Do-Sik Kim

Notched Strength and Fracture Criterion in Fabric Composite Plates Containing a Circular Hole

The effects of the hole size and the specimen width on the fracture behavior of several woven fabric composite plates are experimentally investigated in tension. It is shown in this paper that the characteristic length (d o ) in the Point Stress Criterion (PSC) depends on the hole size and the specimen width. A modified PSC for predicting the notched strength is proposed. An excellent agreement is found between the experimental results and the analytical predictions using the modified PSC. When the unstable fracture occurred, the equivalent critical crack length (a c ) corresponding to the damage zone size is about twice the characteristic length. The characteristic length decreases with an increase in the notched strength. The critical energy release rate (G c ) corresponding to the unstable fracture criterion is independent of the hole size for the same specimen width. G c increases with an increase in the specimen width. These results can be explained by the correlation between the G c and the notch sensitivity factor (k).

Lifetime Estimation of an Axle Drive Shaft by Calibrated Accelerated Life Test Method

In this paper, a method to predict the fatigue life of an axle drive shaft by the calibrated accelerated life test (CALT) method is proposed. The CALT method is very effective for predicting lifetimes, significantly reducing test time, and quantifying reliability. The fatigue test is performed by considering two high stress and one low stress levels, and the lifetime at the normal stress level is predicted by extrapolation. In addition, in this study, the major reliability parameters such as the lifetime, accelerated power index, shape parameter, and scale parameter are determined by conducting various experiments. The lifetime prediction of the axle drive shaft is verified by comparing the experimental results with load spectrum data. The results confirm that the CALT method is effective for lifetime prediction and requires a short test time.

Study of the Accelerated Life Test Method for Power Train Components Under Cyclic Loads Using Weibull-IPL (Inverse Power Law) Model

This study was performed to develop the accelerated life test method using Weibull-IPL (Inverse Power Law) model for power train components that are running on cyclic loads. Weibull-IPL model is concerned with determining the assurance life with confidence level and the accelerated life test time. From the relation of weibull distribution factors and confidence limit, the testing times on the no or some number of failure acceptance criteria are determined. The power train components under cyclic loads generally represent wear and fatigue characteristics as a failure mode. IPL based on the cumulative damage theory is applied effectively the mechanical components to reduce the testing time and to achieve the accelerating test conditions. The proposed Weibull-IPL model is an approach to improve the reliability assessment on the statistical distribution and wear out failure mechanism concepts. As the actual application example, accelerated life test method of agricultural tractor transmission was described. Life distribution of agricultural tractor transmission was supposed to follow Weibull distribution and life test time was calculated under the conditions of average life (MTBF) 3,000 hours and 90% confidence level for one test sample. According to IPL, because test time can be shorten in case increase test load, test time could be reduced by 482 hours when we put the load 1.1 times of rated load than 0.73 times of rated load that is equivalent load calculated by load spectrum of the agricultural tractor. This time, acceleration factor was 11.7. This Weibull-IPL model can be used to develop accelerated test method of gear reducer, hydraulic hose, bearing and etc.Copyright

Accelerated Life Evaluation of Propeller Shaft for Forklift Truck

* Reliability Assessment Center, Korea Institure of Machinery & Materials (Received February 21, 2014 ; Revised August 14, 2014 ; Accepted August 18, 2014)Key Words: Propeller Shaft(추진축), Rotary Fatigue Test(회전 피로 시험), Load Spectrum(부하 스펙트럼), Accelerated Life Test(가속수명시험), Calibrated Accelerated Life Test(교정가속수명시험)초록: 본 논문은 가속시험법을 이용하여 추진축의 피로 수명을 예측하는 것이다. 가속시험방법으로는 교정가속시험법을 적용하였으며, 이 시험법은 시험시료 수량이 적을 경우에 시험시간 단축과 신뢰성을 높이는 매우 효과적인 방법이다. 두 수준의 고부하 스트레스 수준에서 피로시험을 수행하고, 외삽법과 가용한 시험시간을 고려하여 저부하 스트레스 시험 수준을 결정하여 피로시험을 수행하였다. 본 논문에서는 가속지수와 형상모수 등의 신뢰성 파라미터를 시험을 수행하여 획득하였고, 실제 사용 환경의 로드 스펙트럼을 분석하고 시험 결과와 비교하여 추진축의 수명을 예측할 수 있었다.Abstract: This paper proposes an approach for predicting the fatigue life of a propeller shaft of a forklift truck by an accelerated life test method. The accelerated life test method adopted in this study is the calibrated accelerated life test, which is highly effective in the prediction of the lifetime and enables significant reduction of the test time as well as a quantification of reliability in the case of small sample sizes. First, the fatigue test was performed under two high stress levels, and then, it was performed by setting low stress levels in consideration of the available test time and extrapolation. Major reliability parameters such as the lifetime, accelerated power index, and shape parameter were obtained experimentally, and the experimentally predicted lifetime of the propeller shaft was verified through comparison with results of an analysis of load spectrum data under actual operating conditions.

Development of the Accelerated Life Test Method & Life Test Equipment for the Counterweight of the Construction Machinery

A large-sized exciter that vibrates a two-ton component is required to simulate the field operating conditions of a counterweight of an excavator. However, it is difficult for a small-medium sized company to obtain a large exciter for the life test of a counterweight which is an equivalent counterbalancing weight that balances a load. Therefore, in this study, we developed life test equipment for evaluating the reliability of construction machinery weighing about two tons. It simulates the field operating conditions using rotational vibrators consisting of electric motors. A failure analysis of the counterweight was also performed for the major components. Field data acquired from various sites were applied to the life test design of the counterweight. Finally, a zero-failure qualification test based on the accelerated life test was designed, and there was no failure during the test, which guarantees a life of 10,000 hours.

Reliability Evaluation of Concentric Butterfly Valve Using Statistical Hypothesis Test

A butterfly valve is a type of flow-control device typically used to regulate a fluid flow. This paper presents an estimation of the shape parameter of the Weibull distribution, characteristic life, and life for a concentric butterfly valve based on a statistical analysis of the reliability test data taken before and after the valve improvement. The difference in the shape and scale parameters between the existing and improved valves is reviewed using a statistical hypothesis test. The test results indicate that the shape parameter of the improved valve is similar to that of the existing valve, and that the scale parameter of the improved valve is found to have increased. These analysis results are particularly useful for a reliability qualification test and the determination of the service life cycles.

Reliability Demonstration Test Method for Electromagnetic Shielding Doorset with a Sub-Unit Subjected to Preventive Replacement

An electromagnetic shielding doorset must satisfy requirements associated with both mechanical strength/durability and electromagnetic shielding. Among the primary components of the doorset, a finger strip—a leaftype spring normally made of beryllium copper—is a core part to shield electromagnetic waves as well as to endure repetitive dry sliding friction. This study presented a reliability demonstration test method for the doorset system and, by a simple and simultaneous implementation, of the replacement interval of the finger strip. A rigorous evaluation for the qualification of maintenance or replacement interval is included in the reliability demonstration test of any series system that holds critical maintenance sub-units. § 이 논문은 2014년도 대한기계학회 신뢰성부문 춘계학술대회 (2014. 2. 26.-28., 제주대) 발표논문임 † Corresponding Author, dohsik@kimm.re.kr C 2014 The Korean Society of Mechanical Engineers 신정훈 · 이효경 · 장 진 · 김도식 · 남태연 · 정동수 1172 resonance imaging, MRI) 시스템이나 전자기 펄스 (electromagnetic pulse) 방호용 구조물 등에 사용되 는 전자파 차폐시설의 유일한 출입구로서, 반복 작동되는 부품들의 기계적 내구성과 전자파 차폐 라는 전기적 성능 요구를 동시에 충족시켜야 하는 융합 기기이다. 본 논문에서 다루는 이 전자파차 폐용 문세트에 대한 제품평가기법은 아직 제한적 이고 고장 해석이나 예측 또한 난해한 실정이다. 제품의 완전한 고장 시점까지 시험하는 것은 있는 그대로의 제품의 성능을 평가할 수 있지만 과도한 시간과 비용이 요구된다. 그래서 개발현장에서는 많 은 경우 와이블 분포(Weibull distribution)를 가정한 적 은 고장 혹은 무고장(fewor zero-failure acceptance) 시 험이 수행된다. 권영일은 무고장 시험의 경제성 을 보다 숙고한 시험방법을 제안하였고, yadav 등은 시스템을 물리특성·기능·시간 차원에서 분류하여 고장과 관련된 핵심적인 요소를 규명한 입증시험법 (demonstration test planning)을 제안하였다. 전자파차폐 용 구조물에 대해서는 미국공병단(united state army corps of engineers)에서 제정한 인증규격이 존재하는 데 전체 규격 중 하나의 장(chapter)에서 문세트에 관 해 기술하고 있다. 하지만 설계단계에서의 신뢰성을 보증하기 위해서는 현재보다 더 철저한 시험이 요구 된다. 기존의 단순한 전자파차폐 문세트의 시험법의 한계를 극복하고자 본 연구에서는 보다 철저한 시 험절차를 개발하였다. 특히 주기적으로 교체가 필 요한 핵심 구성품인 베릴륨동(Beryllium-Copper) 재 질의 핑거스트립(finger strip)을 가진 전자파 문세 트 시스템의 신뢰성을 인증(qualification)하기 위한 수명 입증시험법을 개발하였다. 본문에서는 신뢰 성 보증에 필요한 시스템 분석과 전체 시험항목도 출을 수행하고 신뢰성에 핵심이 되는 수명시험설 계에 관해 상술한다. 일반 건축용 문세트에 대해 서는 이미 ISO 나 KS에서 그 시험법이 규격화 되어 있고, 전자파차폐용 문세트에 대해서는 미주 지역의 MIL-STD-188-125 혹은 유럽의 IEEE 299 규격에서 상세히 규정하고 있으므로 본 논문의 각종 신뢰성시험항목 역시 기존 규격들에 기술된 방법들을 상당부분 그대로 채용하였다. 최종적으 로 설계단계에서의 문세트의 전방위적 신뢰성입증 절차를 MRI용 문 세트를 예시로 설명한다.

Low Temperature Effects on the Strength and Fracture Toughness of Membrane for LNG Storage Tank

Tensile and fracture toughness tests of the cold-rolled STS 304 steel plate for membrane material of LNG storage tank were performed at wide range of temperatures, 11 IK(boiling point of LNG), 153K , 193K and 293K(room temperature). Tensile strength significantly increases with a decrease in temperature, but the yield strength is relatively insensitive to temperature. Elongation at 193K abruptly decreases by 50% of that at 293K, and then decreases slightly in the temperature range of 193K to 111K. Strain hardening exponents at low temperatures are about four times as high as that at 293K. Elastic-plastic fracture toughness() and tearing modulus() tend to decrease with a decrease in temperature. The values are inversely related to effective yield strength in the temperature range of 111K to 293K. These phenomena result from a significant increase in the amount of transformed martensite in low temperature regions.