Fatigue Design Curves Research Articles

鋼道路橋の疲労設計曲線に関するパラメトリック解析

Fatigue design curves of steel highway bridges were analyzed parametrically under computer simulated variable amplitude stresses. Crack propagation life of 4 types of welded joints was estimated by applying the constant amplitude da/dN-ΔK relationship and the liner cumulative damage rule. The configuration of fatigue life curves under variable amplitude stresses varied with the traffic constitution and the beam span particularly in the longer life region. The fatigue limit under variable amplitude stresses for each joint could be defined by using estimated fatigue life curves and Miner's rule.

Elastic creep-fatigue damage evaluation: less restrictive design rules for SNR 2 components

To achieve less conservative design rules for LMFBR Class 1 components in elevated temperatures service, two modifications of the elastic creep-fatigue evaluation in use are proposed: (1) In the creep-damage evaluation, the relaxation of residual stresses during hold-times shall be taken into account; (2) In the fatigue-damage evaluation, the creep effects on the cyclic stress strain hysteresis loop shall be considered by adding the net relaxation strain during holdtimes to the elastic-plastic strain range, and design fatigue curves without hold-time shall apply.

Protection against fatigue damage with respect to the environmental influence of LWR operating conditions

As nuclear power plant operation proceeds, a certain number of isolated leakages and cracks are found in pipes and components. Considering the extensive design and analysis efforts made, it is at least the early occurrence of the incidents in relation to the design lifetime of the material that is surprising. In the Federal Republic of Germany, a tremendous effort has been directed towards the development of optimized steels with respect to fracture toughness and weldability, the manufacturing of integral forged products (forged-on nozzles, seamless pipes and elbows), and the reduction of peak stresses by an advanced design (basic safety concept). The design of the areas of cold water injection has been improved to prevent temperature stratification. The safety of pipes and components has been considerably improved by these developments. Pipes and components previously affected by material, design and manufacturing problems have been replaced by components built to the new high standard of quality. An area which still deserves attention with respect to the lifetime of a plant is the protection against fatigue damage. The present design fatigue curve (ASME Section III or KTA 3201.2) only allows a generic assessment of the safety against cyclic loadings. In most cases, this concept has proved its worth and is sufficiently safe. In some cases, however, and especially in the presence of a corrosive environment, a detailed investigation is necessary. The design fatigue curve can no longer be applied if cracks form in the protective magnetite layer and, owing to this, strain-induced corrosion cracking occurs. The concept developed in this paper defines a number of correction factors which take into account the effects of a high-temperature water environment as a function of the duration of the strain cycle. It is based on work presented by General Electric. The concept developed allows a more detailed assessment of the local conditions and, consequently, serves as a useful tool for both the designer and the operator of the plant.

実働荷重疲労に対する信頼性設計手法の提唱

A reliability-based design method has been proposed for fatigue under service loads. The method is a practical one because it does not require to conduct fatigue tests on a large number of specimens under the relevant loading condition. In order to develop such a practical method, literature survey was carried out concerning the statistical nature of the cumulative cycle ratio D≅∑ni/Ñi where Ñi was the fatigue life under constant amplitude loading as given by the S-N curve. The survey revealed the following common tendencies: (i) D follows a log-normal distribution approximately, and (ii) ηD, the coefficient of variation of D, is roughly equal to ηN, the coefficient of variation of the fatigue life N under constant amplitude loading. Based on (i) and (ii) above, a reliability based design method was proposed. In this method, S-Ñ/k curve is used as the design fatigue curve, and ∑ni/(Ñi/k)≤1 is used as a design criterion. The factor k is determined from ln 1/k=ln(μD/√1+ηN2)+[Φ-1(Pa)+n-1/2Φ-1(γ)]√ln(1+ηN2) where, μD is the estimate of the mean value of D≅∑ni/Ñi, n is the number of specimens tested to obtain μD, 1-Pa is the reliability goal, 1-γ is the confidence coefficient, and Φ-1 is the inverse function of the standard normal distribution function. The above equation was derived from the condition that the reliability of 1-Pa is attained with the confidence of 1-γ. The most remarkable point in employing this design method is that fatigue tests to be conducted under the relevant loading condition is simply for obtaining μD, hence a large number of samples are not required. A simple design diagram for obtaining k easily was also proposed.

Loading conditions in horizontal feedwater pipes of LWRs influenced by thermal shock and thermal stratification effects

In recent years, a particular form of crack formation has occurred in a number of pressurized and boiling water reactors on the internal surfaces of horizontal lengths of feedwater piping upstream of the steam generators and reactor pressure vessels. The fractographic evaluation and the orientation of the cracks show that these are to be attributed to cyclic stressing in the axial direction. Comparison of the stresses due to thermal shock and thermal stratification reveals that, on account of the associated load cycles, the cracks were in essence caused by thermal stratification. This is also indicated by the orientation of the cracks. The present results of the corrosion tests show that with high oxygen content (450 ppb) and temperature level (210°C) the strain rate at thermal stratification exerts an essential influence on the number of cycles to crack initiation. With the conservative test conditions described, the values may fall below those given in the ASME fatigue design curve. In the case of strain rates that apply to thermal shock, the cycles to crack initiation are on the safe side of the curve. The remedial measures taken by KWU by the installation of the siphon reduce the frequency of stress amplitudes. It can be concluded from corrosion tests simulating these conditions that with the strain rates occurring in this case, the number of cycles to crack initiation are on the safe side according to the ASME fatigue design curve.

Analyses of Fatigue Test Data of Offshore Tubular Joints

A paper-surveying on fatigue tests of tubular and foundamental joints for offshore structure design was carried out and tested data were collected. The collected data were arranged as two sets of data base : the data base on tubular joint tests and the data base on foundamental joint tests including cruciform joints, butt-welded plates and so on. Based on these data bases, the regression analyses were conducted in order to evaluate effects of factors on the fatigue strength of these joints. Plate thickness, material, load ratio, random loading, corrosive environments, grinding, TIG-dressing etc. are included in these factors.Correlations between the effect of each of these factors on the fatigue strength of foundamental and tubular joints were discussed and it was, generally speaking, found that the correlations are good. Finally, the current fatigue design curves were compared with the collected data and the extent of safety of the curves were confirmed.

A Bayesian method for establishing fatigue design curves

From a reliability viewpoint, the simple conventional procedure for establishing design S− N curves from laboratory fatigue test data suffers from two important limitations. Firstly, the calculated fatigue reliability on the design curve only reflects the observed “physical” uncertainty associated with the fatigue process itself. “Statistical” uncertainties, connected with estimating the parameters of the fatigue model, are not considered. Secondly, it is not possible to account for fatigue run-outs (non-failures) in a rational manner. A new procedure, based on Bayesian statistical inference, is presented which is capable of handling both the above problems. Its use is illustrated via the analysis of some typical fatigue data sets and a number of general points arising from the results are discussed.

Low Cycle Fatigue of Commercial Piping Steels in a BWR Primary Water Environment

This report summarizes results of a comprehensive experimental program on the low cycle fatigue behavior of commercial grade piping steels in a high-pressure, high-temperature, oxygenated water environment. A total of 110 specimens of special design were tested in a boiling water reactor (BWR) primary water environment (500°F, 1040 psig (or 260° C, ∼ 7 MPa). Four piping steels, in several different metallurgical conditions, were involved (Types-304 and –304L stainless steels, Inconel-600 and A-516 carbon steel). Companion tests, employing the same types of specimens, were conducted in 500° F (260° C) air. The results of this work confirm the adequacy of the current ASME Section III fatigue design curves to account for the effect of a BWR primary water environment on the low cycle fatigue behavior of the four materials tested.

Comparative Stress Corrosion Behavior of Welded Austenitic Stainless Steel Pipe in High Temperature High Purity Oxygenated Water

Abstract An alloy for boiling water reactor (BWR) welded pipe applications that is not subject to intergranular stress corrosion cracking (IGSCC) has been qualified for service in high purity high temperature oxygenated water. This alloy provides extra margin against cracking as compared to reference Type 304 stainless steel. Several commercially available alloys were studied in the form of welded 4 inch (10 cm) diameter schedule 80 pipe. Basic resistance to IGSCC was evaluated utilizing cyclic load stress corrosion tests conducted on 4 foot (1.2 m) lengths of welded pipe. Once the basic IGSCC resistance was established, the corrosion fatigue resistance of the alloys was evaluated to verify the adequacy of the ASME Code fatigue design curve. Results of all the testing were analyzed in terms of a model that considers chemical composition, degree of sensitization, stress and phases present. The two optimum alternate alloys were selected.

Fatigue Threshold Stress Intensity and Life Estimation of ASTM—A106B Piping Steel

Fatigue design curves extending to 1010 cycles do not exist for A106B and A155KC70 piping steels. Therefore, a fracture mechanics approach which uses the concept of threshold stress intensity factor was utilized to determine a fatigue limit extending to 1010 cycles. The effects of microstructure, composition and wet steam environment on threshold stress intensity factor were investigated. The material properties were integrated with service loading conditions to determine maximum allowable strain amplitudes in a vibrating pipe.

An Analysis on Applicability of Fatigue Data to Design of Offshore Tubular Joints

Fatigue strength of welded tubular joints is one of the most important items in the design of offshore structures. Various design formulas have been proposed both on the stress concentration factors and on the fatigue failure life. However, most of these design curves do not involve any informations on the relationship between the crack initiation life Nc and the failure life Nf, nor on the relationship between the fatigue design curves and the detailed procedure of hot spot stress estimation.This paper presents a reanalysis of fatigue data of various welded tubular joints obtained in Japan. The relationship between the fatigue crack initiation life Nc and the failure life Nf was examined in wide range of fatigue life. Scatters in fatigue life estimation were also examined in relation with various procedures of hot spot stress estimation : 1) measurement, 2) finite element method, 3) Beale's formulas and 4) Kuang's formulas. Applicability to design is discussed with reference to the confidence limits of fatigue lives Nc and Nf.

A Study of Fatigue Design Procedure for Tubular Connections of Offshore Structures

It is the most important thing to consider the fatigue strength of tubular connections at the design stage of offshore structures. Some design curves have been provided to estimate the fatigue strength of the tubular connections. One of the most popular curves is the X-curve in AWS Structural Welding Code. The ordinate of this curve is defined as the hotspot stress or strain. However, the definition of the term “hotspot” is ambiguous, and a procedure to estimate the hotspot stress or strain seems not to be established even now.In this paper, the meaning of “hotspot” was clearly defined as the location where the fatigue crack initiates, and the value of strain at the hotspot was estimated by a simple procedure. Namely, the factors of stress concentration at a tubular connection were devided into two components due to structural deformation and the existence of weld bead, and Stowell's equation was used to evaluate the plastic strain concentration factor. Subsequently, more than 100 pieces of fatigue test specimens were reanalyzed by this procedure and a fatigue design curve was obtained.The main conclusions are as follows : (1) A simple procedure to estimate the value of strain at a hotspot was developed.(2) The value of stress concentration factor due to structural deformation can be estimated by Kuang's equations with a fairly good degree of accuracy when it is compared to the results obtained by a finite element method.(3) The following fatigue design curve of tubular connections was proposed on the crack initiation life basis.ea= 0. 0180 Nc-0.270A similar expression for a failure life was as follows.ea=0.0235 Nf-0.252where ea : estimated strain amplitudeNc : crack initiation lifeN f : failure life

Development of Fatigue Design Curves for Pressure Vessel Alloys Using a Modified Langer Equation

The Jaske and O’Donnell [1] curve-fitting procedure for analyzing fatigue data generated between room temperature and 427° C (800° F) for several pressure vessel alloys is reexamined in the present paper. Substantial improvements over their best-fit curves to the data are found to result from two proposed modifications to their procedure, namely 1) the use of a variable exponent in the Langer equation, and 2) minimization of the sum of the squares of the errors in the logarithms of the cyclic-stress amplitudes rather than in the stress amplitudes directly. Likewise, important differences are observed for the resultant allowable stress-amplitude values for design purposes. In particular, the present analysis permits higher allowable stress amplitudes in the critical low-cycle fatigue-life region for the austenitic stainless steels, alloy 800, and alloy 600. Two best-fit curves and the associated sets of allowable stress amplitudes, corresponding to the inclusion or deletion of load-controlled data, are obtained for alloy 718.

Background to the elastic creep-fatigue rules of the ASME B&PV Code Case 1592

Recent modifications associated primarily with elastic analysis have been made to the creep-fatigue rules of the ASME B&PV Code for use in design of elevated temperature nuclear components. The changes involve: (1) a modified equation for predicting the total strain range per cycle based on a more accurate approximation to the Neuber equation; (2) a variable Poisson's ratio in calculating stresses and strain using elastic equations to account for local thermal strain concentrations; (3) a modified approach for determining the equivalent strain range and; (4) an adjusted design fatigue curve. The modified Neuber approach accounts for the beneficial effects of a residual stress related to the relaxation strength of the material and includes a strain concentration factor greater than the elastic stress concentration factor for geometrical notches for mechanical loads. Notch concentration factors are taken equal to the elastic stress concentration factors for peak thermal and creep strains. The background and intent for these rules are discussed. Correlations of the strain range predictions to those obtained using more rigorous detailed inelastic analysis and test data are presented.

Mechanical and physical properties of 2$sup 1$/$sub 4$ Cr--1 Mo steel in support of Clinch River Breeder Reactor Plant steam generator design

Mechanical and physical property tests on annealed 2$sup 1$/$sub 4$ Cr-- 1 Mo steel were conducted in an effort to define behavior in support of the design of the Clinch River Breeder Reactor Plant steam generator system. True- stress true-plastic strain tensile data between the 0.2 percent offset yield stress and the ultimate tensile strength for temperatures between 25 and 593$sup 0$C and strain rates between 6.7 x 10$sup -6$ to 6.7 x 10$sup -3$/sec were fit with an equation. Creep data were analyzed and an analytical expression formulated defining strain-time behavior for primary and secondary creep. Similarly, an expression was obtained defining the time to the onset of tertiary creep as a function of stress and temperature. Strain- and load-controlled fatigue data were analyzed and proposed. American Society of Mechanical Engineers (ASME) fatigue design curves were formulated for temperatures ranging from 25 to 593$sup 0$C. Dynamic strain aging was found to lower the fatigue strength of this material at 371$sup 0$C, and this was attributed to dynamic strain aging. Results of subcritical crack growth tests are also reported. Comparisons between thermal conductivity and diffusivity values and those found in the ASME Code indicated that the new values were significantlymore » higher than those presently found in the Code. (auth)« less

Elevated Temperature Fatigue Behavior of 2 1/4 Cr-1 Mo Steel

Results are reported for a number of load and strain controlled fatigue tests conducted over the temperature range of room temperature to 1000°F (538°C). Cyclic hardening and softening characteristics for a single heat of 2 1/4 Cr-1 Mo steel in the isothermally annealed condition are discussed. Comparisons of the data generated in this effort are made with data available from the literature and from these compilations possible ASME design fatigue curves were prepared covering continuous high and low cycle behavior over the temperature range of room temperature to 1100°F (593°C). Equations for these design curves are also given.

Evaluation of Low Cycle Fatigue Design Curve Based on Life Distribution Shape

The safety factor prescribed in codes and regulations is not always based on any reasonable background. Many researchers have endeavored to give to the safety factor a background which is based on probability of failure of structure and statistical nature of applied load.In order to obtain some information of probability of fracture in low cycle fatigue, experimental investigation was made into life distribution in strain controlled low cycle fatigue tests of quenched and tempered high tensile steels, HT 60 and HT 80, by testing 18 to 40 specimens at each of four strain amplitudes.Test results were examined by using normal, logarithmic normal, and Weibull distribution functions. It was found that distributions of low cycle fatigue lives agreed fairly well with logarithmic normal and Weibull distributions rather than normal distribution, and that scatter of crack initiation lives was, in general, larger than that of failure lives. Following relations were obtained between visible crack initiation life Nc and failure life Nf; Nc=0.467N1.043f for HT 60, and Nc=0.657N0.953f for HT 80. The curve with 50% probability of fracture on Nc criterion was used, as a basis, for constructing _??_-N curves for 0.0001% probability of fracture, and modified _??_-N curves were also obtained by applying fatigue life reduction factor of 1/20 or strength reduction factors of 2/3 and 1/2 to the _??_-N curve of 50% probability of fracture. The concept of fatigue design curves were discussed by examining the strength and life reduction ratios.

Low-Cycle Fatigue of Large-Diameter Bolts

New constant-deflection amplitude fatigue data on bolting materials are presented. The relationship of cyclic yield strength to static yield strength is examined. Several series of full-size stud fatigue tests are analyzed and compared to the basic failure curve to obtain experimental fatigue strength reduction factors for bolts and studs. The variation of fatigue strength reduction factors with thread root radius, stud size, thread taper, and other variables is noted. A new design fatigue curve for bolts is proposed which reflects the additional information available at the present time.