PSI - Issue 7

4

A. Brueckner-Foit et al. / Procedia Structural Integrity 7 (2017) 36–43 A. Brückner-Foit/ Structural Integrity Procedia 00 (2017) 000–000

39

Fig. 3 Fatigue curve of die cast specimens; classification according to fracture origin; red circles: gas pores, green circles: interior and subsurface shrinkage pores, blue circles: interaction between gas pores and shrinkage pores, black circles: crack initiation from MgO inclusions on surface; small symbols: surface damage or non identified fracture origin. Firstly, in the cases in which gas pores are located close to the surface in the absence of disproportionally large shrinkage pores, the fatal crack tend to initiate from this type of pores instead of from shrinkage pores. Fig. 4 shows an example in which the mortal crack initiated from a gas pore near the surface even though a shrinkage pore was situated in the vicinity. In this type of specimens, relatively long lifetimes were obtained, what suggests that the influence of single shrinkage pores on fatigue life is limited.

Fig. 4 Fracture surface of specimen failed at 140 MPa, 74,283 load cycles; blue arrow: gas pore at fracture origin, adjacent shrinkage pore in white rectangle. The size of the shrinkage pores varied due to the dispersion of the cooling rate during the casting process, leading to specimens with giant shrinkage pores. Even in these cases, the drop on lifetime was not dramatic and a substantial amount of internal flaw extension occurs prior to failure. The fracture surfaces containing similar disproportionally large shrinkage pores are generally very rough, which can be explained by the fact that actually multiple crack initiation takes place (see Fig. 5).