PSI - Issue 23
Sascha Gerbe et al. / Procedia Structural Integrity 23 (2019) 511–516 Author name / Structural Integrity Procedia 00 (2019) 000 – 000
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Fig. 3. (a) S-N Woehler diagram of both investigated parts (grey: high solidification rate; black: low solidification rate); (b) Kitagawa-Takahashi threshold diagram with multiple El Haddad threshold-curves for different numbers of cycles to pass N max , derived from Basquin equation.
As shown in Fig. 3a, the bearable stress amplitude under fully reversed cyclic loading is significantly higher for lower SDAS and respective finer microstructure (cf. Gerbe et al. (2019)). However, the observed scattering is quite stronger for the bearing seat set than for the coarser stud bolt material. To evaluate the strong scattering, the critical, maximum diameters of the found crack initiating features d p,max (mainly pores) were investigated by fractography and plotted as data points in the Kitagawa-Takahashi diagram (cf. Fig. 3b). Information about experiment and fractography is given in Table 1. Figure 4 shows representative regions of fracture surfaces of the two marked specimens (marked red in Fig. 3b and bolt in Tab. 1), which lay significantly above the expected Kitagawa-Takahashi threshold (with respect to applied stress amplitude, found defect size and the number of cycles to failure; cf. Fig 3b). Table 1. Exemplarily results of fractographic analyzation linked to the test parameters for the bearing seat set of AlSi8Cu3. Stress amplitude σ a [MPa] Number of cycles to failure N f [-] Max. diam. of initiating feature d p,max [µm] Remarks 90 1.9 ∙ 10 9 (run through) 20 (biggest not propagating defect found) no fatigue damage 110 1.8 ∙ 10 9 19 high amount of striations 110 3.3 ∙ 10 6 103 surface cutting pore 120 3.9 ∙ 10 8 - (no significant defects observable) Initi. / propag. on facet
Fig. 4. Fracture surfaces from VHCF experiments of AlSi8Cu3 bearing seat specimens. (a) σ a = 110 MPa, N f = 1.8∙10 striations in small, separated sections around the origin of initiation; (b) σ a = 120 MPa, N f = 3.9 ∙ 10 plane) where surface initiation and further crack propagation occurred (cf. Gerbe et al. (2019)).
9 cycles, high amount of
8 cycles, big crystallographic facet ({111}-slip
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