Modares Mechanical Engineering
Year:2015 | Volume:14 | Issue:15|Papers Count:51

Elliptical subsurface cracks are one of the probable types of cracks that occur in engineering structures, especially under rolling contact fatigue. Due to the non-symmetrical geometry, coupling of the fracture modes occurs in an elliptical subsurface crack and the crack under shear loading will experience all fracture modes. This paper investigates the coupling of the fracture modes of elliptical subsurface cracks under uniform shear loadings in two directions. First, a three-dimensional parametric finite element model of a crack in an infinite space has been developed and validated. Then, by moving the crack close to surface, mixed mode stress intensity factors (SIFs) have been calculated for cracks with aspect ratios of a=0.2-1.0 and ratios of crack depth to crack length of b=0.05-1.0. Based on the results, coupling of the fracture modes occur considerably when the crack depth becomes less than crack length. By decreasing of the crack depth from b=¥ to b=0.05, shear SIFs and KI,max/KII,max ratio increase at least up to 65% and 90%, respectively. Six equations for SIFs of the subsurface cracks under uniform shear loadings in two directions have been obtained by fitting to the finite element results. These equations can be used efficiently in high accurate calculation of the SIFs for subsurface cracks with any a and b under uniform shear loading with any direction.

This is a study of the effect of synchronous combustion of gas-gasoil, achieved through the injection of gasoil droplets into natural gas flame, on the flame luminosity and radiative heat transfer. Droplets were injected by a single-hole micro-nozzle with a hole diameter of 100 mm and injection pressure of 9 bars. A photovoltaic cell was used to determine the luminous radiation and the total radiation of flame was measured by a thermopile. Also, the combination of chemiluminescence and IR photography of flame was employed to determine the qualitative distribution of soot particles in flame. The results show that the synchronous combustion of gas-gasoil raises the soot content of flame, leading to an increase of the luminosity and volume reaction of flame 38 and 2.5 times in comparison to the non-injection mode. Also, for the synchronous combustion of gasoil and gas with a mass fraction of 10%, the flame temperature changed only 95oC, whereas the flame radiation rose as much as 52%. The improvement of flame radiation in synchronous combustion of gas-gasoil is due to the enhancement of flame emissivity coefficient in the IR region of electromagnetic wavelengths. Meanwhile, the injection of gasoil droplets increased the CO and NO pollutants by 4 ppm and 35 ppm in comparison to the non-injection mode; due to the low mass flow rate of injection, however, the increase does not exceed the allowable limit for outlet pollution.

This work is the result of a research on the lifting forces during upward bird flight via modeling and manufacturing dynamical structures resembling bird wings of sizes between half to about 2 meters. The variables in this work included the wings sizes and their oscillation frequencies. In the presented formulations the lifting force and the consumed power at the beginning of a bird flight in a fixed frequency is proportional to the fourth and fifth power of the wings sizes, and for fixes sizes is proportional to the second and third power of the frequency, respectively. The lift force here is taken to be of two different forms. The first is the very form relevant to the manufactured and used wing systems in the present work. In the second form the wings are assumed to stay horizontal during their vertical periodic motion. The extent of validity of these formulations when practicing for our manufactured wings, and for the real functioning of bird wings as well, has been the most important question in the present research. As far as the lifting force is concerned, the extrapolation of final results seems to be in consistence with the sizes relevant to human “bird-like” flight. However, provision of the needed power necessitates requirements to be thought of deliberately for restoring the energy in an effective way.

The present study deals with nonlinear vibration analysis of the plate having at least one free edge. The plate has a part-through central crack with arbitrary limit length which is parallel to one side of the plate. Due to complexity of the governing equation of motion, the Galerkin method is used for solving the problem. Therefore, the appropriate admissible functions satisfying free edge conditions in the cracked plate, must be employed. The beam functions can not satisfy free edge conditions in the plate, nevertheless these functions used in many researches in the literature, which lead to high numerical errors in computing the frequency and mode shapes. Therefore, in this research, new admissible functions are proposed which can obviate incapability of the beam functions to accurately estimate natural frequency of the intact and cracked plate and reduce the related computational errors. The effects of changes in thickness and non-dimensional crack length on natural frequency of the cracked plate are investigated using proposed functions, and frequency response curves indicating the dependence of frequency on amplitude is derived for different boundary conditions. Also, the influence of crack length on the changes in the nonlinear behavior of the cracked plate is investigated. The results are compared with those of the available results in the literature.

The significance of research on the specifications of the supercritical fluids becomes more evident with respect to the increase of their application in different food, chemical, polymer, oil, and gas industries. One of the major specifications, is the coefficient of thermal expansion (b) where the ideal gas model was used in most of the processes in which this component is applied; the weakness of this model is that it is unable to make an accurate prediction of this parameter within the range of critical point. For this reason, in this study to determine the coefficient of thermal expansion, Redlich-Kwong equation of state is used and a new relation as a function of temperature, pressure, and compressibility is obtained. Comparing behavior of the curves obtained from this relation with experimental data, exhibits a favorable consistency. Moreover, natural convection heat transfer of the supercritical fluid in a vertical channel at constant temperature wall conditions was considered numerically. The governing equations were solved using the finite-volume method (FVM) and based on the SIMPLE Algorithm. after validation with earlier studies. Then, the flow and heat transfer characteristics based on the obtained coefficient of thermal expansion were compared with the ideal gas assumptions. Finally, the trend of change in heat transfer coefficient away from the critical point was studied.

In this research, accumulation of plastic strain and softening behavior of stainless steel SS316L cylindrical shell under cyclic bending and combined loads (bending-torsion) is studied. Cyclic bending was under force-control and displacement-control but Combined loading was under displacement-control. Experimental tests were performed using an INSTRON 8802 servo-hydraulic machine. Under force-control loading with non-zero mean force, plastic strain was accumulated in continuous cycles called ratcheting. Based on experimental results, linear relation was observed between plastic energy and rate of plastic deformation, which shows the rigidity of fixtures used in experimental tests. Under displacement-control loading, softening behavior was observed due to growth of ovalization and the rate of softening increased by use of the higher displacement amplitude. The crack growth up to failure is oblique in combined load due to torsion and bending loads whereas the crack growth is peripheral in bending load. The numerical analysis was carried out by ABAQUS software and nonlinear isotropic/kinematic hardening was compared with isotropic hardening; it was observed that the nonlinear isotropic/kinematic hardening model simulates the softening behavior and accumulation of plastic strain of cylindrical shells under cyclic bending accurately.

Superalloys are extensively used in various industries like aerospace, chemical and petrochemical industries due to their properties such as high strength at elevated temperature and good corrosion resistance. On the other hand, owing to these properties superalloys are classified as difficult to cut materials. In the present work, the effect of cutting parameters on tool life in turning of N-155 iron-nickel-base superalloy is investigated. Cutting speed and feed rate, each at five levels, were selected as cutting variables. Relationship between cutting parameters and output variable, i.e. tool life, was modeled using response surface methodology (RSM). The results showed that there was good agreement between the experimental results and the predicted values using the developed mathematical model. Additionally, analysis of variance was implemented to evaluate the adequacy of the regression model and respective variables. ANOVA results indicated that the cutting speed had more effect on tool life than feed rate. Moreover, wear mechanisms and failure modes of the cutting edges were analyzed by using the images of scanning electron microscope (SEM) at different cutting speeds and feed rates. It was observed that abrasion and adhesion were the most dominant wear mechanisms in this study. Finally, desirability function was used so as to predict optimum cutting parameters for achieving maximum tool life. The results of optimization process showed that 50 m/min cutting speed and 0.2 mm/rev feed rate are the optimum cutting parameters which result in 7.526 min tool life.

The Unscented Kalman filter (UKF) is the popular approach to estimate the recursive parameter of nonlinear dynamical system corrupted with Gaussian and white noises. Also, it has been applied to train the weights of the multi-layered neural network (MNN) models. The Group method of data handling (GMDH)-type neural network is one of the most widely used neural networks having high capacity in modeling of the complex data. In many researches, different approaches are used in training of neural networks in terms of associated weights or coefficients, such as singular value decomposition and genetic algorithms. In this paper, the Unscented Kalman filter is used to train the parameters of GMDH-type neural network when the experimental data are deterministic. The effectiveness of GMDH-type neural network with UKF algorithm is demonstrated by modeling using a table of the multi input-single output experimental data. The simulation result shows that the UKF-based GMDH algorithm performs well in modeling of nonlinear systems in comparison with the results of using traditional GMDH-type neural network and is more robust against the model and measurement uncertainty.

Nowadays, a great number of researches are being done by scientists to provide various models that can predict the passenger injuries in crashes. In this paper, a hybrid model of vehicle and passenger is proposed to predict the head acceleration in the front crash. A lumped mass model with 12-degree-of-freedom (DOF) is first used to predict the behavior of vehicle in front crash. In this model, any member of vehicle is modeled as a lumped mass and connected to the other members through some springs and dampers. The unknown coefficients of such model are obtained using genetic algorithm to minimize the deviation between the results of experimental and suggested model. The parameters of model are established by experimental results of a real world car, namely the HONDA ACCORD2011, in an accident velocity of 48 km/h. Also, the validity of the proposed model is checked by experimental results of the mentioned vehicle at two other crash velocities of 40 km/h, and 56 km/h. The results show that the proposed model is an efficient framework for preliminary design of both structure and parameter design of vehicle to improve its crash worthiness. Moreover, a multi-body dynamic model of driver is proposed to predict the head injury in front crash. The seat acceleration which has been calculated using the vehicle model is considered as input of this model.

In this paper, a novel method for solving consensus problem in a multi agent system consisting of single link manipulators with flexible joint is presented. This method is based on linear matrix inequalities and the objective is to design a dynamic fixed order controller that can fulfill consensus by using output feedback information and Laplacian Matrix of the network of manipulators. The exact model of a single Link manipulator is assumed thus a nonlinear Lipchitz term emerges. Each manipulator as an agent in the corresponding network obtains only its neighbors output information, therefore the controller is decentralized. To guarantee consensus in this method, first the multi agent system should become one augmented system. Then, based on considered conditions on nonlinear terms, using appropriate structure conversion is necessary. The unknown controller coefficients of the closed loop system can be achieved by using Lyapunov stability theorem. Applying special conditions on symmetric positive definite matrix in Lyapunov quadratic function results in an LMI form, thus iterative methods of solving nonlinear matrix inequalities with less accuracy is prevented. Finally, to demonstrate the effectiveness of this algorithm and compare with similar earlier researches, a numerical example on a multi agent system consisting of three single link flexible manipulators is investigated.

In this research, three dimensional grain burn-back of solid rocket motors is simulated based on level set method and its accuracy is increased according to marching cube algorithm (MCA). To that end and according to requirements of level set method, grain burn-back is simulated during three steps including grid generation, distance function determination, and calculation of burn-back parameters. In this article, with focus on the last step, the strengths and weaknesses of grain burn-back analysis for common methods such as captured cell, three dimensional cut cell, sectional, and Heaviside Delta Dirac are compared and in the following, we introduce and run MCA. For validation, first, three simple grains such as cylindrical, quad and hexahedron are considered and the performance of capture cell, 3D c ut cell and sectional methods are compared with MCA in terms of accuracy and CPU time. Then, to evaluate the new method facing complex and practical grains, burn-back results of conocyle and NAWC N.13 grains are compared with MCA and heaviside/dirac delta methods. The obtained results show that MCA has a better performance at CPU time and accuracy.

In recent years, the need for low power electronic circuits like sensors and wireless systems has been considered by many researchers. Excessive weight, limited lifetime of the batteries and also the problem of replacing them, are the main reasons for harvesting energy from ambient vibrations. Among the various sources of environmental energy, mechanical vibrations have gained in popularity due to their availability. Between the different methods of ambient vibration energy harvesting, piezoelectric method, is one of the good ways to harvest energy due to the favorable effects of electromechanical coupling. The most common means of harvesting energy from vibrations is a cantilevered beam with one or more piezoelectric layers. In the present paper, electrical energy harvesting from Euler-Bernoulli trapezoidal cantilevered unimorph beam with base excitation using distributed parameter method has been considered. First, equations of motion are analytically obtained and then using Assumed modes method (for rectangular beam), system’s natural frequencies are calculated and output voltage, current and power diagrams are presented. For verifying results, presented voltage, current and power output diagrams for trapezoidal configuration close to rectangular configuration, the results of which are published in references, will be compared. Then, functional parameters for trapezoidal energy harvester with resistance value changes for energy consumer have been analyzed.

The lack of an accurate equation of state for predicting the thermodynamic properties of materials in a wide range of temperature and pressure encouraged the researchers to study new equations. Fundamental equations of state are the equations that are explicit in terms of the Helmholtz energy. This form of equation of state has high applicability, especially in cryogenics ranges. In this investigation, a reverse Brayton cryocooler was simulated as a system that prepares the sub-cooled liquid nitrogen supplying the operation condition for high temperature superconductor cables. A computational code was developed for predicting thermodynamic properties of Helium and Neon using fundamental equation of state. Comparing the results with experimental data validates the accuracy of these equations in predicting the thermodynamic properties. Then, using the developed computational code, a reverse Brayton cycle with 10 kW cooling capacity was designed and simulated and the effect of various parameters on its performance was evaluated. Performance characteristic curves were plotted to illustrate the sensitivity analysis under different operation conditions, and the influence of various parameters such as compression ratio in compressor, maximum pressure, working fluid, efficiency of the heat recovery exchanger and efficiency of expander on the performance of the cycle were addressed. The results showed that the use of neon as a refrigerant gives a better performance than helium. Efficiency of heat recovery exchanger has a significant effect on the performance of cycle, so that 3 percent increase of this parameter increases 11 percent figure of merit (FOM) of the cycle.

Spin-bonding is a method for fabrication of bilayer tubes based on flow-forming process. This new method is a process with high potential in production of seamless thin-walled tubes. Utilizing this process, aluminum tube (as the inner layer or clad layer) has been bonded into steel tube (as the outer layer) to fabricate tubular laminate composites. As important parameters for creating a suitable bond, effects of thickness reduction, initial aluminum thickness and strength on bonding strength were investigated. The bond strength was evaluated by peel test and the peeled surfaces were examined using scanning electron microscopy (SEM). Results indicated that thickness reduction has great influence on strength and quality of the bond. After a threshold reduction (about 35%) the bond strength increases rapidly with the amount of deformation until it approaches the weaker metal strength, and samples fracture from the base metal in the peel test. Approaching the strength of the two metals and decreasing the initial thickness of the clad layer, with a high amount of deformation increased the bonding strength. Fracture surface images showed that the surface fraction of bonding area was increased when deformation increased. It was also increased with the reduction of the initial thickness of the clad layer and when the strength of the two layers approached each other. Additionally, distribution and shape of the fracture area changed from a disordered fibril structure to an approximately straight area, with an increase in the deformation.

The productivity of a part is assessed based on factors such as dimensional and geometrical tolerances. In fact, tolerance features are the most important factors in shop drawing of an industrial part. The aim of the present study is to empirically investigate the precision of holes created by helical milling method on AISI 4340 alloyed steel. This method refers to creating a hole using milling tool, which moves along a helical path. By using helical milling, a high quality hole has been produced and there is no need for boring. Taguchi design of experiment was used to study the effects of process parameters including; cutting speed, feed rate, axial depth of cut and work piece hardness on dimensional and geometrical tolerances of the created hole. In addition, effect of minimum quantity lubrication method with two different oils and dry milling methods was studied. Results showed that the helical milling can be a suitable replacement for conventional drilling. In addition, cutting speed as the main parameter had significant effect on quality improvement of the created hole. On the other hand, in the helical milling, minimum quantity lubrication method using vegetable-based oil showed the best performance compared to mineral oil or dry cutting.

Mesh segmentation and partitioning of 3D models have always been significant as one of the most structural tools used in many applications of CAD and computer graphics. One of the most versatile of these algorithms, which is capable of optimum segmentation of model, is the iterative algorithm. It is a parametric method based on Lloyd algorithm, which segments the model in an optimized way by plotting the Voronoi diagram through the points cloud data. The main disadvantage of this method is the time-consuming problem which limits its application. In this paper, by employing the nature of fuzzy segmentation a solution has been proposed to specify the number of regions required for model’s partitioning and to carry out the nonparametric segmentation without the need for user’s initial settings. Additionally, utilizing the approximate Voronoi diagram and fuzzy regions construction, a novel method for obtaining the optimized segmentation in a shorter time interval in comparison with other iterative algorithms has been presented. The proposed method has been implemented in a standard model for validation. It has been observed that the obtained results have remarkable improvements relative to the results from the iterative algorithm, which demonstrates the efficiency of this method in segmentation of 3D models.

Size dependent behavior of materials appears for a structure when the characteristic size such as thickness or diameter is close to its internal length-scale parameter. In these cases, ignoring this behavior in modeling may lead to incorrect results. In this paper, strong effects of the size dependence on the static and dynamic behavior of the electrostatically actuated micro-beams have been studied. The equilibrium positions or fixed points of the gold and nickel micro-beams have been determined and it is shown that for a given DC voltage, there is a considerable difference between the fixed points gained using the classic beam theory and the modified couple stress theory. In addition, it is also seen that the static and dynamic pull-in voltages gained using the couple stress theory are several times higher than those gained using the classic beam theory. Some previous studies have applied the classic beam theory in their models and introduced a considerable hypothetical value of residual stress to match their experimental and incorrect theoretical results. Using the modified couple stress theory considerably decreases the difference with the experimental results.

Multi-effect distillation is one of the thermal desalting systems. MEDs have recently come to notice more than other systems because of their high energy utilizing and performance. High complicity and possessing different heat transfer mechanisms have distinguished them from other desalination systems such as Multi-stage flash. In MEDs although formation of thin falling-film layer on horizontal rows of tube-bundle increases heat transfer, however the risk of precipitation will be high especially on lower rows where film thickness is the least. Falling-film evaporation is a self-compensation phenomenon; the more the evaporation, the thinner the film, and subsequently the more the evaporation. In present work, an applicable algorithm is proposed and applied for thermohydraulic design of tube bundle, and heat transfer surface. Flow and heat distribution on tube-bundle is numerically simulated with the advantage of given algorithm. Results show that more concentration and precipitation risk will occur on outer surfaces, near the entrance of last rows. Uniform distribution of feed on tubes will result in non-uniform vapor generation throughout the tube length. Steam quality increases almost linearly inside tubes, whereas its cross-section is occupied by gas phase predominantly; and condensate film will experience an annular regime. Steam quality reduction and non-uniform vapor generation decrease thermal performance of the last zone of the tube-bundle.

Fluid-Elastic Instability is the most important mechanism among the vibration excitations in heat exchanger tube bundles subjected to cross flow. Flows through the heat exchanger are mostly two phase flow like air-water, vapor-water or Freon. Two phase numerical methods are quite complicated because of some parameters like VOF and interaction between two phases. Experimental studies are another problem and costly. Therefore, numerical methods are very important for studying two phase flow. In this article, threshold of vibration has been numerically predicted by simulation of incompressible, viscose, and unsteady cross flow through a tube bundle in normal triangular arrangement. Interactions between the fluid and the structure have been counted in a fully coupled manner. HEM was used for analyzing two phase flow. In HEM method there is no difference in velocity between gas and fluid. In this study, two phase flow with HEM was solved around a single flexible cylinder surrounded by rigid and flexible tubes of bundle. Eventually, the flow through tube bundle was simulated and analyzed by monitoring critical reduced velocity. Result shows that with increasing VOF, amplitude decreased and the critical velocity increased.

Nowadays, grid stiffened composite shells have many applications in aerospace. These structures include an external shell in which some helical and circumferential ribs placed in the inner surface of the shell are being used to reinforce it. Conical shells are one type of these structures that is used in the construction of space projectile body. In this study, buckling behavior of grid stiffened composite conical shells under axial loading have been investigated. For this purpose, both smeared and finite element methods have been used and effects of external shell winding, helical ribs angle, ribs number and vertex angle of cone parameters on the buckling load of these structures were investigated. In analytical method, stringers by a shell that have equivalent stiffness were smeared. Based on this analysis, the extensional, coupling and bending matrices associated with the stiffeners were determined. Then, by use of Ritz method, buckling load was calculated. Also, in the finite element method, conical shells by use of ANSYS software were modeled and analyzed. In finite element analysis, two kinds of mode shape for these structures were observed. Also, the results from smeared method showed that the structure with ribs of 30 degrees and shell winding angle between 70 to 80 degrees can be a modify case for design.

In this research, influence of foam filling technique in honeycomb cores by using lightweight rigid polyurethane foam is investigated. Plastic mechanical behaviors of honeycomb cores are studied by mean of numerical method. Four types of Aluminum honeycombs (Al 5052 alloy) both bare and foam-filled were subjected to unidirectional compression. Analyses were carried out for different impact velocities to survey quasi-static and dynamic crushing response of the honeycomb cores. Crushing strengths and specific absorbed energy were obtained for honeycomb cores which have different relative densities. FEM results showed that foam filling technique can increase crushing strength of honeycomb core up to 24 times, and its specific absorbed energy up to 11 times. However, it was found that in heavier honeycombs the effect of foam filling technique decreases significantly. Furthermore, it was found that in quasi-static situation, foam filling technique can enhance desired core mechanical properties’. Meanwhile, in dynamic states of impact it was illustrated that polyurethane foam have not significant role in improving crushing strength and specific absorbed energy of honeycomb cores. For instance, in crushing with impact velocity of 100 m/s, amount of specific absorbed energy for bare and foam-filled cores were approximately equal.

In this study, the nonlinear vibration of a sandwich FG plate resting on a nonlinear Pasternak foundation which is simultaneously subjected to transverse harmonic forcing excitation and inplane static force is investigated. Based on the Modified First-Order Shear Deformation Theory (FSDT), applying the von-Karman nonlinear strain–displacement relation and the Hamilton’s principle, the governing nonlinear coupled partial differential equations are derived. Then, the Galerkin’s procedure is used to reduce the equations of motion to nonlinear ordinary differential equations. In the absence of foundation, the validity of the formulation for analyzing the modified shear correction factors for shear stresses is accomplished by comparing the results with those reported in the literature. By applying the multiple scales method and considering the second order nonlinear approximation of solution, the primary resonance of the system under the transverse forcing excitation is analyzed. Under the steady-state condition, the frequency response, the force-response and the damping-response equations are derived. Then the conditions of existence and stability of multiple coexisting non-trivial solutions for amplitude of the responses are discussed and the saddle node bifurcation points of the characteristic curves are derived. It is shown that, the variation of the system parameters in the resonance boundary may cause the jump phenomenon. Moreover, the effects of the system parameters including, excitation frequency, foundation parameters, damping, and amplitude of the harmonic and inplane forces on the system nonlinear dynamics are investigated. Also it is shown that the presence of the foundation has a considerable influence on the resonance characteristic curves.

This paper presents the investigation of general formulation and numerical solution of the dynamic load carrying capacity (DLCC) of flexible link manipulator. The proposed method is based on open loop optimal control problem. A two point boundary value problem (TPBVP) is taken from the Pontryagin's minimum principle. The indirect approach is employed to derive optimality conditions. The system’s dynamics equation of motion is obtained from Gibbs-Appell (G-A) formulation and assumed mode method (AMM). Elastic properties of the links are modeled according to the assumption of Timoshenko beam theory (TBT) and its associated mode shapes. As TBT is more accurate compared with the Euler-Bernoulli beam theory, it is utilized for mathematical modeling of flexible links. The main contribution of the paper is to calculate the maximum allowable load of a flexible link robot while an optimal trajectory is provided. Finally, the result of the simulation and experimental platform are compared for a two-link flexible arm to verify the introduced technique. The efficiency of the proposed method is illustrated by performing some simulation studies on the IUST flexible link manipulator. Simulation and experimental results confirm the validity of the claimed capability for controlling point-to-point motion of the proposed method and its application toward DLCC calculation.

Because of high accuracy and low weight-to-force ratio, servo hydraulic systems are widely used in various branches of industry. Simultaneous improvement of accuracy and time response are among the ever increasing needs for these systems. Rapid movement commands to hydraulic actuator excite attached mechanical components and consequently produce undesired vibrations. The recommended solution to overcome the above mentioned problem is to design and implement an advanced controller which takes into consideration the high frequency uncertainties. In this research a two-degree-of–freedom (2DOF) position controller has been designed and implemented for undesirable vibration regulation and robust performance achievement on a servo hydraulic table. In this regard various elements of the system are modeled and then the servo hydraulic table nominal system and uncertainty are identified using grey-box method. The 2DOF robust controller is designed using general H∞ framework and analyzed by structured singular value, Mu. The feedback block of controller is used to reduce the effect of uncertainty, measurement noises and reject disturbances, whereas the forward controller shapes the command signals to improve the performance. The designed controller has been implemented on the servo hydraulic test rig in order to track sine and trapezoid position command signals. It has been observed that the controller has a more accurate performance and faster time response than the common robust controller with just one feedback block. Extensive experimental results of the developed controller indicate robust performance and acceptable response to disturbance and measurement noise rejection in the defined uncertainty range.

In this paper, using the Armstrong-Frederick nonlinear kinematic hardening model, the ratcheting behavior of carbon steel piping elbows is described under conditions of steady internal pressure and dynamic moments out-of-plane at frequencies typical of seismic excitations. The elbows had an outside diameter of 60.3 mm and thicknesses of 3.91 and 5.54 mm. For each thickness two bend radius geometries (long and short) were studied. A pure out-of-plane bending moment applied at one end of a 90° welding elbow is reacted by a purely torsional moment at the other end. Three-dimensional elastic-plastic analyses by Armstrong-Frederick nonlinear kinematic hardening model are carried out to evaluate structural ratcheting behaviors. Initially, the rate of ratcheting is high and then decreases with the increasing cycles. While there is practically no strain accumulation in the axial direction, the direction of highest ratcheting is along the hoop direction. The cyclic strain accumulation against response moment for each component is assessed. By Armstrong-Frederick model, the predicted ratcheting of low moments is near to the experimental results, while for the high moment, this model will over-predict the ratcheting strain.

In this paper, the effect of the reaction wheel dynamics as controller actuator in multi axis attitude maneuver of a 3D nonlinear flexible spacecraft is considered. In modeling of the actuator dynamic, friction, inertia and electrical subsystems are considered. The nonlinear robust control approach is composed of dynamic inversion and m-synthesis schemes. To overcome the non-minimum phase characteristics, the controllers are designed by utilizing the modified output re-definition approach. In the design of controllers, actuator saturation is considered. It is assumed that only three reaction wheels in three directions on the hub are used. To evaluate the performance of the proposed controllers, an extensive number of simulations on a nonlinear model of the spacecraft are performed. The performances of the proposed controllers is compared in terms of nominal performance, robustness to uncertainties, vibration suppression of panel, sensitivity to measurement noise, environment disturbance and nonlinearity in large maneuvers. In the disturbance modeling all terms such as constant, sinusoidal and impulse are considered. Simulation results show the effects of actuator dynamics and confirm the ability of the proposed controller in tracking the attitude trajectory while damping the panel vibration.

Accuracy and numerical calculation time are the two main challenges of continuum robots dynamics modeling. In fact, the numerical calculation times of exact models are so long, that they are not practical in applications such as real-time control. This paper presents a new method for dynamics modeling of continuum robot backbones. In this method, the backbone shape is considered as an arbitrary number of constant-curvature (circular arc) elements, and the dynamics model is derived using Lagrange energy methods. First, kinetics and kinematics of one element are derived. Then, the robot kinematics is obtained as a series of such elements. Finally, the robot dynamics model is derived using Euler-Lagrange method. This paper is focused on dynamics of the flexible body of continuum robots, and the proposed model is independent of actuation systems. Besides, the numerical singularity of the constant-curvature elements is avoided, which occurs when an element is straight. The model is validated using experimental results. Comparison of simulation and experimental results shows the accuracy of the proposed method on dynamics modeling. Furthermore, the calculation time of the model is short enough to make it practical for applications such as real-time control.

Measurement of wave velocity in materials is very important .It has applications in ultrasonic thickness gauging as well as estimating the elastic constants of materials. The aim of the current paper is to improve the accuracy of wave velocity measurements by signal processing techniques. For this purpose, the SAGE algorithm, which is a model-based estimation technique, is implemented. Using SAGE, the overlapping echoes are separated and consequently the time-delay between these echoes is estimated more accurately. The signal processing scheme reduces the adverse effects of noise too. To demonstrate the effectiveness of the proposed technique, an AISI 4140 steel block with four steps of thicknesses 10, 15, 20, and 25 mm was tested by the immersion ultrasonic testing technique. The time-delay between echoes obtained from each step was measured fifty times and by averaging these measurements, the actual time-delay and its uncertainty were estimated. The thickness of the block at each step was also measured by a micrometer. Using the time-delay and thickness data, the wave velocity and its uncertainty were estimated for each of the four thicknesses. The results show that this technique can reduce the uncertainty of wave velocity measurements significantly.

The erosion of core boxes caused due to sand shooting in core molding process is one of major concerns of foundry industry. This paper study the effect of impact angle, blasting pressure of sand particles and the type of heat treatment on erosion of AISI H13 tool steel that is widely used in producing core dies. The workpiece material used in this study was AISI H13 tool steel. The tests were performed on a sand shooting machine which simulated the core molding process experimentally. The results show that the erosion of samples is a function of impact angle, shooting pressure and heat treatment as the erosion increases with the increase in shooting pressure. Among the heat treated samples the highest level of erosion has been observed for quench tempered, martempered, carburized and nitrated samples respectively. The Scanning Electron Microscopy (SEM) of surface of templates show that by changing the impact angle from 20 to 90 degrees, the material removal type changes from cutting mechanism to fracture. Further analyses revealed that with increase in shooting pressure from a threshold value abrasive particles trap on the surface of the samples that makes the Mechanically Mixed Layer (MML) and so that decreases the erosion rate. Results express that silica sand causes more erosion than chromite sand; also the erosion increases with increase in size of sand particles.

In this study, commercially pure copper samples were severely deformed by equal channel angular pressing (ECAP) up to 8 passes in room temperature. The effect of sever plastic deformation on the microstructure, mechanical properties, electrical conductivity and electrical wear resistance of the cupper were investigated. In addition, the effect of induced strain on mechanical properties of the extruded cupper in each pass was studied. Field emission scanning electron microscope micrographs show the extreme evaluation of the microstructure after 4 to 8 ECAP passes, in which a large amount of nano and ultra-fine grains are observable. The mechanical properties of the pure cupper in each pass were estimated by compression testing and Brinell hardness method at room temperature. Yield strength and hardness increased by ~390 MPa and 75HB respectively after 5-pass ECAP due to finer boundary spacing. Increasing the strength of pure copper led to only a minor decrease of the electrical conductivity. Hence, by applying ECAP, the ultra-fine pure copper that can improve the mechanical properties without impairing the electrical conductivity is obtained. By reducing the applied strain in each pass (25%) of the ECAP process, the pure copper with higher strength can be obtained. The electrical wear behavior of the samples was investigated by electrical discharge machining (EDM). The results indicate that, electrical wear of the extruded samples is reduced compared to the original samples.

The nose and nasal cavity and sinuses are parts of the upper respiratory system and study of the air passage into the upper component of human airway is important to improve or cure deficiency in human respiration cycle. The nose performs many important physiological functions, including heating, humidifying and filtering inspired air, as well as sampling air to smell. Previously, numerical modeling of turbulent flow in nasal cavity, sinus, pharynx and larynx was rarely employed Since the 1990s, with the development of computed tomography technology and computational fluid dynamics, a number of numerical studies on gas and particle flows in realistic nasal cavities have been conducted and provide precise data for deeper insight of the nature of nasal airflows. Also, most of the pioneering studies in this field have been developed to investigate only the nasal cavity without sinuses, especially maxillary sinus So, this research attempts to study details of turbulent airflow through all spaces in human head that air can flow through. For this purpose, this study is based on computed tomography scans image of a 26-year old female head, neck and chest without problems in her respiratory system from Shahid Chamran hospital, Shiraz, Iran. It is found that, nasal resistance contributed up to half of the total airway resistance within the first 2-3 cm of the airway and the majority of the flow in this region remained close to the septum wall and only a small proportion reached the olfactory region.

Residual stress is stress that remains in the body, such as some specific operations like welding, and is available when body is not under external loads. Different methods are available for residual stress relieved in any welded samples. In this research, samples one, two and three have been prepared and welded. Stress of samples one and two have been relieved using heat treatment method and ultrasonic method, respectively. Also, no operation has been taken on the sample three. For different amounts of residual stress of each sample, hole drilling method was used and amount of residual stress in welding heat affected area of each sample was determined. Results show that residual stress in ultrasonic method is less than heat treatment method. To validate the results the x-ray diffraction method was used and results of this method show that in the sample with ultrasonic stress, stress relief had no peak at 2θ angles, but for sample with heat treatment stress releif had a peak at 137o, which shows some residual stress.

With respect to special conditions applying to the gas turbine, its blades are affected by many different factors such as, hot corrosion, oxidation, wear, impact of external particles, etc. and are destroyed. Due to the reduction of their working life time, the turbine efficiency reduces and ultimately the heavy costs of periodic repairs are needed, and new replacements of their blades are unavoidable. The aim of this study is investigation of the effects of corrosion and blade damage on flow field and gas turbine performance, by numerical simulation. In this research, a two stage turbine is modeled in the form of three dimension and the results are validated with experimental data. To analyze of the behavior of entire flow, conservation of mass, momentum, and energy equations are solved. The numerical simulation of the turbine is done with ANSYS CFX software. Then the increased rotors tip clearance effects with decreasing thickness due to corrosion in both nozzles and blade leading edge and trailing edge were separately studied on turbine flow field and its performance in five actual different pressure ratios. The results showed that the most important factor in reducing the efficiency of gas turbine is due to rotor tip clearance increasing. Also, corrosion of the blade edge with respect to the trailing edge damage is slightly more affected by reducing efficiency and increasing loss coefficients.

This paper describes a simple method for determining the critical buckling load of composite lattice conical structures under axially compressive load. To reveal the critical buckling strength of conical lattice structures, an analytical method based on the classical beam-column theory was applied. Characteristic equations were built according to the equilibrium equations. Furthermore, the buckling behavior of the conical composite shells under axial compression were investigated using experimental method. The specimens used in the experiment were made from glass/epoxy by winding the continuous glass fibers wetted with epoxy on a die with helical and circumferential grooves adopting a simplified manufacturing process. A relatively new flexible tool was developed for forming the grid-structures die, and the specimens were tested by using a universal testing machine. The diagrams of axial load versus displacement were recorded in real time during the tests. The experimental results describe failure modes that are present in the structures such as rib crippling, and general buckling. Axial buckling tests were carried out and the results were compared with the analytical method. The results have been summarized to verify the analytical method. Also, the proposed model was verified with the aid of finite element analysis. The proposed model suggests the possibility to improve the preliminary design solution with respect to the fully analytical approach.

In this paper, free vibration of two-dimensional functionally graded (2D-FG) annular sectorial plate surrounded by Winkler-Pasternak elastic foundation has been investigated. It is assumed that the plate properties vary continuously through its both circumference and thickness according to power law distribution of the volume fraction. Primarily, we calculate the forces and resultant moments and then the total potential energy of system. Then, by applying the Hamilton’s principal any by regarding the first order shear deformation plate theory (FSDT) the governing differential equations have been derived. The numerical differential quadrature method, (DQM), has been employed for solving the motion equations. Two different boundary conditions such as simply supported and clamp supported are considered. Initially, the obtained results were verified against those given in the literature and by ANSYS software and we confident from the obtain results. The effects of geometrical and elastic foundation parameters along with FG power indices effects on the natural frequencies have been studied. The study of results shows that, elastic foundation and FG parameters have significant effects on natural frequencies. By doing this research for 2D-FG materials the characteristic vibration of structure can be controlled by more parameters than 1D-FG materials.

The aim of this study is the feasibility study and design of hydraulic hybrid power train system for refuse truck in order to regenerate and store kinetic energy to be reused for supplying propulsion power of vehicle. The hydraulic hybrid propulsion system includes a conventional internal combustion engine, a hydraulic pump/motor and also the accumulator as the energy storage device. Here, the parallel configuration has been chosen for implementing this powertrain. In the first part of the paper, regarding the unique driving trends of refuse trucks, a driving cycle for refuse truck in Tehran has been extracted to improve the reliability of the designed powertrain. Also, AXOR 1828, one of the trucks used as refuse vehicles in Tehran, has been chosen as the base vehicle. The driving cycle is extracted by performing observations on the base vehicle operation during a period of several days several days. In the second part of the paper, the components of hydraulic hybrid powertrain have been designed to recoup as much kinetic energy as possible in the refuse truck driving cycle. The initial computations show 17 percent reduction in fuel consumption of the refuse truck.

With the increasing development of the pharmaceutical industry and producing drugs with specific performance, its transfer into cells is also of great importance. Cell membranes are effectively impermeable to hydrophilic compounds unless the permeation is facilitated by dedicated transport systems. As a consequence, there is much interest finding ways to facilitate the transport of molecules across cell membranes. Cell-penetrating peptides (CPPs) in particular have shown much promise as potential delivery agents. That have been claimed to penetrate cell membranes in an energy- and receptor-independent manner. In the present investigation, the translocation of PENETRATIN into the cell membrane is carried out applying constant velocity steered molecular dynamics via MARTINI coarse grain approach. In order to study the orientation of peptide as it get closer to the membrane, equilibrium simulation is carried out and it is shown that to investigate the penetration process, steered molecular dynamics simulation must be applied.  Energy barrier upon the insertion is calculated and its diffusion in the membrane is considered. It is shown that pore formation phenomenon breaks down the energy barrier and facilitates the translocation process which is in agreement with previous researches. Furthermore, 110 kJ/mol energy barrier is obtained from simulations for this peptide.

In the present study the free vibration analysis of functionally graded rectangular nanoplates in thermal environment is investigated. The modified coupled stress theory based on the first order shear deformation theory has been used to obtain the natural frequencies of the nanoplate. Modified coupled stress theory is a non-classical theory. In this theory material length scale parameter is applied to capture the size effect of the microstructures that the earlier classical plate theories were not able to explain. Functionally graded material properties are varied continuously and smoothly along the thickness. The Poisson’s ratio of the FGM plate is assumed to be constant in the whole plate. In order to validate the present method, the natural frequencies of both the functionally graded rectangular plate and the rectangular nanoplates are compared with those reported in the literature, separately. Finally, the effect of various parameters such as the power law index a, the thickness to length scale parameter ratio h/l, aspect ratio a/b, thickness ratio a/h on the natural frequencies of plates in thermal environments with different temperatures are presented and discussed in detail.

Dissimilar welding between Inconel 625 nickel base super alloy and high strength quenched and tempered A 517 Gr.B steel was investigated by pulsed ND: YAG laser beam welding equipment. This joint has special application in submarine components. After welding, the optimized joint microstructure including the weld metal and heat affected zones were characterized by optical and scanning electron microscopy (SEM). The results showed a fine dendritic structure and existence of large amount of Niobium carbide and Laves eutectic phase in the weld metal. Energy-dispersive X-ray spectroscopy (EDS) analysis showed Nb and Mo segregation to interdendritic zones at the weld metal. Grain growth in the heat affected zone of Inconel 625 did not occur, however, ultrafine precipitations were deposited at the heat affected zone. An approximately 65 mm wide transition zone was observed at the steel and weld zone interface; consisting of a martensitic layer (10-20mm) along the weld interface and the austenite phase region with a small amount of ferrite adjacent to the base metal. The tensile test and micro hardness test of the optimized sample was investigated. The electrochemical behavior of the weld metal was investigated at room temperature in 3.5% NaCl solution using potentiodynamic polarization. The results show that the corrosion resistance of weld metal is more than that of Inconel 625 and less than that of 517A Gr.B. It can be concluded that a proper selection of laser beam welding parameters provides sound, fully-penetrated welds.

The purpose of this paper is to reach the maximum energy recovery or maximum cold stream outlet terminal temperature in a plate fin heat exchanger (PFHE) with constant volume and heat transfer area for a specified maximum pressure drop. This paper presents a methodology in surface selection and design of PFHE where full pressure drop utilization is taken as a design objective in constant heat exchanger volume and heat transfer area. Several kinds of PFHE with different fin type and geometries and different heat exchanger width, length and height could satisfy the constant volume and area condition. Setting maximum pressure drop could reduce these heat exchangers. While the fin type and dimension of each heat exchanger is extracted due to constant volume-area and pressure drop conditions respectively, the terminal temperature of the heat exchanger would be calculated utilizing thermo-hydraulic modeling of the PFHE. A typical gas turbine regenerator is chosen as case study. The methodology is applied to this case study and results are shown. The surfaces which result in maximum energy recovery are specified. In the cases in which energy recovery of some surfaces would be approximately the same, other parameters such as frontal area and flow length will be considered.

Offshore platforms are exposed to random cyclic loads imposed on the structure by natural phenomena including waves, sea currents, wind and etc, so fatigue analysis of these structures is one of the most important design steps. Hot spot method is one of the most common techniques for evaluation of the fatigue life of offshore platforms. In this approach, the stress adjacent to the weld is estimated by extrapolation from the stress distribution approaching the weld, as obtained by finite element method or perhaps from strain measurements on the surface. In order to calculate the fatigue life of Amir Kabir semi-submersible drilling platform, first a model of platform is created. Then according to the environmental conditions of the Caspian sea, hydrodynamic forces exerted on the platform are calculated. The simulated hydrodynamic forces are then applied to the platform structure for calculating the stress and strain field in the whole structure. It is found that the intersection of column and pontoon is the critical section of the platform and hence the fatigue life of the structure is predicted in terms of conditions of this location.

Intense pressure pulsations, which are caused by the vortex rope in the runner cone and the draft tube of pump-turbines, result in vibrations and noise under partial load conditions in turbine mode and also reduce the machine’s efficiency. The most common method for reducing these fluctuations is injecting air through the shaft. This method has some disadvantages such as, negative influence on efficiency, high cost, and technical difficulties. In the present paper, the concept of locating grooves on the conic surface of runner has been investigated. In this regard, the runner and the draft tube geometry have been designed according to the specifications and the accessible information of Siah-Bishe project. Afterwards, the 3-dimensional flow field has been solved numerically, using Ansys CFX package. The numerical results have been verified by investigating their independency from grid size and comparing the results with experimental ones. Maximum difference between the proposed and the existing design’s performance is than 2 percent. The results indicate that locating grooves on the conic surface of the runner results in an increase in the flow velocity beneath the runner cone. Moreover, pressure pulsations have been decreased and the low-pressure area at the beginning of the draft tube shrank. The maximum amount of decrease in pressure pulsations has been recorded in two opening positions of the guide vanes (lower than 60% and more than 90% of design point). In addition, maximum efficiency drop in the revised design is less than 0.3 percent. Furthermore, because of the rotational direction change in the pump mode, the magnitude of the tangential velocity is increased.

Disjoining of spot welds in steel sheets because of fatigue failure is one of the main reasons for noise in used cars. Fatigue life of spot welding connections of ST12 steel sheets, used in car industry, with 6mm nugget diameter and subjected to uniaxial dynamic shear load is studied and determined in this research. Sheet thickness and other spot welding parameters including weld pitch are considered and compared according to C-G006 standard utilizing the test sample selected according to DIN50165 standard. ABAQUS and FE-SAFE software is used for static and fatigue life analysis respectively. To generate Stress-Number of life cycle (S-N) diagram, the equivalent fully reversed load is determined using Goodman and Gerber theories since the applied load is unidirectional. The resultant finite element results were verified experimentally by a fatigue test machine designed and manufactured during this study by the authors. The finite element results showed a good agreement with Gerber criterion and it was concluded that with increase in sheet thickness, both static strength and fatigue life would be increased. The results also indicate that the weld pitch has no effect on fatigue life. Stress-life graph for 6mm nugget diameter spot weld on ST12 sheet is the most important output of this research that can be used in automobile industry.

In this paper, an upper bound analysis of novel backward extrusion was presented. Initially, deformation zone was divided in four separate regions and an admissible velocity field for each was suggested. Then, total power in this process was calculated for each region, therefore, extrusion force was obtained. Moreover, investigation of relevance of extrusion force and process powers (friction, deformation, velocity discontinuity) with process parameters revealed better understanding in load estimation and process efficiency in this method. Finite element analysis by DEFORMTM3D was utilized for validation of upper bound results. Upper bound analysis showed increasing initial billet diameter enhances extrusion force by nonlinear relation. In addition, big billet size remodels novel backward extrusion to conventional backward extrusion and proves lower requirement extrusion load in novel backward extrusion in comparison with conventional backward extrusion. Moreover, increasing the first region’s thickness in this process diminishes extrusion force by exponential relation and no considerable change in extrusion force can be seen in a particular thickness domain. Investigation of process parameters in power efficiency showed that increasing the extruded part’s diameter created a critical condition in process efficiency because of high friction power. But increasing the thickness enhances power efficiency. Finally, upper bound analysis results had good agreement with FEA.

In general, in dynamic analysis of mechanical systems joints are assumed to be ideal. However, due to errors in fabrication and assembly of components, existence of joints clearances is an inevitable problem that causes frequent collisions between the journal, and bearing and stable periodic behavior of the system becomes chaotic. Degradation of the dynamic performance of the system, reduction in fatigue life of components and producing undesirable vibrations are all factors resulting from impact-contact forces due to joint clearance. First, different contact force models for two surfaces have been introduced and dynamical models of revolute joint with clearance for two modes, namely, dry contact model and lubricated joint model are then presented. In this paper, the dynamic behavior of a slider-crank mechanism with a revolute joint clearance between the slider and connecting rod, using the Lankarani-Nikravesh contact force model is studied and compared to the ideal case. Considering the effect of friction between journal and bearing, governing equations of motion of the system for two phase, contact and non-contact modes are extracted and it is shown that system exhibits chaotic behavior under specified size of clearance. A fluid lubricant is used in clearance between journal and bearing for stabilizing an unstable periodic orbit embedded in the chaotic attractor.

Soft tissue abnormalities are often correlated with a change in the mechanical properties of the soft tissue. New developing non-invasive techniques with the ability of early detection of cancerous tissue with high accuracy is a challenging state of art. In this paper, a new method is proposed to investigate the liver tissue cancers. Hyperelastic behavior of a porcine liver tissue has been extracted from the in vitro stress-strain experimental tests of the tissue. Hyperelastic coefficients have been used as the input of the Abaqus FEM software and the palpation of a physician has been simulated. The soft tissue contains a tumor with specified mechanical and geometrical properties. Artificial tactile sensing capability in tumor detection and localization has been investigated thoroughly. In mass localization we have focused on deeply located tumor which is a challenging area in the medical diagnosis. Moreover, tumor type differentiation which is commonly achieved through pathological investigations is studied by changing the stiffness ratio of the tumor and the tissue. Results show that the new proposed method has high ability in mass detection, localization and type differentiation.

In this paper, by expanding the Lekhnitiskii’s solution the stress distribution around quasi-rectangular hole has been studied. Lekhnitiskii used complex variables analytic method for stress analysis of anisotropic plates with circular and elliptical hole. In order to extend the Lekhnitiskii’s analytical method for stress analysis of perforated symmetric laminates with non-circular holes, by means of conformal mapping, the area external to the hole can be represented by the area outside the unit circle. In this paper an attempt has been made to study the effect of different parameters such as aspect ratio, stacking sequence, rotation angle of hole, bluntness and load angle on stress distribution around quasi-rectangular hole. The finite element method has been used to check the accuracy of analytical results. The analytical results are in good agreement with the numerical results. The results presented herein, indicate that the presented method can be used to accurately determine the stresses and stress concentration in composite plates with special shape cutouts. The results obtained clearly demonstrate the effect of these parameters on maximum stresses in perforated plates subjected to uni-axial tensile load. Appropriate selection of bluntness and rotation angle of hole can decrease stress concentration.

A homogenous 2D plate with simply support boundary conditions is assumed. The effect of plate curvature and nonlinear deformation effects with cylindrical shell model and von Karman’s relation, has been introduced. Linear and Non-linear Frequency analysis with the effect of curvature and in-plane load has been investigated for the first time. Curved plate panel flutter, with the effect of supersonic aerodynamic and in-plane load has been studied for the first time. First and third order piston theory aerodynamic (PTA) is employed to model supersonic aerodynamic loading. Equations of motion have been derived by the use of Hamilton’s principle and resultant nonlinear PDEs have been transformed into nonlinear ODEs via Galerkin’s method. Forth and fifth order rang-kutta numerical method has been used to solve ODEs and define panel behavior. Results show that, structural linear frequencies increase with panel curvature, while, it is more complicated for non-linear frequencies due to the effects of in-plane loads. Fuethermore, 3rd order PTA theory has more critical effect on the instability boundary in comparison with 1st order PTA. The effect of in-plane load in aeroelastic phenomena make limite cycle osilation to chaotic motion for curve plates.

In this study, single-objective and multi-objective optimization of curved sandwich panel with composite face sheets and magneto-rheological core have been done to maximize the first modal loss factor and minimize the mass by using genetic algorithm. The studied sandwich panel was curved with simply support boundary condition. In order to derive the governing equations of motion, an improved high order sandwich panel theory and Hamilton's principle were used for the first time. The face sheet thickness, core thickness, fiber angles and intensity of the magnetic field have been considered as optimization variables. In single-objective optimization, the optimized values of variables were calculated. The results showed that the structures tend to have thick core and thin face sheets which seems physically true. As the magneto-rheological fluid placed in the core, it has a significant effect on the increasing of the modal loss factor. For the multi-objective optimization the Pareto front of optimal technique was presented. Then for the first time at this field, the set of optimal points are selected based on TOPSIS method and it was showed that in the case of similar size and mass, modal loss factor of double-curved panel is more than sigle-curved.

In this paper, the problem of multiple scattering of obliquely incident elastic waves from fibers encased in a solid viscoelastic medium is studied. This problem has applications in ultrasonic nondestructive testing of composite materials. The fibers could be either isotropic or transversely isotropic. Based on the existing theory of elastic wave scattering from a single fiber, a mathematical model was developed for the multiple scattering of elastic waves from a number of parallel fibers embedded in a viscoelastic matrix. This model incorporates all three types of longitudinal, horizontally polarized shear, and vertically polarized shear waves. The incident wave angle can be arbitrarily chosen and the receiver can be at any desired location in space. The model is capable of handling any order of scattering from any number of cylinders. To validate the numerical results, a number of experiments were carried out on two steel cylinders embedded in a polymeric block. Using an ultrasonic probe, the scattered field from this block was measured and analyzed. The analytical and experimental results showed good agreement, particularly at their resonance frequencies.