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Ⓒ 출구압력(Pa) Key Words: Shape Optimization(형상최적화), Cavitation(공동현상), 3-Way Reversing Valve(3 방향 절환밸브), Automotive Washing Machine(자동차부품 세척장치), Cavitation Index(공동화지표) 초록: 자동차부품 세척장비는 엔진과 변속기 블록 등의 가공과정에서 잔류하는 기름때를 제거하며, 잦은 수류방향 전환과 고압수의 분사를 위해 한 쌍의 2 방향 밸브를 사용한다. 그러나 정교한 밸브제어장치 없이 2 방향 밸브를 사용하는 경우 급격한 수류방향 전환에 따른 맥동현상이 발생하여 사용에 어려움이 따른다. 대안으로 하나의 3 방향 절환밸브를 사용하는 방법은 정교한 제어장치 없이도 정확한 수류방향 절환이 원활히 이루어져 이러한 문제점을 해결할 수 있다. 그러나 복잡한 유로 및 바텀플러그 형상으로 인해 유속변화가 심하게 발생하여 공동현상이 나타날 수 있다. 본 연구에서는 3 방향 절환밸브 내의 유동특성을 해석적으로 평가하였으며, 바텀플러그 하부에서 나타나는 공동현상을 공동화지표와 POC(Percent of cavitation)를 도입하여 정량화하였다. 공동현상의 저감을 위해 바텀플러그 형상을 매개변수화하고, 해석의 수렴성 개선과 해석시간을 단축시킬 수 있는 단순 유한요소모델을 이용하여 유동해석을 통한 형상최적설계를 실시하였다. 완전요인배치법을 통한 실험계획법과 인공신경망 기반 반응표면모델을 적용하여 공동현상이 발생하지 않는 POC 가 30% 미만인 바텀플러그의 형상을 제시하였다. 얻어진 최적해는 POC 27%에 대하여 바텀플러그의 허리길이와 꼬리길이가 각각 6.42mm 및 6.96mm이다. Abstract: A pair of two-way valves typically is used in automotive washing machines, where the water flow direction is frequently reversed and highly pressurized clean water is sprayed to remove the oil and dirt remaining on machined engine and transmission blocks. Although this valve system has been widely used because of its competitive price, its application is sometimes restricted by surging effects, such as pressure ripples occurring in rapid changes in water flow caused by inaccurate valve control. As an alternative, one three-way reversing valve can replace the valve system because it provides rapid and accurate changes to the water flow direction without any precise control device. However, a cavitation effect occurs because of the complicated bottom plug shape of the valve. In this study, the cavitation index and percent of cavitation (POC) were introduced to numerically evaluate fluid flows via computational fluid dynamics (CFD) analysis. To reduce the cavitation effect generated by the bottom plug, the optimal shape design was carried out through a parametric study, in which a simple computer-aided engineering (CAE) model was applied to avoid time-consuming CFD analysis and difficulties in achieving convergence. The optimal shape design process using full factorial design of experiments (DOEs) and an artificial neural network meta-model yielded the optimal waist and tail length of the bottom plug with a POC value of less than 30%, which meets the requirement of no cavitation occurrence. The optimal waist length, tail length and POC value were found to 6.42 mm, 6.96 mm and 27%, respectively.

일반적으로 기계 또는 구조물은 응력집중에 의해 노치선단에 발생하는 균열 및 제작과정에서 발생한 균열이 다양한 형태의 피로하중에 의해 전파되어 제품의 안정성과 수명을 감소시키며 최종적으로 구조물 전체의 파괴를 야기한다. 제품에 발생하는 파괴의 대부분은 항복강도 이하의 반복하중으로 발생하며, 이러한 현상을 규명하기 위하여 반복적인 피로하중을 받는 환경에서 균열진전거동Key Words: Fatigue Crack Growth(피로균열진전), Stress Intensity Factor(응력확대계수), Crack Path(균열경로), Mixed Mode(혼합모드), Compact Tensile Shear Specimen(CTS 시편), Single Edge Notched Bend Specimen(SENB 시편), Eccentric 3 Point- Bending (편측 3점 굽힘) 초록: 혼합모드 피로하중을 받는 균열을 갖은 CTS 시편에 대하여 균열경로 예측이론과 Tanaka 의 등가응력확대계수식을 적용하여 피로균열진전거동을 평가하였다. 새롭게 생성되는 균열선단의 응력확대계수 산정은 ANSYS 를 이용한 유한요소법을 통해 이루어졌고, 균열경로와 균열증분은 마이크로소프트 엑셀에 프로그래밍한 균열경로예측식과 Paris 식으로 계산되었다. 균열증분으로 새롭게 생성된 균열선단의 기하학적인 정보는 엑셀의 기능을 이용해 ANSYS 의 KSCON 명령어가 인식할 수 있게 변화시켜 균열모델링을 용이하게 하였다. 반복적인 균열해석을 위해 유한요소법과 엑셀을 결합한 FECTUM(Finite Element Crack Tip Updating Method)을 개발하였다. 개발된 FECTUM을 편측 3점 굽힘을 통해 혼합모드의 구현이 가능한 SENB시편(Single Edge Notched Bend Specimen)에 적용해본 결과, 균열경로는 물론 파단될 때까지의 피로하중 반복수의 차이가 3% 미만으로 잘 일치하는 모습을 보여, 개발된 기법의 타당성을 검증하였다. Abstract: To estimate the fatigue crack propagation behavior of compact tension shear (CTS) specimen under mixed-mode loads, crack path prediction theories and Tanaka’s equation were applied. The stress intensity factor at a newly created crack tip was calculated using a finite element method via ANSYS, and the crack path and crack increment were then obtained from the crack path prediction theories, Tanaka’s equation, and the Paris’ equation, which were preprogrammed in Microsoft Excel. A new method called the finite element crack tip updating method (FECTUM) was developed. In this method, the finite element method and Microsoft Excel are used to calculate the stress intensity factors and the crack path, respectively, at the crack tip per each crack increment. The developed FECTUM was applied to simulate the fatigue crack propagation of a single-edge notched bending (SENB) specimen under eccentric three-point bending loads. The results showed that the number of cycles to failure of the specimen obtained experimentally and numerically were in good agreement within an error range of less than 3%.

A robust optimization is only one of the ways to minimize the effects of variances in design variables on the objective functions at the preliminary design stage. To predict the variances and to formulate the probabilistic constraints are the most important procedures for the robust optimization formulation. Though several methods such as the process capability index and the six sigma technique were proposed for the prediction and formulation of the variances and probabilistic constraints, respectively, there are few attempts using a percent defective which has been widely applied in the quality control of the manufacturing process for probabilistic constraints. In this study, the robust optimization for a lower control arm of automobile vehicle was carried out, in which the design space showing the mean and variance sensitivity of weight and stress was explored before robust optimization for a lower control arm. The 2nd order Taylor expansion for calculating the standard deviation was used to improve the numerical accuracy for predicting the variances. Simplex algorithm which does not use the gradient information in optimization was used to convert constrained optimization into unconstrained one in robust optimization.

(s) : 플랜트 전달함수 Key Words: Optimization(최적화), Wind Turbine(풍력발전기), Pitch Controller(피치제어기), Auto-tuning(자동동조), Ziegler-Nichols Step Response(지글러-니콜스 계단응답법), Latin Hypercube(라틴 하이퍼큐브), Neural Network(신경망), Genetic Algorithm(유전자 알고리즘) 초록: 풍력발전기의 안정적인 전력생산은 정격풍속 이상에서 피치제어와 스톨제어와 같은 일정속도제어로 이루어지고 있다. 최근, 효율적인 전력생산을 위하여 정격풍속 이하의 변동풍속 조건에서 최대 출력을 얻기 위한 가변 속도제어가 적용되고 있는 추세이다. 기존의 피치제어기에서는 지글러-니콜스 계단응답법에 의한 제어기 최적화가 이루어지고 있으나, 가변 속도제어의 요구로 보다 정확한 최적화가 필요하게 되었다. 본 연구에서는 기존의 지글러-니콜스 계단응답법을 개선하기 위하여 라틴 하이퍼큐브 샘플링을 통한 신경망모델을 구축하고, 구축된 PID 제어 계수 신경망모델에 유전자 알고리즘을 적용하여 피치제어기를 최적화하였다. 유전자 알고리즘으로 구한 최적해가 지글러-니콜스 계단응답법의 초기해 보다 평균제곱근 오차가 13.4% 향상되었고, 응답특성을 나타내는 상승속도와 정착시간은 각각15.8% 및 15.3%으로 개선되었다. Abstract: Wind energy is becoming one of the most preferable alternatives to conventional sources of electric power that rely on fossil fuels. For stable electric power generation, constant rotating speed control of a wind turbine is performed through pitch control and stall control of the turbine blades. Recently, variable pitch control has been implemented in modern wind turbines to harvest more energy at variable wind speeds that are even lower than the rated one. Although wind turbine pitch controllers are currently optimized using a step response via the Ziegler-Nichols auto-tuning process, this approach does not satisfy the requirements of variable pitch control. In this study, the variable pitch controller was optimized by a genetic algorithm using a neural network model that was constructed by the Latin Hypercube sampling method to improve the Ziegler-Nichols auto-tuning process. The optimized solution shows that the root mean square error, rise time, and settle time are respectively improved by more than 7.64%, 15.8%, and 15.3% compared with the corresponding initial solutions obtained by the Ziegler-Nichols auto-tuning process.

또한 원자력 발전소에서 작동되는 기기가 안전등급에 해당하는 경우 기기검증의 단계를 거쳐야 하며, 기기 검증에는 열 노화와 방사선 노화로 구성되는 내환경검증과 내진해석과 가진 실험으로 구성되는 내진검증으로 구분된다. 기존에는 구조물 전체에 대하여 지진력에 해당하는 힘으로 Key Words : Butterfly Valve(버터플라이밸브), Nuclear Power Plant(원자력 발전소), Seismic Analysis(내진해석) , Static Seismic Analysis(정적내진해석) , Dynamic Seismic Analysis(동적내진해석) 초록: 밸브는 지진하중 하에서 구조안전성이 요구되는 원자력 발전소 파이프 라인 시스템에 설치되는 중요한 장비 중 하나이다. 밸브의 성능향상을 위한 형상최적설계에서 지진하중조건을 고려한 밸브의 구조안전성 검증이 반드시 필요하다. 최근, 이론적인 내진검증 기법과 절차가 체계화되어 있어 지진하중하에서 설계조건을 만족하는 대상체의 적절한 설계변수가 이론적으로 얻어지고 있다. 본 연구에서는 원자력 발전소용 200A 버터플라이밸브를 대상으로 KEPIC MFA에서 제시하고 있는 이론적인 정적내진해석과 동적내진해석 절차를 통하여 내진검증을 수치해석기법과 실험을 병행하여 수행하였다. 자중, 운전조건 및 안전정지지진하중 조건을 모두 고려한 정적내진해석을 통해 밸브의 스템과 바디 접촉부에 작용하는 최대 작용응력이 135MPa으로 도출되었다. 또한 동적내진해석시 적용한 응답스펙트럼 해석법과 모드조합법으로 계산된 최대응력은 183MPa 이었다. 이는 밸브 소재의 허용강도 대비 안전계수가 1.7 및 1.3 수준임을 확인하였다. Abstract: Valves are one of the most important components of a pipeline system in a nuclear power plant, and it is important to ensure their structural safety under seismic loads. A crucial aspect of structural safety verification is the seismic qualification, and therefore, an optimal shape design and experimental seismic qualification is necessary in case the configuration of the valve parts needs to be modified and their performance needs to be improved. Recently, intensive numerical analyses have been performed before the experimental verification in order to determine the appropriate design variables that satisfy the performance requirements under seismic loads. In this study, static and dynamic numerical structural analyses of a 200A butterfly valve for a nuclear power plant were performed according to the KEPIC MFA. The result of static analysis considering an equivalent static load under SSE condition gave an applied stress of 135 MPa. In addition, the result of dynamic analysis gave an applied stress of 183 MPa, where the CQC method using response spectrums was taken into account. These values are under the allowable strength of the materials used for manufacturing the butterfly valve, and therefore, its structural safety satisfies the requirements of KEPIC MFA.

In the current design process for the lower arm used in automobile parts, an optimal solution of its various design variables should be found through exploration of the design space approximated using the response surface model formulated with a first- or second-order polynomial equation. In this study, a multi-level computational DOE (design of experiment) was carried out to explore the design space showing nonlinear behavior, in terms of factors such as the total weight and applied stress of the lower arm, where a fractional-factorial orthogonal array based on the artificial neural network model was introduced. In addition, the tolerance robustness of the optimal solution was estimated using a tolerance optimization with six sigma constraints, taking into account the tolerances occurring in the design variables.

Ball joint is a rotating and swiveling element that is typically the interface between two parts. In an automobile, the ball joint is the component that connects the control arms to the steering knuckles by playing a role of bearing. The ball joint can also be installed in linkage systems for motion control applications. This paper describes the simulation strategy for a ball joint analysis, considering manufacturing process. Its manufacturing process can be divided into plugging and spinning. Then, the interested response is selected as the stress distribution generated between its ball and bearing. In this paper, a commercial code of NX DAFUL 2.0 using an implicit integration method is introduced to calculate the response. In addition, the gap analysis is performed to investigate the fitness. Also, the optimum design is suggested through case studies.

Key Words : TOPSIS(Technique for Order Preference by Similarity to Ideal Solution), Multi-objective Shape Optimization(다중목적 형상최적설계), CRT Funnel(CRT 후면유리), Design of Experiments(실험계획법), Rational Bezier Curve(베지어 곡선), MADM(Multi-Attribute Decision Making: 다속성 의사결정) 초록: TOPSIS(Technique for Order Preference by Similarity to Ideal Solution)는 상충되는 다수의 속성이 존재하는 상황에서 의사결정이 요구되는 다속성 의사결정법(Multi Attribute Decision Making) 중 하나이다. 이는 선택된 대체안이 최선의 이상적 대체안으로부터 가장 가까운 거리에 위치해야 하고, 동시에 부정적으로 이상적인 대체안으로부터는 가장 멀리 위치해야 한다는 논리에 입각한 의사결정 기법이다. TOPSIS 는 최소화와 최대화가 공존하는 다목적함수 형상 최적설계에 적용이 가능하다. 본 연구에서는 TOPSIS 와 베지어 곡선(Rational Bezier Curve)을 적용하여 CRT(Cathode Ray Tubes) 후면유리의 다중목적 형상최적설계를 수행하였다. 무게와 1 차 주응력의 두 가지 다중목적 함수를 최적화하기 위하여, 다중목적 함수의 성능지표를 TOPSIS의 상대적 근접도로 정의하고 이를 반응표면모델로 구성하여 다중목적 형상최적설계가 가능한 방법론을 제안하였다. 이를 통해 하나의 최적해가 아닌 최적해의 군이 선정되어, 무게와 주응력 최적해의 모순관계를 확인하면서 다양한 설계요구 스펙을 만족시켜줄 수 있는 방안을 설계자가 스스로 선택하도록 하였다. Abstract: The technique for order preference by similarity to ideal solution (TOPSIS) is regarded as a classical method of multiple attribute decision making (MADM), often used to solve various decision-making or selection problems. It is based on the concept that the chosen alternative should have the shortest distance from the positive ideal solution and the farthest distance from the negative ideal solution. The TOPSIS can be applied to a design process for carrying out multi-objective shape optimization wherein the best and worst alternatives are to be decided. In this paper, multi-objective shape optimization using the TOPSIS and Rational Bezier curve was applied to the funnel of a cathode-ray tube (CRT). In order to minimize the weight and first principal stress, a new multi-objective shape optimization methodology is proposed, wherein the relative-closeness coefficients of the TOPSIS are defined as the performance indices of a multi-objective function and evaluated by response surface models. This methodology enables the designer to decide on the best solution from a number of design specification groups by examining the various conflicts between the weight and the first principal stress.

In a practical design process, the method of extracting the design space information of the complex system for verifying, improving, and optimizing the design process by taking into account the design variables and their shape tolerance is very important. Finite element analysis has been successfully implemented and integrated with design of experiment such as D-Optimal array; thus, a response surface model and optimization tools have been obtained, and design variables can be optimized by using the model and these tools. Then, to guarantee the robustness of the design variables, a robust design should be additionally performed by taking into account the statistical variation of the shape tolerance of the optimized design variables. In this study, a new approach based on the use of the response surface model is proposed; in this approach, the standard normal distribution of the shape tolerance is considered. By adopting this approach, it is possible to simultaneously optimize variables and perform a robust design. This approach can serve as a means of efficiently modeling the trade-off among many conflicting goals in the applications of finite element analysis. A case study on the robust optimal design of disc brakes under thermal loadings was carried out to solve multiple objective functions and determine the constraints of the design variables, such as a thermal deformation and weight.

초록:터빈블레이드와같은시스템의피로수명은형상설계변수의변화에따라비선형적으로복잡하게나타난다.방대한계산시간이요구되는이러한시스템의CAE기반파라메트릭설계최적화문제에근사기법인크리깅메타모델방법이활용되고있다.본연구에서는터빈블레이드의피로수명을향상시키기위하여,설계변수변화에따른피로수명의비선형성을고려함은물론직교성과균형성을모두만족하는다수준전산실험계획법을수행하여크리깅메타모델을구축하였다.크리깅메타모델로부터만족도함수를적용하여피로수명을최적화하였고,몬테카를로모의실험법을적용한식스시그마최적설계를수행하여피로수명의결함률을향상시킨최적해의강건성을확보하였다.Abstract:Acomplexsysteminvolvesalargenumberofdesignvariables,anditsoperationisnon-linear.Toexplorethe characteristics in its design space, a Kriging meta-model can be utilized; this model has replaced expensivecomputational analysis that was performed in traditional parametric design optimization. In this study, a Krigingmeta-model with a computational orthogonal array for the design of experiments was developed to optimize thefatiguelifeof aturbinebladewhosebehavior under cyclicrotational loadsissignificantlynon-linear. Theresultsnot only show that the maximum fatigue life is improved but also indicate that the accuracy of computationalanalysisisachieved.Inaddition,therobustnessoftheresultsobtainedbysix-sigmaoptimizationcanbeverifiedbycomparisonwiththeresultsobtainedbyperformingMonteCarlosimulations.

The spring constants of primary suspensions for a railway vehicle are optimized by a robust design process, in which the response surface models(RSMs) of their dynamic responses are constructed via the design of experiment(DOE). The robust design process requires an intensive computation to evaluate exactly a probabilistic feasibility for the robustness of dynamic responses with their probabilistic variances for the railway vehicle. In order to overcome the computational process, the process capability index  is introduced which enables not only to show the mean value and the scattering of the product quality to a certain extent, but also to normalize the objective functions irrespective of various different dimensions. This robust design, consequently, becomes to optimize the  subjected to constraints, i.e. 2, satisfying six sigma. The proposed method shows not only an improvement of some   violating the constraints obtained by the conventional optimization, but also a significant decrease of the variance of the .

Fatigue failures are often occurred at welded joints where stress concentrations are relatively high due to the joint geometry. Although employing good detail design practices by upgrading the welded detail class enables to improve the fatigue performance, in many cases, the modification of the detail may not be practicable. As an alternative, the fatigue life extension techniques that reduce the severity of the stress concentration at the weld toe region, remove imperfections and introduce local compressive welding residual stress, have been applied. These techniques are also used as definite measures to extend the fatigue life of critical welds that have failed prematurely and have been repaired. In this study, a hammer peening procedure for using commercial pneumatic chipping hammer was developed, and the effectiveness is quantitatively evaluated. The pneumatic hammer peening makes it possible to give the weld not only a favorable shape reducing the local stress concentration, but also a beneficial compressive residual stress into material surface. In the fatigue life calculation of non-load carrying cruciform specimen treated by the pneumatic hammer peening, the life was lengthened about ten times at a stress range of 240MPa, and fatigue limit increased over 65% for the as-welded specimen.

Multiple surface crack distributed randomly along a weld toe influences strongly on the fatigue crack propagation life of welded joint. It is investigated by using statistical approaches based on series of systematic experiments. From the statistical results, initial crack numbers and its locations follow the normal distribution, and the probability of initial crack depths and lengths can be described well by tile Weibull distribution. These characteristics are used to calculate the fatigue crack propagation life, in which the mechanisms of mutual interaction and coalescence of the multiple cracks are considered as well as the Mk-factors obtained from a parametric study on the crack depths and lengths. The automatic calculation is achieved by the NESUSS, where the parameters such as the number, location and size of the cracks are all treated as random variables. The random variables are dealt through the Monte-Carlo simulation with sampling random numbers of 2,000. The simulation results show that the multiple cracks lead to much shorter crack propagation life compared with those in single crack situation. The sum of the simulation and tile fatigue crack initiation life derived by the notch strain approach agrees well with the experiments.

An evaluation of fatigue life of welded components is complicated due to various geometrically complex welding details and stress raisers in vicinity of weld beads, ego under cuts, overlaps and blow holes. These factors have a considerable influence on the fatigue strength of welded joints, as well as the welding residual stress which is relaxed depending on the distribution of local stress at the front of the stress raisers. To reasonably evaluate fatigue life, the effect of geometries and welding residual stress should be taken into account. The several methods based on the notch strain approach have been proposed in order to accomplish this. These methods, however, result in differences between analytical and experimental results due to discrepancies in estimated amount of relaxed welding residual stress present. In this paper, an approach that involves the use of a modified notch strain approach considering geometrical effects and a residual stress relaxation model based on experimental results was proposed. The fatigue life for five types of representative welding details, ego cruciform, cover plate, longitudinal stiffener, gusset and side attachment joint, are evaluated using this method.

The fatigue life of welded joints is sensitive to welding residual stress and complexity of their geometric shapes. To predict the fatigue life more reasonably. the effects of welding residual stress and its relaxation on their fatigue strengths should be considered quantitatively, which are often regarded to be equivalent to the effects of mean stresses by external loads. The hot-spot stress concept should be also adopted which can reduce the dependence of fatigue strengths for various welding details. Considering the factors mentioned above, a fatigue life prediction model using the modified Goodman's diagram was proposed. In this model, an equivalent stress was introduced which is composed of the mean stress based on the hot-spot stress concept and the relaxed welding residual stress. From the verification of the proposed model to real welding details, it is proved that this model can be applied to predict reasonably their fatigue lives.

It is well known that the strength and the fatigue life of welded steel components are affected extensively by welding residual stresses distributed around their weldments under not only monotonic but also cyclic loads. The externally applied loads are to be superimposed with the welding residual stresses, so that unexpected deformations and failures of the components might occur. These residual stresses are not kept constant, but relaxed or redistributed during in service. Under monotonic loads the relaxation takes place when the sum of external and welding residual stress exceeds locally the yield stress of material used. By the way, it is shown that under cyclic loads the welding residual stress is considerably relieved by the first or the early cycles of loads, and then gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon and mechanism of the stress relaxation are still not clear, and there are few comprehensive models to predict amount of relaxed welding residual stress. In this study, the characteristics of the welding residual stress relaxation under monotonic and cyclic loads were investigated, and a model to predict quantitatively amount of welding residual stress relaxation was proposed.