مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 6 (شهریور 1400)

In this paper, finite-time active fault tolerant control based on adaptive back-stepping nonsingular fast integral terminal sliding mode control is proposed to control a lower limb exoskeleton in the presence of actuator fault. In order to detect, isolate and accommodate the actuator fault, a third-order super twisting sliding mode observer is used. To eliminate the chattering of conventional sliding mode, supper twisting sliding mode algorithm is applied, which leads to finite-time convergence and high precision in tracking the desired trajectories. Back-stepping term guarantees global stability based on Lyapunov theory. Upper limb motion is used to provide stability to robot's motion based on zero-moment point criterion. In order to attain maximum stability based on zero-moment point, minimize error in tracking the desired trajectories, increase the tolerance of the controller against actuator fault, controller, observer and upper limb trajectory parameters are optimally tuned based on harmony search algorithm. Performance of the proposed controller is compared with the performance of sliding mode controller with/without fault information. Simulation results reveal the effectiveness of the proposed controller in the presence of actuator fault, uncertainty and disturbance in comparison with sliding mode controller.

Since the maintenance and repairing costs of mechanical systems, such as structures and rotating machines are significantly high, one way to reduce these costs is to consider some approaches before any operational work to check for damages in such systems. In this study, a new method is presented for damage detection of offshore jacket structures in the presence of various uncertainties, such as modeling errors, measurement errors and environmental noises, based on the simulated model and intact state of the real model. In the proposed method, real intact structure data is used to update the simulated model parameters. Some parts of the signal that are not related to the nature of the system are removed using the complete ensemble empirical mode decomposition method. Frequency data is extracted from the vibrational signals using the frequency domain decomposition method. A deep auto-encoder neural network is designed to learn the damage-sensitive features from the frequency data and to damage detection of the structure. In order to train the proposed deep network, frequency data of the simulated model and real intact state are used; then the frequency data of the real structure is used to test the proposed deep network. The results show that the proposed method is capable for damage detection of the offshore jacket structure with more accurate results than the other comparative methods.

Exploiting the nonlinear nature of structural damage through piezoelectric patches is one of the latest concepts of early detection of damage. Support loosening as one of the prevalent defects in engineering systems is a source of contact acoustic nonlinearity. Vibro-acoustic modulation has proven to be a promising method for revealing structural nonlinearity characteristics. Requiring a large number of cycles to be solved in the time domain to reach the steady-state before evaluating the results in the frequency domain, makes using existing computational methods such as finite element method to model this phenomenon very expensive. This paper is concerned with a novel numerical method called Fourier spectral element to investigate the vibro-acoustic modulation in the Euler-Bernoulli beam caused by contact nonlinearity. Three piezoelectrics were attached to the beam as the probe, pump and sensor. The numerical outcomes are compared against the experimental results to check the validity of the developed method. The results show that the Fourier spectral element not only provides a high convergence rate but is also capable of simulating the phenomenon of modulation with sufficient accuracy.

This paper deals with the problem of fault detection and isolation for discrete time-varying systems with stochastic and bounded uncertainties, and in presence of noises in the plant and sensors. Faults can occur simultaneously or sequentially, so the designed filter has the ability to detect and isolate these faults, and handle the challenges posed by uncertainty and the effects of noises. In solving the problem of fault diagnosis, fault detection and isolation filter based on the robust Kalman filter are presented. For this purpose, a time-varying threshold is defined based on the upper bound of covariance of the residuals. This threshold helps in better performance and prevents misdiagnosis. In the design of the fault detector, due to the number of outputs, fault detectors are designed. Moreover, by examining the residuals of the system, some conditions are obtained, which, by applying these conditions, a robust fault isolator is achieved. Finally, using three examples, the efficiency and performance of the proposed method are shown. In the first example, the performance of the proposed method is studied in the presence of uncertainty and noise, and in the second and third examples, the performance of the method is compared with other methods and the superiority of the proposed approach in the presence of uncertainties is shown.

The use of microfluidic systems for various applications such as lab-on-chip, drug delivery, and micro chemical reactors is increasing day by day and several sensors have been provided to control and develop these microsystems. In this paper, a biosensor based on microelectromechanical systems was introduced with direct application in liquid environment for digital microfluidic systems that have the ability to be integrated with various fluidics components such as delivery, separation, and mixing. The proposed sensor comprises  semi-sized coupled microresonators which vibrate in parallel to the substrate so that it can be integrated between the plane electrodes in digital microfluidics systems. Active area of the sensor is located in the center of the structure and immobilized for capturing any special biological targets. Due to in-plane vibration of the sensor, the viscous damping is low enough to achieve measurable quality factor by resonator. The total system is simulated by finite element methods and the results demonstrate that the appropriate vibration frequency for in-plane motion of the sensor is 16.5kHz. In addition, the quality factor and mass sensitivity are 49 and 100Hz/µg, respectively, which are comparable to sensors with similar fluidics applications.

The most important factor in car accidents and even traffic congestion is the driving behavior of human. Analyzing the driving behavior and applying restrictions on risk-making drivers can reduce probable causalities and improve transportation safety. A hardware can collect the data related to the driving conditions and processes that information that may be useful in this regard. By analyzing the information recorded by the hardware, one can obtain an approximate prospective of the driving behavior. This prospective can distinguish a careful driver from a careless one. By collecting various data in different conditions, the accuracy of these criteria will be improved. On the other hand, the accuracy of different components of the hardware is crucial since the unexpected noises deteriorate the performance of data collection. In this research, a data analysis module is developed to record the acceleration and angular velocity of a sample vehicle to identify the driving behavior and propose quantitative criteria to distinguish aggressive and cautious drivers. In this way, one can figure out if his/her driving habits are dangerous or not.

In this investigation according to the nonlocal nonlinear Euler-Bernoulli beam theory, the instability and frequency sensitivity analysis of single-walled carbon nanotube conveying fluid is studied. The thermomagnetic field, residual stress and surface elasticity, viscoelastic foundation and small-scale effects on the governing equation of single-walled carbon nanotube are taken into account. The Galerkin decomposition method with the trigonometric shape functions corresponding to the standard boundary conditions including simple-simple, clamped-simple and clamped-clamped at two ends of carbon nanotube are employed to solve. The eigenvalues and critical fluid velocity in the threshold of instability of system are computed by extracting the mass, stiffness and damping matrices. The magnetic intensity, change of temperature in the cases of high and low-temperature conditions, length of nanotube, outer diameter of nanotube and small scale parameters are conceded as the input factors for sensitivity analysis. The qualitative and quantitative effects of input factors on the critical fluid velocity and natural frequencies of single-walled carbon nanotube are computed and compared with together by normalization. The results of sensitivity analysis of present work can be used for optimal or target design of single-walled carbon nanotube for different applications especially for drug delivery to kill metastatic cancer cells.

Vibration problems are of great importance in design and construction of electric machines which affect both of mechanical and electrical properties of these machines. In this paper, the effects of both perfect and perturbed conditions of cyclic symmetry on the vibration behavior of spinning ring under moving electromagnetic loading have been investigated. Euler-Bernoulli beam assumptions have been implemented in the modeling of structure and electromagnetic loading has been modeled with discrete springs. Using Hamilton’s principle, the governing equations of in-plane vibrations of spinning ring have been extracted. Eigen analysis of the system has been extracted using perturbation methods. The obtained results show the condition of instability for a precise value of ring angular speed versus support speed in different vibration modes. The effects of time variation of spring’s stiffness and the variation of connection angle between springs and ring on the in-plane vibration of spinning ring, have been investigated extensively. It is shown that the deviation from cyclic symmetry could eliminate the mode splitting phenomenon in some cases. The obtained results are expected to offer better predictions of the vibrational behavior of spinning rings structures under moving loads in general, and in the design of electric machines, in particular.

In this study, a method for energy harvesting from sloshing of fluids has been proposed. In the first part, voltage and electrical power are measured experimentally. A magnet is floated on the liquid in the coiled container and the system is placed on a shaker table. According to Faraday's law of induction, the movement of the magnet inside the container induces voltage in the coil. The results show that the induced voltage increases with increasing frequency and reaches its maximum value at the natural frequency of the structure and container and decreases again. Also, the inductive voltage has increased with increasing both magnet strength and height of the liquid inside the container. The highest inductive voltage and power output in this study were 850mV and 400µW, respectively. To evaluate the efficiency of the proposed method, the system was installed on a shaper machine and the induced voltage and electrical power were measured. Also, the numerical method is used to simulate and analyze the proposed system. The results show that variations of interface parameters including its pressure, are consistent with experimental data and therefore this method can be used to design and predict the performance of the energy harvester.

Composite materials are widely used in modern aerospace flight vehicles, especially because of their high specific strength and lightweight than other materials. Since the study of reliability and uncertainty of composite material design variables in aeroelasticity has received less attention, in the present work the reliability of the laminated composite plate due to uncertainty in variables including elastic model, Poisson coefficient, density, thickness, and length is examined. The composite laminated plate is symmetric with different boundary conditions subjected to supersonic airflow. The classical plate theory and the first-order piston theory are utilized to derive the equation of motion. The differential quadrature method has been used to discretize and analyze the aeroelastic equations. The governing equations after discretization are solved by calculating and analyzing eigenvalues and the occurrence of the flutter phenomenon for the laminated composite plate is obtained. To examine the reliability, the distribution of random variables as a normal distribution has been used. Finally, the Monte Carlo simulation method was used for five different boundary conditions to obtain the reliability of the plate flat threshold. According to the presented results, the reliability value of the composite plate flutter threshold for the full hinge boundary condition (SSSS) will be higher than other boundary conditions and the all-bound boundary condition (CCCC) will be lower than other boundary conditions. Also, according to the studies on the condition of the fiber angle of the composite plate, it can be concluded that increasing the angle of the fiber of the composite plate increases the reliability of the occurrence of the filter threshold.

Microtubules, polymer tubes stretched from the cell nucleus to the cell membrane, are the major parts of the cytoskeleton that provide the mechanical rigidity, organization, and shape retention for the cytoplasm of eukaryotic cells. These structures play a key role in some cellular processes such as cell division, intracellular transport, and the internal organization of cells. In all the above applications, the network structure of microtubules is the main reason for the importance of in-depth studies of their mechanical properties. In this paper, the propagation of elastic waves in periodic networks based on two-dimensional fractal microtubules of fixed mass-center triangles is analyzed. This study begins with the selection of a suitable beam model for a microtubule and examines the dynamic behavior of microtubules by creating periodic structures. To obtain dispersion curves, finite element models of microtubules and their networks are developed, and the bandgap equations are calculated based on Bloch's theory. The results show that depending on the topology of the selected unit cells as well as the considered periods, it is possible to design a frequency gap in specific ranges for the application of low and high-frequency bio-filters. This study helps researchers control or absorb some unwanted vibrations using periodic structures, and thanks to their better biocompatibility, these networks can be used in next-generation nanomechanical devices such as implantable biosensors.

Discrete concrete targets show more resistance to penetration against shaped charges. The purpose of this study is to simulate the penetration of shaped charge in discrete concrete targets using LS-DYNA and ANSYS-AUTODYN and compare the results. For this purpose, the simulation process for one of the experimental results is performed and the results obtained from both software are validated. Finally, the results of two software in the fields of jet velocity, penetration depth, entry diameters, middle and exit crater, and run time are compared. Application of the ALE method for jet elements and the RHT concrete model to simulate the concrete behavior at high strain rates yielded good results. Differences in numerical solution method and command differences in the interaction of the Lagrangian and Eulerian elements in two software caused the depth of penetration in ANSYS-AUTODYN to be less than the LS-DYNA and the diameters of entry, middle and exit crater in ANSYS-AUTODYN become larger than the LS-DYNA and closer to the experimental results. The results of the two software are in good agreement with the experimental results. Continuous concrete was also simulated and it was found that the penetration depth of discrete concrete was lower than continuous concrete.

In this study, the synergistic influence of reduced graphene oxide and multi-walled carbon nanotubes  on the mechanical properties of epoxy nanocomposites was investigated. In the first step, the epoxy nanocomposite specimens reinforced with 0.02, 0.04, 0.06, 0.08 and 0.1 multi-walled carbon nanotubes weight percentages fabricated using direct homogenization technique. The mechanical properties were obtained via a tensile test setup. The results showed the 35.7%, 21.7% and 12.47% increase in Young's modulus, ultimate stress and yield stress of the 0.04% multi-walled carbon nanotubes reinforced specimen. In the second step, the epoxy reinforced with 0.2, 0.4, 0.6, 0.8 and 1 reduced graphene oxide weight percentages fabricated. For the 0.6% reduced graphene oxide reinforced specimen, 37.6%, 18.1% and 13.14% increase in Young's modulus, ultimate stress and yield stress were seen. Next, the effect of different reduced graphene oxide content on 0.04% multi-walled carbon nanotubes reinforced epoxy was investigated. The obtained results demonstrated the increase in the mechanical properties of 0.04% multi-walled carbon nanotubes -0.4% reduced graphene oxide (Mixing ratio 1: 10). Due to this mixing ratio for 0.06% multi-walled carbon nanotubes -0.6% reduced graphene oxide specimen, 42.2%, 25.88% and 18.97% increase in Young's modulus, ultimate stress and yield stress were seen. The analysis of specimens' fracture surface was performed to observe the failure modes and dispersion of nanoparticles in the epoxy matrix. The results revealed that mechanical properties can change significantly by adding two different nanoparticles, simultaneously.

Auxetic structures (negative Poisson’s ratio) are a group of materials that expand (contract) under tensile (compression) longitudinal loading. In this work, the effect of re-entrant auxetic structure geometry on Poisson’s ratio was investigated under large tensile loading experimentally and numerically and showed that the location and stiffness of rotation joints are two new important parameters affecting the value of Poisson’s ratio. Poisson's ratio increases as the rotation joints tighten and move away from the center of the structure. Therefore, by changing the location and stiffness of the rotation joints, it will be easy to obtain re-entrant auxetic structures with different Poisson's ratios, which makes it possible to build piezoresistant auxetic sensors with different sensitivities. A highly sensitive, stretchable piezoresistant auxetic sensor made of silicon rubber and chopped carbon fibers is proposed for low strain values. The main feature of this sensor is its high sensitivity for strains less than 6%, which previous works have been unable to detect this range of strains. Shifting strain is the value of strain in which the Poisson's ratio of the structure changes from negative to positive. The provided auxetic sensor performs exceptionally well until the shifting strain and then performs as conventional sensors. This improvement in sensing performance is about 150% (in terms of gauge factor).

In this paper, the effect of three different types of nanostructured reinforcements on the mechanical and tribological properties of nanocomposites are investigated. Carbon nanotube, nanoclay and nanographene oxide with equal weight percentages and under similar environmental conditions are added into the epoxy resin. To achieve uniformly dispersed nanoparticles within the epoxy matrix, mechanical stirring with ultra-sonication is utilized. After degassing process, tensile and wear test specimens were made according to the relative standards. Three samples of each nanocomposite were prepared and tested. Young's modulus, ultimate tensile strength, strain at break point and toughness of the specimens were extracted from the stress-strain curve using the tensile test. Dry wear test was performed at 20N, 60N and 100N loads using a disk on pin testing machine at room temperature. The results showed that in the sample containing carbon nanotubes, the ultimate stress increased by 16% and the strain at the break point increased by 27% compared to pure epoxy. Also, carbon nanotube/epoxy and nanoclay/epoxy nanocomposites showed the highest wear resistance compared to other samples, so that in the sample containing nanoclay reinforcement, a 60% decrease in the amount of wear rate was observed compared to the pure epoxy sample.

In the current research, the plastic behavior of circular plates under dynamic loading was investigated. In the experimental section, a ballistic pendulum apparatus and aluminum alloy plates were used. To investigate the deformation profile and the failure pattern of the specimens, dynamic loads were applied in a wide range from 6.49 to 24.69 Ns. To test the behavior under successive loading, each experiment was continued up to 5 loadings. Experimental observations indicate a large plastic deformation of the structure along with the thinning of the test specimens at fully clamped boundaries and the rupture of some in the same area. The results showed that with increasing the number of explosions and the charge mass, the maximum deflection increases, but the progressive deflection decreases exponentially. In the modeling section, some closed-form relations were proposed using the dimensional analysis approach to predict the maximum permanent deflection plates. In these relations, the effect of various parameters such as the plate geometry, inertia of the applied load, and strain rate sensitivity of material was considered. Comparing the results of the model with the experimental results showed that there is a very good agreement between the experimental results and the predictions of the model.

In this paper, fluorine doped tin oxide layers are created as electrodes and seeds for the growth of titanium dioxide nanorods by spray pyrolysis coating based on silicon microarrays. The coating of this layer on the fluorine-contaminated tin oxide layer is of great importance for the fabrication of nanogenerators based on barium titanate. In this paper, two methods nebulizer and hydrothermal have been used. The effect of coating temperature as well as nebulizer volume on the surface resistance of the layers has been investigated by four-point probe device. Also, the samples were subjected to identical hydrothermal conditions for the growth of nanorods and the morphology of nanostructures grown on fabricated layers by scanning electron microscope has been investigated.  The adhesion of the layers is also very important in this area and has been studied as one of the important parameters in the hydrothermal process. The results show that with this method, nanorods have grown on the side walls of the micro-machining structure, which can increase the quality of devices based on these structures. In this paper, low surface authorities up to 11 Ω / cm2 were achieved using the nebulizer method.

Shearography is one of the advanced methods of non-destructive techniques based on the interference of laser beams that have been reflected from the surface of the specimen. This method, which has high speed and accuracy, can evaluate the displacement derivative on the sample surface at once. In this paper, the possibility of sub-surface cracks detection with different lengths and angles in composite samples was investigated by shearography method and thermal stimulation system. For this purpose, in composite samples, controlled cracks were created with different lengths and angles. After calibration of the performance of the shearography setup, two heat sources of radiation were used to load samples. Loading quantity, the amount and direction of the shear, the cracks length, and their angles were chosen as the studied parameters. The results of this paper showed that the optimum loading amount plays a more critical role in the quality of the results than shear amount, and this value is related to the materials of the samples. To achieve the best results in crack detection on the selected specimens, optimum thermal loading was obtained between 12 and 15 seconds from in front of the specimen. Also, the optimum shear amount in the composite specimens was estimated at about 0.1 image width recorded by the camera. With the optimized values, all sub-surface cracks were identified.

Finite element method based on crystal plasticity becomes a powerful tool in investigating mechanical properties of ferritic steel and dual phase steel. In this work, a three-dimensional, microstructure-based representative volume element method is employed for simulating the mechanical properties of the alloy steel 42CrMo4, a typical spheroidized ferrite–cementite steel. A computer program has been developed to generate automatic models considering microstructural features such as grain size, volume fraction, distribution of particles, ferrite texture and carbide bands in ferrite–cementite steel. The spheroidized cementite is generally without plastic deformation under the normal tensile test even at the fracture moment. Crystal plasticity constitutive law is employed to model the ferrite grains employing Huang’s code in Abaqus software. Material hardening parameters are determined and calibrated by comparing simulated tensile tests with experimental stress-strain curves. To study the influence of microstructure features and the capability of this method to predict the material mechanical behavior, several 3D samples including different microstructural features are modeled. The results show that when the proportion of cementite in the steel increases, the strength of the steel increases accordingly. Although in this study a random texture is assigned to crystalline aggregates, the code is capable of working with any texture data. Also, effects of ferrite grain size and carbide band which leads to the microstructure inhomogeneity and stress concentration are studied.

4D printing is a relatively new branch of 3D printing. Material extrusion, as one of the most common 3D printing processes, has been recently receiving increasing attention within this area of research. There are, however, many aspects of material extrusion-based 4D printing processes that are not yet well understood. In this study, we investigated the effects of different processing parameters including the activation parameters, printing parameters, and material parameters on the curvature of self-folding bilayer specimens. All the specimens were printed using three different types of Poly-lactic acid filaments. We found that the activation time and activation temperature, as well as the printing speed, have significant effects on the resulting curvature. Moreover, the shape-shifting behavior of the specimens depended to a great extent on the type of Poly-lactic acid filament used for their printing. Characterization of the filament materials showed no significant difference in terms of mechanical properties. However, considerable differences in thermal and thermomechanical properties of the different types of filaments were observed. According to the results of differential scanning calorimetry and dynamic mechanical analysis, the differences between the different types of filaments could be traced back to their compositions including the amounts and types of additives. The results of the current study have important implications for the design of material extrusion-based 4D printing processes.