K.H. Yoon

Fretting wear of laterally supported tube

The fretting wear of a tube, which is in contact with a lateral support, is examined experimentally. A fretting wear tester is specifically designed. Elastic springs are used as the support, which can simulate the contact between a spacer grid and a fuel rod in pressurized water reactor fuel. The tubes and the springs are made of Zircaloy-4. The experiments are conducted in air at room temperature. The experimental conditions, i.e. the normal and shear forces on the contact, the slip range and the number of cycles, are set to be the same. To investigate the influence of the contact geometry on the wear, the spring supports have a concave, a flat or a convex contour. The influence on the axial and transverse slip directions is investigated to incorporate the actual tube motion caused by such a flow-induced vibration in the reactor. The wear on the tube is examined by the surface roughness tester, which measures the depth, and the contour of the worn surface of the tube. Since the shape and the distribution of wear are found arbitrary, a method for evaluating the wear volume is proposed using the signal processing technique. It is found that wear can be restrained when the slip direction is transverse, and if the support has a concave contour.

A study on occupant neck injury in rear end collisions

Abstract The stiffness of the automobile seat back mostly affects neck injuries in rear end collisions. The effects of the stiffness have been experimentally investigated. A sled simulator was utilized with varying speeds and fixed stiffness. Another simulation was performed with a fixed speed and varying stiffness. A stopper was installed behind the seat back to keep a certain stiffness. The neck injuries for various cases are calculated equations from industrial standards. Using the results of the test, the effect of seat back stiffness in minimizing occupant neck injury in rear end collisions is discussed.

An orthogonal-array-based design-of-experiments method for designing a vehicle hood and bumper structure

Although the numbers of pedestrian fatalities and injuries are steadily declining worldwide, pedestrian protection is still an important issue. Extensive research has been carried out for pedestrian protection in order to establish appropriate regulations for pedestrian safety. The automobile hoods and bumpers are fairly important for pedestrian protection because pedestrians frequently collide with them during accidents, and they should be designed for the safety of the pedestrians. A new design method for hoods and bumpers is proposed to enhance pedestrian protection by using an orthogonal-array-based design-of-experiments method. Two analysis methods, namely a real experiment and a computer simulation, are utilized to design safe structures of the hood and the bumper. A real experiment is very expensive while computer simulation has modelling imperfections. A design method which uses an experiment and simulation simultaneously is developed. Orthogonal arrays are employed to link the two analyses. The minimum number of experiments is allocated to some rows of an orthogonal array and the simulations are allocated to the rest of the rows to save the cost. Real experiments and computer simulations are conducted for the rows of orthogonal arrays. Design problems are formulated and the orthogonal arrays are directly used to find the design variables by performing the analysis-of-means process in a discrete space. Mathematical error analysis is conducted. Based on the proposed method, a hood and a bumper are designed to protect pedestrians. The results show that the errors are distributed uniformly and a precise design is obtained.

Vibration Characteristics of a Dummy Fuel Rod Supported by Spacer Grids

The spacer grid is one of the main structural components in the fuel assembly, which supports the fuel rods and maintains coolable geometry from an external load. A vibration test and a finite element analysis using ABAQUS on a dummy fuel rod continuously supported by Optimized H type(OHT) and New Doublet (ND) spacer grids arc performed to obtain the vibration characteristics such as natural frequencies and mode shapes an(1 to verify a finite element model. The results from the test and the finite element analysis are compared by modal assurance criteria (MAC) values. It is resulted that MACs for the first, the third and the fifth mode shapes are relatively good as compared with those of the second an(1 fourth ones. The natural frequency differences between two methods as well as the mode comparison results for the rod with OHT spacer grid are better than those with ND spacer grid. It is judged that the FE model for the ND spacer grid spring should be modified to consider the long contact length which actually happen when the spring supports the rod.

Design of a Spacer Grid Using Axiomatic Design

Recently, much attention has been focused on the design of the fuel assemblies in the Pressurized Light Water Reactor (PLWR). The spacer grid is one of the main structural components in a fuel assembly. It supports fuel rods, guides cooling water, and maintains geometry from the external impact loads. In this research, a new shape of the spacer grid is designed by the axiomatic approach. The Independence Axiom is utilized for the design. For the conceptual design, functional requirements (FRs) are defined and corresponding design parameters (DPs) are found to satisfy FRs in sequence. Overall configuration and shapes are determined in this process. Detailed design is carried out based on the result of the axiomatic design. For the detailed design, the system performances are evaluated by using linear and nonlinear finite element analysis. The dimensions are determined by optimization. Some commercial codes are utilized for the analysis and design.Copyright