Seung-Wook Han

Reliability-based topology optimization using reliability index approach

Reliability-Based Topology Optimization (RBTO) based on the Evolutionary Structural Optimization (ESO) using Reliability Index Approach (RIA) is developed. When the uncertainties such as the elastic modulus, applied load and dimensional variation exist in design process, the optimum topology obtained from the Deterministic Topology Optimization (DTO) can not satisfy the design constraints since it is obtained without consideration of uncertainties related to the random variables. To alleviate the possible degradation of performance in design process, these uncertainties must be considered during the process of topology or design optimization. In the case that the RBTO is used, it is possible to perform the topology optimization where the uncertainties of the random variables should be considered since RBTO can deal with the probabilistic constraints in topology optimization. Reliability Index Approach (RIA) and Performance Measure Approach (PMA) calculate the reliability index as a measure of the probability of failure. In this study, the RIA, which has the probabilistic constraints that are formulated in terms of the reliability index, is adopted to evaluate the probabilistic constraints. In the RBTO based on ESO, the sensitivity number is defined as the change of the reliability index due to the removal of the each element.

Technical Review of Specimens under Ultrasonic Fatigue Test

An accelerated ultrasonic fatigue test(UFT) has been used for analyzing very high cycle fatigue( VHCF, > ). This study reviews how the test specimen is to be determined. We focus on UFT using a resonance of 20 kHz. The specimen geometry is determined by selecting test materials by using a dynamic Youngs modulus and wavelength of 20 kHz. The dynamic Youngs modulus is calculated at the resonant frequency. Through a resonant vibration test at 20 kHz, the length of the specimen is calculated. By determining the shape of the specimen, the stress during the UFT is calculated. The UFT results should be comparable at the test frequency and the specimen geometry obtained by the conventional fatigue tests.

Electrochemical degradation tests on a carbon-black-filled ethylene propylene diene monomer rubber under tensile strain:

Rubber hoses play an important role as a path for carrying the coolant liquid between the radiator and the automotive engine. However, the rubber hoses are apt to deteriorate because of the formation of a galvanic circuit between the rubber hose, the coolant water and the fitting metal part of the engine. Such electrochemical degradation can damage the inner skin of the hose in contact with the coolant liquid heated by the working engine, which often leads to leakage of the coolant water and thereby to hose failure. An ethylene propylene diene monomer rubber has mostly been utilized as the material for the coolant hoses of automotive engines because of the strong resistance to various environmental stresses. This study conducted experimental simulation tests and evaluation of the electrochemical degradation behaviours of the carbon-black-filled ethylene propylene diene monomer rubber used for the coolant hoses. The tests were performed by considering the actual circumstances together with the combined thermal, tensile strain and electrostatic voltage loadings on the rubber hoses. The behaviour of the change in the electrical resistance of a specimen and, thus, the apparent decrease in the current were analysed quantitatively. An Arrhenius life prediction by using the decrease in the current was reasonably expressed for the accelerated life tests. Many microhole networks were thus formed in the skin part of the specimen by the microbreaks in the carbon-black phase and the scission of the sulphur cross-links. The coolant liquid penetrated significantly into the skin, which resulted in a considerable swelling of the skin. The larger the strain application to the rubber, the lower are the strength and the elongation at the breaking point. An exponential index parameter can represent quantitatively the behaviours of the decreases in the mechanical properties used for accurate life prediction.

Reviews on Very High Cycle Fatigue Behaviors of Structural Metals

Abstract : The paper presents an overview of the present state of study on the fatigue behaviors at very high number of cycles (N f > 10 7 ). A classification of materials with typical S-N curves and influencing factors such as notches, residual stresses, temperatures, corrosion environments and stress ratios are given. The microstructural inhomogeneities of materials and micro-cracks played an important roles in very high cycle fatigue behaviors. The failure mechanisms for the fatigue design of materials and components are mentioned. Key words : Very high cycle fatigue(초고주기피로), S-N curves(응력-수명 곡선), Fatigue strength(피로강도), Fatigue fracture(피로파괴), Failure mechanism(파손 메커니즘) 1. 서 론 1) 실제 산업현장의 많은 기계부품들은 오랜 시간동안 지속적인 반복하중을 받고 있다. 모터, 자동차의 내연기관 부품들, 고속철의 차륜, 그리고 항공기 및 발전기의 블레이드와 같이 심하게 압력을 받는 장치들은 피로손상 누적에 의해 피로파괴를 유발하여 대형사고로 연결될 수 있으므로 내구성, 안정성, 신뢰성이 크게 요구된다. 때문에 최근 산업현장의 기계부품들은 10 7 이상의 초고주기피로수명을 갖도록 요구되고 있다. 1) 이에 따라 피로수명 연구에서는 저주기피로(Low Cycles Fatigue, LCF, 10 4 < N f <10 5 cycles)와 고주기피로(High Cycles Fatigue, HCF, 10