PSI - Issue 18

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

800

9

Table 6. Parameters for calculation of roughness factor K R,σ according to FKM-guideline [FKM (2012)]

Parameters for calculation of K R,σ

R z [µm]

K R,σ [-] 0.91

R m [MPa]

a R,σ [-]

R m,N,min [MPa]

EN-GJS-400-15

400

0.16

80

400

Table 3 shows that the nominal stress amplitude σ a,n at the knee point, calculated for EN-GJS-400-15 according to the FKM guideline, is comparable to σ a,n of the specimen with surface roughness. Only the slope k of both S-N curves deviates from the obtained cyclic material behavior and, as a result, the fatigue strength is overestimated in the low cycle fatigue regime. Fig. 10 shows that the FKM guideline is not able to represent the obtained data correctly and, furthermore, overestimates the fatigue strength determined on the fatigue specimens with the graphite degeneration. Nevertheless, the fatigue strength calculated by FKM approaches quite well the fatigue strength of the specimens having only surface roughness. In this case, the nominal stress amplitude at the knee point N k =1 ∙ 10 6 derived by the FKM guideline reaches σ a,n,k = 166 MPa, more or less exactly the nominal stress amplitude for the specimens with surface roughness (σ a,n,k,50% = 165 MPa).

Fig. 10. Comparison of the experimentally determined S-N curves of EN-GJS-400-15 for the specimens with surface roughness only, with a pearlitic or a ferritic rim and a S-N curve calculated according to the FKM guideline 4. Discussion Firstly, it can be stated that, as expected, the preserved surface roughness reduces the fatigue strength compared to the completely machined specimens made of EN-GJS-400-15 (Fig. 4). Furthermore, the graphite lamellae in the DGL have a negative effect on its fatigue strength. Concerning the microstructure of the matrix of the DGL, on the other hand, it makes no difference whether this is pearlitic or ferritic. The negative effect of the graphite lamellae seems to superimpose the favourable effect of a stronger pearlitic microstructure on the fatigue strength, due to a high internal notch effect for the sharp lamellar graphite (F ig. 6 , Fig. 7). This notch effect is the reason for the lower fatigue strength of lamellar cast iron in comparison to nodular cast iron (Fig. 9).