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Research Papers

Estimating Lifetime of Notched Components Subjected to Variable Amplitude Fatigue Loading According to the Elastoplastic Theory of Critical Distances

[+] Author and Article Information
Luca Susmel

Professor
Department of Civil and Structural Engineering,
The University of Sheffield,
Mappin Street,
Sheffield S1 3JD, UK
e-mail: l.susmel@sheffield.ac.uk

David Taylor

Professor
Department of Mechanical Engineering,
Trinity College,
Dublin 2, Ireland
e-mail: dtaylor@tcd.ie

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received October 1, 2013; final manuscript received October 22, 2014; published online November 11, 2014. Assoc. Editor: Toshio Nakamura.

J. Eng. Mater. Technol 137(1), 011008 (Jan 01, 2015) (15 pages) Paper No: MATS-13-1176; doi: 10.1115/1.4028927 History: Received October 01, 2013; Revised October 22, 2014; Online November 11, 2014

The present paper is concerned with the use of the elastoplastic theory of critical distances (TCD) to perform the fatigue assessment of notched components subjected to in-service variable amplitude (VA) fatigue loading. The elastoplastic TCD takes as its starting point the assumption that the detrimental effect of stress/strain concentrators of any kind can efficiently be taken into account by directly postprocessing the entire elastoplastic stress/strain field in the vicinity of the notch apex. Thanks to its specific features, the TCD can be formalized in different ways by simply changing size and geometrical features of the domain used to calculate the required effective stress. The so-called point method (PM) is the simplest form in which this theory can be applied. This formalization of the TCD postulates that the elastoplastic stress/strain state to be used to estimate fatigue damage has to be determined at a given distance from the tip of the notch being assessed. According to the TCD's philosophy, such a distance is treated as a fatigue property. Therefore, given the material, this critical length does not change as either the features of the assessed stress/strain concentrator or the profile of the investigated loading path vary. In the present study, the above design strategy is attempted to be used to estimate lifetime of notched component subjected to VA loading, the required critical distance being determined under constant amplitude (CA) loading. The accuracy and reliability of the devised approach were checked by using a number of experimental results generated by testing, under both concave upward and concave downward spectra, notched samples containing geometrical features having a different sharpness. Such a validation exercise allowed us to prove that the elastoplastic TCD, used in the form of the PM, is highly accurate in estimating fatigue damage also in notched components subjected to in-service VA loading.

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Copyright © 2015 by ASME
Topics: Fatigue , Stress
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Figures

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Fig. 1

Accuracy of the classical stress based approach (a) versus accuracy of the theory of critical distances applied in the form of the PM (c)—data taken from Ref. [11]. Smith and Miller's diagram (b)

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Fig. 2

In-field use of the PM to estimate fatigue lifetime of notched components

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Fig. 3

Summary of the experimental procedure suggested to be followed to determine critical distance LPM

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Fig. 4

In-field procedure to apply the elastoplastic PM to estimate fatigue lifetime of notched components subjected to VA fatigue loading

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Fig. 5

Geometries of the investigated notched samples (a), stabilized stress/strain curve (b), and fully reversed plain Manson–Coffin curve (c)

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Fig. 6

Investigated load spectra (a); examples showing the experimental load histories employed to test the notched samples of C40 (b)–(d)

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Fig. 7

Examples of local stress and strain histories at LPM/2 = 0.06 mm for notched samples of C40 subjected to a CUS (a) as well as to a CDS (b) nominal load spectrum

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Fig. 8

Examples of rain-flow matrices calculated, in terms of strains, from the performed VA FE analyses run by using the real load histories applied during testing

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Fig. 9

Accuracy of the elastoplastic PM in estimating the VA fatigue lifetime of the tested notched samples of carbon steel C40 by taking into account the mean stress effect according to SWT parameter (a), Morrow's formula (b), and Morrow's modified formula (c)

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Fig. 10

Summary of the experimental results generated by Costa et al. [30]

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Fig. 11

Examples of rain-flow matrices (calculated in terms of strains) and accuracy of the PM in estimating the fatigue results generated by Costa et al. [30] by testing notched plates of aluminum 6082-T6

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