An analytical solution for nonlinear dynamics of a viscoelastic beam-heavy mass system

The aim of this study is to develop an approximate analytic solution for nonlinear dynamic response of a simply-supported Kelvin-Voigt viscoelastic beam with an attached heavy intra-span mass. A geometric nonlinearity due to midplane stretching is considered and Newton’s second law of motion along with Kelvin-Voigt rheological model, which is a two-parameter energy dissipation model, are employed to derive the nonlinear equations of motion. The method of multiple timescales is applied directly to the governing equations of motion, and nonlinear natural frequencies and vibration responses of the system are obtained analytically. Regarding the resonance case, the limit-cycle of the response is formulated analytically. A parametric study is conducted in order to highlight the influences of the system parameters. The main objective is to examine how the vibration response of a plain (i.e. without additional adornment) beam is modified by the presence of a heavy mass, attached somewhere along the beam length.     

Buckling limit evaluations for reactor vessel of ABTR

We evaluated the buckling limit of a conceptually designed reactor vessel of the Advanced Burner Test Reactor (ABTR) for a horizontal safe shutdown earthquake (SSE) seismic load. In this evaluation, both seismic isolation and non-isolation designs were considered for thin reactor vessels subjected to elevated temperature services. For calculating the buckling load, two kinds of methods, a numerical simulation method using finite element analysis and an evaluation formula driven from a theoretical basis, were used. To consider the material aging effect caused by a 60-year design lifetime and 510°C normal operating temperature, an isochronous stress-strain curve corresponding to these conditions was used for the nonlinear elastic-plastic buckling analysis method. From the evaluation results of the buckling load, it was found that plasticity behavior significantly affects the buckling strength, but that the initial geometrical imperfections have little effect. Also, the non-seismic isolation design does not satisfy the buckling limit rules of the ASME BPV Section III, Subsection NH, but the seismic isolation design does satisfy it with sufficient margins.     

Influences of post weld heat treatment on fatigue crack growth behavior of TIG welding of 6013 T4 aluminum alloy joint (Part 1. Fatigue crack growth across the weld metal)

The present study evaluates the influences of PWHT on FCG behavior and tensile properties of TIG butt welded Al 6013-T4 sheets. Crack propagation tests were carried out on compact tension (CT) specimens. The T82 heat treatment was varied in three artificial aging times (soaking) of 6, 18 and 24 hours. The results of T82 heat treatment with artificial aging variations were tested for their fatigue crack growth rates at the main metal zone, the heat-affected zone (HAZ), and the welded metal zone. It has been observed that the various agings in heat treatment T82 are sensitive to the mechanical properties (fatigue crack growth rate test, tensile test). The results show that PWHT-T82 for 18 hours aging is the highest fatigue resistance, while the aging 18 hours provided the highest tensile test result.     

Inverse estimation of properties for charring material using a hybrid genetic algorithm

Fire characteristics can be analyzed more realistically by using more accurate material properties related to the fire dynamics and one way to acquire these fire properties is to use one of the inverse property estimation techniques. In this study an optimization algorithm which is frequently applied for the inverse heat transfer problems is selected to demonstrate the procedure of obtaining fire properties of a solid charring material with relatively simple chemical structure. Thermal decomposition is occurred at the surface of the test plate by receiving the radiative energy from external heat sources and in this process the heat transfer through the test plate can be simplified by an unsteady one dimensional problem. The input parameters for the analyses are the surface temperature and mass loss rate of the char plate which are determined from the actual experiment of from the unsteady one-dimensional analysis with a given set of eight properties. The performance of hybrid genetic algorithm (HGA) is compare with a basic genetic algorithm (GA) in order to examine its performance. This comparison is carried out for the inverse property problem of estimating the fire properties related to the reaction pyrolysis of some relatively simple materials; redwood and red oak. Results show that the hybrid genetic algorithm has better performance in estimating the eight pyrolysis properties than the genetic algorithm.     

Friction and Wear Properties of MoS<Subscript>2</Subscript>-Overcoated Laser Surface-Textured Silver-Containing Nickel-Based Alloy at Elevated Temperatures

The solid lubricant that is coated on a flat surface is easily removed during friction. Surface texture dimples, which act as reservoirs of solid lubricant, can prolong the wear life of solid lubricant films. We textured silver-containing nickel-based alloys by a pulse laser and filled the micro-dimples with molybdenum disulfide powders. The tribological properties of the alloys were tested by rubbing against alloyed steel on a ring-on-disk tribometer at temperatures ranging from room temperature to 600°C . After laser surface texturing, the friction coefficients of the silver-containing nickel-based alloy smeared with molybdenum disulfide powders were reduced at temperatures ranging from room temperature to 400°C. With increasing dimple density, the wear life of the MoS₂ film increased while the wear rate of the nickel-based alloy decreased. The wear life of the textured surface with a dimple density of 11.2% exceeded 10,000 m at room temperature. We conclude that molybdenum disulfide and its oxides stored in the micro-dimples play a role in lubrication at room temperature and high temperatures, respectively.     

Analytic study on structural behavior of the string bed in a tennis racket

The structural behavior of string bed of tennis rackets was investigated subjected to transverse force perpendicular to the string bed. The mathematical model developed for the string bed was implemented into a computer programming code. This code was used to conduct extensive parametric studies on the structural behavior of the string bed for various parameters, including string tension, axial rigidity of the string, string spacing and head size. The analysis results showed that while the transverse stiffness of the string bed is proportional to the string tension, the transverse stiffness of the string bed is inversely proportional to string spacing and head size. In addition, the axial rigidity of the string significantly amplifies the transverse stiffness of the string bed for relatively large transverse deflection of the string bed.     

Calculation of water droplet impingement using the coupled method of rigid body dynamics and the moving particle semi-implicit method

When a liquid droplet impacts on a solid surface, it recoils to the center of that surface after reaching its maximum spreading diameter. The mechanism of droplet recoiling is not fully understood. To simulate this recoiling of a droplet, a particle method is a good choice because it does not require grids for simulating fluid motions, and can easily handle a large deformation of fluid. In this study, the coupled method of rigid body dynamics and the moving particle semi-implicit (MPS) method (Park and Jeun, 2011) was used to calculate three-dimensional droplet impingement. Also, the previous surface tension model for MPS (Nomura et al., 2001) was revised to get a more realistic surface tension force. A two-step calculation was performed. In the first step, a MPS calculation was performed with particles that were considered to have no mass or volume. In the second step, rigid body dynamics came into the calculation and considered the diameters of particles being slightly lesser than the initial distance between particles. In this study, the calculated results were compared with the measured data (Kim and Chun, 2000) and the recoiling lengths of droplets for the various initial impingement speeds were estimated.     

Numerical study on the hydrodynamic control derivatives of a high-speed underwater vehicle with X-stern configuration

A numerical study using a computational fluid dynamics approach is conducted on the control effectiveness of a high-speed underwater vehicle with an X-stern configuration. The calculation method is verified by comparing its results with the calculation results and measurements of control derivatives of an underwater vehicle with a cruciform stern configuration. The results of the study show that the control effectiveness of a high-speed underwater vehicle is improved by about 40% in terms of control derivatives, when the cruciform stern configuration is replaced with an X-stern configuration.     

Enhanced friction model for high-speed right-angle gear dynamics

The modeling of elastohydrodynamic lubrication friction and the analysis of its dynamic effect on right-angle gears, such as hypoid and spiral bevel types are performed in the present study. Unlike the classically applied empirical constant coefficient of friction at the contacting tooth surfaces, the enhanced physics-based gear mesh friction model is both spatial and time-varying. The underlying formulation assumes mixed elastohydrodynamic lubrication (EHL) condition in which the division and load distribution between the full film and asperity contact zones are determined by the film thickness ratio and load sharing coefficient. In the proposed time-varying friction model, the calculation of friction coefficient is performed at each contact grid inside the instantaneous contact area that is being subjected to mineral oil lubrication. The effective friction coefficient and directional parameters synthesized from the net frictional and normal contact forces are then incorporated into a nonlinear time-varying right-angle gear dynamic model. Using this model, the effect of friction on the gear dynamic response due to the transmission error and mesh excitations is analyzed. Also, parametric studies are performed by varying torque, surface roughness and lubrication properties to understand the salient role of tooth sliding friction in gear dynamics. The simulation results are included. But experimental verification is needed.     

Thermal analysis on the letter mark spot of the corvette Cheonan-hit torpedo

Dispute is going on over the letter mark on the torpedo that is believed to have sunk the Korean corvette Cheonan in March, 2010. This research is made to investigate if the spot has experienced temperature high enough to burn, which can be proof of later fabrication. Conditions to maximize the heating are introduced and the temperature is calculated numerically as the explosion ‘bubble’ expands underwater, the gas stream heats the disk front surface and heat is conducted to the letter-written rear surface. Results show less than 20°C rise everywhere in the torpedo propulsion part and essentially no temperature rise on the letter mark over extensive tests. Therefore, the assertion that the letter mark should have been burnt turns out to be fallacious.