جستجوی مقالات مرتبط با کلیدواژه « نرخ کرنش » در نشریات گروه « فنی و مهندسی »

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.

In this research, the tensile behavior of polyethylene matrix nanocomposites reinforced with different percentages of clay nanoparticles (0, 1, 3, 5 and 10) at two strain rates (0.1 and 0.5 min-1) and two different temperatures (25 and 50 degrees Celsius) has been investigated.  

For this purpose, standard samples were prepared using injection molding method. Also, the fracture surface of the deformed samples along with the side surface of the stretched samples were examined using scanning electron microscope and light microscope, respectively.

The results obtained from the tensile test showed that the tensile properties of polyethylene strongly depend on the content of nano clay. Examining the Eyring model showed that the addition of clay nanoparticles up to 10 wt% causes to increase the amount of enthalpy changes from 64.97 to greater than 100 kj/mol and activation volume from 0.0049 to 0.01

Evaluation of flow velocity in a porous medium is one of the important subjects in geotechnical engineering. Mine tailings with high water content larger than the liquid limit in the form of slurry have high compressibility, hence, a suitable improvement method such as vacuum preloading is required to use materials in the tailings dam. The present study investigates the effect of sample water content and suction pressure level on time-dependent changes in flow velocity. The tests are performed on saturated samples of fine-grained tailings from the Sungun copper mine. The results show that the graphs of drained water volume and vertical strain of samples follow an exponential function of time with a very good approximation. Assuming Darcian flow through the test materials, relationships are presented to determine the flow velocity based on two approaches including the pore water discharge rate and vertical strain rate of samples. Then, results of the two approaches in calculating the flow velocity and interpreting laboratory observations are compared. The results also demonstrate that the flow velocity increases with sample water content and suction pressure. In addition, vacuum preloading continues until samples reach a certain void ratio. Furthermore, a new relation is proposed to calculate the average degree of consolidation for the layer based on the flow velocity ratio. Finally, the proposed relationship is accurately verified with the estimation of the volumetric compressibility coefficient for the data of the present study using the Oedometer test and a vacuum preloading improvement project on Dalian coast of China.

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 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.

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.

The z-pins in spite of their small sizes and small weights, but they have been expanded to reduce the disadvantages of composites and sandwich panels, increase their resistances and strengths, and prevent crack growth. In this paper, using a finite element method, the behavior of multi-functional sandwich panels with z-pin cores in average strain rates (between 164-327 1/s) under Split Hopkinson pressure bar (SHPB) test have been investigated. The effects of strain rate and mechanical behavior of multifunctional sandwich panel subjected to dynamic loading were investigated and the results compared with the published experimental tests. Good agreements were found between them. Also, effective parameters such as embedding angle of z-pins, their materials and their diameters were investigated. In conclusion, the results show that the foresaid parameters have effective roles in increasing flow stress and ultimate strength of multifunctional sandwich panels. Furthermore, the mechanical properties of sandwich panel with z-pin and honeycomb were compared. The results show that in the same weight and Volume core condition, the ultimate strength and flexibility of z-pins are respectfully smaller and larger than honeycomb cores.

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.

Cellular materials are widely used in aerospace industries due to high energy absorption and strength-to-weight ratio. In this paper, the dynamic response of these materials is numerically investigated in order to assess the effects of porosity, strain rate and various pore morphologies on the mechanical response. To do so, two-dimensional finite element models, with different pore morphologies at various porosities and strain rates, are developed utilizing Johnson-Cook strength and failure model through ABAQUS finite element package. The obtained results show that the mechanical responses of these materials strongly depend on the pore morphologies. Among the different morphologies, the highest energy absorption is associated with the vertical ellipse and vertical rectangle morphology. In addition, the energy absorption of each morphology is a function of the porosity value and, depending on the porosity, an appropriate morphology can be selected for the sake of maximum energy absorption. The collapse stress of the material increases by increasing the strain rate until reaching a specific value and then decreases due to the high stress wave.

Casing collapse is one of the costly and safety-threatening incidents in the oil industry. For example in Kupal oil field, Iran, approximately, a third of the wells have been removed from the production cycle due to casing collapse, which in some cases have led to the total loss of those wells. Since most of these cases have occurred in Gachsaran Formation, it is necessary to use an appropriate method to determine the geomechanical characteristics of this formation, especially, the layer considered as caprock for the underlain Asmari reservoir under the real conditions of temperature and pressure at real depths. In this paper, after discussing the effect of temperature, loading rate, and confining pressure on general geomechanical behavior of rocks, and in particular, the caprock of the Asmari reservoir, the results of geomechanical experiments performed under different temperature and pressure conditions on this type of rock are presented, and their comparison with the corresponding results determined by other researchers are discussed. The obtained results from the experiments demonstrate that under constant loading rate, each 1.0 MPA increase in confining pressure increases the strength of the caprock about 1.0 MPa. Conversely, every 1 °C increase in temperature decreases the caprock strength almost 0.2 MPa.

In this paper, the performance of the rigid polyurethane (PU) foams used in composite armors to mitigate the impact pressure and energy of a projectile has been studied numerically through nite element methods. The armors of this study comprise two layers, namely, a metal layer and a PU-foam layer. A sophisticated FE-Model was developed to simulate the dynamic performance of the composite armor under the impact of a projectile using the software package ANSYS-Autodyn. The FEModel was veri ed with the results of a previous experimental study in which a projectile impacted a certain two-layer armor comprising a steel front face that was attached on a layer of PU-foam. The main objectives of this research study are to study the eects of design parameters such as the thickness and density of the PU-foam and the impact velocity of the projectile (representing the strain rate) on the performance of a composite armor. To achieve the research objectives, PU-foams of dierent densities of 80, 160, 288 and 320 kg/m3, at six dierent thicknesses of 1.25, 2.5, 5, 7.5, 10 and 12.5 mm were modeled. Additionally, the armors were subjected to projectiles with dierent impact velocities of 400, 450, 509 and 550 m/s. Results of the FE-Analysis runs conducted in this paper indicate that the response mitigation of a composite armor is signi - cantly aected by both the density and thickness of its PU-foam layer. For a given thickness of foam layer, it was observed from the FE-analysis runs that the optimum value for the foam-density that results in the maximum response mitigation is approximately 150 kg/m3. The impact pressure that is dissipated by a composite armor is signi cantly decreased as the foam-density diverts from its optimal value. For a given foam-density, it was found that the response mitigation of the armor was improved with an increase in the thickness of the foam layer. For the composite armors investigated in this study, the rate of improvement in the response mitigation with increased foam-layer thickness was found to be very slow for the thickness values beyond 5 mm. The impact velocity of the projectile, as an indicator of strain rate, was also investigated in this paper. The dynamic response of foam layers of relatively lower thickness and density was found to be more sensitive to the variations of strain rate.

At present paper, an equivalent model with different dimensions and also with different dimensions and material in comparison with main body for strain rate sensitive structures subjected to high rate loading is presented by using the novel finite similitude method. The finite similitude method provides performing a test on the model instead of the original sample. This method is used to obtain the properties of model and to reverse the obtained results for model to main body by using the principles of nature (the law of conservation of mass, the law of conservation of momentum, the law of conservation of energy and the law of conservation of entropy) which is always true for any system. The relationships for both pure dimensional and simultaneously dimensional/material scaling of strain rate sensitive structures are presented. To evaluate the efficiency of the proposed relationships, the numerical results are obtained for impacted circular plates. It should be mentioned that the numerical results are obtained by using the finite element software LS-Dyna in which the strain rate effects are considered into account by using the Cowper-Symonds and Johnson-Cook constitutive equations. The results indicate that the scaled plate to one tenth of its original dimensions and also made of different material in comparison with original plate predicts the response characteristics of the original plate with a very good accuracy.

Elastomers are a group of polymeric materials that have unique properties, including time-dependent behavior and time-independent, the mechanical behavior of this material is affected by various factors. In this study, the effect of increasing the silica nanoparticles and strain rates in two quasi-static and dynamic states on the tensile behavior of HDPE / POE has been investigated. For this purpose, an elastomeric material was first created with 40% HDPE and 60% POE mixing ratio. Then with increasing Nano silica particles, 4 sample types including 3 samples 0.7%, 1% and 1.4%, and one sample of HDPE/POE was fabricated. The samples were loaded at strain rate of 0.04 1⁄s, 0.07 1⁄s , 0.1 1⁄s , 0.14 1⁄s , 0.17 1⁄s in a quasi-static tensile state. In dynamic mode, tensile load with a strain rate of 160 1⁄s and 100 1⁄s was applied to the specimens using a new fixture designed on the low velocity impact test machine (Drop weight impact test machine). In the dynamic loading, the behavior of the elastomeric material is extremely dependent on the strain rate, with increasing the strain rate the level of stress and forces in both quasi-static and dynamic loads will be increase. The increase in force levels in dynamic loading is much more than static. Also, the new designed mechanism provides access to dynamic tensile data at different strain rates in a low velocity impact machine. On the other hand, with increasing Nano silica percentage, the tensile strength of the samples is noticeably increased.

FLDs, in fact are the range of strain combinations which identify the beginning of local necking. Different parameters such as sheet thickness, structural defects, temperature, loading direction, forming speed and etc. have influence on these diagrams and one of the most effective parameters is forming speed and it has a direct connection with press speed in sheet forming. In this research FLD are calculated for aluminum 6061 sheets with 3mm thickness in rates of 20, 100 and 200 mm/min experimentally and simulated in the rates of 20, 100, 200, 500 and 800mm/min. In order to do the experimental tests, bulge test is conducted in sheets in six different sizes according to standard by hydraulic press and built steel die. Also numerical modeling was done using the Abaqus finite element software and the maximum strain gauge criterion by entering the Johnson Cook data. Experimental and modelling results verify is studied by surveying the tearing location and errors between FLDs and result showed that experimental and numerical data are compatible with acceptable errors. It was observed that by increasing forming speed FLD increases, in a way that by increasing the press speed from 20 mm/min to 200 mm/min, FLD increases for 30 percentage. This variation can have different reasons such as friction effect and interaction effects between die and sheets, because at the low forming speed (of less than 100 1/s for strain rate) and at the room temperature, the effect of strain rate and mass inertia are minimal.