We designed and validated a rotary magnetorheological (MR) damper with a specified damping torque capacity, an unsaturated magnetic flux density (MFD), and a high magnetic field intensity (MFI) for unmanned vehicle suspension systems. In this study, for the rotary type MR damper to have these satisfactory performances, the roles of the sealing location and the cover case curvature of the MR damper were investigated by using the detailed 3D finite element model to reflect asymmetrical shapes and sealing components. The current study also optimized the damper cover case curvature based on the MFD, the MFI, and the weight of the MR damper components. The damping torques, which were computed using the characteristic equation of the MR fluid and the MFI of the MR damper, were 239.2, 436.95, and 576.78 N·m at currents of 0.5, 1, and 1.5 A, respectively, at a disk rotating speed of 10 RPM. These predicted damping torques satisfied the specified damping torque of 475 N·m at 1.5 A and showed errors of less than 5% when compared to experimental measurements from the MR damper manufactured by the proposed design. The current study could play an important role in improving the performance of rotary type MR dampers.
During the expansion forming of aluminum tube, the efficiency of heat exchanger diminishes due to the adhesion of groove and expansion ball inside the tube. Despite its importance, a limited number of researches on the adhesion problems in aluminum tube expansion have been published. This study aims to analyze the adhesion occurring during the expansion forming of aluminum tube for heat exchanger and to identify its location. For this, the method of using the statistical analysis of geometry by image processing and the slope of force measured from expansion forming was suggested. The new method discovers the adhesion location from the standard deviation of groove height measured before and after expansion of tube and the differentiation of force measured from expansion forming. To prove this method, the area with deviation of groove height above average was discovered, and it was confirmed that from the actual expansion of tube cross-sectional images, the height of some grooves was abnormally shorter due to adhesion. Also, from this method, it was confirmed that the changes in differentiation of force occurring from the expansion of the tube also include the information on adhesion location.
The purpose of this study is to analyze the differences between the effects of robot welding and manual welding on the low- and high-cycle fatigue lives of the weld zones for T-shaped weld structures fabricated from SM50A carbon steel using a CO2 gas arc welding method. Fatigue tests were conducted using a three-point bending method, and the S-N curves of the manual welding and robot welding crossed each other at approximately 3 × 104 cycles. The robot welding weld zone had better high-cycle fatigue lives than the manual welding. The results are attributable to the fact that the more uniform and higher welding speed of the robot welding leads to smaller weld zone area (i.e. ~12% smaller than the manual welding) and also smaller grain size than the manual welding. Because a smaller grain size in the robot welding weld zone results in a higher hardness than the manual welding and material brittleness increases with increasing hardness, the robot welding weld zone shows better high-cycle fatigue lives but poorer low-cycle fatigue lives than the manual welding.
The purpose of this study is to identify the causes of the continuous failure of center-link chains for a trolley conveyor system by analyzing their failure characteristics under static and dynamic loading conditions and to propose methods for securing their mechanical integrity. For this purpose, center-link chains were minimally processed to prepare testing specimens with their original surface property and structural dimension maintained. These specimens were used to measure the tensile strength (1172.1 MPa), elongation (8.7%), yield strength (1073.4 MPa), hardness (44.1 HRC), and surface roughness (R a = 10.5 μm). A three-point bending fatigue test (R = 0.1) resulted in a fatigue limit of 746.7 MPa. The results confirmed that the rough surface of the center-link chain caused a reduction in the elongation and fatigue limit and an increase in the scattering of the surface hardness and fatigue life, which were the direct causes of frequent chain failure in the field. To improve the fatigue life of the center-link chain, two methods were reviewed in this study. The first method was to reduce its surface roughness through surface machining of the center-link chain, and the second was to modify its shape design to allow improved structural integrity, even if the current surface roughness is retained. In the case of the first method, the results of the fatigue test using the specimen with reduced surface roughness (from R a = 10.5 to 0.9 μm) by surface machining demonstrated reduced scattering and increased fatigue limit (from 746.7 to 920.3 MPa). As for the second method, a stiffener was added and two design variables (slope angle and fillet radius) of the stiffener were selected to propose a new design that can reduce the maximum stress ~ 1.6 times compared to the conventional design. While both methods for improving the structural integrity of the center-link chain were effective enough to improve the fatigue life of the chain, the second method, which requires only the initial mold manufacturing cost for modifying the shape design of the chain, was finally selected, because it exhibited better economic feasibility than the first method that increases the product cost and overall process time due to addition of the surface machining process.
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