نشریه مهندسی مکانیک مدرس
سال بیست و دوم شماره 6 (خرداد 1401)

Machining of hard workpieces is one of the most important challenges of the manufacturing industry. Hence, new methods were added to traditional machining. Ultrasonic vibration machining is one of these methods. The advantages of using ultrasonic vibrations compared to traditional machining include reducing machining forces, reducing tool wear and friction, increasing tool life, creating intermittent cutting conditions, increasing surface quality, and so on. To vibrate the tool, a horn with a resonant frequency of 20,633 Hz was analyzed by Abacus software. In this study, the effects of cutting speed, feed rate, conventional machining conditions, and vibration machining conditions at three different hardness of 15, 30, and 45 Rockwell C for the workpiece on surface roughness and tool wear were evaluated. The experiments were designed at full factorial, and a total of 54 experiments were performed. The results showed that at higher workpiece hardness by applying vibration the surface roughness was reduced. The surface roughness (Ra) in machining by means of ultrasonic vibrations is up to about 36% less than conventional machining in various machining parameters. In addition, the temperature in vibration machining is lower about 15% at higher stiffness of the workpiece. Also, with the increase in the hardness of the workpiece, the tool wear was increased, which is less by applying ultrasonic vibrations. Also, by applying vibrations, tool wear was reduced in total, which can be minimized by selecting cermet tools and applying vibrations in 4140 AISI steel machining.

In this study, bending of titanium tubes using steel balls in 0.5 and 0.85 mm sizes and resistance heating with experimental and simulation methods have been investigated. In order to apply temperature in rotatory draw bending of tubes, electric current cables were connected to both sides of the tube, and experiments were performed at room temperatures, 100℃, 200℃, 300℃ and 400 ℃ with a bending ratio of 1.8 and a bending angle of 90 ° was done. After the experiments, cross-sectional distortion, wrinkles, cracking and thickness distribution of bent tubes were investigated. The results of this study showed that in the case of bending at room temperature with and without metal balls, the tubes could not be bent. In the bending process with a constant speed of 0.8 Rad/s, by placing metal balls inside the tube and increasing the temperature 100℃, 200℃ and 300℃, the thickening in the intrados of ​​the bent tube decreased by 9.8% and the thinning at the extrados of the bent tube increased by 8.4%. Also, by changing the bending speed from 0.8 to 0.4 Rad/s the cracking defect was eliminated at 400℃. Due to increased pressure due to steel balls in bending area, cross section distortion in tubes decreased by 10.4%. The best bending conditions and the least amount of defects were obtained at 300℃ with steel balls.

Equal Channel Angular Pressing (ECAP) is one of the methods of refining and fine-graining metal materials. In this research, ECAP operation was performed on samples of 5182 alloy in 1 to 4 passes at ambient temperature. After implementation of the specimens through ECAP, prepared to obtain mechanical properties such as hardness, tensile and metallography. The results of these experiments showed that the mechanical properties of the packed materials through ECAP have improved compared to the normal state. Using a scanning microscope, it was observed that the average grain size decreased from 131 μm in the initial state to 745 nm after the ECAP process after the fourth pass. The results of hardness test also showed a 213% increase compared to normal.  The increase in yield stress after 4 passes is about 3 times. Finally, the crack growth of these materials under fatigue loading was compared with the non-ECAP mode by creating a suitable pre-crack. It was observed that crack growth is faster in ECAP materials and the failure surface is smoother compared to normal. Also, the deviation of the crack from its path in microstructure materials is less than normal. Finally, by comparing the Experimental results of crack growth with the results of numerical analysis, the accuracy of the numerical results is validated and confirmed.

In this research, the dielectric barrier discharge plasma driven channel flow with the applied magnetic field has been proposed for use as a thruster in propulsion applications and studied experimentally. Measurements of the thrust and consumed power of thruster for different values of the barrier thickness have been performed and the data have been compared with the corresponding ones without magnetic field. It is found that consumed power and thrust of the thruster in the presence of magnetic field are respectively little reduced and increased than that without the magnetic field. The measurements show that the effectiveness increases to a maximum and then drops as the operating voltage monotonically increases over a range from 12 to 26 kV. A power law analysis for revealing the relationships among the effectiveness, thrust, consumed power, and operating voltage has been presented for the thruster with and without the magnetic field. It is seen that the applied magnetic field and thicker dielectric barrier can lead to a higher effectiveness at the point of transition from the glow regime to the filamentary regime. The effects of micro-discharge channels on the effectiveness in the both regimes have been discussed. The observations indicate that in the presence of magnetic field, the additional micro-discharge channels are generated and develop along the magnetic field lines and the diffuse background emission of the discharge is stronger in plasma. The underlying physical mechanisms of mentioned phenomena have been explained and mainly ascribed to the enhanced ionization by applying the magnetic field.

To use higher capacities in Iran's energy transmission systems, API standardized pipes made of API X65 steel have been utilized (made of thermo-mechanically controlled rolling process, TMCR steels). The TMCR inherently increases the anisotropic properties of steel coils and plates used for pipe manufacturing. In addition, the production of helical welded pipe involves steps that can lead to different mechanical properties in different directions. The aim of the present study is to measure the orientation dependence of the Charpy fracture energy. Therefore, the effect of changing the angle of specimens relative to the rolling direction and also the effect of changing the notch orientation (three notch A, B and C in total) on the fracture energy in API X65 steel has been experimentally determined. The maximum change in the average Charpy fracture energy at different angles relative to the rolling direction is a maximum of 13% (in notch B), but the largest change in the average Charpy fracture energy between different notches is a maximum of 12.2% (at an angle of 0 °). As a result, the effect of changing the angle of the specimen relative to the rolling direction is greater than the effect of changing the notch orientation on the Charpy fracture energy. Also, at an angle of 67.5 degrees to the direction of rolling (equivalent to the diagonal direction (D-D)), the most fracture energy for all notches was obtained. To quantitatively compare the fracture energy changes in different notches, an index called anisotropy index has been presented.

One of the most proven ways to reduce satellite launch vehicles fuel consumption is to use multistage separation systems. These separation systems based on mechanism of operation divided into two category, explosive and non-explosive. Nowadays, non-explosive separation systems have been considered due to their features such as ease of maintenance, reduction of explosion hazards and reduction of shock to payload. One type of non-explosive separation systems is separation systems based on CBOD (Clamp Band Opening Device) releaser. In this study, changing the dimensions (geometry and thickness) of the flywheel which is one of the effective parameters in the CBOD release performance, have been investigated. Adams software (ADAMS) was used for simulation and the Results have been validated using experimental test on sample made by 3D printer. The parameters of separation time, linear acceleration of Counter-clockwise screw along separation axis and angular acceleration of flywheel around separation axis has been selected as output parameters. It was found that the increase in diameter and thickness of the flywheel is directly related to the separation time and inversely related to the angular acceleration of the flywheel around the separation axis and the linear acceleration of the Counter-clockwise screw along separation axis. Also, with increasing moment of inertia and mass of the flywheel, the amplitude of acceleration fluctuations has decreased.