مجله مکانیک سازه ها و شاره ها
سال دوازدهم شماره 4 (مهر و آبان 1401)

In the present paper, the size-dependent vibrations and stability of rotating microbeams with axial motion embedded in a viscous medium with different supported boundary conditions in humid-thermal-magnetic environments under gravitational and axial loads are studied based on the coupled stress theory and Rayleigh beam model. The dynamic equations of the system are derived using the Hamilton principle. Using the Galerkin method and solving the eigenvalue problem, the backward and forward vibrational frequencies and the instability thresholds of the system are obtained. Comparative studies are performed to validate the results of the present study. The effects of various key parameters such as rotary inertia factor, substrate damping, flexural stiffness ratio on system dynamics are examined. The results showed that magnetic fields improve system performance in contrast to humid environments. Also, when the axial motion of the system is in the opposite direction of gravitational acceleration, gravitational forces reduce the instability threshold of the system and can change the system stability evolution. It is also shown that increasing the rotary inertia factor reduces vibrational frequencies and system stability. The modeling and the results of the present study can be useful in the optimal design of microswitches.

In this paper, the green’s functions resulting from wave propagation in a porous isotropic foam monolayer with the boundary condition of the foam end as a rigid-bonded are investigated. This article aims to obtain the stresses and displacements resulting from wave propagation of harmonic forces located on isotropic foam. Foams are of the saturated porous material type due to the hollow pores in which fluid moves. The governing equations of wave propagation for saturated porous material are complex partial differential equations converted into two separate equations using two unknown potential functions. The obtained equations are transformed into simpler equations using Henkel integral and Fourier series transformations. By utilizing the governing boundary condition of the problem, two unknown potential functions are obtained in the transformed space, and with Henkel's integral inverse operations, the resulting solution is obtained in the frequency space. One of the most important results of this study is that in the real part of Green’s functions, point loading causes maximum stress and displacement in the vertical direction. Ring load and low porosity have the maximum frequency angle from the wave propagation in Green’s functions.

Sandwich structures with cores made of porous materials such as various types of metal and polymer foams are effective in absorbing the energy of explosive loads and due to their lightness can be used well in various industries. In this paper, deformation and energy absorption of sandwich panels with aluminum and steel face sheet and cores made of polyurethane foam, which is filled with two types of mineral pumice of different sizes (so-called chickpea size and almond size), under experimental free explosion loading. And numerical simulation has been studied with the help of Abaqus finite element software. In this study, the mechanical behavior of the core was first studied by making samples of polyurethane foam and mineral pumice under compression testing and the results were used in software behavioral models. After comparing the numerical results with the results of experimental test, and validating the results of numerical methods, numerical parametric studies were performed and the effect of pumice type, core thickness, face sheet material and thickness on the rate of deformation and energy absorption of sandwich panel components have been studied. The results show that the panel with a thicker back face sheet has a better performance in absorbing explosion energy. Also, as the strength or thickness of the face sheets increases, the role of the core in energy absorption decreases.

Diffusion welding is a solid-state welding process in which contacting surfaces to be joined under pressure at elevated temperatures with minimal macroscopic deformation. This study aimed to investigate the effect of diffusion welding parameters on the microstructural characteristics and mechanical properties of the dissimilar joint between 17-4PH steel and Ti6Al4V alloy using Ni interlayer with a thickness of 150 µm. The experiments were performed in a vacuum furnace at three temperatures of 900, 950 and 1050 °C for 45 and 60 min under the pressure of 5 MPa. The results indicate that at the temperatures of 900 and 950 °C, Ni3Ti, NiTi and NiTi2 intermetallic compounds are formed at the nickel-titanium interface, while with increasing the temperature to 1050 °C, NiTi2+Fe2Ti, β-Ti and β-Ti+α-Ti phases are formed at the interface. The minimum microhardness (562 HV) is achieved at the temperatures of 900 °C and time of 45 minutes and the maximum microhardness (700 HV) is obtained at the temperatures of 900 °C and time of 60 minutes. By increasing the time of diffusion welding from 45 to 60 minutes at two constant temperatures of 900 and 950 ° C, the shear strength increases %4.8 and %23.6, respectively. Also, by increasing the temperature of diffusion welding from 900 to 950 °C at fixed times of 45 and 60 minutes, the shear strength decreases %55.3 and %31.7, respectively. The best shear bond strength was 303 MPa, which was obtained at the temperature of 900 ° C and the time of 60 minutes.

Superimposing high power ultrasonic vibration on metal forming processes changes the mechanical behavior of the material and reduces its flow stress. These phenomena improve the ductility of material and reduce forming forces. To investigate the influence of ultrasonic vibration on various materials, standard compression and tensile tests under ultrasonic vibration could be performed. This research investigates the compression behavior of Ti-6Al-4V alloy under superimposed ultrasonic vibration. For this purpose, a special set-up was designed, fabricated and tested. Ultrasonic vibration is transmitted from the ultrasonic transducer to the booster and then guided by horn (punch) to apply to test sample. Horn geometry, vibrational mode shape and ultrasonic power were selected as input variables. The effect of specimen position in system vibrational mode shape was studied by using different ultrasonic transmitters, which allowed the specimen to place at node and anti-node positions. It is found that ultrasonic vibrations reduced the formability of this alloy. Acoustic hardening phenomena do not observe in this material. Also, flow stress of the material reduced under ultrasonic vibration up to 17.63% which has direct relation to ultrasonic power. In addition, the most efficiency was observed when the specimen placed in the node position.

Stretching of sheet metals due to the high applied tensile stress and elimination of springback leads to the production of high-strength parts with high dimensional accuracy. In this research, an analytical-experimental study of stretching of the DC04 steel sheet as one of the most widely used sheets in the automotive industry has been considered. In the experimental stage, by performing uniaxial tensile test at angles of 0 °, 45 °, and 90 ° with respect to the rolling direction of the sheet, the plastic anisotropy coefficients, the constants of the power-law hardening model, and the other mechanical factors of the material were obtained. Then the strain components in different areas of the specimens were calculated by using the Hill-48 anisotropic yield criteria based on the associated flow law. Using equilibrium equations at the contact surfaces of the sheet with the tooling components, the effect of frictional forces on the stress-strain distribution was determined. After designing and fabricating the apparatus and performing experimental tests, the values of strain, stress and thickness were measured with three displacements of punch stroke at 10.5, 15.75, and 21 mm on the specimens. Finally, comparing the results of analytical method with experimental observation, the magnitude of the errors for three mentioned punch strokes were obtained 15.8%, 4.7%, and 0.07%, respectively.

Alloying elements and temperature are non-uniformly distributed in the wall of a steel pipe during the manufacturing process. In this study, the effects of these non-uniform distributions on mechanical properties were evaluated macroscopically. Plate-type tensile specimens were cut from the wall of a steel pipe in a way that each specimen was separated from a specified location along the wall thickness. The results of tensile tests showed significant variation of mechanical properties in the thickness direction. To investigate the effect of this variation on the steel deformation, a thick tensile specimen with a thickness equal to the pipe wall thickness was then modeled as a multilayer structure based on the measured mechanical properties. First of all, with the help of finite element simulation and according to the obtained stress-strain curve from the tension test, GTN damage parameters have been calibrated for every single layer, and then, these parameters were used in modeling the multilayer specimen. According to the results, the numerical load-displacement curve obtained from multilayer modeling did not exactly match the experimental curve of the thick specimen; this can be due to different governing stress states in thin and thick specimens and also the effect of residual stresses.

The Markov chain model is one of the statistical-probabilistic approach that can predict the failure status of samples in higher cycles by using experimental results in primitive cycles and also it can be used instantaneously to control fatigue failure of parts in working. In this paper, double lap adhesive joints are subjected to cyclic loading at three different charge levels in the form of tensile load, and the purpose of this work is investigating the fatigue failure process of adhesive joints. In this study, ratcheting changes have been introduced as a failure indicator that shows the growth trend of fatigue failure. It is observed that fatigue damage occurred after 18% growth in the initial ratcheting size. These experimental results are consistent with the data obtained from the Markov chain model. Therefore, this forecasting method can predict the remaining life of double lap adhesive joints based on strain evaluations regardless of their loading history.

The aim of this research is investigation of surface enerfy effet on free axial vibration of cracked functionally graded nanorods modelled based on the Rayleigh theory of rods. In Rayleigh theory, the effect not only of the axial inertia but also of the inertia of lateral motions are considered. It is assumed that the material of nanorod is functionally graded in its length direction and varies as power low relation. The crack is also modelled as a linear spring in which its stiffness is proporsional to crack severity. The surface energy is included effects of the surface density, surface stress and surface Lame constants parameters. Due to considering the effect of surface energy parameters, the governing equations of motions and corresponding boundary conditions become inhomogeneous in which to solve them, they are converted to homogeneus ones using an appropriate change of variable, firstly. Then, the natural frequencies of fixed-fixed and fixed-free nanorods are extracted using the method of harmonic differential quadrature. In addition to type of boundary condition, effects of parameters like length and radius of nanorod, severe and location of crack, and mode number on natural axial frequencies of cracked functionally graded nanorods in presence of surface energy are investigated.

Using Fire Dynamic Simulator (FDS) software, this study investigates the effects of sprinklers, branches and blowing fan on fire in a 40-meter-long coal mine tunnel corridor. This study has shown that nozzles, despite reducing the visibility due to water evaporation, will reduce the temperature throughout the corridor and reduce the concentration of carbon dioxide by 15,000 ppm at the farthest point. A branch in a corridor and providing an evacuation route from the damaged area will increase the time for different parts of the mine to remain safe in terms of temperature and the concentration of harmful gases. On the other hand, if the branch is close to the fire, it will cause the fire to spread due to providing the required oxygen. The presence of a blowing fan for ventilation in the tunnel corridor, despite faster fire extinguishing and a 46% reduction in the average temperature on the fire source, will increase the concentration of carbon monoxide by 7.5 ppm at the farthest point. Also, the nozzles connected to the thermocouple, due to the lack of Time Response Index (RTI) compared to sprinklers, are more effective in fire controlling and will reduce the temperature on the fire source by 69%.

The most common method of simulating liquid spray in a gaseous environment is the Eulerian Lagrangian approach where the gas phase is solved by the Euler method and the liquid phase by the Lagrange method. In the dense regime, the effect of the liquid phase on the gas phase and also the effect of the droplets on each other are of great importance. Due to the strong coupling between the gas phase and liquid phase, numerical solution of these equations is one of the main challenges of this approach. In this study, two SIMPLE based algorithms for solving these equations are presented. To do this task, a Lagrangian-Eulerian computer code was developed for the two-phase flow equations. To evaluate the proposed approaches, the diesel fuel spray was solved and by comparing results, the appropriate algorithm was selected. The results showed solving the liquid phase equations at the beginning of the SIMPLE iteration loop and then making two corrections in the appropriate position inside the algorithm, provides a suitable method for solving governing equations of the spray with the assumption of four-way coupling in the Eulerian-Lagrangian approach.

Adsorption desalination system employs low temperature waste heat, which is available in renewable energy sources. A three-dimensional transient model has been developed for modeling heat and mass transfer in adsorbent bed silicagel/water pair and validated by the experimental results. Evaporator is one of the main adsorption desalination parts and in this study, the effects of shell and tube evaporator pipe lenght on the performance ratio (PR) and specific daily water production (SDWP) are numerically investigated. According to modeling results, it was found that the performance ratio increases by increasing the evaporator pipe length, while aproaching 0.66. SDWP has an optimum cycle time for each evaporator pipe length, which increases with increasing the evaporator size. However, it was also found that the SDWP remains almost constant for evaporator pipe length larger than 4m. Specific daily water production increases by 11% for the pipe length increase from 2m to 4m, while it only increases by 6% when the pipe lenght increases from 4m to 8m.

The air humidification-dehumidification method is a good option for decentralized freshwater production because of no need for high temperature operation. The main disadvantage of these systems is their dependence on direct sunlight. to solve this problem, using the sun's thermal energy storage during the day and using this energy during the night can be a very good option that can be achieved using phase change materials. In the present work, it is also possible to continue the desalination process after sunset, by equipping the solar collector with phase change material from two different types of paraffin wax. MATLAB software has been used to solve the equations governing the components of the desalination system. According to the results, the temperature of the output water from the solar collector plays a significant role in the amount of fresh water produced. The results also confirm that the use of phase change material leads to more than 9% increase in fresh water production. Another important result is that in the phase change materials the melting process speed is substantially higher than the freezing process speed.

In this paper, using experimental studies, the effect of volumetric filling ratio and evaporator and condenser length ratio on the performance of the pulsating heat pipe has been investigated using the response surface methodology. The surface methodology is a response to a set of statistical techniques and applied mathematics to construct experimental models with the aim of reducing experiments. In order to improve the performance of the pulsating heat pipe, the present study studies the minimum thermal resistance under the influence of two parameters. The pulsating heat pipe with general dimensions of 3 × 210 × 200 mm consists of four rounds of copper pipe with internal and external diameters of 2.4 and 3 mm, respectively. Experimental results show that the changes in thermal resistance in terms of the parameters of the volume filling ratio and the ratio of the length of the evaporator to the condenser, has a turning point before which the relationship between thermal resistance and each of the parameters will be inverted and then direct.