• 한국자동차공학회논문집
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• 제18권3호
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• pp.133-142
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• 2010

The present work focuses on the characterization of material parameters of the Overlay(multilinear hardening) model for analyzing the non-isothermal cyclic deformation. In the previous study, all the parameters were especially based on the Overlay theories, and a simple method was suggested to find out the best material parameters for the isothermal cyclic deformation analysis. Based on the previous research this paper f dther improves the isothermal parameters and suggests how to apply the isothermal parameters to the non-isothermal conditions especially for the description of TMF(Thermo-Mechanical Fatigue) hysteresis behavior. The parameters are determined and calibrated using 400 series stainless steel test data in the reference papers. For the implementation into ABAQUS, a user subroutine is developed by means of ABAQUS/UMAT. The finite element results show good agreement with test for the case of uniaxial non-isothermal cyclic loading, signifying the proposed method can be used in the TMF analysis of the converter-inserted heavy duty muffler system and the stainless steel exhaust-manifold system which are to be done in our future research.

• 비파괴검사학회지
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• 제29권1호
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• pp.10-14
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• 2009

초음파 적외선 비파괴 열화상 검사기술의 발열 메커니즘은 정확히 규명되지 않았으나, 열-기계 연성효과와 결함 계면 사이의 마찰효과가 주요한 원인인 것으로 추정되고 있다. 본 논문에서는 피로균열을 갖는 알루미늄 합금 시험편에서 결함을 검출하고, 실험조건으로부터 각각의 메커니즘에 따라 온도 변화를 수치 예측하였다. 시험결과와 수치예측 결과로부터 발열의 주요한 원인이 마찰이라는 것을 밝혔다.

• Smart Structures and Systems
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• 제6권1호
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• pp.73-85
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• 2010

The potential offered by the thermo-mechanical properties of shape memory alloys (SMA) in structural engineering applications has been the topic of many research studies during the last two decades. The main issues concern the long-term predictability of the material behaviour and the fatigue lifetime of the macro structural elements (as different from the one of wire segments). The laboratory tests reported in this paper are carried out on bar specimens and they were planned in order to pursue two objectives. First, the creep phenomenon is investigated for two different alloys, a classical Ni-Ti alloy and a Cu-based alloy. The attention is then focused on the Cu-based alloy only and its fatigue characteristics at given temperatures are investigated. Stress and thermal cycles are alternated to detect any path dependency.

• 대한기계학회논문집A
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• 제25권11호
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• pp.1713-1719
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• 2001

Estimation of fatigue strength on the spot welded joint is very Important for strength design of spot welded steed sheet structures. In this paper, the effect of residual stress on the fatigue life of resistance spot weldment was studied. Residual stress fields of weldment were calculated by using thermo elastic plastic finite element analysis and equivalent fatigue stress considering residual stress effect was obtained. And then we predicted fatigue life, which included the effect of the residual stresses and the actual loading stresses. The calculation and experimental results were in good agreement. Therefore, the proposed calculated model can be considered to be sufficiently powerful for the prediction of fatigue life.

• 한국수소및신에너지학회논문집
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• 제29권5호
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• pp.538-548
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• 2018

The thermo-acoustic instability in the combustion process of a gas turbine is caused by the interaction of the heat release mechanism and the pressure perturbation. These acoustic vibrations cause fatigue failure of the combustor and decrease the combustion efficiency. This study is to develop a segmented dynamic thermo-acoustic model to understand combustion instability of gas turbine. Therefore, this study required a dynamic analysis rather than static analysis, and developed a segmented model that can analyze the performance of the system over time using the Matlab/Simulink. The developed model can confirm the thermo-acoustic combustion instability and exhaust gas concentration in the combustion chamber according to the equivalent ratio change, and confirm the thermo-acoustic combustion instability for the inlet temperature and the load changes. As a result, segmented dynamic thermo-acoustic model has been developed to analyze combustion instability under the operating condition.

• 대한기계학회논문집A
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• 제34권7호
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• pp.911-917
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• 2010

본 연구에서는 열적-기계적 주기하중을 받고 있는 엔진 배기매니폴드에 대해서 열응력 해석방법과 열피로수명 예측과정을 제시하였다. 즉, 파손현상이 복잡한 배기시스템의 효율적인 유한요소 모델링 방법과 온도 의존성 재료의 시험결과를 이용한 해석 데이터 구성, 그리고 열사이클 하중에 대한 열응력 및 파손 예측방법을 디젤엔진의 배기매니폴드에 대해서 나타내었다. 일반적으로 배기매니폴드의 파손 취약부에서는 고온영역에서 큰 압축소성변형이 발생하고 냉각시에는 인장의 잔류응력이 나타난다. 따라서 이같은 응력과 변형률의 이력곡선으로부터 소성변형의 진폭 또는 소성에너지의 크기를 얻을 수 있으며 이를 통해서 피로수명을 예측할 수 있다.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2005년도 춘계학술발표회
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• pp.479-480
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• 2005

• 대한금속재료학회지
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• 제47권11호
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• pp.707-715
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• 2009

This paper deals with cyclic stress and strain responses during isothermal low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) loadings on Nb and Mo containing 15Cr stainless steel, which is used for exhaust manifolds in automobiles. The test temperatures ($T_{i}$) of the isothermal LCF were 600 and $800^{\circ}C$. The minimum temperature of the TMF test was $100^{\circ}C$ and the maximum temperaures ($T_{p}$) were varied between 500 and $800^{\circ}C$. In both loading conditions, weak cyclic softening is observed at $T_{i}=T_{p}=800^{\circ}C$, but the transition to strong cyclic hardening is completed with the temperature decrease below $T_i=600{\sim}700^{\circ}C$ for LCF and $T_{p}=500{\sim}600^{\circ}C$ for TMF. The stress-strain hysteresis loops in the TMF loading show a significant stress relaxation during compressive (heating) half cycle at $T_{p}>500^{\circ}C$, which develops tensile mean stress during cycling. Due to the stress relaxation, the TMF test sample reveals much lower dislocation density than the isothermally fatigued sample at the same temperature with $T_{p}$. A detailed correlation between fatigue microstructure and cycling deformation behavior is discussed.

• 한국재료학회지
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• 제19권6호
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• pp.337-343
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• 2009

In this study, we optimized Pb-free Sn/Ni plating thickness and conditions were optimized to counteract the environmental regulations, such as RoHS and ELV(End-of Life Vehicles). The $B_{10}$ life verification method was also suggested to have been successful when used with the accelerated life test(ALT) for assessing Pb-free solder joint life of piezoelectric (PZT) ceramic resonator. In order to evaluate the solder joint life, a modified Norris-Landzberg equation and a Coffin-Manson equation were utilized. Test vehicles that were composed of 2520 PZT ceramic resonator on FR-4 PCB with Sn-3.0Ag-0.5Cu for ALT were manufactured as well. Thermal shock test was conducted with 1,500 cycles from $(-40{\pm}2)^{\circ}C$ to $(120{\pm}2)^{\circ}C$, and 30 minutes dwell time at each temperature, respectively. It was discovered that the thermal shock test is a very useful method in introducing the CTE mismatch caused by thermo-mechanical stress at the solder joints. The resonance frequency of test components was measured and observed the microsection views were also observed to confirm the crack generation of the solder joints.

• 대한금속재료학회지
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• 제49권2호
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• pp.112-120
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• 2011

The out-of-phase thermo-mechanical fatigue (OP TMF) in a <001> oriented single crystal nickel-based superalloy CMSX-4 has been studied. OP TMF life was less than a half of low cycle fatigue(LCF) life in spite of a small hysteresis loop area of OP TMF compared to that of LCF. The failure was caused by the initiation of a crack at the oxide-layered surface followed by its planar growth along the <100> ${\gamma}$ channel in both LCF and OP TMF. However, deformation twins appeared near the major crack of OP TMF. The multiple groups of parallel twin plates on {111} planes provided a preferential path for crack propagation, which caused a significant decrease in OP TMF life. Additionally, the analysis on the surface crack morphology revealed that the tensile strain at the minimum temperature of OP TMF was found to accelerate the crack propagation.