• Tribology and Lubricants
• /
• v.18 no.6
• /
• pp.402-410
• /
• 2002

In this study, a hydrodynamic analysis of the reciprocating compressor crankshaft considering a finite bearing modelling of the journal bearings used in small refrigeration compressors is performed. In the problem formulation of the compression mechanism dynamics, all corresponding hydrodynamic forces and moments are considered using the finite bearing analysis in order to determine the crankshaft trajectory at each step. The solution of the Reynolds' equation is determined numerically using a finite difference method and a Newton-Raphson procedure was employed in solving the dynamic equations of the crankshaft. The crankshaft orbits fur the finite bearing model and short bearing theory were used to compare the effect of the hydrodynamic farces of the journal bearings on the dynamic and lubrication characteristics of the crankshaft-journal bearing system. Results show that the finite bearing model for the journal bearings must be considered in calculating for the accurate dynamic characteristics of the reciprocating compressor crankshaft.

• Computers and Concrete
• /
• v.14 no.2
• /
• pp.163-174
• /
• 2014

Damping as a material property plays an important role in decreasing dynamic response of structures. However, very little is known about the evaluation and application of the actual damping of Fiber Reinforced Polymer Confined Reinforced Concrete (FRP-C RC) material which is widely adopted in civil engineering at present. This paper first proposes a stress-dependent damping model for FRP-C RC material using a validated Finite Element Model (FEM), then based on this damping-stress relation, an iterative scheme is developed for the computations of the non-linear damping and dynamic response of FRP-C RC columns at any given harmonic exciting frequency. Numerical results show that at resonance, a considerable increase of the loss factor of the FRP-C RC columns effectively reduces the dynamic response of the columns, and the columns with lower concrete strength, FRP volume ratio and axial compression ratio or higher longitudinal reinforcement ratio have stronger damping values, and can relatively reduce the resonant response.

• Proceedings of the Korean Society of Broadcast Engineers Conference
• /
• 2009.01a
• /
• pp.207-210
• /
• 2009

An efficient algorithm to compress high dynamic range (HDR) videos is proposed in this work. We separate an HDR video sequence into a tone-mapped low dynamic range (LDR) sequence and a ratio sequence. Then, we encode those two sequences using the standard H.264/AVC codec. During the encoding, we allocate a limited amount of bit budget to the LDR sequence and the ratio sequence adaptively to maximize the qualities of both the LDR and HDR sequences. While a conventional LDR decoder uses only the LDR stream, an HDR decoder can reconstruct the HDR video using the LDR stream and the ratio stream. Simulation results demonstrate that the proposed algorithm provides higher performance than the conventional methods.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2001.10a
• /
• pp.301-304
• /
• 2001

Hot-compression tests were carried out to investigate the hot workability of Ti64 and Ti6246 alloys at different temperatures and strain rates. Processing maps were developed on the basis of the dynamic material model unifying the relationship among constitutive behavior, hot workability and microstructure development. Stable regions, defined on the basis of four stability criteria 0${\delta}log(m)/\frac{\bot}{\varepsilon})<0$, s<1 and ${\delta}log(s)/\frac{\bot}{\varepsilon})<0$, were found to be associated with dynamic recovery and recrystallization.

• Transactions of Materials Processing
• /
• v.12 no.4
• /
• pp.308-313
• /
• 2003

High temperature deformation and softening behavior of SAF 2507 super duplex stainless steel (SDSS) has been investigated in connection with an FEM analysis of hot forging process. Flow curves at various strain rates and temperatures were determined first from compression tests, and the kinetics of dynamic recrystallization were also formulated through the analysis of load relaxation test results. Using the dynamic materials theory proposed by Prasad, the deformation behavior was effectively determined for various conditions. Constitutive relations and recrystallization kinetics formulated from the test results were then implemented in a commercial FEM code. The forming load as well as the distribution of recrystallized volume fraction after forging was successfully predicted by means of the flow stress compensation formulated upon the volume fraction of recrystallization and adiabatic heating.

• Journal of Institute of Control, Robotics and Systems
• /
• v.8 no.2
• /
• pp.158-166
• /
• 2002

A mathematical model and dynamic characteristics ova continuously variable damper for semi-active suspen- sion systems are investigated. After analyzing the geometry of a typical continuously variable damper, mathematical models fur individual components including piston, orifices, spring, and valves are first derived and then the flow equations for extension and compression strokes are investigated. To verify the developed mathematical model, the dynamic response of the model are simulated using MATLAB/SIMULINK and are compared with experimental results. The proposed model can be used not only for mechanical components design but also for control system design.

• Transactions of Materials Processing
• /
• v.11 no.6
• /
• pp.538-546
• /
• 2002

Hot deformation behavior of Udiment720Li was characterized by compression tests in the temperature range of 10$25^{\circ}C$ to 115$0^{\circ}C$ and the strain rate range of $0.0005 s^{-1};to;5 s^{-1}$. The combination of dynamic material model (DMM) and Ziegler's instability criterion was applied to predict an optimum condition and unstable regions for hot forming. A dynamic recrystallization model coupled with FEM results was used to interpret the evolution of microstructures. In order to verify the reliability of the present coupled model, isothermal forging was performed in the temperature range 1050~115$0^{\circ}C$ at strain rates of $0.05 s^{-1};and;0.005 s^{-1}$. The present model was successfully applied to the hot forming process of Udimet720Li.

• Composites Research
• /
• v.31 no.5
• /
• pp.221-226
• /
• 2018

The Split Hopkinson Pressure Bar(SHPB) has been the most widely used apparatus to characterize dynamic mechanical behavior of materials at high strain rates between $100s^{-1}$ and $10,000s^{-1}$. The SHPB test is based on the wave propagation theory which was developed to give the stress, strain and strain rate in the specimen using the strains measured in the incident and transmission bars. In this study, the SHPB was directly designed and fabricated for the dynamic mechanical properties of fiber reinforced plastic (FRP) composites. In addition, this apparatus was verified for the validity by comparing the strain data obtained through the high speed camera and Digital Image Correlation(DIC) during the high strain rate compression test of the self-reinforced polypropylene composite (SRPP) specimen.

• Journal of the Korean Vacuum Society
• /
• v.12 no.2
• /
• pp.93-104
• /
• 2003

For evaluation of durability of low vacuum pumps, it is required to examine the performance and degradation of low vacuum pumps. Pump degradation may result from abnormalities associated with the performance in many areas of pump operation. The diagnostics method can be used to monitor the pump performance in the semi-conductor process line. Based on the mechanical defect of the pump, the dynamic response and reliability of the system for performance test, and the dynamic characteristics of the pump were experimentally assessed. The theoretical work rate for the compression process in the pump was calculated, and then the efficiency of the pump associated with the power consumption was evaluated. This analysis will be useful in detecting pump degradation with increasing the power consumption. To determine the predominant factors of pump degradation, it is important to evaluate the entire pumping system. We studied vibration, dynamic pressure, pumping speed, and power consumption of low vacuum pumps. Our results can be utilized for the future research on the evaluating technology of durability of low vacuum pumps.

• Geomechanics and Engineering
• /
• v.30 no.5
• /
• pp.401-411
• /
• 2022

To obtain the dynamic mechanical properties, fracture modes, energy and brittleness characteristics of Furong Baijiao coal rock, the dynamic impact compression tests under 0, 4, 8 and 12 MPa confining pressure were carried out using the split Hopkinson pressure bar. The results show that failure mode of coal rock in uniaxial state is axial splitting failure, while it is mainly compression-shear failure with tensile failure in triaxial state. With strain rate and confining pressure increasing, compressive strength and peak strain increase, average fragmentation increases and fractal dimension decreases. Based on energy dissipation theory, the dissipated energy density of coal rock increases gradually with growing confining pressure, but it has little correlation with strain rate. Considering progressive destruction process of coal rock, damage variable was defined as the ratio of dissipated energy density to total absorbed energy density. The maximum damage rate was obtained by deriving damage variable to reflect its maximum failure severity, then a brittleness index BD was established based on the maximum damage rate. BD value declined gradually as confining pressure and strain rate increase, indicating the decrease of brittleness and destruction degree. When confining pressure rises to 12 MPa, brittleness index and average fragmentation gradually stabilize, which shows confining pressure growing cannot cause continuous damage. Finally, integrating dynamic deformation and destruction process of coal rock and according to its final failure characteristics under different confining pressures, BD value is used to classify the brittleness into four grades.