Nam-Gyu Park

Vibration of Initially Stressed Beam with Discretely Spaced Multiple Elastic Supports

Vibration behavior of an initially stressed beam on discretely spaced multiple elastic supports has been studied and a theoretical formulation of the system is derived using the variational principle. Unlike beams on an elastic foundation, discretely spaced supports can distort the beam mode shapes when the supports have rather large stiffness, i.e. usually expected beam modes cannot be obtained, but rather irregular mode shapes are observed. Conversely, irregular modes can be recovered by changing initial stress. Since support location is closely associated with the dynamic characteristics, this work also discusses eigenvalue sensitivity with respect to the support position and some numerical examples are investigated to illustrate the above findings.

Preliminary Study on Optimization of a Bulge Tool for Nuclear Fuel Manufacturing

A nuclear fuel assembly, manufactured through several processes, is composed of a range of components. Guide thimbles and spacer grid sleeves, among others, are particularly important to maintain the integrity of fuel assembly, and they are connected through a bulge forming process. The purpose of the bulge process for fuel assembly is to connect guide thimbles to spacer grids. The connecting load between tubes is affected by bulge joint strength, and this strength depends on bulge design. While reaching to a specified load, bulge equipment endures high working loads during the working process, and a cracked tool can be seen infrequently. The equipment consists of a bulge tool and a taper pin, and the neck of the bulge tool is most susceptible to damage. A crack may appear in fuel assembly when the bulge tool is broken during the bulge process. The optimization of bulge equipment is difficult because the bulge process has geometric nonlinearity, boundary nonlinearity, and material nonlinearity. The work velocity of the bulge process for nuclear fuel manufacturing is relatively slower than that of general forming processes, but a stroke is very important. Therefore, nonlinear analysis should be required in the optimization process. In this research, the design of experiments using an orthogonal array and the finite element analyses are employed to determine the optimal shape and material. Design variables are the material and three types of local shapes of a bulge tool, and the level of the design variables is three. The objective of the optimal design is to reduce the maximum stress imposed on a bulge tool. The commercial software, ABAQUS, is utilized for nonlinear static analysis of the bulge process, and L9 orthogonal array is used for the optimization of the bulge tool.

Harmonic Analysis on a Lévy Plate and Its Application to Fatigue Analysis

This paper discusses a harmonic response estimation method on the Levy plate with two opposite edges simply supported and the other two edges having free boundary conditions. Then, the harmonic response is processed to evaluate fatigue damage. Since the equation of motion of the plate is not self-adjoint, the modes are not orthogonal to each other on the structure domain. Noting that the Levy plate can be expressed using one term sinusoidal function that is orthogonal to other sinusoidal functions, this paper suggested the calculation method that is equivalent to finding a least square error minimization solution of the finite number of algebraic equations. Example problems subjected to a distributed area loading and a distributed line loading are defined and their solutions are provided. The solutions are compared to those of the commercial code, ANSYS. The plate motion due to high frequency vibration can be seen in the nuclear fuels subjected to highly turbulent coolant flow. The dominant exciting frequency is dependent on the coolant velocity and Strouhal number, a dimensionless number describing oscillating flow mechanism. This paper also discusses fatigue damage considering the high frequency vibration using the Dirlik equation.