Effect of the orientation on the fatigue crack growth of polyamide 12 manufactured by selective laser sintering

Abstract

Purpose Polyamide 12 (PA12) properties meet specific requirements for various applications in the automotive and aerospace industries. Bulk specimens made of PA12 and processed via the additive manufacturing technique such as selective laser sintering (SLS) present a layered structure. In case of structural applications, the fatigue performance of SLS PA12 parts is of vital importance and fatigue response studies in these type of materials are still scarce. Therefore, the purpose of this paper is to analyse the effect of the applied load orientation on the fatigue crack propagation behaviour of the layered structure of SLS PA12. Design/methodology/approach With the aim of understanding the effect of the applied load with respect to the layer orientation on the fatigue crack growth of SLS PA12, fatigue crack growth tests were carried out at two orientations. The specimens called PARA were orientated in such a way that the applied force direction belongs to the layer plane while in the group called PERP, the tensile force direction is coincident with the build direction, that is, perpendicular to the slice. Besides, special attention has been paid to the analysis of the fracture surfaces of the specimens, linking the micromechanisms of failure with the microstructure of the material. Findings The SLS PA12 specimens tested with the load applied parallel to the layered structure show a little better fatigue response than those tested at perpendicular orientation. The fracture surfaces of the specimens tested at perpendicular orientation are slightly smoother than those tested at parallel orientation. Crazes are the main micromechanism of failure with a crater size of 50 microns, which coincide with the spherulite size. This indicates that the void nucleation of the crazes takes places between lamellae inside the spherulites, and consequently, the craze growth and rupture occurs mainly in a transspherulitic mode. Originality/value PA12 parts manufactured via SLS are becoming more valuable in structural elements in the automative and aeronatical fields. In such applications, fatigue performance is vital for design. Fatigue studies are scarce in literature and even more when dealing with fatigue crack growth behaviour. The value of this work is the analysis of the fatigue crack growth response of these materials taking into account the anisotropic microstructure and to get a better understanding, this behaviour is explained taking into account the micromechanisms of failure and the microstructure of the material.

Fatigue life assessment of polyamide 12 processed by selective laser sintering. Damage modelling according to fracture mechanics

“…Therefore, the crack nucleated at some point of the surface, favored by the high roughness, and once generated progressed through the spherulites and in many cases, going through their equatorial plane. This mechanism of failure was also observed by Amel et al (2014) and by the present authors in the very same materials when analyzing the fracture surfaces obtained from fatigue crack growth tests (Cano et al, 2019) and from fracture toughness tests (Cano et al, 2018). The only difference between parallel and perpendicular orientation in the crack growth zone is that this area seemed to be rougher in the samples tested at parallel orientation that in the samples tested at perpendicular orientation.…”

“…The authors analyzed the physical property as the density, ρ, the morphology of the microstructure as the spherulite form and size, trueD¯, the mechanical properties via tensile tests as the Young’s modulus, E; the yield strength, σ T ; the elongation at break, ε R ; and the Poisson ratio, ν; and the fracture parameters in terms of the energy at crack growth initiation, J IC , in Cano et al (2019, 2018). The main results are gathered in Tables 1 and 2.…”

Comparison of the bending fatigue performances of selective laser sintered and injection moulded nylon spur gears

“…Fatigue test results, by nature, are prone to significant scatter due to the variation in material properties and test parameters. 27 However, the scatter was minimal in this case. In summary, the experimental campaign comprised of a maximum of 24 tests (including both SLS and IM gears), entailing 461 h of test duration.…”

Polyamide 12 Filled with Cross-Linked Polyethylene Waste: Processing, Compatibilization, and Properties

“…Typically, PA12 compounds are more expensive than other polyamides. Thus, its use is restricted to specific applications, such as fuel pipes and other fluids for the automotive and aerospace industries or applications that require dimensional accuracy in water distribution systems [20][21][22] . In addition, recently, PA12 has become the most used polymer material in the manufacture of prototypes for industries in different segments using multi-jet fusion technology 23 .…”