JOURNAL OF SOLID AND FLUID MECHANICS
Year:2017 | Volume:7 | Issue:3 |Papers Count:15

Welding residual stresses, especially longitudinal ones, have a significant role in the fatigue and fracture of the structures. The common residual stress measurement methods have an important restriction that they are incapable in measuring the residual stresses versus depth. In this study, the combination of FEM simulation and X-ray diffraction experimental method has been used for residual stress measurement along specimen depth. Residual stresses were induced by the TIG welding of two 5086 aluminum plates with a thickness of 8 mm. The welding process was simulated in Abaqus and then the simulation results were validated with X-ray diffraction ones. It was observed that in weld centerline, by increasing depth, maximum longitudinal residual stresses decreased by 37 MPa while this reduction in heat affected zone and in the distance of 10 to 25 mm from the weld centerline was 70 MPa. At farther points from the weld centerline, the longitudinal residual stresses decreased and the compressive residual stresses were formed in the specimen depths. With respect to significant variation of residual stress along the depths of the specimen, residual stress measurement along specimen depths seems to be necessary for performing accurate analysis.

Shape Memory Alloys (SMAs) are a group of alloys, which have distinct and superior properties compared to conventional alloys. They demonstrate two particular non- linear stress- strain behavior, Shape Memory Effect and Psedoelasticity (Superelasticity). In this paper, we have studied transverse vibration and buckling of pretwisted cantilever SMA tubes subjected to the phase change. All analyzes were done using the ABAQUS finite element software. Since the modeling of SMA is not available in ABAQUS software, the UMAT subroutine in the software is used to implement the three SMA models, Boyd & Lagoudas, Liang & Rogers and Tanaka, to FE analysis. Preparing a user material subroutine enables the designer to analyze any structure that made of shape memory alloys in the software. The effect of internal and external diameter of the constant weight tube on phase change delay, critical buckling load and natural frequency is discussed.

In this paper, a truck seat backrest is proposed which can be tuned the backrest stiffness (using a special air bag) and torsional backrest stiffness with respect to the driver specification. This driver based tuning can reduce the fore-aft vibration of the human body. The coefficients of the backrest stiffness and the torsional backrest stiffness are optimized by full searching method in the proper intervals. For optimization cost function calculation, the fore-aft vibration transmissibility function in the range of 0.1 to 20 Hz are determined in ADAMS/ Vibration using ADAMS/ Insight toolkit. The root mean square of vibration transmissibility function is weighted using a frequency filter by MATLAB software. The frequency filter is introduced in ISO 2631-1: 1997 standard for the fore- aft vibration of a seated human body. This weighted RMS is assigned as the cost function of the stiffness optimization. The human body and truck seat model is augmented in ADAMS/Truck on a full truck model. As a case study, the truck simulation results are compared for a conventional seat and an optimized seat in a section of a garbagetruck driving cycle, when the driver's weight is 73 kg. The optimized seat backrest has 35% lower fore and aft vibration RMS than the conventional truck seat. Then this Procedure is Repeated for drivers have 40 kg and 100 kg Mass.

Now piezoelectric vibrational energy harvesters can produce more than 300 microwatts per square centimeter and this has led to a suitable method for power energy harvesting for electronic devices with low power problem. The use of new technology and materials in manufacturing energy harvesting devices can greatly fix the problems in the current energy harvesting structures. In this work, using EAPap piezopolymer films, the cantilever energy harvesters are made and are split into equal parts with smaller widths. Once a beam is divided into two equal parts and once again is divided into three equal parts. Then the smaller cantilevers are connected together in series and parallel and the effect of binding were also noted. It was observed that splitting a beam into several beams with less width and connecting the beams in series or parallel, can increase the current, voltage and the resulting power output. Parallel connection create more current flow and series connection, causes more output voltage.

In this paper, the effect of friction stir spot welding process parameters in AA 7075-T6 sheets has been investigated. Also, the effect of different rotational speeds and plunge depths of the tool with different backplate materials on shear failure load of the welding area has been studied. Joints were created in two rotational speeds of 800 and 1600 (rpm). Also, different plunge depths of 55.3 5.5, 5.7 and 5.9 (mm) with two backplate of AA1100 and Ti6Al4V were used. Shear strength test was performed to evaluate the quality of the weld. Results indicated that the backplate material has a direct effect on the shear strength and geometry of the stir zone. In rotational speed of 1600 (rpm) and by using Ti6Al4V backplate with increasing plunge depth, upper sheet of the base metal zone has been teared. Also, it was found that high rotational speeds is caused a drop in shear strength of welds compared with low rotational speeds. Maximum load for shear failure obtained in 800 (rpm) and 5.9 (mm) with Al1100 backplate.

The chatter phenomenon is a kind of unstable self-excited vibration which occurs in most machining processes including grinding. A 3-D nonlinear model of chatter vibrations in grinding process has been presented in this paper. The workpiece has been modeled as a continuous rotating shaft with transverse vibrations and wheel is regarded as a one degree of freedom damped spring-mass system. The equations of motion have been dimensionless by p-Buckingham theory. Then using assumed modes method, partial differential equations of workpiece changed to ordinary differential equations and then the method of multiple scales is adopted for the approximate solution. Stability regions for workpiece and wheel in various workpiece rotary speeds and various wheel positions drawed and then the effect of various parameters such as rotational speed of the wheel, and the radius of the workpiece have been investigated. Results showed that vibrations in workpiece and in wheel are quite different from each other. When grinding occurs in the middle of the workpiece the possibility of chatter phenomenon is more, while the position of wheel has no effect on the vibration of the wheel.

In this paper, the issue of robust control of the spacecraft formation flying will be discussed. For this purpose two high order sliding mode controller (HOSMC) are designed for nonlinear spacecraft formation flying in leader-follower structure. The SMC- super twisting algorithm is applied due to chattering reduction and so SMC- nonsingular terminal is applied due to finite time convergence. In the design of both controllers, the actuator saturations is considered.To reduce the control effort, the adaptive gains are applied. The gains of robust controllers are adapted due to the error measurement or conditions. Both adaptive super twisting and terminal sliding mode are proposed on the PROBA-3 Space mission. The controllers' performances are examined in formation flying maneuvers with large eccentricity considering uncertainty and J2 disturbance.

In this manuscript, the mechanical properties of hybrid laminates with content of Epoxy/ Carbon Fiber/ clay/ TiO2 and Epoxy/ Carbon Fiber/ clay/ CNT have been investigated separately and comparative. For this purpose, the samples with and without nano particles: (Epoxy/ CF) and (Epoxy/ CF/Clay/ Tio2 & Epoxy/ CF/ Clay/ CNT) have been fabricated. In this study, in order to decrease the number of experiments, costs and optimum tensile and bending strength, Taguchi as one of experimental design methods is employed. The results showed that the samples with 0.5 wt% of clay, 1 wt% TiO2 and 0.5 wt% CNT have the highest tensile and bending strength.

Hybrid Reinforced Concrete (HRC) is a composite material with a combination of rebars and fibers to reinforce concrete. In this study, the behavior of HRC beams under flexure is studied. To model HRC, steel and concrete under compression were assumed to be elastic-perfectly plastic. For concrete under tension, a three-linear model including elastic zone, transition zone, and residual tensile strength zone were used. Analytical relations to determine the depth of neutral axis and moment-curvature diagram at each level of loading were studied. In order to verify the model, the results obtained from the model were compared with those of published papers. It was realized that the relations obtained from this study, can predict the behavior of Fiber Reinforced Concrete (without rebar reinforcement), HRC, and traditional Reinforced Concrete under flexure with good accuracy. The proposed model, could predict the maximum load carrying capacity of HRC beams with an accuracy of more than 93% compared to experimental results. Taking into account the mentioned assumptions and proposed procedure it was found that the model is capable of predicting the behavior of all types of cement based composites: materials with strain- hardening, strain- softening, deflection- hardening and deflection- softening behavior.

In this paper, the energy absorption performance of double- walled structures with reinforcements that joint the inner and outer members together were studied under axial and oblique quasi- static loading. These structures were assumed to have square and circular cross- sections. In the first step of this research, the finite element simulations performed in LS-DYNA were validated by comparing with experimental results. This validated code was then used to simulate energy absorption behavior of the double-walled square and circular structures with different scales (0, 0.25, 0.5, 0.75, and 1) under different axial and oblique quasi- static loading. In the second step, a multi- criteria decision- making method namely complex proportional assessment (COPRAS) was employed to select the best possible structure in view point of crashworthiness. The results demonstrated that the cross-sections with the scale of 0.5 were the best energy absorbers. Moreover, the circular cross-section was found to be the best energy absorber due to having higher number of sides. In addition, both energy absorption and peak crushing force reduced by increasing the loading angles. Finally, the response surface (RSM) and central composite design methods were used to optimize the design parameters including the thickness and radius of the double- walled circular structure.

Underwater blast is a destructive phenomenon for offshore and onshore structures, so it is highly important to design these structures to bear explosive loads. This study has evaluated the performance of fiber reinforced concretes against air blast and underwater blast loadings. For this puropse, the performance of fiber reinforced concretes against explosive loading was analyzed while including the strain rate effect. In this way, two numerical models CONWEP explosion function and arbitrary Euler- Lagrange method were used for air blast, and results were then compared with data from full- scale blast test. A single degree of freedom system was used to validate data for underwater blast obtained through the arbitrary Euler- Lagrange method. Panels displacement results showed that the CONWEP, Euler- Lagrange and single degree of freedom system numerical modeling methods had a difference smaller than 13.3% with experimental results. Additionally, results showed underwater blast led to larger displacements in reinforced concretes up to about 74 mm and more destructive damages to concrete panels than air blast. The amount of rebars had a direct contribution compared to the fiber percentage.

The noisy environment of the underwater nonlinear under-actuated dynamic system of the Autonomous Underwater Vehicle (AUV) turns out the design of the self-tuning controller, more challenging. In this paper, the Model Reference Adaptive Control (MRAC) along with the Artificial Neural Network (ANN) compensator of the 6 Degree of Freedom (DOF) AUV is illustrated.4 Input-6 Output (4I6O) nonlinear under-actuated dynamic system is divided into first, 4 subsystems and the partial or inverse linearization technique and the coupled linearized model are employed for each one. The stability of the closed- loop subsystems, and hence the complete controlled model is insured according to the Lyapounve's stability theory. To increase the robustness of the closed loop system, an ANN compensator benefiting online backpropagation learning algorithm to tune the network's parameters is incorporated with each controllers. The results of the simulations of the hybrid MRAC along with ANN compensator in Matlab Simulink environment, clearly indicates the outperformance of the ANN compensated control method versus its non-ANN compensated counterpart in terms of increasing the robustness as well as more accurate trajectory tracking performance of the control system subjected to the continual applied noises for both coupled and decoupled dynamical systems.

In this paper, the energy equations of a manipulator, including two flexible links, is extracted based on the Euler- Bernoulli beam assumptions. The robot can carry a specified mass as the load on its end- effector. Applying assumed modes method and considering a finite number of modes, the dynamical motion equations of the arm is achieved using Euler- Lagrange equations. The control and the vibration damping of the arm links is implemented using an active controller. This matter is done by applying a specified voltage on two considered piezoelectric layers, the embedding location of which is external layers of beam. The algorithm of vibration control is designed according to the Lyapunov's second method. In addition, the path planning of the end- effector is performed by making use of an equivalent two link rigid arm. Regarding the fact that the robot weight is included as well, and also it starts its own motion from the rest, the static deflection is calculated from Castigliano's method and is used as the initial conditions of beam. The performed simulation shows that the path of the controlled motion of flexible robot follows that of equivalent rigid manipulator end-effector with a good approximation.

In the present paper, three dimensional turbulent mixed convection inside the ventilated cavity is solved using by Large Eddy Simulation (LES) and the results are validated with available numerical and experimental ones. In the present work, the forced convection and free convection are due to the injected flow from ventilation system and temperature difference among floor and other walls of cavity, respectively. The effects of obstacle on flow characteristics such as velocity, temperature and turbulent kinetic energy are investigated for cavity flow containing obstacle with three different height. In continuation, effect of obstacles on coherent structures are investigated by applying the proper orthogonal decomposition (POD) algorithm on velocity fluctuation field in x direction. From the results, firstly, obstacle causes to increase the energy of coherent structures, and secondly, obstacle with large height break the coherent structures to smaller ones, but for obstacles with medium and small height dominant structures are enlarged.

In the current research, a CFD simulation has been carried out to investigate the simultaneous effects of injection pressure and Exhaust Gas Recirculation (EGR) on the engine performance and the amount of pollutant emissions in a modern High Speed Direct Injection (HSDI) diesel engine. For this purpose, the computational results have been firstly compared to the measured data and a good agreement has been achieved in regard to predicting the in-cylinder pressure and the amount of NOx and soot emissions. Then, the effects of injection pressure has been studied at the various EGR rates. The results show, in the case of a constant injected fuel and without using the EGR, decreasing the nozzle diameter causes increase of Indicated Mean Effective Pressure (IMEP) and reduction of Indicate Specific Fuel Consumption (ISFC). However, the amount of NOx emission has been largely increased and the amount of soot emissions has been decreased. By applying the different percentage of EGR rates, a potential to decrease the amount of NOx emission have been provided while the engine operating conditions (including the ISFC and IMEP) remain constant. However, the increase of soot emissions should be considered as a negative parameter in this case.