مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 12 (اسفند 1400)

The contact stiffness between workpiece and fixture locating system is one of the decisive factors for maintenance of the stability of workpiece during the machining process. In order to estimate the contact stiffness, it is needed to determine the locator reaction forces. These forces are created by the clamping forces, cutting forces, workpiece weight and friction effects of the contact between the workpiece and fixture locating system. Some analytical approaches have already been presented for calculating the location reaction forces. However, there are six equations for a (3-2-1) locating system, but 18 unknown parameters. Therefore, an optimization solution is proposed in the literature to obtain the reaction forces which involves several simplifying assumptions which result in considerable errors in the solution. In this study, in addition to presenting the mathematical model of the total system, an experimental approach has been proposed in order to determine the locator reaction forces. This can provide a suitable means for evaluating the optimization solutions and analytical models for determining the locator reaction forces and contact stiffness and diminishing the errors. an experimental approach has been proposed in order to determine the locator reaction forces. This can provide a suitable means for evaluating the optimization solutions and analytical models for determining the locator reaction forces and contact stiffness and diminishing the errors.

Machining of hardened steels, which their hardness is generally higher than 45 Rockwell C, is called hard turning. These components usually work under dynamic loading conditions and require a high level of surface finish (in the order of 0.15 μm Ra) which cannot be achieved by sequential hard turning and burnishing processes. However, there are serious concerns about this complimant grinding operation; grinding process, on one hand increases tensile residual stresses and on the other hand, increases crack nucleation regions. Therefore, these two factors might decrease workpiece fatigue strength. So, in this paper, the effects of adding a grinding operation before ball burnishing process, have been experimentally studied on final surface roughness and burnishing forces; at the same time, in order to consider the possible destructive effects of grinding process, a set of experimental measurements including surface residual stresses and endurance limit measurement, have been done for AISI 4130 fatigue samples. Based on the achieved results, adding a grinding operation before burnishing process, has lead to 91.56% improvement in surface finish and 39.52% reduction in burnishing forces. In addition, surface residual stress are compressive and there is slight difference in the magnitude of compressive residual stresses in comparison to burnished hard turned samples. Due to these positive findings, endurance limit of produced samples show 10.95% improvement in comparison to burnished hard turned samples.

This paper aims to calculate and compare normal anisotropy coefficients in 3D-printed and hot-compression molded micro ABS specimens. To achieve this goal, micro specimens of additively-printed and compression-molded ABS were fabricated and tested using a micro-tensile testing apparatus integrated with an optical microscope while the deformation of the specimens was recorded by a camera. Frames from this video were selected and strain distribution on a micron-sized area of interest was obtained using digital image correlation (DIC) analysis. It was shown that there exists a close agreement between DIC results and in situ optical observations. The plastic anisotropy coefficients (R-values) were then calculated from the surface strains as a function of applied strain. For this purpose, through-thickness strain component was obtained assuming plastic incompressibility condition. Results showed that both micro samples revealed an anisotropic response during plastic deformation. At low plastic strains, the printed micro specimen exhibits a more anisotropic behavior than the monolithic micro specimen. As the deformation proceeds, the normal anisotropy coefficient increases for the additively-manufactured micro specimen and decreases for the hot-pressed micro specimen.

The purpose of present paper is the experimental study of the effects of hydrogen embrittlement with residual stress on mechanical properties of GTD450 alloy. For this purpose, 0.5 M sulfuric acid solution was used to create one- and two-hour hydrogen embrittlement (two levels) and the indentation with a cylindrical surface was used to create a residual stress proportional to the force of 5 and 9 kN (two levels) in the specimens. Based on findings, changes in ductility and percentage of reduction area, compared to baseline conditions (no residual stress and hydrogen embrittlement) in the four test levels ranged from 42.69% to 74.68% and 11.78% to 39.58%, respectively. The results of statistical analysis also estimated the contribution of residual stress and hydrogen embrittlement in ductility as 1.15% and 67.05%, respectively. For the residual stress related to the 5 kN force, with increasing the hydrogen charging to two hours, the toughness value has decreased by 60.54%. It is also observed that the maximum changes made compared to the baseline conditions for yield stress are equivalent to 1.68% of the specimen with one hour hydrogen embrittlement and the residual stress generated by the 9kN force. Regarding the baseline conditions, necking is associated with 36% reduction area. However, when the specimen is charged for two hours, the failure occurs with minimal necking, low strain and a slight reduction area.

The present work, deals with the Gurson-Tvergaard-Needleman (GTN) micromechanics based damage model ‎to add the ability to predict and calculate damage under shear loads; and use it in modeling damage and failure ‎under shear dominated loading conditions. In the development of GTN model, since different damages have ‎different physical concepts and attenuation effects, so an independent shear damage parameter was presented as a ‎function of equivalent plastic strain of the matrix. The modified GTN damage model was implemented by ‎developing the VUMAT code in the Abaqus software environment. To use the GTN model developed in VUMAT ‎format for damage analysis, 16 input parameters of the model were determined for the material under study. ‎After developing the model, the code and determining the input parameters for it, the modified model was first ‎tested on a single element. The results of the developed model showed complete agreement with the results of the ‎basic GTN model and analytical solutions under tensile and shear loads, respectively. Finally, the developed model ‎was tested in shear loading on a shear sample. It was observed that the modified model under shear load ‎eliminates the weakness of the base GTN model and well predicts the occurrence of damage and weakening of the ‎mechanical properties of the material under the prevailing shear conditions.‎

The theory of continuum damage mechanics with a phenomenological approach is used in predicting the phenomenon of material failure, and since each material has its own behavior, it is especially important to find out the damage behavior of common used metals. In this study, the constants of two damage models, Ganjiani and Bonora, for analyzing the elastoplastic damage behavior of several metals by writing VUMAT subroutine in Abaqus software, simulation and comparison with experimental data in literature, were determined and the ability and efficiency of models in the analysis of metals damage behavior was ensured. In finding the constants of the models, stress-strain, damage-strain and failures train-stress triaxiality diagrams were used and after the simulation, with the help of force-displacement diagram, which had a good agreement with the experimental diagram for each metal, the accuracy of the performance was confirmed. In general, the constants during a trial and error process were determined to be the optimal fit. The studied metals were steel 1045, aluminum 2024-T351and HY130 steel. For the first two metals, only simple tension test is simulated, and for HY130 steel a more general test, three point bending test, is simulated and the obtained force-displacement diagram was compared with the experimental diagram. Also, because the strains were in the range of large strains, the relationships related to large strains have been used.

In this research, the effects of graphene oxide (GO) as an additive to Kevlar fabric impregnated with nanosilica/PEG shear thickening fluid (STF) on the ballistic performance were investigated. In order to understand the influence of STF, pull-out tests were accomplished to assess the friction between yarns. The energy absorption in the high-impact ballistic test for the fabric impregnated with STF increased by 25.8% compared to that for the neat Kevlar fabric. This parameter for the fabric impregnated with STF-0.2 wt.% GO was 23.3% as compared with that of the neat fabric, demonstrating the deteriorating effect of graphene oxide additive. The results of the pull-up tests were in agreement with ballistic tests, meaning that the increase or decrease in the maximum forces in pull-up tests was followed by the increase or decrease in the energy absorption in ballistic tests. Compared to the sample impregnated with STF, adding graphene oxide causes the decrease in the maximum force in the pull-up test, resulting in a reduction in restriction of yarns movement, consequently facilitating their movement inside the fabric.

Bolt connections often loosen under environmental loading conditions and system vibrations, which can lead to disaster risks during its operation. In this study, the behavior of nonlinear vibrations of an aluminum single-lap joint has been studied analytically and experimentally. Accordingly, considering the effects of nonlinear behavior at the bole joint, a nonlinear two-degree of freedom model for this type of connection is proposed. Then, in order to determine the unknown parameters of the proposed model, the vibrational and dynamic properties of this structure have been estimated using experimental modal analysis and model updating method. Finally, the effect of amplitude of the excitation force and the preload force of the bolts on the dynamic behavior of these systems has been studied analytically. Examination of amplitude-frequency curves shows that reducing the preload force of the bolts reduces the natural frequency and also distorts the amplitude-frequency curve to the left side, which indicates the softening nonlinear behavior of the system with decreasing applied bolt preload force. In addition, the comparison of the theoretical and experimental natural frequencies shows that the proposed model predicts the vibrational characteristics of these systems with a good accuracy and using the proposed model can study the dynamic behavior of these systems for different parameters.

In sheet metal forming, thickness stress may affect the formability of sheet metal. In this paper, effect of through-thickness pressure stress on the formability of aluminum alloy AA6022-T4 sheet is investigated based on crystal plasticity finite element analysis. A self-hardening behavior is considered for the strain hardening behavior of the slip systems. Further, for prediction of necking initiation and growth, maximum shear strain criterion is used for damage initiation and evolution. In order to implement the model in Abaqus finite element package, a VUMAT was developed based on the discretized equations and forward Euler integration scheme. After verification of the developed code, the parameters of the model were calibrated against the tensile test results. For simulating tensile test of 1 mm thick sheet, a representative volume of 3x1.5x0.5 mm3،was partitioned into 14790 grains through a python code in ABAQUS/CAE environment and then discretized using 50 μm tetrahedral linear elements. Using the experimental data available in literature and considering appropriate texture for the simulation domain, the crystal orientations were assigned through Euler angles. Then, tensile tests were performed on the sample in the presence of the thickness pressure stress. The results show that application of the through thickness stress increases the strain corresponding to the necking initiation and thus postpones necking. Correspondingly, a decrease in tensile load is observed in this case.

Breast cancer is one of the most important cancers in the field of medicine due to its high prevalence. Understanding the mechanical properties of cellular tissue, including the Young's modulus, and comparing the differences created after the onset of the disease, lead to the development of new methods in recognizing, controlling, and treating the disease. Nanomanipulation is one of the processes used in the field of nanotechnology, which explores the properties of cellular tissues by exploring the surface of biological particles and analyzing information in two different phases. Atomic force microscope is a tool used during this process that examines the properties of cellular tissue by measuring the tip of the probe with the cellular tissue during the movement of the cantilevers and by measuring the changes due to displacement and force. In this study, nanomanipulation of MCF-7 breast cancer cell was performed experimentally using atomic force microscope with the aim of finding the Young modulus of cell tissue. After extracting the experimental results, modeling and calculating the critical force and time by considering different contact models including Hertz contact model, PT and COS, has been done. According to the comparison of experimental and simulation results, the Young's modulus of MCF-7 breast cancer cell was obtained in the range of 800 Pa. Also, the COS contact model was more in line with the experimental results.

This paper is an attempt to integrate computer vision techniques and MAV guidance to design and optimize an automated mission performed by a light MAV such that automating the mission becomes reasonably more efficient than performing it manually. A system is provided for warehouse management using an MAV equipped with a front camera. Computer vision algorithms makes it possible for the MAV to locate packages, verify the presence or absence of a specified package and list the entire warehouse inventory in a short time. An innovative method is provided to detect shelves and their packages by the camera image, which enables the system to instantly plan the shortest path for the MAV while performing a shelf inventory listing. Then, following the planned path completes the mission faster than conventional guidance methods. The guidance algorithm is designed such that the efficiency of automatic operations compared to human operations is significantly increased. The system is first simulated and then implemented and the test output data is provided. The tests indicate the success of the system in securing automated operations, while decreasing mission time.

In this paper, the aeroelastic model of a 3-D box wing configuration is derived using a semi-analytical approach and its aeroelastic behavior is studied. Modeling and aeroelastic analysis of box wings were carried out in previous published papers by 2-D models or professional software. Here, the box wing structure is considered as two front and rear wings connected by a winglet and plunge and pitch motions are considered for each wing. The winglet is modeled by two longitudinal and torsional linear springs. The governing equations are extracted via Hamilton's variational principle. In order to apply the aerodynamic force and moment, Wagner unsteady model is considered. For the purpose of model validation, the box wing flutter speed and frequency are compared with MSC NASTRAN software results and good agreement is reported. The effects of different design parameters such as sweep angles and the winglet rigidity on the stability boundary are investigated. The results reveal that increasing the sweep angles and the chord ratio enhances the flutter speed, remarkably. Furthermore, increasing the torsional rigidity of the winglet is more significant than the longitudinal rigidity on the BWA stability boundary.

The Creation of reference trajectories and the ability to track them in the presence of disturbances and uncertainties are important issues in investigating the exoskeleton performance. One of the methods of trajectory planning is the central pattern generation algorithm. This algorithm will behave in a limit cycle and the temporal disturbances have quickly removed the system and create harmonious trajectories. In this paper, for the creation of reference trajectories of each joint, a combination of seven modified Hopfield oscillators is used which provides the ability to change the frequency and domain of walking. Online modification of robot joint reference trajectories is done by using the feedback error signal between desired zero momentum point and zero momentum point of the robot at any moment. In order to cope with the disturbances and uncertainty with the uncertain domain and achieve maximum efficiency in tracking robot reference trajectories, an adaptive dynamic fast terminal sliding mode controller is used due to the elimination of chattering phenomena, and finite-time convergence. Also, by moving the Upper link the maximum stability of the robot based on zero momentum point criterion is guaranteed. To achieve maximum performance, controller parameters, oscillator coefficients, and connections between them are optimized. Finally, the performance of the proposed method is compared with a sliding mode controller. The results demonstrate the superiority of the proposed method.

Nowadays, the use of wind as one of the main sources of low carbon and renewable energy is expanding rapidly all around the world. Recently, with the development of wind farms and the consequent increase in the size of wind turbines, their maintenance has become more difficult. Therefore, researchers have deeply focused on the analysis and the control of their vibration. In this study, a wind turbine blade with a type of nonlinear absorber, called highly efficient nonlinear energy sink is analyzed, and the interaction between the heavy and long blade and the nonlinear energy sink (NES), under the influence of gravity in the vertical plane and wind force varying periodically due to height dependence is examined. For this purpose, the equations of motion for the blade connected to the NES are extracted using the energy method and solved numerically. Finally, the sensitivity of the parameters affecting the performance of the NES is analyzed and the performance of the system under rotation of the standalone blade and also the rotation of the blade in the presence of the optimal NES is studied.

Cylindrical shells are used tremendously in many engineering fields such as ships, submarines and fuel tank in airplanes. Stiffeners which are beam elements are used for increasing stiffenes of shells. In many cases shells are exposed to dynamic loads. One of dynamic loads in shells is internal moving pressure. Analysis of cylindrical stiffened shells under moving internal pressure are investigated in this research. Equations of motion are based on classic shell theory and derived from Hamilton’s method. Boundary conditions is assumed simply support. Displacement components are assumed Fourie double series based on boundary conditions. Equations of motions are solved by Galerkin weighted functions method for calculation of natural frequency and dynamic response of cylindrical shells under moving internal pressure. Codes in Fortran are used to derive natural frequency and dynamic response of cylindrical shells. Results are compared with other references and Abaqus software. The effect of geometrical parameters on natural frequency and dynamic response of cylindrical shells under moving internal pressure are investigated finally and results for stiffened shells and unstiffened shells with different stiffeners are compared.