جستجوی مقالات مرتبط با کلیدواژه « رفتار مکانیکی » در نشریات گروه « مکانیک »

Welding connection of 316L stainless steel has great importance in the oil and gas industry. In this research, welding of 316L stainless steel was investigated using GTAW with the aim of evaluation of the mechanical behaviour of filler metals. Three filler metals of ER309L, ER316L and ER309LMO were used for this purpose. The microstructure results of different regions of welding sections showed that welding structures of three types of filler metals are similar and have structure of Austenite and Ferrite delta. Also, there is a slight difference in the distribution amount of Ferrite delta in the Austenite background. Hardness evaluation results showed that the average hardness of filler metal of ER309LMO is higher than average hardness of filler metals of ER316L and ER309. Also, hardness of impacted region by connection heat of 316L steel in each of filler metals had been increased. The final strength of connection of all three filler metals was higher than 545 MPa. It was in such a way that sample of tensile test was failed and ruptured in the base metal location. The average of impact energy was (156J) for weld metal of ER316L which was higher than average of impact energy (96J) for weld metal of ER309L and average of impact energy (58J) for weld metal of ER309LMO. This reduction in impact energy in two weld metal of ER309 is due to microstructure of Ferrite delta. Using filler metal ER316L with V connection design is better for this connection due to improved impact energy.

In this article, the mechanical behavior of triangular origami DNA, which has a special appearance and functional characteristics, has been investigated. Triangular origami DNA can be introduced as a nanodevice with several degrees of freedom. To investigate this issue, molecular dynamics modeling and simulation have been done and then its performance is analyzed. The first step in knowing a nanodevice is its structural analysis. In this report, the structural changes of the nanodevice due to the change in temperature have been investigated. The results show that the amount of structural changes has reached its lowest level in approximately 800 picoseconds. After this period of time, it can be said that the triangular origami DNA has relatively reached structural stability. Is. The mechanical behavior of Origami DNA will be such that it is suitable for taking different volumes of cargo. In other words, it can be used to carry nano drugs with various dimensions. In fact, the most interesting parameter in this nanodevice is the flexibility of its arms. This flexibility can be properly used to carry different types of cargo.

In this study, the effect of functionalized graphene nanoplatelets (GNPs) on the mechanical properties of vinyl ester/glass fiber composites was investigated. In order to properly distribute the graphene in the matrix and create a stronger bond between surface modified GNPs, vinyl ester and glass fibers, surface treatment of GNPs was performed using (3-aminopropyl) trimethoxysilane. The nanocomposites with different weight percentages of treated GNPs (0, 0.2, 0.4 and 0.6) were fabricated by hand lay-up method. The nanocomposite with 0.4 wt.% of modified GNPs showed the best mechanical properties. The results of bending test of 0.4 wt.% showed 63 and 28 percent increase in flexural strength and modulus, respetivily compared to composite without GNPs. Also nanocomposite containing 0.4 wt.% surface modified GNPs exhibited 26% and 11% improvement in tensile strength and modulus of elstisty compared to the FMLs containing 0.0 wt.% GNPs, respectively. According to the SEM images, the increase in mechanical properties could be related to the improvement in the adhesion between glass fibers and vinyl ester resin, and also the toughening mechanism of treated GNPs.

In this research, nanostructured TiAlN coatings were applied on HSS substrate using cathodic arc evaporation method (CAE) in the different duty cycle values. Then the effect of duty cycle on the coating surface properties including surface morphology and structure, coating thickness and mechanical behavior of nanostructured coatings were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the surface coatings. Also, micro indentation and adhesion test were utilized to evaluate the mechanical behavior. The results show that by changing the duty cycle, the macro-particles size and amount change which is effective on the roughness and morphology of the coatings. It is attributed to the electrical charge of macro-particles that are produced in the process which can be influenced by the structure. Also, the changes in grain size depend on the changes of duty cycle value. Furthermore, the mechanical properties of the coatings are affected by altering the duty cycle related to the deposition mechanism. The hardness value of TiAlN coatings increases from 3168 HV to 3817 HV when the duty cycle increases from 25% to 50%. But whit an increase in duty cycle from 50% to 75%, hardness reduced to 3582 HV. Consequently, it can be possible to find an optimum duty cycle value to achieve the best mechanical properties. Also, the minimum friction coefficient (0.44) and the minimum wear rate were determined for the TiAlN coating with the duty cycle of 75%, which it can be attributed to better smoothness and higher density of the coating.

The friction stir spot welding of Al 6061-T6 alloy is experimented to determine the behavior of micro-hardness, static tensile strength and fatigue behavior of the lap shear specimens by changing of rotating speed at three different speeds. Micro-hardness results show similarity in the regions far from shoulder indentation region. By the static strength and fatigue results, The optimal behavior of the rotational speed is determined. Therefore, the speed of 1000 rpm shows much better mechanical behavior than the other conditions of this research. The fatigue results of different welding conditions demonstrate, in the same cycles, more divergences in the lower cycles than, the higher ones. Two different failure modes have been observed at different load levels. At high load levels, final failure was nugget pull out. At low load levels, the final failure was as separation from the plate. At medium load levels, although the final failure was similar to high load levels, the growth of the crack in the sheet outside the stir zone, just like low load levels, was also observed.

One of the new approaches to produce nanoscale metals with ultera fine grains is applying severe plastic deformation on initial sample with coarse grains. In this method, by applying intense strain to the sample in several steps, the size of the grain decreases to a nanoscale, which results in the improvement of the mechanical and physical properties of the metals. One of the most important methods for this purpose is the constrained groove pressing (CGP) method. Due to the need for a small weight of space structures, sheets of aluminum alloys, aluminum7075-T6, and steel 4130 were selected. The mechanical behavior of the sheets was studied experimentally. The simulation of the interaction between the fluid and the structure was performed for a curved fin model with three different alloys and the deformation of the flying rocket was compared. The results show that the size of the aluminum7075-T6 block decreases from 60 microns to 270 nm with increasing the stages of the process, while the yield strength in the fourth pass increases compared to the annealed sample by 38%. The tensile strength increased by 34%, and the length elongation in the fourth passes reduced by 40%. The total deformation in the fin of the aluminum 7075-T6 improved to 99.9% with the CGP process. However, the amount of deformation in the steel 4130 fin compared to the CGPed aluminum7075-T6 is less than 0.1% of the total deformation.

For the first time in this paper, by using semi-analytical scaled boundary finite element method (SBFM), a perfect nano garphene sheet or defected ones were simulated and their mechanical behavior had been investigated. In this analysis, the atomic carbon bonds were modeled by simple bar elements with circular cross- sections and then the scaled boundary finite element relations were formulated based on the geometry of the model. The obtained results from SBFM were compared to those obtained from molecular dynamic method which showed that the SBFM can be used as a continuum mechanics model with high accuracy in mechanical analysis of both perfect and defected nano graphene sheets. Existence of structural defects in nano grapheme sheets decrease the strength as well as fracture strain in a considerable manner. It can be noted that in a defected nano grapheme sheet, the fracture stress decreases more than 34% while fracture strain decreases more than 50%. In the cases that instead of using bar elements, whole area is considered as a continuum sheet and in order to obtain a similar geometry to those problems have bar elements, no material zone be modeled by zero elastic properties, the results show considerable errors.

The bipolar plate is one of the most important components in a PEM (Proton Exchange Membrane) fuel cell .In this paper the mechanical behavior of the composite bipolar plates are studied and compared with Department of Energy (DOE) criteria. All specimens were prepared by casting under pressure and under the hot press. Phenolic resin powder was used as matrix of composite in manufacturing of bipolar plates. To increase the flexural strength and impact strength, the carbon fibers and carbon fabrics were used as filler. electrical conductivity of the samples was analyzed by adding natural graphite powder in the composite. In for the mechanical behavior of bipolar plates, samples with different weights of phenolic resin were produced and tested. The morphology of samples was conducted using a Scanning Electron Microscope (SEM). Finally, the results showed that increasing the percentage of resin, increases the flexural strength and impact strength of composite bipolar plates however the electrically conductivity reduces compare with DOE criteria.

This paper aims at investigating the effect of heat input in resistance spot welding on microstructure and mechanical behavior of 2304 duplex stainless steel, as a promising candidate for automotive application. The results showed that due to rapid cooling rate inherent to resistance spot welding, the ferrite-austenite phase balance is destroyed and nitride-type precipitates are formed within the ferrite grains. The amount of austenite in the weld nugget was a function of welding current, as the most important factor affecting welding heat input. Increasing welding current increased the austenite volume fraction from 4 to 18%. Moreover, the nitride precipitation was reduced upon using higher welding currents. Investigation of weld mechanical performance during the tensile-shear loading showed that increasing welding current enhances both load bearing capacity and energy absorption capability. The maximum achievable peak load and energy absorption of 2304 duplex stainless steel resistance spot welds were 25 kN and 40 J indicating a superior weldability.

This paper deals with studying and developing a proper constitutive model for liver tissue. For this purpose, deformation of liver in uniaxial compression, for two different strain rates, is analytically and numerically studied, based on both hyperelastic and hyperviscoelastic constitutive models. Both of the models are based on a polynomial-form energy function. The stress-strain curves, for uniaxial compression, obtained from these models, have been fitted to the existing experimental data to determine the model coefficients. Moreover the models are examined in uniaxial tension and pure shear loadings. ABAQUS commercial software, in which both of the models are available, has been used for numerical simulations. Then, to evaluate the computational analyses, analytical and numerical results have been compared with each other and also with the existing experimental data. The results show that the presented analytical solution and FE simulation are very close together and also both are accurate enough, compared with the experimental data and an acceptable stability is observed. Furthermore the effect of friction coefficient between the sample and the compressing plate in uniaxial compression test has been investigated. FE simulation results show that the stress will increase with increasing friction coefficient. This implies that friction coefficient must be carefully selected to accurately describe the tissue’s response. Compared with previously published researches on other tissues, the constitutive models adopted here to predict liver behavior is mathematically more complex due to non-zero material constants. Analytical solution of these constitutive models is, in fact, the main challenge and innovation of this paper.

This paper deals with atomistic modeling of nanomechanical behavior of actin monomer. The major cytoskeletal protein of most cells is actin, which is responsible for the mechanical properties of the cells. Actin also plays critical mechanical roles in many cellular processes which gives structural support to cells and links the interior of the cell with its surroundings. Based on the accuracy of atomistic-based methods such as molecular dynamics simulations, in this paper, we perform a series of steered molecular dynamics simulations on both ATP and ADP single actin monomers to determine their intrinsic molecular strength. The effect of virtual spring of steered molecular dynamics on the mechanical behavior of actin monomer is investigated. The results reveal increasing the virtual spring constant leads to convergence of the stiffness. The stiffness of ADP actin and ATP actin calculated as 215.16 and 228.24 pN/Å, respectively. The results also show higher stiffness and Young’s modulus for ATP G-actin in comparison to ADP G-actin. In order to compare the behavior of ATP and ADP G-actins, the number of hydrogen bonds and nonbonded energies between the nucleotide and the protein is analyzed. The obtained persistent length is 15.61 µm which is in good agreement with the other reported literature values.

Due to their accuracy and reliability, atomistic-based methods such as molecular dynamics (MD) simulations have played an essential role in the field of predictive modeling of single layered graphene sheets (SLGSs(. However, due to the computational costs, applications of these methods are limited to small systems. Additionally, according to the discrete nature of SLGSs, conventional continuum-based methods cannot be utilized to study the mechanical characteristics of them. To overcome these issues, here, a new Atomic-scale Finite Element Method (AFEM) based on the Tersoff-Brenner potential has been developed. Efficiency of the proposed method is evaluated through a numerical example analyzed by both of the proposed method and MD simulation. The results show that the computational cost is much reduced (~100 times), while the accuracy of MD simulation is kept. Furthermore, the effects of initial C-C bond length and number of atoms on the speed of the proposed method is investigated. To mimic the MD simulation completely, periodic boundary conditions have been implemented in the extended AFEM. It is demonstrated that there is a noticeable deviation from MD results without considering this type of boundary conditions.

Energy absorption capability of thin-walled structures with various cross sections has been considered by researchers up to now. These structures as energy absorbers are used widely in different industries such as automotive and aerospace and protect passengers and goods against impact. In this paper, mechanical behavior of thin-walled aluminum tubes with and without polyurethane foam filler subjected to axial impact has been investigated. The tubes are very thin so that (D/t) ≈ 550 governs for cylindrical specimen. Structure behavior was analyzed through finite element analysis by LS-DYNA. Circular, hexagonal, and square cross sections with the same length, thickness, and circumference of sections were studied. The results show that circular cross section has the highest energy absorption while experiences the lowest change in length compared to hexagonal and square cross sections. Besides, the effects of stress concentration in hexagonal and square sections can be observed on the corners of walls. Also under the dynamic loading circular structure was crushed more symmetric, while hexagonal and square structures tended to the buckling. Also an Artificial Neural Network is introduced to predict load & energy Absorption behavior. The Neural Network's data obtained from LS-DYNA. The introduced model could present acceptable results in comparison with analysis of LS-DYNA.

In this paper the macromechanical behavior of dual phase steel based on actual microstructure has been predicted. In order to prepare dual phase steelsc (DP) of different percent phase combinations، a low carbon steel (C-Mn) was subjected to intercritical annealing treatment (ICT) and quenched in water. Then، the actual microstructures of dual phase steels were obtained by metallographic analysis and optical microscopy. A 2D representative volume element (RVE) was generated by finite element code Ansys on the basis of actual microstructure which was obtained by image processing code in Matlab software. The individual single-phase flow curves were obtained based on the dislocation theory and the local chemical composition of constituent. The results of 2D micromechanical RVE models under periodic boundary conditions and tension loading were compared with the experimental results. It is shown that the 2D micromechanical model can predict both strength and ductility for low volume fraction of martensite in dual phase steels. The 2D micromechanical modeling may then be used to portray the local strain evolution of the individual phases in the DP microstructures.

This research aims to provide new information about the mechanical behavior of double-stranded DNA (dsDNA). For this purpose, a series of extended atomic resolution molecular dynamics (MD) simulations of DNA dodecamer is performed. The MD calculations are carried out using Generalized Born solvent-accessible surface area method and Langevin dynamics. The stress-strain curves of DNA obtained under various pulling rates and pulling angles are analyzed, and the role of pulling angle and velocity in determining biomechanical properties of short dsDNA is discussed. The results illustrate that how much the behavior of DNA under action of tensile forces could be complicated. By means of at base pair level analyses of the molecule conformation during the stretching processes, the structural stability of the DNA molecule subjected to the angled pulling with different pulling rates and different pathways to the dsDNA rupture are studied. The structural stability of dsDNA can be dependent on the pulling velocity and pulling angle. Whereas the DNA stability can decrease significantly with the reduction of pulling velocity, stretching the DNA under different angles has different unpredictable effects on its structural stability.

This paper investigates the high cycle fatigue behavior of the AZ91 magnesium alloy. The feasibility study of replacing this alloy with the A356 aluminum alloy for a diesel engine cylinder head is discussed. For this purpose، the AZ91 alloy was casted in a permanent mold and the microscopic examination with optical and scanning electron microscopes (SEM) and mechanical tests include tensile، hardness and high cycle fatigue tests were performed on samples which were prepared based on standards. Results are compared to the A356 aluminum alloy. High cycle fatigue tests were carried out at a stress ratio (R) of -1 and a frequency of 50 Hz at room temperature in the air. The microscopic investigation demonstrates that the microstructure composed of alpha and beta (Mg17Al12) phases. Fatigue test results show that the fatigue strength of the AZ91 alloy is lower than the A356 alloy، but still in the range of required values for the application in cylinder heads.