PSI - Issue 18

Rainer Wagener et al. / Procedia Structural Integrity 18 (2019) 490–500 Rainer Wagener, Andreas Maciolek, Heinz Kaufmann/ Structural Integrity Procedia 00 (2019) 000–000

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Figure 5: Comparison of the Strain-Life curves and the stress-strain curves derived by the compatibility conditions EN-AW5754

Comparing the strain-life models, no significant differences occur up to 1 ∙ 10 6 cycles to crack initiation, i.e. in the range of the test results. Deriving the cyclic stress-strain curve by the usage of the compatibility conditions, two different graphs occur. The stress-strain curve, which has been derived by the properties of the conventional strain life curve, does not match the stress-strain test results. On the other hand, the stress-strain curve derived from the Fatigue Life Curve represents the cyclic stress-strain behaviour of the test results. The differences may be explained by the formulation of the compatibility conditions, eq. 4 and 5, and the used strain-life properties. � � � � (4) � � � �� � � � � � (5)

Figure 6: Elastic strain-life curves

Basquin (1910) assumed that the elastic strain-life relation could be described by a line in a double logarithmic coordinate system. For a long time, the results of strain-controlled tests were limited to less than 1 ∙ 10 5 and mainly 1 ∙ 10 4 cycles to crack initiation. The Fatigue Life Curve based on test results up to 1 ∙ 10 6 cycles and the elastic strain life line is divided into three parts. The compatibility conditions enable the derivation of the properties of the stress strain curve, i.e. the properties of the plastic strain – stress behaviour. In contrast to the Fatigue Life Curve, properties