PSI - Issue 18

Christoph Bleicher et al. / Procedia Structural Integrity 18 (2019) 46–62 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 11. Stress-strain curve for EN-GJS-400-15

Fig. 12. Strain-life curve for EN-GJS-400-15

The results for the stress-strain behaviour show a strong cyclic hardening effect, which is also typical of other nodular cast iron materials as shown in Bleicher (2016). Nevertheless, the fatigue results also show that the cyclic stress-strain curve does not meet the single test results, both in the strain-life and, even more, in the stress-strain region. This can be referred to as the missing compatibility for nodular cast iron materials as discussed in Bleicher (2016), Wagener (2007) and Niesłony (2008). If the compatibility requirement is valid, the parameters of the stress-strain curve can be calculated using the parameters of the total strain-life curve, according to Equations (6) and (7). That this assumption is not valid is denoted by the fact that the cyclic stress-strain curve determined is not able to cover the single test results. This can be improved by applying a tri-linear approach for the elastic strain–life curves, based on the investigations of Wagener (2007) on aluminium wrought alloys. In contrast to the single linear approach made by Basquin (1910) for the elastic part of the strain-life curve, the tri-linear approach divides the elastic strain-life curve into three parts: one for the low cycle, one for the high cycle and one for the very high cycle lifetime regime. Investigations in Bleicher (2016) show that this approach is also valid for nodular cast iron materials. The advantage of the tri-linear approach is that fatigue results are much better met and that the method enables the definition of a knee point accordingly to stress-