Finite Element Creep Analysis Research Articles

Internal Pressure Creep Rupture Test and Finite Element Analysis of a Tube with a Circumferential Crack on the Outer Surface

Internal Pressure Creep Rupture Test and Finite Element Analysis of a Tube with a Circumferential Crack on the Outer Surface

Distinctive role of plastic and creep strain in directional coarsening of a Ni-base single crystal superalloy

The distinctive role of plastic and creep strain in directional coarsening of a single crystal Ni-base superalloy, CMSX-4, was experimentally and analytically investigated. First, solid cylindrical samples were subjected to uniaxial tensile plastic strain at 900°C followed by thermal aging at 1080°C. Microstructural observation revealed that the extent of directional coarsening became more pronounced with increasing plastic strain until it reached 0.2%; no further coarsening was induced with the introduction of additional plastic strain. Second, several combinations of plastic and creep strain were applied to the samples. The magnitude of the compressive creep strain at 1080°C was varied with the plastic strain fixed at 1.5% at 900°C. The results indicated that the effect of 0.15% creep strain was completely balanced with that of the 1.5% plastic strain; hence, the effects of the plastic and creep strain on the morphological changes differed quantitatively. Finally, elastic-plastic and creep finite element analyses were conducted to rationalize the experimental results. The analytical investigations suggested that the directional coarsening was dependent on the inelastic strain gap between the γ and γ’ phases, which was attributed to an inhomogeneous relaxation of the local misfit stress field. The difference in the roles of plastic and creep strain in the coarsening was observed to quantitatively correspond to the difference in the localized inelastic deformations.

Evaluation of Simplified Creep Design Methods Based on the Case Analysis of Tee Joint at Elevated Temperature

Based on a tee joint, the simplified creep design methods of ASME code, elastic-perfectly plastic (EPP) analysis of Code Case N-861 (CC N-861) and the inelastic analysis by isochronous stress strain (ISS) curve, were evaluated and compared to nonlinear finite element creep analysis (FECA). Results indicate that both EPP and FECA lead to a greater inelastic strain than ISS curve-based results. By contrast, the ISS-based analysis induces a smaller usage fraction (damage) than FECA due to the underestimated inelastic strain. In addition, the smallest usage fraction (damage) is obtained by using EPP analysis of CC N-861 in which case the remarkable bending strain is involved.

Time-scales for the redistribution of stress in the course of pressuremeter creep tests in permafrost

This study deals with nonlinear creep and the time taken for completion of the stress redistribution process within a borehole scenario. Specifically, the pressuremeter test, with emphasis on a step loading of long duration, in ice saturated frozen soils is analyzed. A closed form stationary creep solution based on the model of a thick cylinder under plane strain condition, that considers large strains, is first developed. Subsequently non-stationary creep finite element analyses were carried out via this model and an assumption-relaxed model that considers the actual geometry of a typical pressuremeter test (where the net cavity pressure is applied only on a portion of the borehole cavity). Finally estimates of the extent to which stress redistribution occurs is numerically considered by revisiting classical theories and proposing a time scale by introducing a redistribution index, RI.

9CrMAG鋼溶接継手のクリープ強度評価

Mod.9Cr steels are widely used as boiler materials in ultra-super critical power plants. In this study, creep tests were conducted using miniature specimens with 1mm diameter machined from a base metal, weld metal and a heat affected zone (HAZ) in a Mod.9Cr-1Mo welded joint. Creep deformation and rupture behaviors of the miniature base metal specimens were almost identical with those obtained from standard size specimens. Material constants of the Norton law were determined from obtained datum by the miniature creep specimens. Creep rupture times of the miniature welded joint specimens were much lower than those of the standard size specimens. The finite element creep analysis results indicated that the triaxiality factor in the HAZ of the miniature welded joint is smaller than that of the standard size welded joint resulting in faster creep strain rate in the miniature specimen under the same applied stress.

CrMoV鍛鋼環状切欠き試験片のボイド成長シミュレーションに基づくクリープ損傷評価

Creep damage preferentially extends at a stress concentration portion in high temperature components such as steam turbine rotors. Therefore development of an accurate damage assessment method for creep damage process at the stress concentration portion under multiaxial stress states is necessary to maintain reliable operation. In this study, creep tests using a plain specimen and two kinds of round bar notch specimens with different notch radius on a CrMoV forging steel have been conducted to clarify effect of stress conditions on creep damage extension process. Three dimensional finite element creep analyses have been performed to discuss relation between the creep damage and stress conditions. Creep rupture time of the plain specimen is shorter than those of the notch specimens. Creep rupture time of the notch specimen with lower stress concentrate factor is longer than that with higher stress concentration factor. In the notch specimens, the maximum stresses occur at notch root at initial loading and portions of the maximum stresses gradually change to inside of the specimens with time due to stress redistribution. Triaxial tensile stress yields at the notch root sections with different distributions of the triaxiality factor depending on the notch radius. Void number densities at the maximum stress portion in the notch specimens are ten times larger than that in the plain specimen. The void growth simulation method developed previously was applied to predict the void number density under multiaxial stress states in the notch specimens. Distributions of the void number density from notch root to center of the specimen both in the notch specimens were quantitatively predicted by the void growth simulation method. An equation, which predicts change of void number density with time under a certain maximum stress and a triaxiality factor, was derived based on the void growth simulation under different multiaxial stress states.

Clamp Load Decay Due to Material Creep of Lightweight-Material Joints Under Cyclic Temperature

Experimental and finite element techniques are used for investigating the effect of cyclic thermal loading on the clamp load decay in preloaded single-lap bolted joints that are made of multimaterial lightweight alloys. Substrate material combinations include aluminum, magnesium, and steel, with various coupon thicknesses. The range of cyclic temperature profile varies between −20 °C and +150 °C in a computer-controlled environmental chamber for generating the desired cyclic temperature profile and durations. Real time clamp load data are recorded using strain gage-based, high-temperature, load cells. Clamp load decay is investigated for various combinations of joint materials, initial preload level, and substrate thickness. Thermal and material creep finite element analysis (FEA) is performed using temperature-dependent mechanical properties. The FEA model and results provided a valuable insight into the experimental results regarding the vulnerability of some lightweight materials to significant material creep at higher temperatures.

Determination of creep properties of P91 by small punch testing

Small punch creep testing was conducted to determine creep properties of P91 at 873 K. The procedure followed the European Code of Practice for Small Punch Tests. Results of small punch creep test and finite element analysis were compared and discussed. Based on the results, a practical method was proposed to determine the parameters of Norton creep law more conveniently for material which obey this law. The validity of the proposed method was verified and it is shown that the parameters of the Norton creep law can be determined from only one experimental curve directly.

Non-destructive prediction method of creep damage and remnant life related to those under creep–fatigue interactive conditions for nickel base superalloys

In order to clarify the microdamage formation behaviour for high temperature brittle IN100 alloy, a polycrystalline nickel base superalloy, in situ observational test was carried out to observe creep damage formation behaviour of a notched IN100 specimen. Furthermore, the experimental findings of creep damage formation were evaluated by an original elastic–plastic creep finite element analysis. As a result, microcracks and damage forming areas were found to develop in the creep damage areas suggested by this analysis. This has resulted in the development of a non-destructive prediction method for creep damage accumulation and material’s remnant life. The application of this method to the cases under creep–fatigue conditions was addressed.

Generation of plastic influence functions for J-integral and crack opening displacement of thin-walled pipes with a short circumferential through-wall crack

Fracture mechanics parameters such as the J-integral and crack opening displacement (COD), are necessary for Leak-Before-Break (LBB) evaluation. The famous two estimation methods, the GE/EPRI and the Reference Stress Method (RSM), have their applicability limit with regard to the ratio of a pipe mean radius to thickness (Rm/t). In order to extend their applicability limit to a thin walled pipe, several finite element analyses are performed for the J-integral and COD, and then new plastic influence functions are developed for thin-walled pipes with a short circumferential through-wall crack. With the newly generated plastic influence functions, the GE/EPRI and the RSM give closer results with those obtained from detailed finite element analyses. In addition, C*-integral and COD rate are estimated by using the new plastic influence functions and they are well matched with elastic–creep finite element analysis results under the power-law creep condition. Since the LBB concept can be applied to a piping system in a Korean Sodium-cooled Fast Reactor (SFR) which is designed to have thin-walled pipes and to operate in high temperature enough to cause creep, this paper can be applied for the LBB assessment of thin-walled pipes with a short through-wall crack in the SFR.

Creep Analysis of Polymer Matrix Composites by Finite Element Method Based on the Shift Factors Method

Creep is one of important properties of polymer matrix composites (PMC).It can be analysis by finite element method (FEM).But creep will be affected by a lot of factors and the creep material parameters are very difficult to be obtained. Shift factors method is one simple method which can be used to obtain the creep properties by fewer tests. Here the basic method analysis of the creep of PMC by FEM was studied. Focus on the analysis of how the material parameters of finite element creep analysis needed to get through the shift factors method. Finally the actual PMC pipe structure is calculated by this method.The error of FEM result and theory result around 2%, the results show that this method has certain rationality and reliability.

Finite element calculation of contour integral parameters for cracked P91 pipe weld

This paper reports the results of elastic–creep finite element (FE) analyses of a P91 steel pipe weld with two external ‘type IV’ circumferential cracks, subjected to internal pressure and end (system) load using creep properties obtained at 650°C. Numerical contour integral calculations have been performed to obtain both transient and stationary high temperature fracture mechanics parameters. A mesh sensitivity analysis was performed in order to ensure the accuracy of FE analyses in the transient creep stage. The effects of load magnitude, the material mismatch near the crack surfaces and the crack depth on the stationary creep C* contour integrals have been investigated, and corresponding analytical correlations are presented.

직관 용접부의 크리프 특성 불균일에 따른 열영향부 정상상태 응력 예측

본 논문은 크리프 파단 수명평가에 주요인자인 정상상태 크리프 응력을 직관 용접부에 대해 정량화한다. 모재와 용접부의 크리프 특성 불균일이 응력에 미치는 영향을 체계적으로 분석하기 위해, 다양한 용접부 불균일에 대해 이차원 유한요소 크리프 탄성해석을 수행하였다. 용접부는 열영향부를 고려하였으며 각각의 재료는 이상화된 탄성-멱 크리프 법칙을 따른다고 가정하였다. 하중에 따른 영향을 보기위해 내압과 인장하중에 대해 연구를 수행하였다. 용접부 크리프 응력의 정량화를 위해 크리프 불균일 지수를 도입하였으며, 무차원화된 단면평균응력과 선형적인 관계를 확인하였다. 불균일 지수로 정량화한 응력과 Type IV 영역을 모사한 용접부의 유한요소해석 결과 및 영국전력의 R5 문헌값의 비교를 통해 연구결과의 유효성을 검증하였다.

Numerical study of local PWHT condition for EB welded joint between first and side walls in ITER-TBM

The stress relaxation behavior of post-weld heat treatment (PWHT) F82H was examined to determine the appropriate PWHT condition for electron beam welded joints between the first and side walls of the International Thermonuclear Experimental Reactor (ITER) test blanket module. Thermal elastic–plastic creep finite element analyses were conducted assuming Norton creep law. Uniform heat treatment and four types of local heating near the weld line were studied. Numerical analyses concluded that the welding residual stress could be reduced by a local heat treatment near the penetration from both the first and side walls, where the cooling area and rate could be controlled. In addition, the stress on the channel surface in the first wall was found to decrease to less than 50MPa with the appropriate local PWHT.

Experience with using the LICON methodology for predicting long term creep behaviour in materials

The LICON methodology is a newly developed approach for predicting the lifetime of materials under creep loading condition. The LICON method has gained attention for application to newly developed materials, for which there is little existing creep-rupture data, and for their welded structures. The LICON method predicts long term uniaxial creep strength using the results from several short duration creep crack incubation tests in conjunction with the outcome of a mechanical analysis (e.g. involving finite element analysis, at least initially for a new material and/or multi-axial specimen geometry). In this study, the sensitivity of results from the LICON method to input conditions for the associated finite element analysis is investigated. Two important considerations for creep finite element analyses are the mesh configuration and the type of creep constitutive model used. In order to evaluate the sensitivity of the outcome of the approach to finite element mesh condition, twelve different configurations in terms of size, geometry and interpolating formulation are examined. Moreover, to examine the sensitivity of the outcome of the approach to the type of creep equation employed, three constitutive models have been evaluated. Finally, the sensitivity of the LICON approach to the evaluated conditions have been discussed with reference to the information contained in a comprehensive creep-rupture database available for a 1%CrMoV steel at 550 °C.

1408 スリット入りつば付き試験片のクリープ変形特性と変形一様性(OS15-2 諸観点からの物質・材料の特性評価II,OS15 諸観点からの物質・材料の特性評価)

Since numerical analyses under elevated temperature usually depend on a constitutive equation of creep, it is necessary to derive a high-precision equation. Creep properties on deformation can be normally obtained using constant load creep machines, but constant stress creep tests are more suitable to acquire high-precision creep data. Conventional uniaxial creep specimens have ridges attached on both sides of gauge length. However, even if the constant stress creep testing machines are used, these ridges prevent area reduction around them, and cause unequal deformation in the gauge part. Authors have proposed cutting slits into the ridge to release the specimen from the circumferential constraint by extensometer ridges. In this study, taking into account a realistic analytic condition, the authors carried out elastic-plastic creep Finite Element (FE) analyses to investigate the performance of uniform deformation. Subsequently, a series of interrupted constant stress creep, tests using SUS304 steel were conducted to examine the effect of introducing slits. The analytical and the experimental results showed that the introduction of these slits into the ridges would affect the deformation during creep, but not improve the uniformity of deformation along the gauge part.

자동차 인스트루먼트 패널에 사용되는 플라스틱의 크리프 거동

자동차 인스트루먼트 패널에 사용되는 플라스틱의 온도와 시간에 따른 기계적 거동을 알아보기 위해 시편에 대한 인장 및 크리프 시험을 수행하였다. 여러 가지 온도에서의 물성치를 구한 결과 온도에 따라 물성치가 크게 변화함을 알 수 있었다. 4 가지 하중조건하에서의 3 점굽힘 크리프시험을 수행하고 지수법칙의 시간경화식을 적용하여 각종 계수를 구하고 크리프 거동을 간단한 식으로 표현하였다. 유한요소 크리프 해석 결과 하중이 커짐에 따라 수치해석 결과와 실험 결과가 차이가 커지지만 대체적으로 두 결과가 유사한 것을 확인할 수 있었다.

G030061 単軸クリープ試験片へのスリット導入と変形一様性([G03006]材料力学部門一般セッション(6):高温・環境)

Specific numerical analyses have been required at designing modern high temperature machines and structures. Since analytical results usually depend on a constitutive equation of creep, it is necessary to derive a high-precision equation. Creep properties on deformation can be normally obtained using constant load creep machines, but constant stress creep tests are more suitable to acquire high-precision creep data. In order to keep the true stress constant, an applied load or the lever ratio should be changed during the test duration. Conventional uniaxial creep specimens have ridges attached on both sides of gauge length to measure deformation outside an electric furnace. For ductile materials, however, even if the constant stress creep testing machine are used, these ridges constrain area reduction around them, and unequal-uniform deformation breaks the constant volume condition in the gauge portion during deformation. Authors have proposed introducing slits into the ridge to release the circumferential constraint by the extensometer ridges. In this study, taking into account a realistic analytic condition, the authors carried out elastic creep Finite Element (FE) analyses to investigate an optimum depth of the slit. Subsequently, a series of constant stress creep tests using SUS304 steel are conducted to examine the effect of introducing slits on the performance of uniform deformation. The analytical and the experimental results show that the introduction of these slits into the ridges would improve uniform deformation effectively.

Pervasiveness of Excessive Segmental Bridge Deflections: Wake-Up Call for Creep

An obsolete standard recommendation for creep design led to significant underestimation of the observed 18-year deflections of the Koror-Babeldaob (KB) Bridge in the island nation of Palau. A search for data on similar bridges revealed that 56 other large-span, prestressed concrete, segmentally erected box girders (66 by the time of proof) have been found to exhibit excessive long-time deflections. There are probably many more in existence. The observed deflections give no sign of approaching a finite bound, as implied in the empirical ACI Committee 209, CEB-fib, and GL models for creep. They were found to evolve approximately logarithmically beginning at about 1000 days after span closing. While sufficient data for the finite element creep analysis of these deflections were not obtainable, comparisons with accurate deflection solutions for the KB Bridge showed that the terminal logarithmic deflection trend can be predicted well by a simple extrapolation of the measured 1000-day deflection under the hypothesis of proportionality to the compliance function increment since the time of span closing. Comparisons of the extrapolations according to various creep models show that the underestimation of long-time deflections is much less severe for the theoretically based Model B3 than it is for the three other models, and that the terminal trend is logarithmic. A simple update of this model that gives the same mean terminal trend as the 56 bridges is devised that should allow for improving the durability of segmental bridges.

Behavior of Branch Cracking and the Microstructural Strengthening Mechanism of Polycrystalline Ni-Base Superalloy, IN100 under Creep Condition

Recently, the development of the high-efficiency technology for gas turbine and jet engine is required to minimize carbon dioxide and nitrogen oxide emission. It is effective way to increase the operational temperature to develop the high-efficiency technology for high temperature instruments. To increase the operating temperature, advanced Ni-base superalloys have been developed as a turbine blade material.Even though a Ni-base superalloy is used for a structural component, creep damages and creep cracks may be caused due to the external tensile load under high temperature conditions. Therefore, a predictive law of creep crack growth life is necessary to maintain operational safety.This study is aimed to clarify the branch cracking behavior due to the microstructural strengthening mechanism of polycrystalline Ni-base superalloy IN100 under the creep condition. The creep crack growth tests were conducted at a temperature of 900°C. The creep crack growth behavior and creep damage formulation were observed by in-situ observational system and FE-SEM/EBSD. Additionally, two-dimensional elastic-plastic creep finite element analyses were conducted for the model which describes the experimental results. The creep crack growth behavior and the creep damage progression were found to be affected by the distribution behaviors of grains and grain boundaries around the notch tip. By comparison of experimental results with mechanical analysis using FEM analyses, mechanisms of the creep crack growth and the creep damage formulation were clarified.