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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However, even those specimens reaching higher lifetimes show a scatter band in fatigue strength of a factor of up to four, Fig. 15. To reduce this scatter, it is possible to use the determined density or virtual Young’s modulus for a classification system to establish S-N curves for different stiffness or density classes, for instance. For this purpose, the nominal stress amplitudes σ a,n for each tested specimen based on the calculated values for the virtual Young’s modulus E f and the applied strain amplitudes ε a,n were calculated. If all determined nominal stress amplitudes are plotted over the lifetime, it is possible to derive a classification for different levels of stiffness. Fig. 19 shows as an example of a classification based on three different threshold values for the virtual Young’s modulus. The classification shows that reducing the scatter band by values for E f is an appropriate method. Nevertheless, in further steps, more fatigue data and measurements of density and stiffness need to be derived to obtain a statistically valid result and to calculate corresponding S-N curves. In Fig. 19, only fatigue tests that reached a lifetime of more than 20,000 cycles to rupture are used. In the case of sound material and for fatigue tests with a higher number of cycles, one can state that only linear elastic material behaviour is present and a transformation from strain to stress amplitude can be done without any errors.

Fig. 19. Stress-lifetime diagram for specimens with Dross

3.3. Metallographic investigations After the fatigue tests, metallographic investigations were also conducted on the test blocks, for both the sound and the Dross-affected material state. An overview of the microstructure and the graphite parameters in the sound condition is given in Table 3. Later investigations were also performed on the Dross to check the microstructure for certain characteristics with the aid of SEM and EDX analysis. The investigation showed that three different forms of graphite regularly occur next to Dross: lamellar, vermicular and nodular graphite, Fig. 20. The three different graphite types are produced when Dross takes all magnesium from the melt so that only lamellar graphite can develop (type A), when nodular graphite develops first (type B) or when graphite and Dross are formed in parallel and not enough magnesium is left to form nodular graphite (type C). Additionally, different forms of Dross were found in the specimens, Table 4, these being string, chunk, cluster and pit-like. It can be found that, especially, the string-shaped Dross always refers to graphite type A and B. A detailed EDX analysis revealed additionally that chunk-shaped Dross consists of high fractions of magnesium, silicon and aluminium with a weight percentage of up to 16 %. Dross formed as surface pits contains high fractions of magnesium, sulphur and manganese. For the Dross form “string”, high fractions of magnesium and carbon can be found.