مجله مکانیک سازه ها و شاره ها
سال دوازدهم شماره 6 (بهمن و اسفند 1401)

Nowadays,due to the increase of patients with low mobility,the presence of rehabilitation robots is necessary.To improve design of rehabilitation devices,musculoskeletal models are widely used to analyze interaction forces between human body and assistive devices.In this study,combined model of rehabilitation system with musculoskeletal model of the body has been discussed using Opensim software.This system has already been designed based on Jensen's 1 DOF mechanism with eight links, and has weight supporter system to support the patient's weight.System uses only one actuator for both the left and right sides of the body,which rotates the crank.Interaction forces between the device and the body are investigated.For this purpose,by using kinematic simulations and direct dynamics,movement of the rehabilitation system has been simulated.Then,by static optimization and performing inverse dynamics simulation,the amount of force and torque applied at the points of contact and each joint have estimated.Results show that interaction forces at the point of contact between the user's hip and the seat of the mechanism are 750 Newtons,which is proportional to the weight of the user,which is considered to be 75 kg.Forces and torques obtained at the connection point of the feet with the pedal are 16 Newtons in the vertical direction,which is caused by the weight of the user and the device,and other components are 2 Newtons, which is insignificant compared to the vertical component, as well as torque produced at the place where the pedal is connected to the foot, is 0.01 Nm. Results of this study can be helpful in designing rehabilitation systems.

In this research, the plastic deformation and failure mechanism of sandwich structures with aluminum face-sheets and waste rubber particles core under low-velocity impact loading have been investigated. The drop hammer testing machine was used to apply the impact load to the sample at seven different energy levels 34.3, 68.6, 102.9, 137.2, 154.3, 171.5, and 205.8 J. To achieve the mentioned energy levels, the weight of the hammer was considered constant and equal to 3.5 kg and the standoff distance of the hammer was changed from 1 to 6.5 m. 10 test samples were considered in two types of layering with and without the core of rubber particles. In this series of experiments, the thickness of aluminum face-sheets was constant at 1 mm and two different thicknesses of 16 and 32 mm were considered for the core. Experimental results showed that, the sandwich panel with 16 mm had a 37 and 18 % smaller deformed area for fall heights of 2 and 3 meters, respectively, due to the less porous space between two aluminum face-sheets. Also, compared to the coreless sandwich structure, the use of a low-mass waste rubber particle cores at low energy levels increased the front sheet perforation diameter by 19 %.

Detection of rotor asymmetry faults (RAFs) in induction machines (IMs) based on stator current signature of machine due to the presence of low-frequency load torque oscillation (LTOs) can cause false alarm (FM). Therefore, isolating the RAFs from the LTOs can improve the condition-based monitoring system. The methods discussed in the past generally require three-phase current and voltage of stator windings information along with machine angular velocity. In this paper, a new method based on single phase machine data is presented. In this regard, a virtual rotating reference frame based on Hilbert transform is provided which do not need the angular velocity of machine. In order to improve the output spectrum resolution and low computational cost, the proposed method is combined with the Gortzel algorithm. The proposed method is tested and evaluated by synthetic data and then evaluated by means of experimental results. The results show that this method can isolate the RAFs indices from LTOs, effectively.

The common dynamic relaxation algorithm (DR) does not have the ability to trace the static path. In these techniques, the jumps occur at the limit points. A variable load factor is used to fix this defect. Here, a new procedure is suggested to calculate the load factor. The authors’ relationship is achieved by minimizing external work and residual energy, simultaneously. It should be stated that the proposed load factor depends only on the DR artificial parameters. To show the ability of the new formulation, several truss and shell structures with nonlinear geometrically behavior are analyzed. All used methods are ranked by the number of iterations, numbers of convergence points and total duration analysis. Numerical solutions show the high efficiency of the new method. In other words, the authors' technique, in addition to good accuracy, has higher convergence rate, in comparison to the other strategies. On the other hand, the time duration of the proposed method to trace the static paths has been reduced appropriately compared to other techniques.

Today, bone machining is known as one of the most important steps in orthopedic surgery. With accurate and effective simulation of machining process can achieve faster patient recovery and high efficiency in surgery. Accordingly, estimating and controlling the production temperature in machining is necessary to prevent excessive heat generation and thermal damage to the bone. The study of the orthogonal cutting process is important because of the basis of other machining processes. Therefore, an experimental and numerical study was performed for orthogonal machining. An elastic-plastic model was used as the material model to predict cortical bone behavior in finite element simulation. The damage model was used for complete material failure, tool penetration and chip formation. The simulation was done for the first time with the damage model and its results were analyzed for the first time with the response surface method and sensitivity analysis using the Sobel method for each parameter separately and the interaction of the parameters. The simulation results are in agreement with the experimental results. According to the optimization, the minimum temperature is about 26ºC when the cutting depth is 0.1 mm, the cutting speed is about 192 mm/s and the rake angle is 12º. This study can be effective in further advancing the bone machining process and lead to progress in orthopedic surgery, optimization of machining parameters and tool design.

The applying of Severe Plastic Deformation (SPD) processes, such as Equal Channel Angular Pressing (ECAP), leads to the introducing of high strains on the metallic material and consequently microstructural evolution. As a result of these microstructural evolution, the grains structure were changed to ultra-fine and leads to the improvement of mechanical properties. In this research, commercially pure titanium, as a difficult deformable metal, was subjected to cold-ECAP process in channel angle of 135º with back pressure, and the effect of back pressure on its mechanical properties and workability was investigated. The experimental results showed that by simultaneously applying back pressure and encapsulating the billet in a pure copper tube, the bimetallic work-piece was processed up to four passes. Considering that there were only two successful passes in ECAP process without back pressure, it can be concluded that the workability of pure titanium billets increased by applying back pressure due to the higher strains in higher passes. It was also demonstrated that by subjecting the back pressure, the ultimate compressive strength of material significantly increased from 899 to 1163 and 1317 MPa and microhardness is enhanced from 163 to 203 and 262 Vickers. In addition, the refining of the grains was occured from 49 to 34 and 24 µm. These results were presented for initial annealed material and the states without and with back pressure, respectively.

In today's era, it is necessary to analyze functionally graded materials. Since the finite element method in the analysis of these materials has limitations and the error is an inseparable part of any numerical analysis, therefore, finding a solution to estimate the error in calculations is important. In the finite element method, modifying or enriching the network to reduce the error is associated with such things as the overlapping of elements during displacement, the formation of elements with zero areas, and the increase of cost in calculations. In this research, the isogeometric method has been used for the first time in the analysis of problems with functionally graded materials in the adaptive solution using the method of moving control points as adaptive correction based on error estimation, with the approach of improving the stress field. By comparing the exact and approximate error norm in the solved examples, the effectivity index is more than 75%, which shows the effectiveness of the proposed error estimator. In addition, improving the network of control points is effective in reducing the error rate by more than 60% and can be used to increase the accuracy of the results.

In this research, the mechanical properties including bending strength and impact resistance for steel composite beams with high performance concrete are discussed. In the construction of the steel part of the specimens, the shield cross-section grade 4 is used in the tensile part of the sheet with a thickness of 5 mm and two widths of 30 and 40 mm and a length of 70 cm. It should be noted that the compression wing of some specimens is made of concrete with Different steel reinforced fibers. Self-compacting concrete with 3 percent of different steel fibers, which are 0, 0.5, and 1 percent, was used in making the concrete of the specimens. The results of the impact tests showed that the amount of energy absorption as well as the initial maximum force increased with the increase in the fibers percentage that the highest energy absorption is related to the w4hc4sf1 beam, which has increased by 0.09% compared to the specimen without concrete. Regarding the results of the bending test, the resistance value increased intermittently with the increase of fibers. The w4hc4sf1 beam had the best performance, and its strength increase compared to the same specimen without concrete was equal to 204%.

Applied study on cancerous tissues for the purpose of treatment is possible only with complete knowledge of cancerous and healthy cells, in terms of mechanical, chemical and geometrical structure. The high sensitivity in the displacement of cellular tissues and their vulnerability when applying large forces has led to the optimal modeling of the process. In the first phase of manipulation, the force applied to the pole of the atomic force microscope must overcome the resistance forces such as friction. Investigations have been done in the first phase of nanomanipulation and in three dimensions. The critical force and time were calculated for all three Persson friction models by comparing the applied forces in all three directions of movement and the resultant force, and in order to ensure the results of this simulation, the necessary verifications were made with Lager, Kolb and Hka friction models, which were obtained in previous researches. was done Finally, the results indicated that the lowest value was in Persson's third friction model, with values of 93 nano newtons for critical force and 78 milliseconds for critical time.

The present study examines the mechanical properties of auxetic stents with Re-entrant structure. Geometries were parametrically modeled, and the design of experiments (DOE) was developed by defining the elastic properties of the stents and using the response surface method (RSM). Finite element (FE) analysis was performed in order to find a polynomial relationship between the geometric parameters as inputs and the elastic parameters as the outputs. Then, the optimal stent was obtained in terms of elasticity parameters by using RSM method. As a result, the optimal parameters of the Re-entrant stent including flexural stiffness, axial elasticity modulus, radial elasticity modulus and Poisson’s ratio were obtained as 40 mPa.m4, 253 MPa, 752 MPa and -0.58, respectively. Moreover, a method was proposed to find an analytical solution for metal elastic stents in order to verify the FEM results. Finally, the blood vessel compliance of the optimal stent was evaluated, showing a good compliance for practical usage.

In high amplitude vibrations when there is an impact damper, effective collisions occur between the damper particle and the main structure. These effective collisions cause the energy of the main system decreases significantly. The use of impact dampers in the free vibrations of a system of one degree of freedom reduces the amplitude of vibrations linearly, where the slope of this line is called the damping inclination.The damping inclination is a criterion of the damper performance, whose number indicates the effect of using the impact damper in reducing the vibrations of the oscillating system. In most of the previous researches, the damping inclination has been extracted by numerical or experimental methods, and no formula has been presented to predict the damping inclination. In this research, the effect of parameters of the coefficient of restitution and the mass ratio on the damping inclination of rigid single mass impact dampers has been studied. Then, by analyzing the data, an analytical relationship has been presented to determine the damping inclination according to the coefficient of restitution and mass ratio. This formula predicts the damping inclination with an error of less than six percent and eliminates the need for numerical and experimental studies to determine the damping inclination.

In this paper, dynamic behavior of API X65 steel under impact loading was studied using an instrumented Charpy machine of 450J capacity. The fracture energy can be measured using this testing machine in two different ways. In the first method, the difference in potential energy before and after impact test is reported via machine dial. In the second method, the output voltage from load-cell (mounted on the machine tup) gives load-displacement data from which energy is calculated via curve integration. Using instrumented data, the initiation energy, propagation energy and total fracture energy for eight series of specimens made from tested steel were measured and found to have exponential behavior versus initial notch depth. Furthermore, the characteristic forces used for the design of dynamically loaded structures (namely general yielding, Fgy, and maximum force, Fm) were determined from the force-displacement diagram. The important innovation of this research is presentation of eight new correction factors for different notch depth of specimens. These new correction factors and their mean value were compared with a similar data from a previous research work for conventional fracture models of energy transportation steel pipelines which showed good agreement.

Droplet coalescence or adhesion occurs when water droplets attach to a bond and form a single drop. In this paper, using numerical simulations, the dynamic behavior of merging two drops of equal and unequal size on a inclined surface is studied. The effect of surface slope and droplet diameter on the behavioral dynamics of the liquid bridge and the three-phase droplet contact line is integrated. During the solution, we constantly see the dominance of gravity and surface tension forces over the fluid flow. The results show that in the integration of small droplets, more intense oscillations occur in the area of the liquid bridge and the displacement of the three-phase line, compared to the larger droplets. In small droplets, with increasing slope level, the fluctuations in the displacement of the beginning and end points of the three-phase line become severe, which weakens the high-frequency oscillations in the liquid bridge area. For merging large droplets, the forward and rear end displacements of the droplet increase uniformly. The results of merging unequal droplets show that the speed of liquid bridge formation in this case is higher than the merger of symmetric droplets, which leads to more oscillations with low oscillation amplitude and high oscillation frequency.

In this paper, the position of the leakage in a pressurized gas pipe is localized. As an experimental example, the leakage in a steel pipe is simulated by different orifices. To decrease the risk of explosion and environmental contamination during experiments, pressurized air was used instead of natural gas. Most of the published papers are based on using high sampling rate sensors placed on both sides of the leakage. In this study, a new method is extended due to leakage localization in a gas pipe. To achieve this goal, attenuation analysis is implementing by installing two sensors on one side of the pipe. In the proposed technique, the dominant frequency of recorded signals is used, to decrease the effect of background noises on final results. Experimental results reveal by increasing the pressure the leakage localization error is decreased. Finally, by implementing signal processing into the data the average value of leakage localization error is reported 13.77 cm.

Heat transfer processes are widely used in many industrial applications, therefore many studies have been conducted in this field. so in this research, the effect of the active vibrations of a piezoelectric vortex generator on the displacement heat transfer rate in a microchannel is investigated. The assumed vortex generators consist of square pins having a flexible splitter plates on their lee side. These plates will be deformed under fluid structure interactions. The Reynolds number, based on the channel’s hydraulic diameter,is set to 1000 to ensure laminar flow. The heat transfer performance, the hydrodynamic friction factor and the overall hydrothermal efficiency for different number of VGs and splitter’s flexural rigidity are investigated. The results showed that softer splitters provide better heat transfer capability and the higher hydrothermal performance. The results also indicated that, by selecting proper configuration, in the expense of 33% decrease in total hydrothermal efficiency with respect to clean channel, 140% increase in the rejected heat, compared to clean channel, can be achieved.

In this paper, thermo-hydraulic effects of the wire-wrap spacer in a hexagonal fuel assembly with seven fuel rods have been investigated. The effects of wire-wrap pitch and Reynolds number on fluid flow, pressure drop, and temperature distribution have been analyzed. Thermal and fluid analysis has been studied for 66, 100, 200, and 400 wire-wrap pitches. K-ε turbulence model is used to perform this numerical simulation. The results are compared with a no-wire-wrap state. The results show that by increasing the Reynolds number, the pressure drop of the assembly with and without wire-wrap increases (100% increase in Reynolds number of 100,000). At a constant Reynolds number, decreasing the pitch of wire-wrap increases the pressure drop (60% increase by reducing the pitch from 200 to 100 mm). Wire-wrap improves flow mixing and heat transfer, and by reducing the pitch of the wire-wrap, the results have a more uniform distribution of temperature in each section of the assembly.