Universal Slope Method Research Articles

Effect of Thinning on Fatigue Life Estimation of Elbows under In-Plane Cyclic Bending Moment

The safety of the piping systems used in any nuclear or thermal power plant is essential to the environment and also to the people involved. Pipe bends are one of the critical components of the piping system. Hence, fatigue life estimation becomes primary focus when the designing pipe bends for their safety limits. The current study uses finite element analysis and the revised manson universal slope method to estimate the life of elbows under in-plane opening and closing cyclic bending moment. The estimated fatigue life was compared with low-cycle fatigue tests in the literature. The displacement of 20 mm was given as input to estimate the number of cycles. The principal stresses were obtained on the highly strained node of the finite element model to locate the crack and its direction accurately. The elbows with thinning have a reduction in the fatigue life of about 19%.

Utilization of motion capture systems for low cycle fatigue tests on induction-hardened steel

Motion capture systems were used to control the low cycle fatigue test of heated and unheated specimens obtained from induction-hardened curved steel braces. The two motion capture systems used in this study demonstrated that the axial-strain control was achieved, and the strain distributions were visualized using digital image correlation. Additionally, the test results demonstrated that the fatigue lives of the unheated and heated specimens can be predicted using Manson's universal slopes method. However, the obtained fatigue lives were slightly shorter. Induction hardening proved to be an effective method to increase the fatigue life of specimens subjected to small strain.

Assessing the Compression Fatigue of the Welded Test Specimens

The focus of this paper is on compression fatigue life in welded test specimen. Considered test specimen is highly complex due to multi-layer welding and production errors affecting its topology. Test specimen is made from S355 steel which is used for both base material and weld. Compression fatigue analysis is seldom performed as compression positively affects fatigue life. At the same time, during MAG welding residual stresses affect the fatigue life, decreasing it significantly. Finite element method (FEM) is applied to estimate compression fatigue life of the welded test specimen and obtained fatigue curve is compared to the experimental curve. Experimental curves are given for different stress ratios: R= -1, and R= - ∞. Analysis consists of thermal and structural analysis using FEM method and fatigue analysis which is based on FEM results. FEM thermal simulation of welding with constant initial temperature is performed first and followed by nonlinear structural analysis for assessment of the residual stresses due to heat expansion. Fatigue parameters are determined using Universal slope method.

Design, analysis and experimental qualification of gripper bellows for FBTR control rod drive mechanism

The design of welded-disc bellows is not addressed in any design codes or standards. This paper discusses the design, analysis and experimental validation of the gripper bellows used in the control rod drive mechanisms of fast breeder test reactor. The material of construction of the bellows is 0.125 mm thick AM350-SCT1000. The design fatigue curve of the material at 803 K is not available in open literature. The fatigue curve of the material (Coffin-Manson's relation) was generated from the tensile testing data using four-point correlation method and universal slopes method. Elasto-plastic finite element analysis of the bellows was carried out and fatigue life of the bellows was estimated numerically. A factor of safety of 20 on number of cycles was used as suggested in codes such as ASME section-III and RCC-MR. Experimental qualification of the bellows as per ASME section-III, appendix-II was carried by testing of the bellows in sodium at 803 K.

Improved accuracy of low cycle fatigue life prediction of elbow

Elbow piping is characterized by complicated stress conditions and easy wall thinning. Therefore, a highly accurate fatigue life prediction method is required to accurately evaluate the soundness of elbow piping. In previous studies, the revised universal slope method was proposed as a life prediction formula for elbow pipes, and the relationship between strain range and fatigue life was obtained. However, with the conventional analysis method, the strain range is overestimated compared to the actual behavior, resulting in a significantly conservative prediction result. In this study, we will improve the prediction accuracy of the strain range by using a more detailed stress-strain diagram than before in the finite element analysis and examine the further improvement of the fatigue life prediction.

The status of the Japanese material properties handbook and the challenge to facilitate structural design criteria for DEMO in-vessel components

This paper summarizes the current status of the material properties handbook for a structural design using Japanese reduced-activation ferritic/martensitic steel F82H. Specifically, the key structural parameters, e.g. time-independent/dependent design stresses and fatigue design curves, were determined by following the French structural design code RCC-MRx. Moreover, under the Japan–U.S. collaboration, tensile data were newly added to the benchmark heavy irradiation data up to 80 dpa, as critical input information in the intermediate check and review in Japan. Furthermore, the status of structural material data and the near-term and long-term issues were clarified by the evaluation using the attribute guides. In parallel, the structural design approaches, which were newly introduced and extended to cope with the structural design issues under the complex environmental conditions peculiar to the DEMO reactor, were noted with the initial R&D results. Of the many design issues, the multi-axial loading conditions due to the complexity of the DEMO reactor as well as the coolant compatibility and the irradiation effect are mentioned. For example, in the paper, multi-axial fatigue–creep testing and evaluation using the modified universal slope method and brittle/ductile fracture testing and evaluation using the local approach are explained toward DEMO.

Computational Fatigue Analysis of Auxetic Cellular Structures Made of SLM AlSi10Mg Alloy

In this study, a computational fatigue analysis of topology optimised auxetic cellular structures made of Selective Laser Melting (SLM) AlSi10Mg alloy is presented. Structures were selected from the Pareto front obtained by the multi-objective optimisation. Five structures with different negative Poisson’s ratios were considered for the parametric numerical analysis, where the fillet radius of cellular struts has been chosen as a parameter. The fatigue life of the analysed structures was determined by the strain–life approach using the Universal Slope method, where the needed material parameters were determined according to the experimental results obtained by quasi-static unidirectional tensile tests. The obtained computational results have shown that generally less auxetic structures tend to have a better fatigue life expectancy. Furthermore, the fillet radius has a significant impact on fatigue life. In general, the fatigue life decreases for smaller fillet radiuses (less than 0.3 mm) as a consequence of the high-stress concentrations, and also for larger fillet radiuses (more than 0.6 mm) due to the moving of the plastic zone away from the edge of the cell connections. The obtained computational results serve as a basis for further investigation, which should be focused on the experimental testing of the fabricated auxetic cellular structures made of SLM AlSi10Mg alloy under cyclic loading conditions.

Proposal of an alternating bending technique for evaluating low‐to‐high cycle fatigue of structural steels

AbstractThis paper proposes an alternating bending technique for evaluating fatigue life in the low‐to‐high cycle fatigue regime. A method was developed for estimating the stress, elastic strain, and plastic strain ranges of a plastically deformed specimen subjected to alternating bending with consideration of stress and strain distributions. To evaluate its effectiveness, fatigue testing was conducted using a specimen made of a steel used for pressure vessels. The stress, elastic strain, and plastic strain ranges could be obtained during cyclic bending. The elastic strain amplitude life and plastic strain amplitude life curves were linear in a log–log plot in the low‐to‐high cycle fatigue regime. Hence, the fatigue life under alternating bending could be evaluated using the proposed strain‐based approach. However, these curves could not be predicted using equations with parameters obtained from tensile testing, such as the universal slope method, due to the strain gradient in the specimen.

Methods for determination of low-cycle properties from monotonic tensile tests of 1.2344 steel applied for hot forging dies

Selected methods for determining fatigue life in the strain-life approach, described by the Manson-Coffin-Basquin (MCB) equation based on results of static tensile tests are presented in this paper. These methods served for determining low-cycle characteristics for 1.2344 steel applied for dies used in forging at high temperatures. The authors presented the results of tensile tests performed at room temperature and at elevated temperatures (300 °C, 500 °C, 600 °C, and 700 °C). Data obtained from these tests enabled to theoretically determine fatigue life characteristics according to the methods: universal slope, 4-point correlation, Mitchell’s, modified universal slope method, Baumel’s and Seeger’s method, modified 4-point-correlation method, Roessle’s and Fatemi’s method, as well as median method. Curves obtained on this basis were then compared to charts obtained from periodic deformation tests. Results obtained in this manner were compared to actual wear of tools (made of 1.2344 steel) used in the hot forging process of a cover-type forging. Fatigue tests covered deformation of specimens with uniaxial, periodically variable load until their cracking, and were performed at temperatures 20 °C, 300 °C, and 600 °C and at four levels of total strain eac: 0.5%, 0.8%, 1.0%, and 2.0%. Clear differences between the values of coefficients and exponents of the MCB equation were observed depending on the applied theoretical method and depending on test temperature. The research goal is description of different methods for determining fatigue life in strain-life approach based on results of static tensile tests and comparison them with the results from experiment. This will allow choosing the best method to predict the fatigue life of tools made of steel 1.2344 and usage of this method in the future, saving time, and costs related to the performance of experimental tests.

Evaluation of Multiaxial Creep-fatigue Strength for High Chromium Steel under Non-proportional Loading

This study discusses the result of creep-fatigue tests of a high-chromium steel, F82H which was designed as blanket structural materials of nuclear fusion reactor, carried out at room temperature to 823K in air. Strain paths applied were a push-pull loading and a circle loading in which normal and shear strain have 90 degree phase difference. The holding times used were 180 s and 600 s. Moreover, an evaluation of failure life by taking into account intensities of creep and non-proportionality is discussed based on both the life evaluation proposed by Itoh, et al and method of modified universal slopes. Availability of the equation for the life evaluation was confirmed by comparison with conventional universal slope method.

Effects of test environment on high temperature fatigue properties of reduced activation ferritic/martensitic steel, F82H

Reduced activation ferritic/martensitic (RAFM) steels are assumed as structural material of blanket design activities. Determining the design limit is essential for the RAFM steels to qualify their use in fusion blankets. This work investigates fatigue properties of a RAFM steel, F82H at elevated temperature up to 823 K in the air and a vacuum condition. The tests were carried out with axial strain controlled condition using cylindrical specimen, and the results were summarized using the Manson’s modified universal slope method, which can describe fatigue lifetime as a function of tensile properties. Manson’s modified universal slope method gives good estimation for lifetime of F82H at temperature below 823 K. However, F82H-IEA demonstrated shorter lifetime than the estimation. The lifetime at vacuum condition was successfully described using universal slope method. On the other hand, the lifetime in the air was 1/3 or shorter than that in vacuum condition. It is notable that a heat of F82H prepared for IEA round robin tests demonstrated 1/5 of shorter lifetime than the latest F82H.

Creep-Fatigue Interaction Life Consumption of Industrial Gas Turbine Blades

This paper presents the creep-fatigue interaction life consumption of industrial gas turbine blades using the LM2500+ engine operated at Pulrose Power station, Isle of Mann as a case study. The linear damage summation approach where creep damage and fatigue damage are combined was used for the creep-fatigue interaction life consumption of the target blades. The creep damage was modelled with the Larson-Miller parameter method while fatigue damage was assessed with the modified universal slopes method and the damage due to creep-fatigue interaction was obtained from the respective life fractions. Because of the difficulty in predicting the life of engine components accurately, relative life consumption analysis was carried out in the work using the concept of creep-fatigue interaction factor which is the ratio of the creep-fatigue interaction life obtained from any condition of engine operation to a reference creep-fatigue interaction life. The developed creep-fatigue interaction life consumption analysis procedure was applied to 8 most of real engine operation. It was observed that the contribution of creep to creep-fatigue interaction life consumption is greater than that of fatigue at all ambient temperatures. The fatigue contribution is greater at lower ambient temperatures as against higher ambient temperatures. For the case study, the overall equivalent creep-fatigue factor obtained was 1.5 which indicates safe engine operation compared to the reference condition. The developed life analysis algorithm could be applied to other engines and could serve as useful tool in engine life monitoring by engine operators.

The Revised Universal Slope Method to Predict the Low-Cycle Fatigue Lives of Elbow and Tee Pipes

The stress states of elbow and tee pipes are complex and different from those of straight pipes. The low-cycle fatigue lives of elbows and tees cannot be predicted by Manson's universal slope method; however, a revised universal method proposed by Takahashi et al. was able to predict with high accuracy the low-cycle fatigue lives of elbows under combined cyclic bending and internal pressure. The objective of this study was to confirm the validity of the revised universal slope method for the prediction of low-cycle fatigue behaviors of elbows and tees of various shapes and dimensions under conditions of in-plane bending and internal pressure. Finite element analysis (FEA) was carried out to simulate the low-cycle fatigue behaviors observed in previous experimental studies of elbows and tees. The low-cycle fatigue behaviors, such as the area of crack initiation, the direction of crack growth, and the fatigue lives, obtained by the analysis were compared with previously obtained experimental data. Based on this comparison, the revised universal slope method was found to accurately predict the low-cycle fatigue behaviors of elbows and tees under internal pressure conditions regardless of differences in shape and dimensions.

Fatigue crack initiation and propagation in lotus-type porous material

The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation (Ni) and crack propagation (Np) period where the total fatigue life (N) is defined as: N = Ni + Np. The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, Ni, required for formation of initial cracks. The number of stress cycles, Np, required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction.

Tensile and fatigue properties of potassium doped and rhenium containing tungsten rods for fusion reactor applications

In this study, large diameter pure tungsten (PW) and potassium doped W (KW) rods fabricated using a swaging process were investigated, in order to develop high performance plasma-facing materials (PFM) for fusion reactor applications. The KW rods show a finer grain size, a more compact [110] grain orientation, and a more uniform grain size distribution than that of the PW rods. Tensile tests of the KW rods along the axial direction indicated higher tensile strength at each test temperature than what is found in PW rods. On the other hand, both types of rod have nearly identical elongation values. Further comparison of the rods with rolled PW and KW plates indicate that the W rods show larger elongation and lower strength than what is observed in W plates. Meanwhile, the longer fatigue life was observed for the W rods, as predicted by Manson's universal slope method. In addition, a K-doped W rod with 3% rhenium (Re) (KW3Re) was investigated in this study, which featured improved recrystallization and grain growth behavior. Suppression of grain growth after recrystallization was observed in KW3Re rod. High performance PFMs based on these materials can be expected with careful consideration of the fabrication process and additional elements used in their creation.

A comparison of methods for predicting the fatigue life of gray cast iron at elevated temperatures

AbstractHigh temperature fatigue life tests were compared with various life assessment methods for gray cast iron in the cylinder liners of marine engines. The plastic strain range‐based methods such as the universal slopes method, Mitchell's method, Bäumel and Seeger's method, and Ong's method were not predicted well. A method employing tensile energy density, which is defined as the sum of the tensile area of the plastic energy and the elastic energy in the hysteresis loop, was suggested as an improved alternative to the plastic strain range‐based methods. The life estimation equation using tensile energy density predicted well within the 3X scatter band at various temperature ranges of the gray cast iron.

OS0912-215 修正共通勾配法によるエルボとティの低サイクル疲労寿動評価 : 配管寸法と形状の影響

The stress state of elbow and tee pipes is complex and different from those of straight pipes. Manson's universal slope method cannot predict the low-cycle fatigue lives of elbow and tee pipes under cyclic bending and internal pressure conditions. In this work, the authors carried out a finite element analysis for simulating the reported experimental results of elbow and tee pipes. The crack initiation area and the crack growth direction were successfully predicted by the analyses. The analytical results showed that the revised Manson's universal slope method can accurately predict the low-cycle fatigue lives of elbow and tee pipes under internal pressure conditions regardless of shape and dimensions.

OS0911-202 修正共通勾配法によるエルボの低サイクル疲労寿命評価 : 配管要素ベンチマーク解析における解析事例

The special task group of the JSME Codes and Standards Committee has planned to make a guideline for excessive cyclic loading of piping. For this purpose, the special task group conducted benchmark analyses to simulate the test results of an elbow pipe under cyclic loading. The authors carried out a finite element analysis and evaluated the fatigue behavior and fatigue life of the elbow pipe under cyclic loading. The following results were obtained. (a) The crack initiation area was successfully predicted by the analyses. (b) The fatigue life of the elbow pipe was predicted conservatively by the analysis based on the prediction curve for piping components proposed by NUPEC and the revised Manson's universal slope method.

GS-3 修正共通勾配法による配管要素の低サイクル疲労寿命評価 : 配管寸法と形状の影響(一般セッション 産業・化学機械のイノベーションにおける課題と対策)

GS-3 修正共通勾配法による配管要素の低サイクル疲労寿命評価 : 配管寸法と形状の影響(一般セッション 産業・化学機械のイノベーションにおける課題と対策)

Multiaxial Fatigue Life Prediction for Steels Based on Some Simple Approximations

The Roessle-Fatemi’s hardness method (HM) and Muralidharan-Manson’s modified universal slopes method (MUSM) were employed to determine the uniaxial fatigue properties of steels from easily obtained tensile properties. Both methods give good life predictions, while the Roessle-Fatemi’s HM is somewhat better. Furthermore, for predicting multiaxial fatigue lives of steels in the absence of any fatigue data, the Li’s modified Wang-Brown model (MWB) was used in combination with the HM method (MWB-HM) as well as the MUSM method (MWB-MUSM), respectively. Correlation between the yield strength and the Brinell hardness was also developed to estimate the multiaxial fatigue lives of steels based only on hardness and elasticity modulus. It is shown that multiaxial fatigue lives were predicted fairly well by all the methods, and the MWB-MUSM method is slightly more accurate. In addition, a computer-based procedure for multiaxial fatigue life assessment incorporating MWB-MUSM approach was proposed and implemented to predict the fatigue life of an intermediate compressor casing. The predicted results are promising.