مجله مکانیک سازه ها و شاره ها
سال ششم شماره 1 (بهار 1395)

Many of energy harvesting devices use piezoelectric elements to convert mechanical vibrations into usable electrical energy. The input excitation is usually assumed to be a deterministic harmonic wave, while in practical situations, the mechanical excitation of the media is a random signal. So, the objective of this research is to study the energy harvesting in piezoelectric devices using the random vibration theory. At the first step a lumped parameter physical model of the device is presented. A mathematical model is then developed by obtaining the normalized differential equations governing the voltage induced in the energy harvesting circuit as well as the length of the piezoelectric material. The random vibration theory is then utilized to derive analytical expressions for the statistical properties of the voltage, power and the length of the piezoelectric material in terms of the statistical properties of the excitation which is assumed to be a band limited white noise. It is shown that with proper selection of the system parameters, the expected value of the harvested power can be effectively maximized. The qualitative and quantitative knowledge resulting from this effort is expected to enable the analysis, optimization, and synthesis of piezoelectric energy harvesting devices.

In this research, homogenous and intra-ply hybrid composite of brittle/ductile fibers (basalt as brittle fiber and nylon as ductile fiber) have been produced in order to investigate the effects of bending deflection on damages caused to the composite structure. The composite samples used in this investigation were of different volume percentage of basalt fiber (0, 50, 66, 75 and 100). All samples including six layers were prepared using hand lay-up method. Three-point bending test with different deflection (10 mm, 20 mm, 30 mm)were performed on the prepare samples. In the following, residual tensile strength was employed in order to study the amount of bending damages caused to the specimens. The results indicate that on samples comprised of high content of basalt fiber, by increasing the deflection, the damage caused to the composite structure increases. With increasing the ductile (nylon) fiber content, increasing the bending deflection has not significant effect on caused damages.

In the present study, measurement and analysis of the dimensionless frequency (Strouhal number) of vortex shedding of an axial compressor cascade in moderate Reynolds numbers was done. Assessment of these effects can help a more precise prediction of wake-induced transition on the downstream blades. To measure the flow field in the wake, the hot film anemometry was used. The measurements were done at three different incidence angles and Reynolds numbers ranging from 240000 to 530000 based on blade chord length and flow velocity. Based on these measurements, there is linear correlation between vortex shedding and Reynolds number and by increasing Reynolds number vortex shedding frequency will increase. The results showed that Strouhal number for Reynolds Number equal or below 360000 had lower scattering compared with the Reynolds Number above 360000. Also, decreasing attack angle will increase wake region. Furthermore, our results showed that vortex shedding frequency at moderate and low Reynolds Numbers displays different behaviors that could be result in different boundary layer formation at trailing edge of blades.

High speed and abscense of a precise control over pressure distribution confine sheet EMF into a die to simple shapes having shallow depth. It is possible to reach a higher depth by applying a convex punch instead of a concave die. In this article, sheet EMF on a punch and sheet EMF into a die have been investigated. The electromagnetic part of the study has been analytically investigated and the its mechanical part has been numerically studied using FEM in Abaqus software. In order to couple electromagnetic with mechanical parts, no-coupling method has been used. The obtained results have been verified by comparing the obtained results with previous experimental ones in literature. Al 1050 has been used in the present research. Rate-dependent and rate-independent hardening have been taken into consideration for the mechanical behavior for material. Using appropriate hardening model for material yields acceptable results. Moreover, a convex punch instead of a concave die is used to reach a higher depth in sheet EMF.

Supports and joints play a basic and important role in the engineering structures. It is necessary to identify the various parameters of supports. The stiffness and damping parameters are the most important parameters of a support. In this paper, an inverse method based on dynamic acceleration measurement data is used to identify and study the stiffness and damping coefficients of the supports of cantilever and doubly clamped beams. To this end, an optimization problem using the least squares method is defined, and solved subsequently. In the cantilever beam, the effect of various parameters such as the magnitude of measurement errors, number of measured data, number of sensors, time duration of the applied load, magnitudes of the stiffness and damping parameters, and time interval of data collection, on the inverse solutions are studied. For the doubly clamped beam, the effects of the magnitude of measurement errors, number of measured data, data type and number of sensors on the results are studied. The results show that the doubly clamped beam problem is much more difficult than the cantilever beam problem. It is very appropriate to use two sensors for problems with the doubly clamped beam. By careful investigation of obtained numerical results, an attempt has been made to answer the questions and difficulties that may occur during practical tests.

Nowadays sandwich sheets with polymer core, have been replaced many applications of single-layer sheets in different industries. Complex behavior of sandwich sheets during deformation and their deformation limits, shows the need to review the formability of this sheets. This research discussed about analysis of effect of geometric parameters of sandwich sheet layers on formability of Aluminum/Polyurethane/Aluminum sandwich sheets. To evaluate the formability of sandwich sheets, the punch stretching test with a hemispherical punch was performed. Also, formability of these sheets obtained using finite element method. After verifying finite element model, it was observed that the difference between the output of the strain on the finite element method and experimental results was 7 to 15% which seems appropriate. The results of experimental method and finite element simulation showed that with increasing the core thickness of the sandwich sheet, the formability decreases. It was also proved that the arrangement of layers plays an important role in the formability of sandwich sheets.

The anisotropy of mechanical property in sheet metals due to their rolling process causes the useful effects of increasing in drawability and harmful effects of earing phenomenon. In the present study, the variation of the anisotropic coefficient during the uniaxial tension of AA6061-O sheet has been considered using digital image correlation technique by two marking methods of regular arrays of circles and irregular accidental pattern. Meshed samples of this alloy are cut in three directions 0, 45 and 90 degrees with respect to the rolling direction and stretched by Instron test machine. During the test, transverse and longitudinal strains are measured and then by using these two parameters anisotropic parameters in different directions as more asthe planar and normal anisotropic coefficients are computed. It demonstrated that, during the straining on samples the anisotropic parameters in 0 and 45 direction showed decreasing trend and in 90 direction it has a wavy manner. The results indicated that after 10 percent elongation the normal and planar anisotropy of sheet decreased by 51% and 63%, respectively. Finally the plastic hardening behavior of sheet in different stages of the tension by using of the Hill’s quadratic yield criterion was studied and appeared that the yield surface grows totally as an anisotropic manner.

In this paper, automatic fault diagnosis of rotating machines using vibrating data measured from different point of machines and an smart fuzzy knowledge-based systems is discussed. To this end, a new vibrations’ identification chart, recently published, is used. This vibration's identification chart contained frequency characteristics and phase angle and is represented for some usual defects such as unbalancy, misalignment, bent shaft and mechanical looseness. Designed fuzzy knowledge-based system has a very simple structure. It do not need any complicated training such as those are used for neural network training. To evaluate the performance of the designed fuzzy system in actual application, it is used for fault diagnosis of some rotating machines in Isfahan Steel Company such as Fans. The effect of different membership functions such as non-linear Gaussian, bell-shaped, sigmoid, s-shape and z-shape function for inputs and outputs of fuzzy rules database is investigated and.the results are compared with the results of some neural networks-based fault diagnosis systems. Results show the designed smart fuzzy system has acceptable performance in detecting fault.

The optimal design problem of systems contains rotating shafts, so that the system has appropriate vibration characteristics, is one of the most important factors in performance and efficiency of the rotor systems operation. Vibration of rotary equipment are usually caused by factors such as unbalance ,Misalignment, Abrasion Parts and Abrasion moving parts and any other reasons. The frequency of this vibration forces are usually integer multiples of rotating speed (spin rate) so if the initial critical speed is as far away as possible from the rotational speed of the rotor,the rotor works in a safer operational area and other parts of system need to a few period of repairs or revision. In this paper Myklestadt-Prohl method has been used to compute initial critical whirling speed of a multi-stepped rotor and genetic algorithm to optimize the location of the bearings in the rotor, so the first critical speed of the system has maximum value for the least number of bearing.

The strength of adhesively bonded joint plays very important role on the behavior of loaded composite structures. In the present paper debonding behavior of the adhesive joint between sandwich panel and pultruded profile is investigated by the finite element method. Cohesive zone model and contact elements are used in order to simulate the adhesive joint between stiffener and the panel. Representing the adhesive material, an embedded layer is modeled between the sandwich panel and profile. The effects of geometrical parameters including the thickness of the adhesive layer and profile on the behavior of joint are investigated. Moreover, the effect of initial defect in the adhesive joint on the debonding results is studied. The results present that in the prefect bonding, increasing in the thicknesses of the profile and adhesive layer improves the joint behavior against debonding. However in defected joint, increasing the profile thickness decreases the debonding load and presence of initial defect decreases the debonding load by 51%. The results also show that presence of embedded adhesive layer has positive effect on the debonding behavior of the joint. Initial debonding load without adhesive layer decreases by 40% in comparison with the joint containing 10 mm adhesive layer.

Buckling failure is a quite common occurrence in plates under compression, in particular when the plate's thickness is small with respect to other plate's sizes. Such a mode of crisis can often precede strength failure. Nowadays, one of the most common types of repairs is done by attaching composite patches on the defeated surfaces. These patches have important advantages, such as high strength and corrosion resistance while their weight is low. In this investigation, experimental studies on the critical buckling load of the notched plates repaired by fiber metal laminate (FML) patches subjected to axial compression is carried out. For this purpose, the rectangular plates made by 2024T4 aluminum with central notches are considered. The desired parameters in this study are layer sequence, thickness and dimensions of the FML patches. The results show that an increase in buckling strength of the repaired parts. Smart choosing of the parameters results in an increase in the buckling load up to 44.13%.

Among the various models of ductile fracture, Gurson-Tevergard-Needleman (GTN) damage model has been widely used due to consider three steps; nucleation, growth, and the coalescence of voids during plastic deformation. Important issues in GTN model is the exact calculation of the model parameters which in experimental methods are very time consuming and costly. Therefore, the finite element simulation method is used for this purpose. The porous metals plasticity model in Abaqus software does not consider the coalescence of voids step and analyzes issues on the basis of two steps of nucleation and growth of voids. This causes an error in the results. In this study, the uniaxial tension test of aluminum alloy 5083-O is simulated with GTN damage model. Simulation is carried out by using finite element software Abaqus through writing code in the UMAT subroutine. The GTN damage model parameters of AA5083-O are evaluated by matching experimental engineering stress-strain curve and simulated curve. The results showed that the written code through UMAT subroutine addition to led to improve the simulation of GTN damage model in Abaqus software, provides an acceptable prediction of the GTN model parameters.

The purpose of this research is to perform experimental study and modeling of the plastic deformation of rectangular plates under the low rate impact loading by drop hammer system. In experimental section, some experiments were conducted on rectangular plates with different levels of energy in order to survey the mechanical behavior of steel and aluminum plates according to applied load. The modeling section consists of presenting an explicit function for experimental data by singular value decomposition (SVD) based on non-dimensional parameters and also multi-objective modeling and design of adaptive neuro-fuzzy inference system (ANFIS) by genetic algorithm. Generally, the aim of modeling is a reliable and satisfactory prediction of deflection – thickness ratio of plates under impact loads. A comparison between modeling results and experimental data is done in order to validate the results. The investigation of training and prediction data errors which has been based on root-mean-square error (RMSE) and coefficient of determination (R2) shows that the obtained results of the optimal design of ANFIS is closer to experimental results than mathematical modeling by SVD, with the exception that a mathematical function based on experimental data is presented by SVD method. Therefore, using these presented models for deflection-thickness ratio of plate under impact loading is desirable.

Investigation of the fatigue behavior of the reinforced polymer composites is necessary in order to design reliable and durable structure. Due to the difference between the fatigue properties of the reinforced polymers and the traditional isotropic engineering materials, introducing criteria for prediction of the fatigue life of the reinforced polymers will be very useful in design and analysis. This paper is aimed to life prediction of reinforced plates which is subjected to bending fatigue loads. The Glass-Epoxy and Graphite-Epoxy plates which are subjected to the distributed and concentrated bending loads are studied based on the strength degradation model. The distribution of the stresses at the plies of the plate subjected to transverse load is obtained using the first shear deformation plate theory. Then the strength degradation model is used to predict the strength reduction of each plies of the plate in each loading cycle. The degradation of plies and reduction of the strength in each loading stage are accumulated and the residual strength of plies is obtained as function of the load cycles. This procedure is repeated until the plies fail and the final catastrophic failure occurs in the plate.

Industrial press units have divided into two primary groups; single action and double action. Single action press has a mechanism that guide drawing slide. Double action unit has blank holder mechanism too. Double action press die has more weight and in conclusion is more expensive than a single action. For the mentioned reasons, sometimes a single action die is utilized for a double action press in which the blank holder mechanism is not used and the unit is changed to a single action one. The main goal of this article is dynamic and stress analysis of this case. In this paper, for kinematic analysis, mathematical model of the mechanism is obtained. Then dynamic analysis of a double action mechanism with standard die and also with small die of single action case, have been provided. This research has ended with stress analysis of these two states. Simulations show that use of single action die for double action press increase the force and stress in the ram mechanism joints. So this usage in double action press, increase the failure chance in the press mechanism.

This research is experimental study of emperature and volume fraction effects and provide a new model for nanofluid viscosity of Multi Wall Carbon Nano Tubes-Ethylene glycol. Multi Wall Carbon Nano Tubes viscosity with 1 to 2 nm diameter in Ethylene glycol is calculated at 30° C to 60° C and in the volume fractions of 0.025 to 0.3 using by viscometer. Type of viscometer is Brookfield and model is DV-I Prime. It is observed that the viscosity decreases while temperature is increasing and at high volume fractions the nanofluids, shows non-Newtonian properties. Because there has not been done studies on the nanofluids, new equation for the first time of viscosity is obtained and Einstein and Bachelor models are compared with empirical relationships and In addition to observing the similarities, a separate formula for the Newtonian offered. It should be noted that non-Newtonian case study in this paper is not considered.

In this study, numerical simulation of the effect of hyperthermia using FePt magnetic nanoparticles with alternating magnetic field for skin cancer treatment is carried out. The numerical solution is presented to analyze bioheat transfer and magnetic induction equations, in cylindrical skin tumor situated in healthy tissue with considering evaporation and convection heat transfer. In order to show the validity of the study, the obtained results are compared with those of the studies already exist in the literature. Bioheat equation is used to predict the temperature rise in terms of characteristics of the magnetic nanoparticles, applied magnetic field and the tissue. The results reveal that nanoparticles diameter has a major effect on the temperature rise. The results also show that the temperature field in axial direction (from surface to depth) of tissue and the effects of hyperthermia decreases by increasing the convection heat transfer coefficient. In other words hyperthermia is more effective in the presence of the environmental natural convection. Moreover, the position of maximum temperature inside the tumor, varies by changing the heat transfer coefficient. Also, the amount of evaporation has a negligible effect on hyperthermia treatment.

In this paper, the flow and temperature fields affected by electric field in the presence of wire collecting electrode are numerically investigated for the two-dimension, incompressible, turbulent, and steady flow conditions with the finite volume approach. The computational methodology includes the use of a structured, non-uniform quadrilateral grid, and the Standard K- model was adopted as the turbulence model. The computed results are compared with the experimental data and the results agree very well. Then, the effect of different parameters such as collecting electrode radius, the applied voltage, Reynolds number, and the distance between emitting and collecting electrodes on the flow pattern and heat transfer coefficient is evaluated. The numerical results show that the influence of electrohydrodynamic phenomenon on the heat transfer enhancement increases with radius of the grounded electrode and the applied voltage but decreases when the Reynolds number and distance between electrodes are augmented. The results indicate that electrohydrodynamic actuator acts as a generator of secondary flow and these vortices were used to enhance the forced convection heat transfer.

Ground source heat pump due to high coefficient of performance (COP) and use of low temperature thermal energy source, is one of the best technologies to use renewable energy resources. In this research at first a geothermal heat pump for heating with economizer was simulated, then the effect of variation of different parameter such as pressure in different parts of cycle, superheating at evaporator outlet, subcooling at condenser outlet and soil temperature on heat pump and total COP and exergy efficiency were analyzed. at the end, Ground source heat pump system was optimized in two manners: total COP is objective function and total exergy efficiency is objective function. In this research total COP and exergy efficiency values for the basic input mode were 3.674 and 0.488 respectively that these values were increased to 5.323 and 0.72 after the optimization..a b c d e f g k l m n o

In recent years, achievement more accurate numerical method in different flow regimes for the discontinuities with less oscillation and numerical errors is the interest of many researchers. What distinguishes this paper is, comparison of the performance of artificial dissipation and upwind methods, in solving the Euler equations for internal compressible flows in a wide range of inlet Mach numbers. In this study we examined the ability of three AUSM upwind method, Scalar and Cusp artificial dissipation methodes from very low Mach up to ultrasound and non-viscous flow in a convergent-divergent nozzle. The ability of AUSM method and Scalar methods in two-dimensional inviscid sonic flow between turbine stator blades at both supersonic and subsonic outlet is also investigated. In this study an excellent performance for AUSM method with more convergence speed and low numerical error in all flow regimes at a converging-diverging nozzle is observed. In the second example, in both flow AUSM method presents the results based on experimental, with low numerical errors and satisfy the mass conservation better than Scalar. It should be mentioned that the AUSM method is highly recommended for higher Mach numbers.

Electrochemical machining process (ECM) is one of non-traditional machining processes used in various industries due to certain advantages of this process. This machining process is based on the anodic dissolution so having no contact between work piece and tool leads to less tool wear and eliminating cutting forces that are some of the most important benefits of electrochemical machining. There is not still an acceptable method to predict the workpiece shape for a specific tool and also desired tool for a given cavity because of the complexity of this process while using conventional trial and error method to extract the shape of the workpiece and the tool is time consuming and costly. Simulation of Electrochemical machining process is a useful method to overcome this problem and let us to design and predict noted parameters while reducing costs and improving time are achievable. In this paper anodic dissolution in each time step has been simulated using finite element method to simulated electrochemical machining process. Then, using results of the simulated model and the sensitivity algorithm extraction of improved tool shape and desired workpiece are possible. The results demonstrate the ability of the new method proposed in this paper for simulation of electrochemical machining process and design tool.

In this paper, diameter ratio of a geothermal borehole heat exchanger is determined in a manner that the entropy generation is minimized. The SST k-ω model is used to model the turbulent flow. Pressure loss calculation for different diameter ratios of the heat exchanger shows that for diameter ratio of 0.7, the total pressure loss is minimum. Results of the bulk flow temperature showes that the outlet temperature increases at higher heat resistance of the internal wall. The total entropy generation for different diameter ratios at different heat resistance of internal wall are also presented. Results show that when the heat resistance of internal wall increases, the minimum entropy generation occurs at higher diameter ratios. On the other hand, reduction in heat transfer coefficient of the outer wall and increment in heat transfer coefficient of internal wall are not favorable. Increment in heat resistance of the internal wall at higher diameter ratios of the heat exchanger, reduces both of these unfavorable effects.

With the development of microelectronics and micromachining technology, micro-heater has found plenty of applications in micro-sensor. Heating electrode material is one of the key factors which affect the power loss, response time and sensitivity of the micro-heater. In this paper, two micro-heaters with same geometry using two various metals have been design, fabricated and characterize on silicon substrate based on micro-electro-mechanical-systems (MEMS) fabrication process. In the first micro-heater, gold and in the second micro-heater, platinum are used as heating electrode, and the effect of heating electrode material on the performance of micro-heater is evaluated. Moreover, to improve micro-heaters efficiency, their design is investigated and the micro-heater with optimum design is chosen. The analytical results exhibit that gold micro-hater has lower response time and higher power loss than platinum micro-heater. The experimental results are in good agreement with the results obtained from the analytical analysis and show that fabricated micro-heaters with optimum design have high performance; as power consumption and response time are 36mW and 1.75ms respectively in gold micro-heater and 30mW and 2.1ms respectively in platinum micro-heater for the temperature variation from 30oC to 450oC. These results demonstrate that with fabrication of gold micro-heater response time improve 16.6% in comparison with platinum micro-heater.

An emulsion is a mixture of two immiscible liquids in which one liquid is dispersed in the form of small drops in the other liquid. The liquid in the form of droplets is called discrete phase whereas the other liquid is termed as a continuous phase. One of the common methods for emulsification is based on using microfluidic devices. Currently, the study of flow in microfluidic devices is highly regarded due to having many applications in various fields including droplet generation in relation with the standard and quality of human life. In this study, the process of droplet generation in microfluidic flow-focusing devices with orthogonal and intersecting flows has been simulated. The effects of viscosity and flow rate of discreted and continuous phases on droplet generation process have been investigated. According to simulation results, with increasing the flow rate of discreted phase (Qd) at a fixed continuous phase leads to the increment of droplet size whereas an increas in the flow rate of continuous phase (Qc) at a fixed discreted phase results in smaller droplet size. Moreover, for a constant Qc/Qd ratio, regardless of whether discrete or continuous phase flow rate is fixed, the equal droplet size is achieved. The higher viscosity of the discrete phase provides a larger droplet size whereas an increase in the viscosity of continuous phase leads to a smaller droplet size. The simulation results in comparison with experimental results show a good agreement confirming the accuracy of the method.

In this paper, an efficient procedure based on the density-based algorithm with explicit solver is presented to solve the compressible Euler equations on a non-orthogonal mesh with finite volume formulation and The fluxes of the convected quantities including mass flow rate are approximated by using the characteristic based TVD and ACM and Jameson methods. the aim of the present study is to introduce a method based on the characteristic variables (Riemann solution) and control of the diffusion term in the classic methods in order to capture the shock waves. Hereby, an inviscid supersonic flow is solved and results are compared together in view of resolution and accuracy of shock waves capturing, and solution convergence. In this paper, the convergence target for mass equations and momentum equations is decrement of residuals from 10-6. Results show in density-based algorithm the characteristics velocities are better converged due to the augmentation of limiter, ACM and TVD methods capture the shock waves with higher resolution relative to the Jameson method. Shock waves include simple waves reflected waves, and waves interaction. Also the ACM method is a useful technique, which prevents the smearing of discontinuities and improves the resolution of shocks and accelerates the convergence rate of the solution in the supersonic flows .