ESDU Engineer

LOW-CYCLE FATIGUE OF STAINLESS STEELS

Following the issue of ESDU 04022, ‘An introduction to low-cycle fatigue phenomena’, in May 2005, two further Data Items have been issued; ESDU 05007, ‘Strain-life data for type 316 austenitic stainless steels at temperatures between –269 and 816°C (–452 and 1501°F)’, andESDU 05008, ‘Cyclic stress-strain response of type 316 austenitic stainless steels at temperatures between –269 and 800°C (–452 and 1472°F)’.

ESDU 05007 presents strain-life data for several type 316 austenitic stainless steels, including the low carbon and high nitrogen variants. The results of more than 300 tests, carried out at temperatures between –269 and 816°C (–452 and 1501°F) and at strain rates between 0.01 × 10–3 and 6.7 × 10–3 s–1, are given. The data are presented as plots of total, plastic and elastic strain amplitude versus the number of cycles to failure and each plot is accompanied by a table giving supplementary information about the test conditions used. The Data Item includes a full discussion of the data and the results of a multiple linear regression analysis of the data are presented, culminating in an expression for predicted fatigue life as a function of total strain amplitude, temperature, strain rate and carbon and nitrogen contents.

ESDU 05007 also contains a summary of the low-cycle fatigue phenomena detailed in ESDU 04022, including the derivation of the strain-life relationship and a discussion of the approximation of fatigue properties from monotonic test data. A worked example illustrating the use of the data and of the expression resulting from the regression analysis is included and the effect on measured fatigue life of using alternative failure criteria is discussed in an appendix to the Data Item.

As an accompaniment to ESDU 05007, ESDU 05008 presents stress-response data and cyclic stress strain data for type 316 stainless steels at temperatures between –269 and 800°C (–452 and 1472°F). The results of tests carried out on type 316 stainless steels, including the low carbon and high nitrogen variants, are examined using the methods described in ESDU 04022. The evolution of stresses per cycle during testing, known as the stress response, is discussed in terms of the initial and peak stress amplitude and the number of cycles to failure. The effects of test temperature, applied strain amplitude and the nitrogen content of the steels tested on the stress response are examined. The hardening and softening behaviour of the steels, including the degree of hardening, which is calculated as the percentage difference between the initial and peak stress, is also considered.

ESDU 05008 also presents half-life cyclic stress-plastic strain curves from tests carried out at temperatures between room temperature and 800°C (1472°F); these curves are analysed to determine the cyclic strength coefficient, K’sc, and the cyclic strain-hardening exponent, n’. An analysis of the effect of test conditions on K’sc and n’ is provided as are the methods used to calculate these constants from fatigue strength and ductility constants (σ’f, ε’f, b and c) which can be found in ESDU 05007. Two worked examples illustrating some of the methods described in the Data Item are also provided.

Queries regarding this article should be directed to Dr Dorothy Downs, Engineer, Strength Analysis Group:dorothy.downs@ihs.com

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