Journal of Computational Applied Mechanics
Volume:46 Issue: 2, Jul 2015

In the present research, nonlinear vibration in a coupled system of Boron-Nitride nano-tube reinforced composite (BNNTRC) oil pipes is studied. Single-walled Boron-Nitride nano-tubes (SWBNNTs) are arranged in a longitudinal direction inside Poly-vinylidene fluoride (PVDF) matrix. Damping and shearing effects of surrounded medium are taken into account by visco-Pasternak model. Based on piezoelectric fiber reinforced composite (PFRC) theory, properties of smart coupled BNNTRC oil pipes are obtained. The equations of motion as well as the boundary conditions are derived using Hamilton’s principle. The effects of various parameters such as volume fraction and orientation angle of fibers, viscosity and density of fluid on stability of coupled BNNTRC oil pipes are investigated. Results indicate that stability of smart composite system is strongly dependent on angle orientation and volume percent of BNNTs. Results of this investigation can be used in oil refineries.

Soil is a nonrenewable source that needs considerable management to prevent physical deteriorationby erosion and compaction. Compacted soil causes low fertility and yield. The purpose of this study isto investigate the effect of viscoelastic properties of soil and to determine important factors oncompaction. Furthermore, stress distribution, prediction of soil compaction and simulation of its effectunder tractor wheels using ANSYS software were also studied. Predicted results using ANSYSsoftware are compared with laboratory and field results. Simulations were carried out by changing andmeasuring effective factors on soil compaction. These factors consist of wheel parameters whichinclude: number of wheel passes, speed and load; and the soil parameters such as soil bulk density andYoung’s modulus. The predicted results indicated that maximum soil compaction in the first trafficwith 512 mm was induced by viscoelastic properties of soil and the minimum soil compaction in thesixth traffic was 8 mm caused by soil elasticity properties. Variation in soil bulk density wasnegligible. Also at each wheel pass, e maximum stress was in the soil surface and this decreased withincrease in depth. The maximum vertical stress on the soil in the sixth traffic was 120.477 kPa at 2.52km/h and the minimum was 117.46 kPa at 5 km/h.

Free vibration analysis of higher-order shear deformation beam resting on one- and two-parameter elasticfoundation is studied using differential transform method (DTM) as a part of a calculation procedure. First,the governing differential equations of beam are derived in a general form considering the shear-freeboundary conditions (zero shear stress conditions at the top and bottom of a beam). Using DTM the derivedequations governing beams, followed by higher-order shear deformation model, Timoshenko model andBernoulli-Euler model are transformed to algebraic forms and a set of recurrence formulae is then derived.Upon imposing the boundary conditions of the beam at hand, a set of algebraic equations are obtained andexpressed in matrix form. Finally, the transverse natural frequencies of the beam are calculated through aniterative procedure. Several numerical examples have been carried out to demonstrate the competency ofthe present method and the results obtained by DTM are in good agreement with those in the literature.Afterward, the free vibration of beams followed up by different models (i.e. Bernoulli-Euler, Timoshenkoand different higher-order models) are taken into consideration.

The present study is the first to analyze the dynamic response of a poroelastic beam subjected to a moving force. Moreover, the influences of attached mass-spring systems and non-ideal supports (with local movements in the supporting points or base due to the presence of factors such as gaps, unbalanced masses, and friction or seismic excitations) on the responses were investigated. Non-ideal support experiences time-dependent deflection and moment. To evaluate the effects of both the theory type and the material properties, three models were investigated for the beam with mass-spring attachment and non-ideal supports: i) elastic Euler-Bernoulli-type beam, ii) elastic Timoshenko-type beam, and iii) poroelastic beam. The governing-coupled PDE equations of the forced vibration of the saturated poroelastic beam were analytically solved via Laplace and finite Fourier transforms. The effects of various parameters on the responses were investigated comprehensively and illustrated graphically. The poroelastic nature of the material properties was found to attenuate the vibration amplitude, and it is assumed that the attached mass can considerably affect the vibration pattern.

In this paper, dynamic transient method and conventional degree-hours method (static) have been compared to estimate heating and cooling loads of building’s wall. All main wall surfaces of various orientations, i.e.South, West, East, North, and horizontal are considered in the climate of Tehran, Iran. In this study, a conventional wall structure, which is comprisedconcrete as main wall material, and EPS (expanded polystyrene), as insulation material, areused. The actual outdoor air temperature (used in dynamic method) was obtained by mean hourly measurementsrecorded in meteorological data over the period of 2006–12. Annual heating and cooling degree-hoursare calculated based on this recent weather data, and results are compared with the values reported in the national building regulations (topic 14). One dimensional transient heat transfer problem for multilayer walls has been solved to obtain temperature distribution within the wall. Annual heating and cooling load resulting from dynamic method have been compared with degree-hoursmethod; the results showed that there is a significant difference between these two estimations.

Microfluidic flow focusing devices have been utilized for droplet generation on account of its superior control over droplet size. Droplet based microfluidics addressed many scientific issues by providing a novel technological platform for applications such as biology, pharmaceutical industry, biomedical studies and drug delivery. This study numerically investigated the droplet generation process of an aqueous flow in oleic acid oil in a microfluidic flow focusing device. A conservative level set method is conducted to numerically model the droplet generation process. The post processing of the simulation results are done using Canny edge detection image processing method, which is a novel approach. Moreover, the results of the numerical simulation were compared to the experimental data provided by Ten et al. on the same device. This method showed a maximum average deviation from the experimental results of 14.6% and a minimum of 6.96%. Also, by means of altering water and oil flows, the influence of parameters affecting droplet size, which lead to a better understanding of droplet generation phenomenon, was investigated in this study. Therefore, it can be concluded that the flow ratio and capillary number are the two primary parameters that affect droplet size, while capillary number showed more dominance in comparison to flow ratio.

It is difficult to develop an algorithm which is able to generate the appropriate mesh around the interfaces in bimaterials. In this study, a corresponding algorithm is proposed for this class of unified structures made from different materials with arbitrary shapes. The non-uniform mesh is generated adaptively based on advancing front technique available in Abaqus software. Implementing several preliminary analyses, the output of each step prepared data source for the next step of mesh generation. After examining several criteria, the mean elemental stress derivative is selected as a suitable criterion to evaluate the performance of current mesh. The convergence indicates non-isometric final mesh with appropriate and optimum distribution. In general, automatic mesh generators determine the mesh density only based on the geometry of the model; however, the developed algorithm modifies mesh after sensing the stress intensity due to various reasons including loading condition and any change in material and geometry. In addition, the proposed algorithm converges to accurate result fast enough if considering the numbers of remeshing steps. An adaptive mesh generator code can be programmed based on the developed procedure to automatically generate mesh if implementing in Abaqus as a subroutine.

In this study, the analysis of transient thermoelastic response of a functionally graded (FG) non-axisymmetric viscoelastic cylinder is presented. The material properties are assumed to be time- dependent and radially and circumferentially non-homogeneous. The finite element (FE) formulations of the thermoelastic problem are obtained using the virtual work method and all the coupling terms are considered. According to material dependencies and nonlinearity of the constitutive equation, an iterative-based FE solution is suggested in order to solve thermo-elastic equations. The effects of material in homogeneities on the time-dependent response of mechanical and thermal components are investigated. From the results of this study, it is concluded that, using appropriate material inhomogeneities can improve the magnitudes of stress components, especially shear stress.

Quadruped robots have unique capabilities for motion over uneven natural environments. This article presents a stable gait for a quadruped robot in such motions and discusses the inverse-dynamics control scheme to follow the planned gait. First, an explicit dynamics model will be developed using a novel constraint elimination method for an 18-DOF quadruped robot. Thereafter, an inverse-dynamics control will be introduced using this model. Next, a dynamically stable condition under sufficient friction assumption for the motion of the robot on uneven terrains will be obtained. Satisfaction of this condition assures that the robot does not tip over all the support polygon edges. Based on this stability condition, a constrained optimization problem is defined to compute a stable and smooth center of gravity (COG) path. The main feature of the COG path is that the height of the robot can be adjusted to follow the terrain. Then, a path generation algorithm for tip of the swing legs will be developed. This smooth path is planned so that any collision with the environment is avoided. Finally, the effectiveness of the proposed method will be verified.

Needle insertion is a minimally invasive technique in diagnosing and treating tumors. However, to perform a surgery accurately, the tissue should have minimum amount of displacement during needle insertion so that it reaches the target tissue. Therefore, the tissue membrane has to move less to decrease rupturing under the membrane. In this study, the effect of different design parameters on displacement of the point where a puncture occurs during needle insertion is investigated. Finite element simulation is used to study the effect of mechanical properties of the tissues (hyper-viscoelastic coefficients) and geometric parameters of the needle (fillet radius, needle tip angle and needle diameter) and friction coefficient. To validate the simulations, the results were compared with previously published results in the literature, i.e. the hyper-viscoelastic properties of brain tissue in neurosurgical procedure and the hyper-viscoelastic properties of liver tissue. The results show that the hyper-viscoelastic constitutive is a suitable model to describe soft tissue behavior. Also, the mechanical properties of the tissue and needle velocity are effective on the displacement of the tissue's membrane and therefore in surgery accuracy.

For many physical systems like vehicles, acceleration can be easily measured for the respective states. However, the outputs are usually affected by stochastic noise disturbance. The mentioned systems are often sensitive to noise and structural uncertainties. Furthermore, it is very difficult to estimate the multiple integrals of the signal, acceleration to velocity and velocity to position. In this study, emphasis was on eliminating the drifting phenomenon caused by the noise disturbance. As a result, it is essential to find a reliable integrator to evaluate the multiple integrals of the signal. The goal of this experiment was to design a continuous low-drift integrator to estimate the integrals of a proposed signal. In addition, the chattering is capable of amplifying the instability of the system and for this reason, it should be avoided. In this study, a solution method was introduced for this problem which is inspired by the designing of experiments based on the Taguchi method and therefore optimizes the parameters which are effective for minimizing the errors. The results show a reliable response in comparison to previous studies.

The main objective of this article is to optimize the walking pattern of a 2D humanoid robot with heel-off and toe-off motions in order to minimize the energy consumption and maximize the stability margin. To this end, at first, a gait planning method is introduced based on the ankle and hip joint position trajectories. Then, using these trajectories and the inverse kinematics, the position trajectories of the knee joint and all joint angles are determined. Afterwards, the dynamic model of the 2D humanoid robot is derived using Lagrange and Kane methods. The dynamic model equations are obtained for different phases of motion and the unknowns, including ground reactions, and joint torques are also calculated. Next, the derived dynamic model is verified by comparing the position of the ZMP point based on the robot kinematics and the ground reactions. Then, the obtained trajectories have been optimized to determine the optimal heel-off and toe-off angles using a genetic algorithm (GA) by two different objective functions: minimum energy consumption and maximum stability margin. After optimization, a parametric analysis has been adopted to inspect the effects of heel-off and toe-off motions on the selected objective functions. Finally, it is concluded that to have more stable walking in high velocities, small angles of heel-off and toe-off motions are needed. Consequently, in low velocities, walking patterns with large angles of heel-off and toe-off motions are more stable. On the contrary, large heel-off and toe-off motions lead to less energy consumption in high velocities, while small heel-off and toe-off motions are suitable for low velocities. Another important point is that for the maximum stability optimization, compared to minimum energy consumption optimization, more heel-off and toe-off motions are needed.

Orthodontic miniscrews are widely used as temporary anchorage devices to facilitate orthodontic movements. Miniscrew loosening is a common problem, which usually occurs during the first two weeks of treatment. Macrodesign can affect the stability of a miniscrew by changing its diameter, length, thread pitch, thread shape, tapering angle and so on. In this study, a 3-D finite element analysis was done to show the effect of thread pitch variant on the stress distribution pattern of the screw-cortical bone interface. While orthodontic forces were applied, stresses were usually concentrated at the first thread of the screw in contact with the cortical bone. The cortical bone provided a significant percentage of stability compared to the trabecular bone against orthodontic forces. Therefore, spongy bone was removed from the finite element analysis. The changes of maximum von Mises stresses were shown on the charts. The results showed that stresses decreased with decrease in thread pitch, but they increase when thread pitch becomes less than a certain value. The pattern of stress distribution differed when the stresses were increased. The results are beneficial for the design of an ergonomic dual miniscrew, with better properties than the commercially available miniscrews and based on the results, a new dual miniscrew is recommended.

Among the several non-conventional processes, electrical discharge machining (EDM) is the most widely and successfully applied for the machining of conductive parts. In this technique, the tool has no mechanical contact with the work piece and also the hardness of work piece has no effect on the machining pace. Hence, this technique could be employed to machine hard materials such as super alloys. Inconel 718 super alloy is a nickel based alloy that is mostly used in oil and gas, power stations and aerospace industries. In this study the effect of input EDM process parameters on Inconel 718 super alloy, is modeled and optimized. The process input parameters considered here include voltage (V), peak current (I), pulse on time (Ton) and duty factor (η). The process quality measures are surface roughness (SR) and material removal rate (MRR). The objective is to determine a combination of process parameters to minimize SR and maximize MRR. The experimental data are gathered based on D-optimal design of experiments. Then, statistical analyses and validation experiments have been carried out to select the best and most fitted regression models. In the last section of this research, genetic algorithm (GA) has been employed for optimization of the performance characteristics. Using the proposed optimization procedure, proper levels of input parameters for any desirable group of process outputs can be identified. A set of verification tests is also performed to verify the accuracy of optimization procedure in determining the optimal levels of machining parameters. The results indicate that the proposed modeling technique and genetic algorithm are quite efficient in modeling and optimization of EDM process parameters.