PSI - Issue 18

Kim Bergner et al. / Procedia Structural Integrity 18 (2019) 792–801 Author name / Structural Integrity Procedia 00 (2019) 000–000

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zone foothills if the crack initiation and propagation occurs in an area of the rim zone that extends further into the bulk than the average penetration depth of the rim.

Fig. 5. Fracture surface with little (10 %, right) and higher (28 %, middle) rim proportion and rim zone foothill (left)

Table 4. Rim zone proportion categorization of the specimen cross section

Fraction of the rim zone on the specimens cross section

Low [%]

Medium [%]

High [%]

Rim zone foothill

2 - 11

12 - 21

22 - 31

-

In addition to the influence of degenerated graphite, the influence of different microstructures on the fatigue strength was investigated by testing two types of microstructures (pearlitic: GE_I and ferritic: GE_II) in the rim zone, Table 1. The evaluation of the test results of the castings GE_I and GE_II shows that the microstructure in the rim has a minor influence on the fatigue strength in the presence of degenerated graphite (Fig. 8). This can be seen in F ig. 6 , since the S-N curves of GE_I (σ a,n,k,50% = 114 MPa) and GE_II (σ a,n,k,50% = 111 MPa) cannot be distinguished from each other (Table 3). Therefore, the test points of both castings were evaluated together (Fig. 7). The single fatigue test points in Fig. 7 are colorized according to their rim zone proportion category (Table 4). It can be seen that, with an increasing fraction of rim zone in the cross section of the bending specimen, the fatigue strength is reduced. An increase in rim proportion from low (σ a,n,k,50% = 125 MPa) to high (σ a,n,k,50% = 95 MPa) leads to a reduction of fatigue strength by 24 % (Table 4, Table 5). The comparison of the S-N curves of the specimens with graphite lamellae in the rim zone and specimens with surface roughness only (Fig. 1) show that the fatigue strength is reduced by 34 % (Fig. 8, Table 3). Fig. 9 serves as an illustration of the difference in fatigue strength of EN-GJS-400-15 and EN-GJL-270. The comparison of these S-N curves shows that the rim zone proportion-dependent reduction of fatigue strength is caused by graphite lamellae. The graphite lamellae in the EN-GJL-270 lead to a lower fatigue strength, due to the higher notch effect of the lamellae compared with that of nodular graphite.