Page 329 - Biaxial Multiaxial Fatigue and Fracture

Influence of Defects on Fatigue Life of Aluminium Pressure Diecastings 313

Fig. 13. Stereographic picture of the fracture surface of an unacceptable sample.

Some of the fracture surfaces were analysed in a SEM microscope. Figures 14 and 15
present some of the images obtained characterised by a brittle facture. In fact, Fig. 14 shows
very clearly this type of fracture. However, small regions with some deformation were also
detected. Figure 14 b) shows a pore interior with a dendrite and small deformation. Figure 15
shows other fracture surfaces presenting some regions of small deformation and large fragile
intermetallic plates.

Fig. 14. Fracture surface, a) the fracture surface presents regions of small deformation, b)
image inside a pore with a dendrite and small deformation.

Figure 16 presents, for both the acceptable and unacceptable sets, the frequency of defects
classified in seven classes of mean pore size diameters. This figure shows the presence of
defects with diameter higher than 250 pm and higher amounts of large size defects in the
fracture surfaces of unacceptable samples, which means that they play an important role in the
fatigue life of these parts.