مجله مهندسی مکانیک امیرکبیر
سال پنجاه و یکم شماره 5 (آذر و دی 1398)

According to their time and rate dependent response to the exterior loads, there was too much complexity to predict mechanical responses. Thus, a thermodynamically consistent viscoelastic-viscoplastic-viscodamage model is considered to predict their mechanical responses. Along this purpose, the explicit time-discrete form of the constitutive model is used in the finite element software ABAQUS by VUMAT subroutine. This method is valuable since its accuracy, low running time and no need of Jacobian matrix calculations. After validating ABAQUS user subroutine with experimental data obtained from creep, creep-recovery and repeated creep-recovery tests on the polymer, constitutive model sensitivity to the material parameters such as viscoelastic, viscoplastic and damage properties has been investigated. Along this purpose, it can be observed that increasing viscoplastic and damage parameters could cause rising in viscoplastic strain and damage variable. Then, the direct effects of loading time and stress level and the indirect effects of unloading time on polymer strain and damage variable were investigated in the two repeated creep-recovery tests with constant and increasing amplitudes. For instance, doubling increasing amplitude cycle number from 50 to 100 could rise service life and decrease about 75 percent of damage level.

In the current study, semi-crystalline polymers and their properties are introduced, and then a mechanical model is precisely presented in order to predict the behavior of these materials. In this model, a material point of the semi-crystalline polymer is considered as an aggregate of inclusions of two phases, namely, the crystalline and the amorphous phase, with the interface plane of these two phases. Constitutive equations of each phase are demonstrated. A numerical algorithm is presented for solving the constitutive equations of each phase, in stages. Moreover, the general behavior of the material is determined in terms of each phase behavior using volume averaging. Because of the availability of the material parameters for polyethylene, this material has been taken into account. Obtained numerical results are reported and compared to that of previous models. After supporting the validity of the presented model, the effect of some material parameters including crystalline phase damage rate, release parameter, amorphous phase damage rate, saturation damage, rubber shear modulus, and amorphous phase strength on polyethylene behavior has been discussed in details.

Acoustic emission is one of the most important technique in None Distructive Test, that is able to evaluate crack growth in steel element of structure and is efficient in order to condition monitoring for deployment on in-service bridge component and these data can be used for schedule planning for maintanance of bridge structure. In line of experiments and empirical research, samples of structural steel (St52) were used as compact tension specimen in axial tensile fatigue test in order to study the crack growth. In this study, application of acoustic emission technique for health monitoring and determination of crack length in components of steel structure which are nuder axial fatigue load are investigated by using experimental test result, for this purpose correlation between acoustic emission parameters such a energy rate, count rate and etc and crack length and fatigue life has been studied. Finally by using the extrcted data from acoustic emission and fatigue test result, and least mean square regression analysis, the constants value for relation between crack length and acoustic emission parameters have been determined.

Non-destructive tests have capabilities to investigate and identify the defects and properties of the test piece without changing the physical and mechanical properties of the sample. Non-destructive ultrasonic test has been used in many researches to study different material properties such as mechanical and structural properties. The ultrasonic wave propagation velocities have been widely used for metal hardness measurement. In this study, for the first time nondestructive ultrasonic testing has been employed to measure the hardness of rubber compounds using Gaussian Process Regression. Eighty seven samples with different formulations were prepared and vulcanized. After the vulcanization the compound hardness of the samples and the longitudinal ultrasonic wave velocity through them was measured. Result of Gaussian Process Regression model show that this model perform well and able to predict tire hardness. Also investigating repeatability shows that this method can be a good alternative for conventional hardness testing method in measuring hardness of rubbers. According to low time of test in this method and no need to sample preparation, the proposed method can be used in tire production lines.

Nowadays industrial developments and applying more dynamic loads on metals have made the issue of fatigue, which approximately cause 90% failure of the metal components to important problem. Some examples of such failures found in aircraft, automotive and high-speed trains and turbines. This resulted in design for industrial for special attention to the issue of fatigue. In the last two decades a new branch called very high cycle fatigue (VHCF) in fatigue life had attend for researchers. Access to this number of cycles (107) and above by using contemporary testing devices due to low frequency, is time-consuming and costly and thus has not been considered in the past. For a lot of material fatigue failure after 107 cycles have been reported, i.e. in the range of very high cycle fatigue. Very high cycle fatigue properties is significant issue for ensuring long life and reliability machines and structural components due to the growing demands of the industry. To achieve this number of cycles the fatigue testing machine with ultrasonic method in frequency about 20,000 Hz was used, and due to high-frequency, time reduction and achieving to high cycle is goal of using this device so can be used to study the properties of very high cycle fatigue. In this study, after describing the process theory, description of components and carry out the design for the vibration transducer and fatigue test specimen fatigue testing machine using ultrasonic method in frequency of about 20 kHz was made in laboratory and then the fatigue behavior of AISI 1045 steel using these device is studied and finally the stress-life curve for this steel was obtained to 108 cycle and The results show that the fatigue life of the steel decreased to 204 MPs. Finally, due to the fracture surfaces the failure of a samples is described.

High ratio of strength to weight in fiber reinforced polymer composites causes to their applications in many of the structures and components. In this study an experimental and numerical investigation on four composite cylinders’ response to low-velocity impact is carried out. The studied cylinders were considered to be carbon-only, kevlar-only, kevlar-outside/carbon-inside, and carbon-outside/kevlar-inside. The experimental impact test was applied on the samples using a drop-weight impact apparatus without initial velocity with a spherical steel impactor. In the numerical part, a finite element (FE) modeling technique has been used in ABAQUS software to investigate the impact behavior. In this paper, various types of modeling methods, meshing process and types of elements such as conventional shell, continuum shell and solid elements were discussed in the software. The issues of concern were the contact force, contact duration, and deflection of the specimens. It was found that kevlar fiber has more ability to absorb energy compared to carbon fiber and that the carbon-only specimen has the greatest contact force and the lowest deflection compared with the other samples. To validate the experimental data, they were compared with the results obtained from FE analysis and a strong concurrence was found between them.

Accumulative roll-Bonding process (ARB) that is a severe plastic deformation (SPD) process is investigated by the authors in order to fabricate ultrafine grained aluminum/brass composite . In this study, aluminum composite is reinforced with brass in seven cycles of ARB at room temperature and without any heat treatment between production cycles. Tensile strength and hardness of the composite increases strongly in the first cycle. These mechanical properties do not change significantly with increasing the number of cycles. After seven cycles of process, tensile strength and hardness of the composite compared to the initial Al sheet respectively increased to 46 and 85 percent. Also elongation of the composite decreases strongly in the first two cycles and slightly increased in the following cycles. This changes in the mechanical properties during the ARB process are due to the strain hardening and cold working mechanism in the primary cycles and the grain refinement in the final cycles. The microstracture results of the composite shows that the average value of grains reached 250 nm and the distribution of brass particles in the composite become more uniform with increasing the ARB cycles.

In this research, the hot pressing of a pure titanium grade 2 was carried out by an equal channel angular tooling. The novel design of die with 90° channel angle eliminated the undesired material flow into the die parting line, increasing forming load and the workpiece sticking. After trials, it was indicated that the combination of the copper foil and molybdenum disulfide lubricant led to the desired product. In this situation, ECAP was successfully carried out up to four passes at 260 ℃. The tensile testing was performed on the sound samples. The results showed after fourth ECAP pass with only 14% decreasing in reduction of area, the ultimate tensile strength increased by 57%. As well, due to decreasing the forming temperature and employing the suitable lubricant the ultimate tensile strength reached 799 MPa which is greater than related articles. In order to examine the microstructure and damage property of ECAPed pure titanium, the SEM images were taken. The results showed that the average initial grain size of 18385 nm decreased to 729 nm after 4 passes of ECAP. Furthermore, it was revealed that after ECAPing the fine dimples with uniform distribution were created indicating homogeneous structure.

Nowadays, in different industries, especially in the automotive industry, the use of lightweight materials such as aluminum alloys has been increased in order to reduce the weight of parts and fuel consumption. These alloys have low formability at room temperature. To overcome this problem, forming at elevated temperatures is proposed. In this paper, formability of 5052 aluminum alloy sheet under hydrodynamic deep drawing assisted by radial pressure process has been studied at warm condition. Initially, after analyzing the effect of geometric parameters on thickness distribution and punch force, the effect of temperature, fluid pressure and punch speed on thickness distribution, punch force and limiting drawing ratio (LDR) were investigated. Also, the effects of forming temperature and punch speed on the minimum thickness of the workpiece were studied using Taguchi method. Based on the obtained results, higher temperature in warm isothermal and non-isothermal states leads to decrease and increase in the part thickness, respectively while higher punch speed helps in thickness improvement. It was found that the LDR increases with increasing temperature in non-isothermal state and with decreasing temperature in isothermal condition. In addition, increase in fluid pressure leads to a higher LDR.

Due to the widespread use of polyamides in various industries and the need to achieve high resistance to surface scratches and optimum surface roughness, in this paper, surface roughness and surface scratch resistance of polyamide 6 reinforced with nanoparticles of calcium carbonate and maleated polyamide (PA-g-MAH) as compatibilizer based nanocomposites specimens have been investigated. For this purpose, components of nanocomposit specimens, with the different weight of nanoparticles of calcium carbonate, by twin coil extruder mixtured and specimens by the method of injection molding were prepared. To perform the hardness scratch test, the nano-coupled hardness device was used in the atomic force microscope, which has a nanoscale test instrument equipped. In order to study the roughness and surface scratch resistance of the specimens, the effect of three variables, vertical force of the scratch, the weight of the reinforcing phase and the compatibilizer have been investigated. The results show that increasing vertical force in all specimens reduces scratch resistance and increases relative height and scratch strength. Addition of calcium nano carbonate particles also increases surface roughness and scratch resistance.

Generally, crack growth in polymers may occur under combined tensile-shear loading. Arcan test specimen was originally designed for use with composite materials, but in recent years has been adapted by many researchers for use with isotropic materials. In this paper, in an attempt to study the fracture toughness, test specimens were prepared in the form of butterfly from Poly methyl methacrylate polymer. Experimental fracture tests were performed in three different crack lengths and first mode, mixed-mode and the pure second mode by changing the loading angle, using a specially developed fixture, based on Arcan. Load versus displacement curves were obtained. Using critical loads of the tests and the dimensionless stress intensity factors, obtained from the finite element analysis by ABAQUS software, fracture toughness of the polymer was determined. As the result, it can be seen that the shearing mode fracture toughness is larger than opening mode toughness. This means that cracked specimen is weaker in tensile loading. Finite element analysis was performed using elastic properties of the Poly methyl methacrylate polymer.

Determination of required force during forging processes. So, in order to increase the accuracy of numerical simulation results at die design process and predicting the required force, it is necessary to do suitable tests to estimate the friction coefficient of the process. In this research, a new test method named open die compression of the cylindrical rod has been presented and the calibration curves have been developed for it. Then, this test and the well-known ring compression test have been used to predict the friction coefficient in isothermal forging process of a sample gas turbine compressor Ti-6Al-4V blade. The friction coefficient determined by ring compression test has been applied for root region and the coefficient determined by open die compression of cylindrical rod test for the airfoil region of the blade. Finally, forging process has been investigated numerically and experimentally. Comparison of the maximum forces form numerical and experimental results shows acceptable agreement. The results show that open die compression of cylindrical rod test is a reliable method for evaluating the friction. The difference between friction coefficients determined by two before mentioned test methods, shows the dependence of friction coefficient value on deformation state of the workpiece.

Resistance spot welding is one of the most common methods of joining especially in sheet metal assemblies. The number of spot welds and their pattern, strongly affects the joint strength, time and costs. So, a hybrid method of finite element and particle swarm optimization was introduced for determining the pattern of spot welds in this paper. At first, one spot weld joint was created on a sheet specimen, and mechanical properties of this joint were determined. The spot welding process of three cases was simulated and verified by three different experimental data. Particle swarm optimization and finite element method were linked to determine an optimum pattern of spot weld joint. This method was utilized for two, three and four spot welds. According to this research, fracture force of two, three, and four spot welds were 1.9, 2.13 and 2.76 times of one spot with the optimal pattern. The results indicate that the best pattern has four spots distributed on a circle with a radius of 18mm and the angle between loading axis and symmetric axis was 66 degrees.

In this research, the determination of curvature and the effect of carbon nanotubes (CNTs) on it in cured three-phase un-symmetric cross-ply composite laminate have been studied. The un-symmetric cross-ply composite laminates is including three different phases: carbon fiber, polymer epoxy and carbon nanotube particles that different volume fraction of CNTs including %0, %1, %2 and %3 have been considered. Different micromechanical models such as Halpin-Tsai, bridging and Schapery models have been used to determine the mechanical and thermal properties. Adding of %1 volume fraction of CNTs has been caused decreasing the longitudinal and transverse coefficient thermal expansions and increasing the longitudinal, transverse and shear modulus ones. Developed Hyer model's has been employed to investigate different parameters such as curvature, critical length and deformed shape in different lengths of square un-symmetric cross-ply laminates. Addition of 1% volume fraction of CNTs decreases the critical length and curvature about 9% and 14%, respectively. In addition, the results of finite element analysis have been compared with the developed Hyer model and showed less than 10% error.

In this study, a semi-analytical method based on the Reisner's mixed formulation is presented for the elastic analysis of anisotropic homogeneous solids with an edge or interior crack. In this method, the displacement and the stress fields are represented as the sum of a known function and a finite series of functions with unknown coefficients. The functions are constructed in such a way that the displacement discontinuity across the crack faces and the exact singular behavior of the stress field at the crack tip are captured, moreover all essential and natural homogeneous and inhomogeneous boundary conditions are satisfied exactly. The equilibrium and the compatibility equations are also applied with the desired accuracy using the Reissner's variational principle. Solution of the variational equation leads to a set of linear algebraic equations in terms of the unknown coefficients. After computing of the unknown coefficients, the displacement and the stress fields are obtained and subsequently the stress intensity factors (SIFs) and crack opening displacement (COD) are calculated. The results show that computing of SIFs has high convergence rate and the results of the proposed approach are in good agreement with those of the analytical solutions reported in the literature.

In this study, a finite element-based numerical method was developed to simulate stress distribution in conventional and functionally graded thermal barrier coatings YSZ/NiCrAlY plasma spayed on the Hastelloy-x. The coatings were characterized using optical microscope and Scanning Electron Microscopy (SEM). Moreover, thermal shock resistance of the coatings was determined. Comparisons of simulation model results with experimental results of nano-indentation stress measurement method showed good agreement. Thermal shock results show that conventional thermal barrier coating is less thermal shock resistant than functionally graded coating. The results of simulation showed that in samples after thermal shock without oxidation time (without Thermally Grown Oxide layer-TGO), the average value of the maximum stress is 29 MPa in duplex TBC (interface of top coat /bondcoat), 3.15 MPa in three-layer FG-TBC system (interface of 50% NiCrAlY - 50% YSZ / YSZ) and 1.8 MPa in five-layer FG-TBC system (interface of 25% NiCrAlY- 75% YSZ / YSZ). Also, the stress distribution is more uniform in five-layer FG-TBC system that helps to increase the performance and extend the life time of thermal barrier system.

Resistance Spot welding (RSW) is a strong coupling process which involves electrical, thermal, and mechanical interactions. These make the whole welding procedure highly non-linear and difficult to model. In this paper, finite element analyzing tool, ANSYS, was used to simulate and model RSW. In order to improve accuracy, material properties were defined temperature-dependent and phase transformation was taken into account in simulation. The steel sheets in this study were AISI 1008 steel. The key parameters of the process including, contact radius, contact pressure, and temperature distribution were investigated. Also development of weld nugget during process was investigated and numerical calculations for nugget size showed good agreement with experimental results. This causes the weld nugget diameter abnormal variations and consequently reduces the weld strength. Therefore the tip of the electrode should be dressed in this process With these results, optimum settings for current, timing, and pressure of the spot welding machine can be formulated for different materials to produce the desired welding quality. In addition to magnitude of welding current, current shunting phenomenon affects the nugget size. In order to investigate the effect of electric current shunting, some tests were designed and simulated.

Abrasive Flow Rotary Machining (AFRM) is a new form of Abrasive Flow Machining process (AFM) which has been recently introduced by the authors as one of the non-conventional finishing and polishing methods. Since in AFM, the negative impact of temperature increase in abrasive paste has been reported on the material removal rate, in this study the role of this parameter has been evaluated on AFRM. For this purpose, a new tool equipped with pressure and temperature measuring instruments as well as temperature control device have been designed and fabricated and the temperature changes of the abrasive paste during the process are measured. The process variables in this case have been selected based on the optimum machining conditions corresponding with the maximum material removal rate from the previous study of the authors. The experimental results with new tool show that material removal rate and surface finish of AFM are obtainable by AFRM process. Also no significant increase in paste temperature occurs in AFRM process during effective machining time which is one of the advantages of AFRM. In the case of the process continuation, material removal rate decreases by increasing the temperature of the paste, which shows similar behavior to AFM process.