Creep processes in silicon nitride ceramics

Frantisek Lofaj,Sheldon M. Wiederhorn 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.3

Recent advances in our understanding of creep deformation in silicon nitride ceramics are reviewed and compared to two different models of creep. The classical models adopted from the metals literature are based on the assumption that creep occurs primarily by diffusion of atoms either through the grains, or along grain boundaries. The cavitation model of creep was developed specifically to explain creep in materials that consist of rigid grains with a mobile secondary phase at the grain boundaries, materials having structures similar to that of silicon nitride. Well-known effects such as creep asymmetry and a very wide range of stress exponents in the early commercial grades of silicon nitride can be fully understood within the framework of the cavitation models. The work discussed includes an identification of critical types of creep cavities in silicon nitride, the evolution of cavities with tensile strain, and an analysis of possible mechanisms involved in cavity formation. The analysis amplifies the cavitation creep model of Luecke and Wiederhorn and assumes that creep occurs via a combination of grain boundary sliding, viscous flow and solution-precipitation of the crystalline secondary phase, resulting in a redistribution of this phase among the multigrain junctions of the solid. The increase in creep resistance in the latest generation of silicon nitride materials was found to be related to the suppression of cavitation and a shift toward non-cavitation creep mechanisms. Differences between volume conservative mechanisms in tension and compression depend on the existence of different driving forces for creep: local tensile/compressive stresses and/or dilatational stresses. Increasing the viscosity of residual glassy films at the grain boundaries is believed to be an effective way to suppress cavitation and increase creep resistance. The addition of Lu+3 and N−3 to the bulk oxynitride glasses, similar to those at the grain boundary films, increases their viscosity. Thus, the suppression of cavitation and the higher creep resistance of the Lu-containing silicon nitride can be explained by the combined effect of Lu+3 and N−3 in the residual glass. Recent advances in our understanding of creep deformation in silicon nitride ceramics are reviewed and compared to two different models of creep. The classical models adopted from the metals literature are based on the assumption that creep occurs primarily by diffusion of atoms either through the grains, or along grain boundaries. The cavitation model of creep was developed specifically to explain creep in materials that consist of rigid grains with a mobile secondary phase at the grain boundaries, materials having structures similar to that of silicon nitride. Well-known effects such as creep asymmetry and a very wide range of stress exponents in the early commercial grades of silicon nitride can be fully understood within the framework of the cavitation models. The work discussed includes an identification of critical types of creep cavities in silicon nitride, the evolution of cavities with tensile strain, and an analysis of possible mechanisms involved in cavity formation. The analysis amplifies the cavitation creep model of Luecke and Wiederhorn and assumes that creep occurs via a combination of grain boundary sliding, viscous flow and solution-precipitation of the crystalline secondary phase, resulting in a redistribution of this phase among the multigrain junctions of the solid. The increase in creep resistance in the latest generation of silicon nitride materials was found to be related to the suppression of cavitation and a shift toward non-cavitation creep mechanisms. Differences between volume conservative mechanisms in tension and compression depend on the existence of different driving forces for creep: local tensile/compressive stresses and/or dilatational stresses. Increasing the viscosity of residual glassy films at the grain boundaries is believed to be an effective way to suppress cavitation and increase creep resistance. The addition of Lu+3 and N−3 to the bulk oxynitride glasses, similar to those at the grain boundary films, increases their viscosity. Thus, the suppression of cavitation and the higher creep resistance of the Lu-containing silicon nitride can be explained by the combined effect of Lu+3 and N−3 in the residual glass.

Experimental study on creep and shrinkage of high-performance ultra lightweight cement composite of 60MPa

Chia, Kok-Seng,Liu, Xuemei,Liew, Jat-Yuen Richard,Zhang, Min-Hong Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.5

Creep and shrinkage behaviour of an ultra lightweight cement composite (ULCC) up to 450 days was evaluated in comparison with those of a normal weight aggregate concrete (NWAC) and a lightweight aggregate concrete (LWAC) with similar 28-day compressive strength. The ULCC is characterized by low density < 1500 $kg/m^3$ and high compressive strength about 60 MPa. Autogenous shrinkage increased rapidly in the ULCC at early-age and almost 95% occurred prior to the start of creep test at 28 days. Hence, majority of shrinkage of the ULCC during creep test was drying shrinkage. Total shrinkage of the ULCC during the 450-day creep test was the lowest compared to the NWAC and LWAC. However, corresponding total creep in the ULCC was the highest with high proportion attributed to basic creep (${\geq}$ ~90%) and limited drying creep. The high creep of the ULCC is likely due to its low elastic modulus. Specific creep of the ULCC was similar to that of the NWAC, but more than 80% higher than the LWAC. Creep coefficient of the ULCC was about 47% lower than that of the NWAC but about 18% higher than that of the LWAC. Among five creep models evaluated which tend to over-estimate the creep coefficient of the ULCC, EC2 model gives acceptable prediction within +25% deviations. The EC2 model may be used as a first approximate for the creep of ULCC in the designs of steel-concrete composites or sandwich structures in the absence of other relevant creep data.

Experimental study on creep and shrinkage of high- performance ultra lightweight cement composite of 60MPa

Kok-Seng Chia,Xuemei Liu,Jat-Yuen Richard Liew,Min-Hong Zhang 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.5

Creep and shrinkage behaviour of an ultra lightweight cement composite (ULCC) up to 450 days was evaluated in comparison with those of a normal weight aggregate concrete (NWAC) and a lightweight aggregate concrete (LWAC) with similar 28-day compressive strength. The ULCC is characterized by low density < 1500 kg/m3 and high compressive strength about 60 MPa. Autogenous shrinkage increased rapidly in the ULCC at early-age and almost 95% occurred prior to the start of creep test at 28 days. Hence, majority of shrinkage of the ULCC during creep test was drying shrinkage. Total shrinkage of the ULCC during the 450-day creep test was the lowest compared to the NWAC and LWAC. However, corresponding total creep in the ULCC was the highest with high proportion attributed to basiccreep (≥ ~90%) and limited drying creep. The high creep of the ULCC is likely due to its low elastic modulus. Specific creep of the ULCC was similar to that of the NWAC, but more than 80% higher than the LWAC. Creep coefficient of the ULCC was about 47% lower than that of the NWAC but about 18% higher than that of the LWAC. Among five creep models evaluated which tend to over-estimate the creep coefficient of the ULCC, EC2 model gives acceptable prediction within +25% deviations. The EC2 model may be used as a first approximate for the creep of ULCC in the designs of steel-concrete composites or sandwich structures in the absence of other relevant creep data.

Modified projection model-based constant-stress creep curve for alloy 690 steam generator tube material

문성인,김종민,권준엽,이봉상,최권재,김민철,한상배 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.3

Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperatureduring an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (q) projection method has been widely used for modeling creep-strain behavior under constantstress. However, many creep test data are obtained under constant load, so creep rupture behavior undera constant load cannot be accurately simulated due to the different stress conditions. This paper proposesa novel methodology to obtain the creep curve under constant stress using a modified q projectionmethod that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology arealso discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novelcreep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied toevaluate the creep rupture behavior of SG tubes. The results of this study show its great potential toevaluate the rupture behavior of an SG tube governed by creep deformation.

Creep 변형을 고려한 압밀해석 프로그램의 개발과 적용성 분석

김수삼,정승용,안상로,Kim, Su-Sam,Jeong, Seung-Yong,An, Sang-Ro 한국지반공학회 1998 지반 : 한국지반공학회지 Vol.14 No.5

본 연구에서는 파괴강도이내의 일정하중 상태에서 장기적으로 진행되는 연약지반의 침하특성을 파악하기 위하여 Creep 변형성분을 고려한 구성방정식을 유도하고, 유한요소해석 프로그램을 개발하여 이를 바탕으로 토공구조물의 장기적인 변형을 합리적으로 예측함으로서 연약지반상 토공구조 물의 설계 및 시공관리에 기여하고자 하였다. 점성토의 탄.소성거동을 표현하기 위해 Modified Cam Clay모델이 사용되었으며, Creep변형을 계산하는데 있어서는 체적 Creep요소를 고려할 경우 2차 압밀계수 C,를 적용하였고, 축차 Creep요소를 고려할 경우 Singh & Mitchel Creep경험식을 통한 m, a, A 상수를 이용한 것으로 개발된 프로그램의 신뢰성을 검증하기 위하여 이론해 및 실험치와 비교하였고, 적용된 각 상수들의 민감도 를 분석한 결과, 개발된 프로그램은 적용성이 좋은 것으로 판단되었다. 또한 국내외 2개의 현장에 적용한 결과, 제방의 변형해석에 있어 Creep을 고려하지 않은 경우보다 Creep을 고려한 경우가 오차가 적게 나타나고 체적 Creep만을 고려한 경우는 약간 과소평가되 고 축차 Creep까지 고려한 경우는 약간 과대평가됨을 알 수 있었다. 따라서 Creep정수를 얻기 위한 실험기의 개발, 적정 토질정수의 선택 등 향후에도 지속적인 연구가 요구된다. This research is focused on the inducement of the constitutive equation considering the creep strain component and on the development of a finite element method program. The purpose of this research was to contribute to the design of construction structures or to the construction management in soft clay ground through predicting the long-term strain of construction structures reasonably bused on the above program. Modified Cam Clay model was adopted to describe the elastic-plastic behavior of clayey soil. And in the calculation of the creep sprain, the secondary coefficient of consolidation C. was applied for considering the volumetric creep element and the constants m, $\alpha$, A were rosed by the empirical creep equation proposed by Singh 8E Mitchell for considering the deviatoric creep element. To examine the reliability of the program which is developed in this study, the estimated values by this program were compared with the theoretical solution and the experimental results. And the applicability of the developed program was found to be reliable from the sensitive analysis of each parameters used in this study. According to the results obtained from the application of the program on the field measurement data, the estimated values by the program were found with be consistent with the actual values. And from the analysis of the displacement of embankments, the case of considering the creep behavior induced much fower errors than the case of neglecting it. But the results obtained from considering the volumetric creep behavior only were slightly underestimated the results from considering the deviator creep behavior showed the slightly overestimated values. Therefore, it remains the task of further studios to develop the laboratory test devices to obtain the reliable creep parameters, and to select the appropriate soil parameters, etc.

Thermal aging of Gr. 91 steel in supercritical thermal plant and its effect on structural integrity at elevated temperature

원민구,정시화,허남수,김우곤,이형연 한국원자력학회 2024 Nuclear Engineering and Technology Vol.56 No.1

In this study, the influence of thermal aging on structural integrity is investigated for Gr. 91 steel. A commercial grade Gr. 91 steel is used for the virgin material, and service-exposed Gr. 91 steel is sampled from a steam pipe of a super critical plant. Time versus creep strain curves are obtained through creep tests with various stress levels at 600 C for the virgin and service-exposed Gr. 91 steels, respectively. Based on the creep test results, the improved Omega model is characterized for describing the total creep strain curve for both Gr. 91 steels. The proposed parameters for creep deformation model are used for predicting the steady-state creep strain rate, creep rupture curve, and stress relaxation. Creep-fatigue damage is evaluated for the intermediate heat exchanger (IHX) in a large-scale sodium test facility of STELLA-2 by using creep deformation model with proposed creep parameters and creep rupture curve for both Gr. 91 steels. Based on the comparison results of creep fatigue damage for the virgin and serviceexposed Gr. 91 steels, the thermal aging effect has been shown to be significant.

증기발생기 전열관용 Alloy 690 소재의 크리프 특성에 미치는 탄화물 형성의 영향

김형규,홍석민,김종민,김민철,이영국 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.4

The creep properties of Alloy 690, used as a steam generator tube material in nuclear power plants,were evaluated at 650oC, 750oC, and 850oC. The parameters of creep life prediction models were derived usingthe Larson-Miller (LM), Manson-Haferd (MH), and Orr-Sherby-Dorn (OSD) models, to use as mechanicalproperties under a virtual severe accident condition like station black out (SBO). The yield strength (YS) andcreep property of Alloy 690 were compared with those of Alloy 600, and the effects of the precipitation behaviorof Cr carbides on creep properties were analyzed. The YS of Alloy 600 decreased rapidly above the temperatureof 750oC, but the YS of Alloy 690 decreased linearly up to the temperature of 850oC because of the formationof M23C6 carbides. The creep stress exponent (n) of Alloy 690 was between 5 and 6, and this indicated thatdislocation creep was the major creep mechanism at the test temperatures. The results of creep tests werewell matched with the LM, MH, and OSD models for Alloy 690, and there were no significant differences inaccuracy between the models. The stress-rupture test results of Alloy 600 and Alloy 690 using the LM modelshowed that the decrease in creep strength with rupture time of Alloy 690 was steeper than that of Alloy 600at high temperatures. This indicated that Alloy 690 was more susceptible to creep degradation under longtermcreep conditions. The precipitation of Cr carbides in Alloy 690 increased YS, benefitting creep propertiesfor short-term creep. However, the Cr carbides coarsened significantly under loading conditions at hightemperature, and this deteriorated the creep properties for long-term creep.

Thermal creep behavior of CZ cladding under biaxial stress state

Jin, Xin,Lin, Yuyu,Zhang, Libin Korean Nuclear Society 2020 Nuclear Engineering and Technology Vol.52 No.12

Thermal creep is a key property of zircaloy cladding. CZ developed by CGN is a new zircaloy used as PWR fuel cladding. This research is devoted to investigating the thermal creep behavior of CZ and build the thermal creep model of CZ. Twenty internal pressure creep tests were conducted, and the ranges of temperature and Tresca stress were 320-430 ℃ and 70-300 MPa, respectively. Real-time creep data were analyzed by separating primary creep and steady-state creep. Based on Soderberg model and creep test data, CZ thermal creep model is derived. As a whole, the mean value and the standard deviation of P/M of CZ saturated primary creep strain are very close to these from steady-state creep rate, however, the predictive effect of primary creep is less satisfactory. Four conditions, where there exists large deviation between predicted values and test data, are 320 ℃ and 300 MPa, 350 ℃ and 190 MPa, 380 ℃ and 160 MPa, 380 ℃ and 190 MPa, respectively. As primary creep was much smaller than steady-state creep in long-time operation, the thermal creep model built can be applied to predict the thermal creep behavior of CZ cladding.

응력수준 및 함수비에 따른 이암의 Creep 특성에 관한 연구

이영휘(Lee Younghuy),정강복(Jeong Ghangbok) 한국지반환경공학회 2012 한국지반환경공학회논문집 Vol.13 No.3

본 연구에서는 포항이암을 대상으로 다양한 경계조건에서의 creep 시험을 수행하여 이암의 시간의존적 특성을 분석하였으며, 이를 바탕으로 기존에 발표된 creep에 대한 점탄성 모델 및 경험식들과 비교?분석함으로써 더 합리적으로 creep 특성을 평가할 수 있는 방법을 제안하고자 한다. Creep 시험결과를 Griggs(1936), Cottrell(1952) 및 Singh(1975)이 제안한 경험식에 대하여 분석한 결과, 전반적으로 응력수준 및 함수비가 증가할수록 creep 상수가 증가하는 경향을 나타내었다. 또한, Singh의 경험식에 의한 예측치는 실측치와 매우 높은 상관성을 보이고 있는 것을 확인할 수 있었으며, 이로부터 이암의 크리프 거동을 해석함에 있어 Singh의 경험식을 적용함이 바람직할 것으로 판단된다. 유동학적 모델인 Burger 모델에 의한 예측치와 실측치를 비교한 결과, 매우 좋은 일치를 보이고 있음을 알 수 있으며, 이로부터 이암의 크리프 거동을 해석함에 있어 Maxwell 요소와 Kelvin 요소를 직렬로 연결한 Burger 모델을 적용하는 것이 바람직할 것으로 판단된다. The time-dependent characteristics of mudstone in this study were analyzed by performing creep tests for the mudstone in Pohang. The creep tests were conducted on various boundary conditions to observe the time-dependent behavior of mudstone and the results were compared with values predicted by established visco-elastic model and empirical equations. As a result of analysis for empirical equations proposed by Griggs(1936), Cottrell(1952) and Singh(1975), the creep constants generally tend to increase as increasing the stress level and water content. Moreover, the values predicted by Singh"s equation were well fitted for the test results. Therefore, it is expected to be reasonable that creep behavior of mudstone is analyzed using Singh"s equation. As a result of analysis for a Burger’s model, the predicted values were well fitted for the test results. Therefore, it is expected be reasonable that the creep behavior of mudstone by a rheological model is analyzed using Burger’s model.

Comparison of Direct and Indirect Measurement of Creep Uncertainty in the Calibration of Force Measuring Devices

마르세스(Manrusces Enot),장진석(Jin-seok Jang),박연규(Yon-Kyu Park) Korean Society for Precision Engineering 2018 한국정밀공학회지 Vol.35 No.7

As per ISO376 : 2011, creep uncertainty can be measured directly or indirectly. In this regard, this paper seeks to provide a comparison between direct and indirect creeps computed from hysteresis. All computations for direct and indirect creeps were done using equations from ISO376 : 2011. Five force measuring devices were experimentally examined for this purpose. Results showed that the behaviors of direct and indirect creeps were quite different. The relative creep that was directly measured was constant. On the other hand, the relative creep that was indirectly estimated varied with the applying force. Therefore, the directly measured creep cannot be replaced by the indirect one. This paper proposes a method to use a representative value for indirect creep, as the maximum of the creep. For the force measuring devices that had good hysteresis characteristics, the directly and indirectly measured creeps were comparable. However, for the force measuring devices with poor hysteresis characteristics, the indirectly estimated creep was much higher than the directly measured creep. Therefore, it is highly recommended to measure the creep directly for the force measuring devices characterized by poor hysteresis.