보일러 헤더 기동시의 탄성 크리프 해석에 의한 열응력 평가

신규인 ( Kyu In Shin ),윤기봉 ( Kee Bong Yoon ) 한국안전학회 2009 한국안전학회지 Vol.24 No.4

Thermal stress and elastic creeping stress analysis was conducted by finite element method to simulate start-up process of a boiler header of 500MW standard fossil power plant. Start-up temperature and operating pressure history were simplified from the real field data and they were used for the thermal stress analysis. Two kinds of thermal stress analysis were considered. In the first case only temperature increase was considered and in the second case both of temperature and operating pressure histories were considered. In the first analysis peak stress was occurred during the temperature increase from the room temperature. Hence cracking or fracture may occur at the temperature far below the operating maximum temperature. In the results of the second analysis von Mises stress appeared to be higher after the second temperature increase. This is due to internal pressure increase not due to the thermal stress. When the stress components of radial(r), hoop(θ) and longitudinal(z) stress were investigated, compression hoop stress was occurred at inner surface of the stub tube when the temperature increased from room temperature to elevated temperature. Then it was changed to tension hoop stress and increased because of the operating pressure. It was expected that frequent start-up and shut-down operations could cause thermal fatigue damage and cracking at the stub tube hole in the header. Elastic-creeping analysis was also carried out to investigate the stress relaxation due to creep and stabilized stress after considerable elapsed time. The results could be used for assessing the creep damage and the residual life of the boiler header during the long-term service.

마찰면의 압력 분포를 고려한 제동디스크의 열응력 해석

이영민(Y. M. Lee),박재실(J. S. Park),석창성(C. S. Seok),이찬우(C. W. Lee),김재훈(J. H. Kim) 한국정밀공학회 2005 한국정밀공학회 학술발표대회 논문집 Vol.2005 No.10월

A brake disk and a pad are important parts that affect the braking stability of a railway vehicle. Especially, because a brake disk stops the vehicle using conversion of the kinetic energy to frictional energy, thermal fatigue cracks are generated by the cyclic thermal load, as frictional heat, on a frictional surface and these cracks cause the fracture of a brake disk. Therefore, many researches for the thermal stress must be performed to improve the efficiency of brake disk and ensure the braking stability. In this study, we performed the thermal stress analysis for a ventilated brake disk with 3-D analysis model. For that, we simplified the shape of a ventilated hole to minimize problems that could be occurred in analysis process . Thermal stress analysis was performed in case that pressure distributions on a frictional surface is constant and is not. To determine pressure distributions of irregular case, pressure distribution analysis for a frictional surface was carried out. Finally using the results that were obtained through pressure distribution analysis, we carried out thermal stress analysis of each case and investigated the results of thermal stress analysis.

Thermal Stress Estimation due to Temperature Difference in the Wall Thickness for Thinned Feedwater Heater Tube

딘홍보,유종민,윤기봉 한국에너지학회 2019 에너지공학 Vol.28 No.3

A major stress determining the remaining life of the tube in feedwater heater of fossil fuel power plant is hoop stress by the internal pressure. However, thermal stress due to temperature difference across the wall thickness also contributed to reduce the remaining life of the tube. Therefore, thermal loading must be considered even though the contribution of internal pressure loading to the stresses of the tube was known to be much higher than that of the thermal loading. In this study, thermal stress of the tubes in the de-superheating zone was estimated, which was generated due to the temperature difference across the tube thickness. Analytic equations were shown for determining the hoop stress and the radial stress of the tube with uniform thinning and for the temperature across the tube thickness. Accuracy and effectiveness of the analytic equations for the stresses were verified by comparing the results obtained by the analytic equations with those obtained from finite element analysis. Using finite element analysis, the stresses for eccentric thinning were also determined. The effect of heat transfer coefficient on thermal stress was investigated using series of finite element analyses with various values of heat transfer coefficient for both inner and outer surface of the tube. It was shown that the effect of heat transfer coefficient at outer surface was larger than that of heat transfer coefficient at inner surface on the thermal stress of the tube. Also, the hoop stress was larger than the radial stress for both cases of uniformly and eccentrically thinned tubes when the thermal loading was only considered without internal pressure loading.

ESPI 장비를 활용한 사형 주조품의 잔류응력 측정 및 주조 열응력 해석

곽시영,남정호 한국주조공학회 2020 한국주조공학회지 Vol.40 No.1

Many studies involving a thermal stress analysis using computational methods have been conducted, though there have been relatively few experimental attempts to investigate thermal stress phenomena. Casting products undergo thermal stress variations during the casting process as the temperature drops from the melting temperature to room temperature, with gradient cooling also occurring from the surface to the core. It is difficult to examine thermal stress states continuously during the casting process. Therefore, only the final states of thermal stress and deformations can be detemined. In this study, specimens sensitive to thermal stress, were made by a casting process. After which the residual stress levels in the specimens were measured by a hole drilling method with Electron Speckle-Interferometry technique. Subsequently, we examined the thermal stresses in terms of deformation during the casting process by means of a numerical analysis. Finally, we compared the experimental and numerical analysis results. It was found that the numerical thermal stress analysis is an effective means of understanding the stress generation mechanism in casting products during the casting process.

Prediction of welding residual stress with real-time phase transformation by CFD thermal analysis

Cheon, Jason,Na, Suck-Joo Elsevier 2017 International journal of mechanical sciences Vol.131 No.-

<P><B>Abstract</B></P> <P>A new method of numerical thermal-metallurgical-mechanical analysis for the gas metal arc welding process is proposed. The latest thermal-metallurgical analysis method, using real-time temperature and phase fraction history was employed in this study. The thermal-metallurgical strain behavior of a wide temperature range of austenization is considered, using dilatometry information and temperature and phase fraction dependent mechanical properties. A simple volumetric strain balance was used to describe the effect of phase transformation in the mechanical analysis. Following validation of the thermal-metallurgical analysis, the stress distribution predicted by the thermal-metallurgical-mechanical analysis was compared with measured directional stresses. In the results, the phase transformed weldment did not always appear to be expanded when a wide range of austenization temperatures was considered in the heating process. Moreover, the thermal-metallurgical-mechanical analysis of the welding process identified stress concentration on the near welding line, and nonlinear deflection by the gradient of total volumetric strain, and phase dependent yield strength. However, the model for this study did not consider the mechanical history reset of the liquid state. Also, the limitation of the stress measurement method made it difficult to make a detailed comparison with the calculated results. The model change method to allow treatment of the liquid state, and a neutron diffraction method for complete validation, will be utilized in future work to overcome these limitations.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A new method of numerical thermal-metallurgical-mechanical analysis is proposed. </LI> <LI> A new thermal-metallurgical strain behavior of welding process is proposed. </LI> <LI> A simple volumetric strain balance was used to describe the strain history. </LI> <LI> The phase transformed weldment is apparently not always expanded. </LI> <LI> The stress concentration was affected due to the phase dependent strain and yield strength. </LI> </UL> </P> <P><B>Graphic abstract</B></P> <P>[DISPLAY OMISSION]</P>

Thermal stress analysis around a cavity on a bimetal

Tugba Baytak,Osman Bulut 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.69 No.1

The plates made of two materials joined to each other having the different coefficient of thermal expansions are frequently encountered in the industrial applications. The stress analysis of these members under the effect of high-temperature variation has great importance in design. In this study, the stress analysis of the experimental model developed for the problem considered here was performed by the method of photothermoelasticity. The thermal strains were formed by the mechanical way and these were fixed by the strain freezing method. For the stress measurements, the method of slicing is applied which provides three-dimensional stress analysis. The analytical solution in the literature was compared with the related stress distribution obtained from the model. Moreover, the axisymmetric finite element model developed for the problem was solved by ABAQUS and the results obtained here compared with those of the experimental model and the analytical solution. As a result of this study, this experimental method and numerical model can be used for these type of thermal stress problems which have not been comprehensively analyzed yet.

Parametric study for optimal design of an air plasma sprayed thermal barrier coating system with respect to thermal stress

Lim, Jang Gyun,Seo, Sangjae,Koo, Jae Mean,Seok, Chang Sung,Choi, Jae Boong,Kim, Moon Ki Elsevier Sequoia 2017 Surface & coatings technology Vol.315 No.-

<P><B>Abstract</B></P> <P>The use of thermal barrier coatings (TBCs) is expected to become more popular in various gas turbines because these coatings provide excellent thermal insulation and damage protection. However, unexpectedly early failure has often discouraged the full use of TBC, resulting in a shortening of the life span of gas turbines because these substrates are directly exposed to harsh operation conditions. The general mechanics of TBC and its complex inner phenomena, mainly related to failure, have been investigated to prolong the lifetime of TBC. However, our understanding is limited because TBC has various specifications and operating conditions, and complex interplays between many factors. The primary goal of this study is to construct an extensive finite element method (FEM) model to evaluate thermal stress of an air plasma sprayed TBC under its operating conditions by considering various inner phenomena, including thermal grown oxide, undulating topology of coating interface, aluminum depletion, creep effect, and elastoplastic deformation. With the proposed FEM model, a parametric study has been conducted with respect to a variety of material properties, as well as the thickness of bond coat and of top coat. The influence of each design factor on thermal stress and the other parameters is fully discussed. Finally, this work leads to optimal design criteria for minimizing thermal stress across a wide range of TBC systems.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A parametric study was carried out for optimal design of thermal barrier coating. </LI> <LI> Elastic modulus of top coat is the most sensitive design parameter to thermal stress. </LI> <LI> Thermal expansion mismatch between top and bond coat was tuned by coating thickness. </LI> </UL> </P>

Thermal stress intensity factor solutions for reactor pressure vessel nozzles

정시화,정경석,마완준,양준석,최재붕,김문기 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.6

To ensure the safety margin of a reactor pressure vessel (RPV) under normal operating conditions, it isregulated through the pressure-temperature (P-T) limit curve. The stress intensity factor (SIF) obtainedby the internal pressure and thermal load should be obtained through crack analysis of the nozzle cornercrack in advance to generate the P-T limit curve for the nozzle. In the ASME code Section XI, Appendix G,the SIF via the internal pressure for the nozzle corner crack is expressed as a function of the cooling orheating rate, and the wall thickness, however, the SIF via the thermal load is presented as a polynomialformat based on the stress linearization analysis results. Inevitably, the SIF can only be obtained throughfinite element (FE) analysis. In this paper, simple prediction equations of the SIF via the thermal loadunder, cool-down and heat-up conditions are presented. For the Korean standard nuclear power plant,three geometric variables were set and 72 cases of RPV models were made, and then the heat transferanalysis and thermal stress analysis were performed sequentially. Based on the FE results, simple engineering solutions predicting the value of thermal SIF under cool-down and heat-up conditions aresuggested

대형 용접구조물의 탄소성 열변형 해석을 위한 용접부의 변형률 경계조건에 관한 연구

하윤석,Ha, Yun-Sok 대한용접접합학회 2011 대한용접·접합학회지 Vol.29 No.4

A thermal distortion analysis which takes strains directly as boundary conditions removed barrier of analysis time for the evaluation of welding distortion in a large shell structure like ship block. If the FE analysis time is dramatically reduced, the structure modeling time or the input-value calculating time will become a new issue. On the contrary to this, if the calculation time of analysis input-value is dramatically reduced and its results also are more meaningful, a little longer analysis time could be affirmative. In this study, instead of using inherent strain based on elastic analysis, a thermal strain based on elasto-plastic analysis is used as the boundary condition of weldments in order to evaluate the welding distortion. Here, the thermal strain at the weldment was established by using a stress-strain curve established from the test results. It is possible to automatically recognize the modeling induced-stiffness in the shrinkage direction of welded or heated region. The validity of elasto-plastic thermal distortion analysis was verified through the experiment results with various welding sequence.

Thermal tolerance mechanism of invasive cotton mealybug parasitoid, Aenasius arizonensis Girault (Hemiptera: Encyrtidae)

Thimmegowda MN.,Suresh Suroshe Sachin,Sagar D. 한국응용곤충학회 2024 Journal of Asia-Pacific Entomology Vol.27 No.1

The sustainability and fitness of biocontrol agents depends on the abiotic factors of which temperature is the most critical factor. Thermal stress affects parasitoid behavior owing to change in physiological activities. So, an experiment was conducted to know the effect of thermal stress on the antioxidant enzymes in Aenasius arizonensis (Girault), a specific parasitoid of cotton mealybug, Phenacoccus solenopsis (Tinsley). Enzyme activity in P. solenopsis and A. arizonensis at each thermal stress treatment was noticed. At 3 h of thermal stress, the tem perature dependent increase in the SOD activity was observed. The maximum activity was noticed at 41 ◦ C in both P. solenopsis and A. arizonensis compared to the 27 ◦ C. The SOD activity was found more in four hours exposure compared to the three hours exposure at all the temperature treatments. Catalase activity was also found to increase with the increase in temperature, the effect of thermal stress was found to be significant in A. arizonensis and P. solenopsis for the activity of catalase. The maximum GST activity was found at 41 ◦ C for both P. solenopsis nymphs and A. arizonensis adults exposed to different temperature treatments for 3 h of duration. However, all the temperature treatments for P. solenopsis nymphs were non-significant, whereas, for A. arizonensis, it was found significant for the GST activity. We observed that thermal stress had no effect on the concentration of MDA in both P. solenopsis and A. arizonensis subjected to different durations of temperature regimes. Elevated levels of SOD, CAT and GST might be providing possible protection against reactive oxygen species generated under elevated thermal stress.