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Optimisation methods for the determination of cyclic material parameters from strain controlled uniaxial fatigue tests

: Fuhrmann, Katrin; Neumeister, Peter; Eulitz, Klaus-Georg


International journal of fatigue 83 (2016), Part 2, pp.93-99
ISSN: 0142-1123
Journal Article
Fraunhofer IKTS ()
cyclic stress-strain curve; cyclic strain-life curve; compatibility terms; linear regression; optimisation

Four new methods are presented for the determination of cyclic strain-life curves according to Manson/Coffin/Basquin/Morrow and of cyclic stress-strain curves according to Ramberg/Osgood from results of strain controlled uniaxial fatigue tests. The material parameters are assessed by linear regression of linearised parts of these curves with the method ofleast squares. The linear regressions are performed in strain-stress-life space as well as in stress-strain and strain-life planes simultaneously for the yet undefined parts of the material behaviour curves. The compatibility terms are applied as constraints. Experimental data from fourteen test series of magnesium die-casting alloys AE42, AM50, and AZ91 at different elevated temperature levels between 30°C and140°C is used exemplarily. The four methods are critically compared to two methods known from literature. For this six different measuresare proposed, which are based on the remaining absolute deviation between material curve and data points. The comparison shows that the method that combines the linear regressions of the plastic part of cyclic stress-strain curve and the elastic and plastic part of the strain-life curve in an optimisation with the compatibility terms as constraints leads to an overall better representation of the experimental data.