Journal of Aerospace Mechanics
Year:2018 | Volume:14 | Issue:2 (52) |Papers Count:9

Parallel kinematic machines are closed-chain mechanisms with high accuracy and stiffness. Kinematics and dynamics of these manipulators are complicated due to the closed-loop structure and the constraints that exist for these manipulators. These mechanisms have some limitations like having erratic workspace, singular points in the workspace and complexity of control systems. These limitations should be studied for suitable usage of parallel mechanisms. This paper presents a novel 4-DOF parallel manipulator with 3 different linear motions and one rotational motion around x direction. Inverse and forward kinematic models of 2-PR(Pa)U-2-PR(Pa)R parallel machine tool mechanism are developed and also simulated by the code written in MATLAB. The results obtained by the mathematical model for the kinematics of the mechanism are verified using motion analysis under Solid Works. Workspace and singularity analysis are done by solving the kinematic relations and using Matlab software. To investigate the results of workspace analysis the structure has been modeled in Solidworks software and the obtained workspace have been validated using the simulation Finally the Newton-Euler method for inverse dynamic analysis of the proposed robot has been used. Therefore, moving path of the end effector, considering the various loads on it can be determined. By defining the path of platform with the specified velocity profile, inverse dynamic model simulated by the code written in MATLAB and diagram of actuator forces obtained and analyzed.

An ultrasonic transducer (probe) is an important part of an ultrasonic testing system. It comprises of a piezoelectric disk, backing material and matching layer. The process of selection and design of these components for any specific ultrasonic transducer is a challenging task. The backing material should be able to attenuate the ultrasonic waves and to reduce the ringing time of the piezoelectric disk. In this paper, a combination of metal particle (tungsten) and epoxy resin is used as backing material in fabricating transducers. The results show that the performance of the ultrasonic transducer is highly dependent on the characteristics of the backing material constituents. A high resolution or a high energy transducer can be produced just by changing the backing material.

Shock tube is an equipment in which by creating a pressure difference between driver and driven sections and bursting membrane has the ability of generating shock wave with very short rise time. The most important applications of shock tube that can be mentioned are the calibration of dynamic pressure sensors, studying of shock wave parameters and the behavior of structures and also the vital tissues under dynamic loading. In this study, manufacturing of a shock tube with the ability of increasing pressure up to 100 bar (in driver section) which is equivalent to the explosion of 21 kilograms of TNT at a distance of 3. 3 meters from the membrane’ s location was placed on the agenda. Other important manufacturing features that was considered as the abilities of this structure are making a shock wave with a maximum speed of 2. 5 Mach and also the possibility of generating a reflection wave with a pressure peak of 2. 6 MPa at the speed of 2 Mach, however by changing the type of gas and make a difference in temperature between the driver and driven sections, these values can be increased considerably. Gas driven shock tube design is based on three main parts, which eventually the structure is made in length and nominal diameter of 4. 5 meters and 3 inches respectively. To ensure the accuracy of shock tube’ s performance and to investigate the system leakage some several static and dynamic experiments by bursting of different membranes was carried out.

Nowadays, using aluminum alloys in construction of transportation vehicles to lighten final product weight and reduce fuel consumption has increased. To increases the formability of aluminum alloys and due to formability problems of these alloys in room temperature using of warm forming process is necessary. One of the forming processes at elevated temperatures is warm hydrodynamic deep drawing assisted by radial pressure. In the paper, after investigating the effect of Geometric Parameters and temperature (isothermal and non-isothermal) on thickness distribution and punch force, the effect of different parameters (temperature, media pressure and forming speed) on the limit drawing ratio was investigated. The result show that in the non-isothermal conditions, with increase in temperature, the maximum thickness reduction remained unchanged but the punch force was decreased. Additionally, the limit drawing ratio was increased with increasing temperature in the non-isothermal conditions, media pressure and forming speed and with decreasing temperature in the isothermal conditions. In this study maximum LDR is 2. 52, in non-isothermal warming condition and temperature of 250c.

Electromagnetic forming is a high energy rate forming process which is applied for manufacturing and assembly of many parts that are used in automobile and aerospace industries. In this process, the electromagnetic body forces (Lorentz forces) are used to produce metallic parts. Joining high electrical conductivity parts by using electromagnetic forming process is as an innovative method. In this research work, the effect of important parameters of process such as discharge voltage, radius and width of rectangular groove on the strength of assembled products were studied by using finite element method and design of experiment. After introducing the governing equations, the output of these equations were applied in simulation as a pressure on work-piece. In this simulation, an axisymmetric model was used in analysis and Johnson-Cook theory was applied due to high strain rate to show the plastic behavior of materials. Finally, the numerical results were compared with the results reported by other researchers. As a result, the contact surface at bottom of the groove increases with the increase of the voltage energy and more filling groove, increasing strength of joint. Also Strength of joint increases, due to create partial shearing of the tube at the groove edge and interference stresses at the tube and mandrel interface.

In spite of increasingly utilizing high speed machining (HSM), the conventional tool path generation methods which are based on linear/circular blocks by applying standard strategies like parallel, z constant and Iso are usually employed in practice. Even though these tool-paths are employed by non-linear interpolators they have inherent limitations for HSM applications. This paper presents a new method for tool path generation in terms of NURBS. In order to increase the continuity of machining in HSM, a helical topology tool path in terms of NURBS is developed. This method reduces the number of CNC blocks up to five times. This algorithm creates a 2D-horizontal guide plane by offsetting the contour edges of the design surface. After that the inside offset of the contour is generated at a computed distance and then the contour and all its offsets are projected on the CL-surface. Finally, the tool-paths are approximated with NURBS approximation algorithm. The algorithm is designed to minimize the number of control points. An example of proposed approach consisting the comparison of the conventional method and verification is given.

In this study, ultrasonic guided waves have been generated by means of a system developed for producing the desired signal needed for excitation of guided waves and a piezoelectric sensor. After propagating, waves are measured using an ultrasonic probe and recorded via a digital oscilloscope. Subsequently, the received signals have been processed and filtered for de-noising in order to be compared. As investigating the parameters influencing the inspection ability of these waves, the receiving probe has been moved along the pipe and by changing number of input signal pulses and sweeping the frequency; signals have been recorded at the frequency in which amplitude is maximum. By sweeping the frequency in the range of 30 to 40 kHz, it is possible to receive an appropriate signal in each point of the test piece.

Tube-shaped equipment is among the most applicant titanium alloys which are widely used in chemical, Marine, and aircraft industries. Most of the titanium tube-shaped products are made of commercially pure titanium or low strength titanium. One of the greatest limits in manufacturing titanium products is low formability of titanium in ambient temperatures. The process of hot tube gas forming is one of the promising methods used to manufacture the tube-shaped products. In the current study, finite element and experimental process of hot tube gas forming were investigated to produce a square cross section product from a commercially pure titanium tube (Grade 2) in 840 ° C. The effect of gas pressure increase rate on forming the tube was also investigated. The results obtained through different experiments showed that the reduction of gas pressure increase rate in tubes, not only provide sufficient time to temperature distribution and uniform forming of products, but also decrease the strain rate in tube walls and consequently decrease the probability of tube tearing. The microstructural investigation revealed that recrystallization occurred during the forming process and final product contains coaxial grains.