PSI - Issue 18

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

796

5

component design of cast iron products [Bleicher (2016), Kaufmann (1998), Schönborn (2011)]. The S-N curve parameters of all curves shown are listed in Table 3 for comparison.

Fig. 4. Determined S-N curves of EN-GJS-400-15 specimen form the bulk and with preserved surface roughness

The surface roughness on the bending specimen reduced the fatigue strength by about 18 % in comparison with the completely machined specimens (Fig. 4). The work of Sonsino (2005) shows that the decline per decade after the knee point should be 5 % (k* = 44.9) for steel and cast iron components. The evaluation of the test points in this project, however, suggest a slope k* = 2.5 % per decade.

Table 3. Overview over S-N curve parameters

σ a [MPa]

T σ [-]

K [-]

N k [-] 1 ∙ 10 6 1 ∙ 10 6 1 ∙ 10 6 2.18 ∙ 10 5 1 ∙ 10 6

Surface roughness

R = 0

Rz [µm]

Ra[µm]

EN-GJS-400-15

Bulk material Surface roughness

12.0

201 165 109

1:1.10 1:1.13 1:1.32

2,5

-

65.7 ± 6.6

11.3 ± 1.1

9.0

-

-

Rim

10.0

EN-GJL-270 *

Bulk material

6.3

48

1:1.25

-

-

FKM-guideline

166

-

80

-

Surface roughness

5.0

* Rausch (2011)

The surface roughness was determined using a Hommel-Etamic roughness-scanning arm. Metallographic examinations on the fractured fatigue specimens were carried out. The fracture surfaces were investigated by stereomicroscopy and, subsequently, one of the fracture surfaces was cut from the specimen, embedded, ground and polished for chemical etching. The pictures, taken by stereomicroscopy, were used to determine the fraction of the rim zone in each specimen in relation to the specimen’s cross section. Therefore, the fracture surfaces were divided into rim zone (red) and bulk (blue) and each was assigned a color (Fig. 5). In this way, it is possible to determine the rim zone proportions and group them into four categories (Table 4). Specimens were assigned to the category of rim