مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 8 (آبان 1400)

In this study, nonlinear vibrations of simply-supported pipes conveying fluid made of multilayer graphene reinforced composite materials have been investigated analytically and based on the Euler-Bernoulli beam theory. The constituent layers of the pipe wall thickness are considered to be made of polymer and graphene platelets and the reinforcing graphene platelets are varied by layers in the pipe wall thickness direction. Four different patterns for distribution of reinforcing graphene platelets, large deformations, and Von-Karman nonlinear strain field are considered. The nonlinear governing equations are derived by the Hamilton principle, they are converted to the ordinary differential equations by the Galerkin method and then are solved analytically using the homotopy analysis method. The variations of the first nonlinear natural frequency of the system with respect to the variation of initial amplitude, fluid velocity, fluid density, pipe length, and also time response of the nonlinear vibrations of the system are presented for different distribution patterns V, X, O and U of the graphene platelets. The results show that the first nonlinear natural frequency of the system for all distribution patterns of graphene platelets is decreased by an increase of pipe length, fluid velocity, and density but increasing the initial amplitude increases the first nonlinear natural frequency and also the distribution pattern V has the highest nonlinear frequency comparing with the other distribution patterns.

Engineering structures are inevitably exposed to various sources of uncertainty. The uncertainty in the parameters led to structures with identical nominal parameters having different vibrational behavior, such as different natural frequencies. Therefore, it is inevitable to consider parameter variability for a robust design. The rotational motion of turbomachinery makes vibration an important issue in their design. Therefore, it is essential to accurately determine the vibrational behavior of rotating systems and the parameters affecting them. No comprehensive experimental study is reported on the sensitivity of vibration behavior of industrial rotating systems to parameter uncertainty in the related literature. Therefore, in this paper, a powerful method of global sensitivity analysis based on variance analysis is presented using an industrial compressor sample to determine the effective parameters in its response uncertainty. The Monte Carlo simulation method is adopted to implement the global sensitivity analysis method. In this method, the uncertainty in the system response quantifiably devotes itself to the uncertainty of its parameters and provides a quantitative analysis along with qualitative predictions to the designer. The presented method in this paper can be very useful in designing rotating machinery and identifying sensitive parameters on the system response for the codification of design and manufacturing instructions, like component tolerance.

In this paper, an active fault tolerant control based on time delay control, sliding mode, and nonlinear disturbance observer is proposed to control the pitch subsystem in the presence of actuator faults and uncertainties. Time delay estimation is applied as a fault estimation algorithm for detection and compensation. Then, a robust control law is synthesized to nullify uncertainty and fault effects using a combination of sliding mode, disturbance observer, and time delay with novel adaptation laws. In order to mitigate chattering which comes from the discontinuous control term, a nonlinear disturbance observer is designed. Through the proposed structure, the discontinuous gain is reduced significantly which leads to chattering reduction. Stability analysis is conducted through Lyapunov Theory. Moreover, wind speed profiles are generated using TurbSim, and simulations are performed based on a nonlinear two-mass wind turbine model and implemented in the FAST environment to verify the validity of the designed controllers. Finally, results reveal the effectiveness of the proposed controller compared to feedback linearization and gain-schedule proportional-integral controllers in the presence of uncertainty and different actuator faults such as hydraulic leakage, pump wear, and high air content in the oil.

In this paper, the nonlinear vibrations of a rectangular hyperelastic membrane resting on a nonlinear elastic Winkler-Pasternak foundation subjected to uniformly distributed hydrostatic pressure are investigated. The membrane is composed of an incompressible, homogeneous, and isotropic material. The elastic foundation includes two Winkler and Pasternak linear terms and a Winkler term with cubic nonlinearity. Using the theory of thin hyperelastic membrane, Hamilton’s principle, and assuming the finite deformations, the governing equations are obtained. Also, the kinetic energy, the work of uniform distributed force and pressure, and the effects of damping are determined, according to the strain energy function for neo-Hookean hyperelastic constitutive law. By applying Galerkin’s method, the nonlinear partial differential equation of motion in the transversal direction is transformed to the ordinary differential equations. Then, utilizing the method of multiple scales, the superharmonic and subharmonic resonances including the 1:3 superharmonic and 3:1 subharmonic, 1:5 superharmonic, and 5:1 subharmonic, 1:7 superharmonic, and 7:1 subharmonic are analyzed. Also, the analytical results are compared with those presented by other researchers. Finally, the effect of the Winkler and Pasternak stiffness, the material properties, and various geometrical characteristics on the superharmonic and subharmonic resonances of the vibration behavior of a rectangular hyperelastic membrane is investigated.

The increasing tendency to soft robots in various applications justifies the reason for studying the behavior of such actuators. The present study investigates the effect of fiber angle on the motion behavior of elastomeric fiber-reinforced actuators with two circular and semicircular sections. Unlike previous researches, this study takes into account the elastomer material used in actuator construction. Furthermore, unlike previous researches in which phase angle variation was studied just in linear actuators, phase angle variation in linear-twisting actuators is also considered. The simulation results showed that the phase change angle is 54.2° in silicone linear actuator and 30° in linear-twisting silicon actuator. The results also showed that the maximum bending in the semi-cylindrical bending actuators occurs at a 90-degree angle of twisting fibers. To verify this behavior, experiments were done. Silicone linear actuators were made with four different fiber angles including 30, 54.2, 54.3, 75, and 85 degrees. Moreover, Linear-twisting actuators were made with two different fiber angles including 30, 55, 65, and 85 degrees clockwise and 45 degrees counterclockwise. At last, one bending actuator with fibers at the angle of 88 degrees was made. All these actuators were evaluated after actuation. The experimental results confirmed the simulation results with a maximum calculated error of 14%.

During the operation of rotating machines in large industries such as power plants, the journal bearing failures are numerous. In order to avoid catastrophic damages in bearings and reducing the causes, evaluation of the root causes of bearing failures is important. In this paper, we investigate the root causes of failure in journal bearings, using one of the powerful methods of maintenance fields, which has been named failure mode and effect analysis. In order to collect bearing failures data, have been referred to the six power plants and failures information is obtained. Using this data and charts related to the occurrence probability, detection probability and severity rates which are the main parameters of this method for determining the risk priority number, the failure mode and effect analysis method has been implemented. According to this method, the main failure has been identified as wear. Then, using the well-known model of bearing wear geometries and computational fluid dynamics analysis for solving Navier-Stocks equations, effects of wear on the bearing load capacity, maximum lubrication pressure, in the different locations of wear, in the lower half of the bearing, has been investigated. Finally, the results of the finite element analysis have been compared to the results of the theory and solving the Sommerfeld-Harrison equation for bearing without wear. Also, to reduce the effects of this failure, another bearing geometry has been proposed in a similar situation with a load capacity greater than the custom one.

The advances in the extended finite element method enable the prediction of crack initiation and propagation without prior knowledge about the crack pattern. In this regard, the purpose of this study was to investigate human femoral fracture location using voxel-based finite element simulation. The simulation was developed in terms of an anisotropic failure mechanism coupled to the extended finite element method to describe the femoral progressive fracture pattern in specimen-specific models. An anisotropic failure mechanism (4 damage criteria) was developed based on the combination of Hashin failure criteria and maximum principal stress criterion to capture femur fracture behavior dependency on femur anisotropy and heterogeneity. Three specimen-specific femur FE models were constructed based on CT-scan images under a particular loading condition. The load was applied to the head of the femur at an angle of -15 degrees relative to the sagittal and coronal planes. To demonstrate the potential of the current approach, a one-to-one comparison of predicted extended finite element method fracture pattern and experimental results were performed. An acceptable agreement was obtained between the predicted and observed fracture patterns suggesting that the proposed failure mechanism in the extended finite element method is capable to simulate femoral fracture type and progressive crack propagation. The presented results indicated that the crack on-set location and subsequent crack trajectories can be correctly captured using the proposed anisotropic failure mechanism in the extended finite element method.

The study of axonal behavior under different environmental conditions can provide a better insight into the development of therapeutic approaches for healing after nerve damages. By modeling of sublayer in the form of a hyperelastic material and applying different pressures, the amount of strains tolerated by the axon was calculated. Strains were applied at three different time intervals to examine the effects of different strain rates. For axon, a model containing microtubules with linear elastic properties, neurofilament, and axolemma with linear viscoelastic properties was considered. The finite element method and COMSOL software were used for the discretization of the sublayer and the substructures of the axon. It was observed that the fluid regime in the channel did not affect the mechanical response of the axon. The strain was close to zero (at most 0.0001) and the stress was also negligible (at most, 70 N/m2). The results showed the major effect of microtubules on resisting mechanical forces and on the overall integrity of the axons. Most of the strains were seen inside the axolemma, indicating the importance of its mechanical response to injury. Regarding the response to the strain rate, the most probable damage to the axon, comparable with the former corresponding reports will occur at the strain of 42% and strain rate of 19.1 s-1, respectively.

In this paper, the energy harvesting by porous beams exposed to the external fluid flow is studied. The electromechanical nonlinear differential equations of the transverse vibration behavior of porous beams exposed to external fluid flow are derived using the Euler-Bernoulli beam theory. A porous beam with concentrated mass which is equipped with a piezoelectric layer at its upper surface is considered energy harvesting. After numerically solving the governing nonlinear equations, the effect of different parameters on the generated energy is investigated. The results show that in the lock-in area, the maximum amount of energy is taken. Also, the porosity distribution has a significant effect on the maximum amplitude of the oscillations as well as the energy harvesting by the porous beam. In addition, for electrical resistance of 1000 kΩ, the maximum voltage generated for the beam with symmetrical porosity distribution in the form of wall stiffness, asymmetric porosity distribution, and uniform porosity distribution is equal to 0.39 V, 0.44 V, and 57 V, respectively, which indicates the highest energy harvesting capability of the beam with the porosity distribution of the third type.

Aluminum/graphene nanolaminated structures have a very proper reinforcing and toughening effect on aluminum composites. Graphene layers effectively prevent the growth and movement of dislocations in the aluminum matrix. Therefore, more and shorter dislocations lines occur in the aluminum matrix between the graphene layers. In this paper, tensile tests have been performed on nanolaminated aluminum/graphene composite using molecular dynamic simulation to study the dislocation-blocking mechanism and its reinforcing and toughening effect. The nucleation, expansion, and displacement of the dislocation in the aluminum matrix were investigated under tension. The results showed that the reinforcement mechanism includes increasing displacement density and shear stress transfer. Besides, the reinforcing and toughening effects were investigated as a function of the distance between the graphene sheets (the spacing of sheets between 4-14 Å). The results showed that the distance between the graphene sheets has an effective role in creating the dislocation-blocking mechanism in the aluminum matrix. Decreased graphene sheets increase the mechanical properties of the aluminum matrix due to the dislocation-blocking mechanism, which can be limited by the onset of graphene sheet aggregation. As the result, stable steps in 10-12 Å distance between graphene sheets were obtained by dislocations with a yield strength of about 14 GPa and yield strain of 0/065.

Increasing product value is a topic that is always a concern for manufacturers. In the case one of the best methodologies is value engineering. There are different steps for performing value engineering in different sources. One of the important and primary phases in performing value engineering is the functional analysis phase. According to this paper, it is possible to perform the product function analysis phase in the proposed improved failure modes and effects analysis framework that is one of the ways of the correct performance analyzing of the products in the National Standard of America. Using of proposed framework not only performs the function analysis process step by step but also reveals one of the hidden factors which are effective on the value of the product functions. In this way, failure risk is revealed in order to calculate the real value and recognize the low-value functions for introducing to the value engineering creativity phase. On the other hand one of the advantages of using the framework presented in this paper is that due to the symmetry of the time to run failure modes and effects analysis and implement value engineering, it integrates a part of the implementation of failure modes and effects analysis and value engineering.

Polyoxymethylene as a thermoplastic or soft plastic material, in addition to its acceptable mechanical strength, has a much lower density comparing to metals. Therefore, it can be a good alternative to non-ferrous metals in the industry. In this study, nanocomposites of this polymer with carbon nanotubes were used to enhance the strength and improve the mechanical properties of the polymer. Experimental analyses of the nanocomposites have limitations due to the high cost. Therefore, using microscopic scale simulation methods can be a good alternative to study the properties and behavior of these nanocomposites. In this study, the molecular dynamics method is used to simulate the mechanical properties of the nanocomposite. The simulation results obtained in this study show that the density and mechanical properties of the pure polymer such as Young's modulus, yield stress, and the ultimate stress are consistent with experimental values. Moreover, with temperature increase, these mechanical properties will be reduced. Also, these properties by reinforcing polymer with carbon nanotubes which functionalized with hydroxyl and fluoro groups in a nanocomposite structure can modulate Young's modulus from 41.31 to 44.6% and yield stress from 20 to 80% respectively.

The aim of this study is microstructure modeling and deformation and damage analysis of aluminum-based metal matrix reinforced by Tic particles by using the Peridynamic theory and experiment. The particulate composite was fabricated by mixing aluminum and TiC powder and then the mixture was hot-extruded. Tensile tests were carried out to validate the Peridynamic model. Four representative volume elements were extracted from surface images of specimens. The location of Particles in the matrix was obtained by image processing. Due to restrictions on bond-based Peridynamic, state-based Peridynamic was utilized for modeling. Overall stress-strain curve, the destitution of equivalent stress and plastic strain, distribution of damage parameter, the total plastic stretch of all interactions, and the number of damaged interactions were used to analyze the results. At matrix surrounded by particles, matrix/particle interface, and narrow particles stress concentration were detected. The damage was initiated at these regions, but the damage was mostly propagated in matrix and matrix/particle interfaces. In the loading process, several damage mechanisms were initiated and propagated, and finally, a principal crack was created that led to the final fracture. By comparing scanning electron microscope images of the fractured surface, modeling, and experimental result, it is shown that the developed Peridynamic model can precisely predict the progressive damage behavior of particulate composites.

In this paper, the sensitivity of the micro-cantilever infrared detector has been increased by optimizing its geometric dimension. The detector's main body consists of an absorbing area, bi-material, and isolation legs, which are made up of silicon dioxide. Bi-material regions (absorbing area and bi-material legs) include a thin film layer of aluminum, which is placed on the main body layer. In this detector, the amount of bending at the end of the tip detector depends on the thickness of insulating and metal layers and the width of isolating and bi-material legs. Furthermore, it has been proved that the detector's displacement and sensitivity are optimized when the thickness of the metal layer is selected the half of the thickness of the absorbing layer. The results of the calculations show that by applying boundary conditions for 100 pW/μm2 constant thermal flux on the absorber, amount of displacement, thermomechanical, power, displacement, and body temperature sensitivities are increased by 26%, 27%, 28%, 2.3%, and 26%, respectively. In addition, the calculation results show that the sensitivities and response time are improved to 4.24, 54, 12.4, 12.8, 54, 4.2, 54.48, 54, and 1.5 times, respectively, in the vacuum environment if the leg’s width, the isolating and metal layers’ thicknesses are selected as 1μm, 0.1μm, and 0.05μm, respectively.

The equal channel angular pressing process is one of the most effective severe plastic deformation processes to produce ultrafine grained steels metals. Also, the upper bound theory is one of the reliable theoretical tools to forecast deformation strain and forming load. In this research, analysis of equal channel angular pressing technique in an arbitrary channel and corner angles using upper-bound theory was performed and a general and user-friendly equation for predicting the forming force is proposed according to the geometry of process and elastic-plastic properties of work-piece material. By comparing the amount of obtained theoretical load with experimental forming force resulted from applying process on 7075 Al alloy, has been observed very good agreement between the results. This guarantees the reliability of the achieved general equation for equal channel angular pressing force. According to the results of the present research, experimental and theoretical forming load of equal channel angular pressing process on this material under conditions of channel angle 135°, corner angle 20°, billet diameter 10 mm, and billet length 90 mm were obtained equal to 48 kN and 55.04 kN, respectively. Furthermore, by increasing the channel angle from 60 to 150 under a constant corner angle of 20°, process load is decreased equal to 41.3% from 86.4 kN to 50.7 kN. In addition, by increasing the corner angle from 0° to 40° under a constant channel angle of 135°, the negligible reduction of a load equal to 2.5% was observed from 56 kN to 54.6 kN.