will be infinitesimal for the lower strain rates as compared to the higher ones.

The aim of this paper is to analyze the strain components in the direct extrusion process to predict the curvature of the exit product. For this purpose, Riemann mapping theory is used to model the deformation zone and create a one-to-one correspondence between the input and output cross sections of the die. With the help of the Bezier curves, flow lines are created between these points and then an upper bound solution is obtained for the velocity field. The process pressure and the distribution of the strain components are determined for the square, hexagonal, and rectangular sections using the obtained velocity field. A theoretical method based on the elastic-plastic bending of beams is presented for calculating the curvature of the exit product for the eccentric dies. In this theoretical method, the distribution of stress components and the bending moments is calculated using the specified strain components. In fact, the amount of bending moments indicates the curvature of the exit product. Finally, the presented theoretical model is validated through comparison with the results of the finite element simulation and the previous studies. The results show that Riemann conformal mapping theory and upper bound method can be used to determine the distribution of strain components and predict the curvature of the output product, in addition to estimate the process pressure.

In the present research, the modulus of a double lap bonded joint in the strain rate ranges of 0 to 1055 S-1 is predicted by a theoretical model. In this approach, using geometrical parameters of bonded joint, elastic young modulus of adherends and shear modulus of the adhesive layer at two different strain rates, strain rate sharing of bonded joint’ constituents can be calculated. Also, using these data, shear modulus of the adhesive layer at its share of strain rate is calculated; Then, using these data, dynamic modulus of double lap bonded joint can be computed. Calculating stiffness of the double lap bonded joint using geometrical parameters of bonded joint and mechanical parameters of constituents without need to do the test is the advantage of this model. Also, in this manuscript, a finite element model (using Abaqus Explicit) is presented in order to predict dynamic modulus of double lap bonded joint using elastic mechanical properties of bonded joint constituents, their densities and bonded joint geometrical parameters. The two presented models show a similar trend in the prediction of double lap bonded joint’ modulus; in a way that, at the two models, bonded joint’ modulus is increased by increasing strain rate. The maximum difference between the two models is about nine percent.

In this study investigated the effects of momentum variations on fracture energy in Charpy impact testing of API X65 steel by experimental and numerical methods. Experimental analysis was conducted in the various speed of impact about 3.50 to 5.72 m/s and impact energy varied about 450 to 1200 J. The experimental results showed that increase of about 63% in impact speed increased the fracture energy about 15%, because of material properties dependence on loading rate. Numeral studies were performed in two categories with ABAQUS software. First mass variation in constant velocity assumed standard quantity about 5.5 m/s in which impact energy varied about 300 to 1200 J and the second, velocity variation with constant mass assumed 50 kg that impact energy varied about 625 to 1600 J. The simulation results showed the variations in mass had not any effect in fracture energy and in all analyses, it was about 265 J. However, increasing the velocity variations with constant mass, caused a slight reduction of about 5% in the fracture energy. The reason for the difference between experimental and numerical results is the lack of consideration of the effect of strain rate on mechanical properties of tested steel in numerical analysis.

Severe plastic deformation processes have been developed for the fabrication of material with proper mechanical properties over the past decade. The Extrusion process with Equal Channel Angular Pressing as a new process is able to apply a severe plastic deformation due to two continuous shear regions with higher strain rates and more homogeneous strain distribution compared to some of the SPD processes like ECAP. In this study, a new process of severe plastic deformation is introduced and fabricating an high strength AA1050 specimen was investigated experimentally and numerically which led to presenting an effective parameter of this process. It has been shown with using this new process, it is possible to fabricate samples with a higher strain rate and more homogeneous distribution compare with conventional ECAP and extrusion, and the limitation of the high length sample was significantly reduced than the conventional ECAP which friction with the die wall prevents it from continuing. It has been shown that the fabricated sample of one pass of this process has a 50% increasing in yield strength than the annealing sample, which indicates the high efficiency of this process.

Forming limit diagram is one of the most applicable methods for prediction of plastic instability in sheet metal forming that temperature and strain rate is among factors effecting on the FLD. In this paper, the effects of temperature and strain rate at quasi-static condition on the true stress-true strain curve and forming limit curve of AA3104 aluminum are analytically investigated using Marciniak-Kuckzynski method and Johnson-cook model. The stress-strain response of numerical results validation using the Ludwik model is performed with experimental results. Theoretical results show a good agreement with experimental results. Then, according to the stress-strain curves obtained based on the Ludwik equation, Johnson-Cook coefficients are calculated for AA3104 and the true stress-true strain respond and forming limit diagram are produced for different temperatures 50°C، 300°C and 400°C and several t strain rates 〖10〗^(-5)، 〖10〗^(-4)and 〖10〗^(-3) s^(-1). The stress-strain curve decreases with increasing temperature and increases with increasing strain rate. Also the FLD increases with increasing temperature and decreases with increasing strain rate. The results exhibit a positive sensitivity of temperature on the limit strain due to the thermal softening and the negative strain rate sensitivity on the FLD AA3104 due to the behavior of crystallographic structure of the material

In this research, the effect of strain rate on the tensile behavior of the graphene/epoxy nanocomposites was investigated. The specimens were prepared for 0.05, 0.1, 0.3 and 0.5 wt.% graphene oxide and were subjected to tensile tests at different strain rates. The experimental results showed that the maximum improvements in the tensile strength, the modulus, and nanocomposite were 9%, 16%, and 0.1 wt.%, respectively. Also, the results indicated that the epoxy and its nanocomposites were sensitive to the strain rate. The rate sensitivity decreased with the increase of the graphene weight percentages. Moreover, it was shown that by increasing the strain rate, the tensile strength and modulus for pure epoxy were improved by 15.8% and 16.8%, respectively. In this study, the appropriateness and applicability of the Johnson-Cook material model for describing the stress-strain relation of the nanocomposites were examined by a combined experimental-numerical-optimization technique. The numerical simulations were carried out using Abaqus commercial program and the optimizations were performed using the Surrogate modeling. The results showed that the Johnson-cook model is not accurate at very low strain rates. However, the accuracy of the model was remarkably improved by increasing the graphene weight percentage or increasing strain rate.