The main conclusion of this work is that the fatigue behaviour of PA-12 processed via SLS and IM was rather similar, which guarantees the use of parts fabricated by SLS in structural applications. Nevertheless, some particular differences were observed due to the presence of defects and microstructural features (spherulite size, anisotropy) induced by the processing technique. To endorse this statement, the following partial conclusions obtained from each of the characterisations carried out are detailed below.
▪ The thermal studies revealed that the degree of crystallinity of SLS samples was 10% lower than that of IM, with slightly higher transition temperatures.
▪ The morphological analysis showed that the spherulite size of the SLS was 50 ± 10 μm while in IM it was significantly lower, 13 ± 3 μm. Moreover, the IM specimens displayed a skin-core morphology, with an amorphous skin layer size of 14% of the total thickness of the specimen.
▪ IM specimens were almost full dense, while SLS specimens exhibited 4% of porosity. The surface roughness of SLS PA-12 at both orientations were 20 times larger than that of IM PA-12.
▪ Regarding the tensile properties, the most pronounced difference is found in the elongation at break, which could reach values in IM PA-12 up to 3 times higher than those measured in SLS at 0° orientation. Comparing SLS orientations, the elongation at break is again the main difference, from 10% at 90° to 30% at 0° orientations.
▪ All samples exhibited a non-linear fracture response with quite similar fracture toughness values independently of orientation and manufacturing technique. Nevertheless, the J-R curves of SLS at 0° orientation were above those of SLS at 90° orientation, being IM curves in between. The energy at crack growth initiation was the highest for IM PA-12, with hardly differences between the two distinct orientations of SLS PA-12. This result is linked to the spherulite size: the higher the fracture toughness, the lower the spherulite size.
▪ The fatigue behaviour at R=0.1 and a frequency of 1 Hz was again very analogous independently of orientation and manufacturing technique, but it is worth pointing out that SLS PA-12 at 0° orientation presented the best fatigue performance of either plain specimens or fracture mechanics specimens, SLS at 90° orientation the worst and IM PA-12 had an intermediate behaviour. Behind this may be the low interlayer strength and the population of defects induced by the SLS technique.
▪ Regarding the fatigue limit and the long crack threshold values, IM PA-12 showed the highest fatigue limit and the lowest long crack threshold. Between SLS orientations, 0° orientation presented highest fatigue limit and threshold values than 90° orientation. These trends observed are again a consequence of the presence of defects, surface roughness and microstructural features such as spherulite size.
▪ The fatigue behaviour of PA-12 was described using the Kitagawa- Takahashi and El Haddad models. The predictions given by El Haddad model were more accurate than those of the Kitagawa-Takahashi diagram, especially for SLS PA-12 samples. The characteristic length that defines the limit between the short and long crack regimes was the lowest for IM PA-12, being scarce the differences between the two orientation of SLS PA-12.
▪ The Kitagawa-Takahashi and El Haddad models have been extended to include finite fatigue lifetime predictions. The tensile strength to fatigue limit ratios showed that in the microstructural short crack region the lower interval of stress range values between integrity and critical crack propagation occurred in IM PA-12. The opposite trend was observed in the long crack regime.
▪ The mechanism of deformation and failure of SLS PA-12 for all testing configurations was the nucleation, growth and coalescences of crazes, promoted by the high population of defects in form of pores and or unmolten powder particles. This very same mechanism was observed in IM PA-12 in fracture mechanics specimens, that is, those used for fracture toughness and fatigue crack propagation characterisations. Nevertheless, for the plain specimens used for tensile tests and the determination of the S-N curves, the dominant mechanism was ductile tearing with necking formation and propagation along the gage length up to catastrophic failure.