Numerical measures of the degree of non-proportionality of multiaxial fatigue loadings
The influence of the non-proportional loadings on the fatigue life depends on the material ductility. Ductile materials react with a shortening of lifetime compared to proportional loading conditions. For a semiductile material there is almost no difference between proportional and non-proportiona...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Gruppo Italiano Frattura
2015-07-01
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| Series: | Fracture and Structural Integrity |
| Subjects: | |
| Online Access: | http://www.gruppofrattura.it/pdf/rivista/numero33/numero_33_art_30.pdf |
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| Summary: | The influence of the non-proportional loadings on the fatigue life depends on the material ductility.
Ductile materials react with a shortening of lifetime compared to proportional loading conditions. For a semiductile
material there is almost no difference between proportional and non-proportional loadings with respect
to the fatigue life. Brittle materials show an increase of the lifetime under non-proportional loadings. If fatigue
life assessment is performed using stress-based hypotheses, it is a rather difficult task to take into account
material ductility correctly, especially the fatigue life reduction as displayed by ductile materials.
Most stress-based hypotheses will compute a longer fatigue life under non-proportional loading conditions.
There are also hypotheses, which already include quantitative evaluation of the non-proportionality (e.g. EESH,
SSCH and MWCM). Anyway in order to improve assessment for ductile materials, some sort of numerical
measure for the degree of non-proportionality of the fatigue loading is required. A number of measures of this
kind (or non-proportionality factors) were proposed in the literature and are discussed here:
- the factor used in EESH is a quotient of stress amplitudes integrals,
- the factor according to Gaier, which works with a discrete stress tensor values in a scaled stress space,
- the factor according to Kanazawa, which makes use of plane-based stress values,
- the factor used in MWCM, which exploits stress values in the plane with the highest shear stress amplitude,
a new non-proportionality factor, which is based on the correlation between individual stress tensor
components, is proposed.
General requirements imposed on the non-proportionality factors are discussed and each of the factors is
evaluated with respect to these requirements. Also application with the stress-based hypotheses is discussed and
illustrated using the experimental data for aluminum and magnesium welded joints under constant and variable
amplitude loadings. |
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| ISSN: | 1971-8993 1971-8993 |