Using Tolerance Bounds for Estimation of Characteristic Fatigue Curves for Composites with Confidence

Fatigue S–N curves provide the number of stress cycles that result in fatigue failure at stress range S and need to be measured for new engineering materials where data are not as readily available as they are for well-characterized and widely used metals. A simple statistical method for the estimation of characteristic fatigue curves defined in terms of lower-tail quantiles in probability distributions of dependent variables is presented. The method allows for the estimation of such quantiles with a specified confidence level, taking account of the statistical uncertainty caused by a limited number of experimental test results available for the estimation. The traditional general approach for estimating characteristic S–N curves by tolerance bounds is complicated and is not much used by engineers. The presented approach allows for calculating the curves with a simple spreadsheet. The only requirement is that the experimental log S data for the S–N curve are fairly uniformly distributed over a finite logS interval, where S denotes the stress range. Experimental fatigue test programs are often designed such that test data fulfil this assumption. Although developed with fatigue of composite laminates in mind, the presented statistical procedure and the presented associated charts are valid for fatigue curve estimation for any material.

Theoretical and Numerical Analysis of the Fatigue Strength of Mechanical Material and Part under Uniaxial Stress

To characterize the fatigue strength of mechanical material and part, a methodology combining the fatigue damage theory with the numerical simulation is proposed under three types of uniaxial loads. The equal-life fatigue curve of a shaft is used for the theoretical and numerical analysis of fatigue damage. Its theoretical fatigue strengths under different loadings are gained through theory equations. Utilizing the mean stress correction method, the stress distribution and fatigue life of the shaft are obtained through the collaborative simulation of ANSYS and nCode software. Consequently, the resultant fatigue lives are all within the value band of basic cycle number as demanded in practice, implying the correction and validation of theory analysis. The proposed strategy provides a new pathway and perspective for the analysis of fatigue damage.

Evaluation of the Japanese Fatigue Test Data in Gr.91 for Elevated Temperature Design

The ASME Boiler and Pressure Vessel Code (ASME BPVC) Section III, Division 5, Subsection HB, Subpart B provided only one design fatigue curve for Grade 91 steel (Gr.91) at 540 °C (or 1000 °F) in 2019 and earlier versions. To overcome this disadvantage, The ASME Section III Working Group on Creep-Fatigue and Negligible Creep (WG-CFNC) had taken an action to incorporate the temperature-dependent design fatigue curves for Gr. 91 developed by Japan Society of Mechanical Engineers (JSME) into ASME BPVC Section III Division 5. As a result, the temperature dependent design fatigue curves are provided in the 2021 edition of the ASME BPVC. To clear the features of the best-fit fatigue curve equation developed by the JSME, 305 data stored in the database were analyzed. Details of the database and relationship between the best-fit fatigue curve equation and the data including the statistic values and the values of 95% and 99% lower confidence bound calculated by failure probability assessment were clarified through analysis. In addition to the best-fit fatigue curve equation, an equation for dynamic stress-strain response showing the behavior of Gr.91 steel under cyclic loading of is also provided based on the same database. Moreover, some additional available data of fatigue and creep-fatigue tests obtained in Japan are also provided for considering the creep-fatigue damage evaluation under elevated temperature condition.

Development of New Design Fatigue Curves in Japan – Treatment of Variable Loading Amplitude Effect

Design fatigue curves of new concept were developed in the Subcommittee on Design Fatigue Curve in the Atomic Energy Research Committee in the Japan Welding Engineering Society (JWES). Also, Working Group on Design Fatigue Curves (WG DFC) in the JSME is studying the validity and the applicability of the design fatigue curves developed in the JWES to incorporate into the JSME Environmental Fatigue Evaluation Method. The developed design fatigue curve consists of the best-fit curves using tensile strength as a parameter, correction of mean stress effect employed the Smith-Watson-Topper approach, surface finish effect and variable loading amplitude effect. This paper discusses the treatment of variable loading amplitude effect in the improved design fatigue curves. The developed design fatigue curves have employed the method of variable loading amplitude effect in EN 13445. This method uses a fatigue life exponent of −0.1 from N = 2 × 106. Research Group on Fatigue Strength in the JSME performed a series of fatigue tests for variable loading amplitude and the test data showed the fatigue life exponent was −0.07. This outcome of the JSME Research Group can support the fatigue life exponent of −0.1 in EN 13445. A survey of the procedure of variable loading effect has been performed on a thermal fluctuation transient at mixed zone of T-junction piping of nuclear power plants for which UF is significantly affected by variable loading effect. Based on this study, the design fatigue curve for variable loading effect that starts from 2 × 106 with the slope of 1/10, which comes from EN 13445-3, can be considered to be applicable to the developed design fatigue curves. Also, the difference of UF between up to 1 × 108 and extension with no limit is relatively small, and the maximum number of cycles of 1 × 108 can be employed for the engineering fatigue analysis.

Perspectives on Fatigue Design of Bellows

In the past couple of decades there have been significant developments in the fatigue design approach for metallic bellows expansion joints. For example, where there were previously separate fatigue curves for reinforced and unreinforced bellows, there is now only one fatigue curve. This paper describes the developments over time and explains the technical reasons for the changes.

Accelerated Locati method for testing machine parts for fatigue resistance

The results of testing specimens and machine parts for fatigue resistance according to the standard method with the construction of the Wöhler curve and the determination of the endurance limit and the accelerated method with the determination of the endurance limit are presented. As an accelerated test method, the method of step-increasing loads (the Locati method) was used, as the conditional fatigue curves — oblique fatigue curves parallel to the oblique fatigue curves obtained during standard tests of specimens with the construction of the Wöhler curve. For all conventional curves, the same inflection point of the fatigue curve in semilogarithmic coordinates was taken. Keywords: fatigue resistance, endurance limit, Wöhler curve, accelerated tests. [email protected]

Environmental Effect on Fatigue Crack Initiation under Equi-Biaxial Loading of an Austenitic Stainless Steel

The lifetime extension of nuclear power stations is considered an energy challenge worldwide. That is why the risk analysis and the study of various effects of different factors that could potentially prevent safe long-term operation are necessary. These structures, often of great dimensions, are subjected during their life to complex loading combining varying multiaxial mechanical loads with non-zero mean values associated with temperature fluctuations under a PWR (pressure water reactor) environment. Based on more recent fatigue data (including tests at 300 °C in air and a PWR environment, etc.), some international codes (RCC-M, ASME, and others) have proposed and suggested a modification of the austenitic stainless steels fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which has to be multiplied by the usual fatigue usage factor. The determination of the field of validation of the application of this penalty factor requires obtaining experimental data. The aim of this paper is to present a new device, “FABIME2e” developed in the LISN (Laboratory of Integrity of Structures and Normalization) in collaboration with EDF (Electricity of France) and Framatome. These new tests allow the effect of a PWR environment on a disk specimen to be quantified. This new device combines structural effects such as equibiaxiality and mean strain and the environmental penalty effect with the use of a PWR environment during fatigue tests.