Jae-Jun Lee

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.

A Preliminary Study on the Optimal Shape Design of the Axisymmetric Forging Component Using Equivalent Static Loads

: s에서의 등가정하중 f : 목적함수 g : 부등제한조건 m : 평균값 S : 표준편차 e : 유효변형률 Key Words : Equivalent Static Loads(등가정하중), Shape Optimization(형상최적설계), Forging(단조), Preform(예비성형체) 초록: 본 논문은 등가정하중을 이용하여 단조공정의 예비성형체 및 빌렛의 형상설계를 위한 최적화 방법을 제안한다. 단조공정에서 예비성형체의 형상은 최종 성형품의 품질을 결정하는데 중요한 역할을 한다. 본 연구는 빌렛 및 예비성형체의 형상을 설계하기 위하여 등가정하중법을 사용하였다. 등가정하중법은 비선형 동적하중을 등가정하중으로 변환하고 여기서 구한 등가정하중을 이용하여 선형 응답 최적화를 수행하는 방법이다. 설계변수의 갱신은 선형 응답 최적화와 비선형 해석을 통하여 이루어진다. 본 논문에 포함된 예제는 원하는 단조품의 생산을 위한 최적의 예비성형체와 빌렛의 형상을 도출하여 제안한 방법의 유용성을 검증한다. 비선형 해석과 선형 응답 최적화는 각각 LS-DYNA와 NASTRAN을 사용하였다. Abstract: An optimization method is proposed for preform and billet shape designs in the forging process by using the Equivalent Static Loads (ESLs). The preform shape is an important factor in the forging process because the quality of the final forging is significantly influenced by it. The ESLSO is used to determine the shape of the preform. In the ESLSO, nonlinear dynamic loads are transformed to the ESLs and linear response optimization is performed using the ESLs. The design is updated in linear response optimization and nonlinear analysis is performed with the updated design. The examples in this paper show that optimization using the ESLs is useful and the design results are satisfactory. Consequently, the optimal preform and billet shapes which produce the desired final shape have been obtained. Nonlinear analysis and linear response optimization of the forging process are performed using the commercial software LS-DYNA and NASTRAN, respectively.

A Preliminary Study on the Optimal Preform Design in the Forging Process Using Equivalent Static Loads

The forging process is the shaping of a workpiece using dynamic loads and typically consists of the multi-step process with the preforming process. Defects such as flash and unfilling occur and the distribution of effective strains is not even in the workpiece after the forging process. An optimized preform is necessary for reduction of the defects in the desired final forging. Optimization of the forging process is nonlinear dynamic response optimization because nonlinearities are involved in the analysis. When the conventional method is utilized in optimization of the forging process, the cost is extremely high due to repeated nonlinear analyses for function and sensitivity calculation. In this paper, the equivalent static loads method for non linear static response structural optimization (ESLSO) is employed to determine the preform shape which leads to reduction of the unfilled area and even distribution of the effective strain in the final forging shape. ESLSO is a structural optimization method where nonlinear dynamic loads are transformed to equivalent static loads (ESLs). ESLs are defined as the loads for linear analysis, which generate the same response field as that of nonlinear analysis. Two kinds of ESLs are proposed and they are the ESLs for the displacements and the ESLs for the effective strains. Examples of the forging process are solved using these ESLs and the results are discussed.

Development of a New Spacer Grid Form to Enhance the Integrity of Fuel Rod Support and the Crush Strength of a Spacer Grid Assembly

A spacer grid is one of the most important structural components in a LWR fuel assembly. The spacer grid, which supports nuclear fuel rods laterally and vertically with a friction grip, is an interconnected array of slotted grid straps welded at the intersections to form an egg-crate structure. Dimples and springs are stamped into each grid strap to support the fuel rods. The form of grid straps and spring form is known to be closely related with the crush strength of spacer grid assembly and the integrity of fuel rod support, respectively. Zircaloy is prevailing as the material of the spacer grid because of its low neutron absorption characteristic and its successful extensive in-reactor use. The primary considerations are to provide a Zircaloy spacer grid with crush strength sufficient to resist design basis loads especially due to seismic accidents, without significantly increasing pressure drop across the reactor core. Generally, the thickness and height of the Zircaloy grid strap have been the main design variables in order to meet the above considerations. Recently, it was reported that a dimple location is also a design variable that affects the crush strength of a spacer grid assembly. In this study, a new spacer grid form was developed in order to enhance the integrity of the fuel rod support and the crush strength of the spacer grid assembly by using a systematic optimization technique. Finite element analysis and crush strength tests on the developed new spacer grid form were carried out to check the performance enhancement compared to commercial spacer grids. The enhancement of fuel rod support was confirmed by comparisons of contact area, peak stresses, plastic deformation and etc. According to the results, it is estimated that the actual critical load enhancement of the spacer grid assembly is approximately up to 30% and the actual contact area, when a fuel rod inserted into a spacer grid cell, is more than double for the developed new spacer grid form. And also, some design variables that effect the crush strength of a PWR spacer grid assembly were classified and their effects on the crush strength were investigated by a finite element analysis and a crush strength test.Copyright

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.

Dynamic Response Optimization of a Mobile Harbor Crane with a Moving Support

The mobile harbor is a new innovative system that delivers containers from a containership to a harbor without good infrastructure. A crane is installed on the deck of the mobile harbor and transfers the containers. The structure of the crane is influenced by the inertia force that occurs from a moving support. Thus an accurate safety verification considering the moving support is required. Lightweight of the crane structure is also significant in the design for low production cost and efficient operation. Dynamic response optimization can be exploited to achieve these two requirements. Equivalent static loads method is employed for dynamic response optimization of the crane. The equivalent static loads method transforms dynamic loads to equivalent static loads, and static response structural optimization with the transformed equivalent static loads are solved. The process proceeds in a cyclic manner. A new method is proposed to consider the moving supports and the structure of the mobile harbor is optimized using the proposed method.

Structural Optimization of the Mobile Harbor Carne Considering Sea State

The mobile harbor is a new concept system to solve the problems of a port. These problems are that container ships cannot be anchored at the dock because they have become larger or the waiting times of anchoring the ships are increased due to heavy container traffic. A new system is designed to carry out the loading and unloading of containers between the mobile harbor and the container ship using the mobile harbor crane at sea. The crane plays an important role when transferring the containers. In this research, various types of the mobile harbor crane are proposed and structural optimization for each type of the crane is carried out. The loading conditions consider the rolling and pitching conditions of the unstable sea state and the wind force are considered. The constraints are mainly the regulations made by the Korean Register of Shipping. The structure of the crane is optimized to minimize the mass while various constraints are satisfied.

A Preliminary Study on the Optimum Parameters in Sheet Metal Forming

The sheet metal forming process is used in many industries because this process is simple and is suited to the purpose of mass production. Since the process typically has some phenomena such as springback, tearing, failure of material and others, the desired final shape is quite difficult to obtain. The problems, which are caused by these phenomena, can be improved by optimization of the sheet metal forming process. The design variables of metal forming optimization are process parameters and structural parameters. The process parameters are the blank holding force (BHF), the drawbead restraining force (DBRF), the friction factor, etc. and the structural parameters are the initial blank shape and others. Metal forming optimization is nonlinear dynamic response optimization because this process has geometric, material and boundary nonlinearities. In this paper, the two groups of parameters are separately optimized by the response surface method (RSM) and structural optimization, respectively. The two optimization process iteratively proceeds until the convergence criteria are satisfied. First, the RSM is utilized to determine the process parameters such as BHF and DBRF. Because BHF and DBRF are input in conventional structural optimization, they cannot be used as design variables. On the other hand, they can be used as design variables in RSM. Moreover, since the number of process variables is small, RSM can be exploited to determine the process variables. An optimization problem is formulated and solved. Second, structural optimization is employed to determine the initial blank shape which can be deformed to the desired final shape after metal forming. The process parameters determined in the first process are regarded as input parameters, and then an optimization problem is formulated. Since the formulated problem is nonlinear dynamic response optimization, a new approach is adopted for this process. This approach is called as the equivalent static loads method for non linear static response structural optimization (ESLSO). Equivalent static loads (ESLs) are defined as the loads for linear analysis, which generate the same response field as that of nonlinear analysis. In ESLSO, nonlinear dynamic loads are transformed to ESLs and the ESLs are used as the loading conditions in linear static response optimization. The design is updated in linear static response optimization. Nonlinear analysis is performed with the updated design and the process proceeds in a cyclic manner until the convergence criterion is satisfied. The existing ESLSO is modified to fit into the metal forming optimization problem. An advantage of ESLSO is that the nodes in the design domain can be controlled easily and exactly because linear static response optimization is utilized. However, various design variables cannot be selected because only the design variables for linear static response optimization can be used in optimization using ESLSO. Several examples are solved by the iterative use of the RSM and ESLSO and the solutions are discussed.