Issue 33

A. Bolchoun et alii, Frattura ed Integrità Strutturale, 33 (2015) 238-252; DOI: 10.3221/IGF-ESIS.33.30 238 Focussed on multiaxial fatigue Numerical measures of the degree of non-proportionality of multiaxial fatigue loadings A. Bolchoun Technische Universität Darmstadt, Chair of System Reliability and Machine Acoustics (SzM), Darmstadt, Germany H. Kaufmann, C. M. Sonsino Fraunhofer Institute for Structural Durability and System Reliability LBF, Darmstadt, Germany A BSTRACT . 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 semi- ductile 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. K EYWORDS . Multiaxial fatigue; Non-proportional loadings; Non-proportionality measures; Welded joints; Stress-based criteria. I NTRODUCTION aterials and structural components subjected to multiaxial fatigue loadings can react differently depending on the non-proportionality of the loading. Some materials (brittle material state) exhibit a longer fatigue lifetime under non-proportional loadings compared to the proportional ones. Other materials (ductile material state) M