Fatigue behaviour of polyamide 12 processed by selective laser sintering
- Autores: Alberto Jesús Cano Aragón
- Directores de la Tesis: Alicia Salazar (dir. tes.), Jesús Rodríguez Pérez (codir. tes.)
- Lectura: En la Universidad Rey Juan Carlos ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Pedro Alberto Poza Gómez (presid.), T. Gómez del Río (secret.), Bamber R.K. Blackman (voc.), Alfonso Carlos Fernández Canteli (voc.), Eugenio Giner Maravilla (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías Industriales: Química, Ambiental, Energética, Electrónica, Mecánica y de los Materiales por la Universidad Rey Juan Carlos
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
In this doctoral thesis, the fatigue behavior of polyamide 12 manufactured by selective laser sintering (SLS) is studied and compared with the same material processed by injection moulding (IM). An extensive experimental program has been carried out, providing relevant data on the behaviour of this material to evaluate the structural integrity of the parts processed using additive manufacturing techniques. Since in these techniques the manufacturing process is carried out by layer-by-layer deposition, the resulting parts are susceptible to exhibit a transversely isotropic behaviour. That is the reason why the mechanical and fatigue characterizations were performed applying the force parallel and perpendicular to the layered structure.
Firstly, the most relevant thermal and microstructural properties of this semicrystalline thermoplastic have been determined, including the glass transition temperatures, the degree of crystallinity, the porosity or the size of the spherulites, together with features typically associated with the manufacturing process, such as roughness or skin-core morphology. Some significant differences between the two manufacturing procedures have been detected, especially in the size of the spherulites and in the surface roughness, both much smaller in the case of IM specimens. Moreover, IM specimens showed a skin layer of amorphous material.
Regarding the mechanical characterization, tensile tests have been carried out, the results do not show great differences either between the SLS and IM specimens or between different orientations in the case of the specimens processed by additive manufacturing. The only and very marked difference is the elongation at break, which is significantly lower in the case of samples processed by SLS, and within these, in the specimens tested with the load applied perpendicularly to the layer wise structure.
Regarding the fracture behavior, the polyamide has presented a non-linear mechanical response in all cases, which has compelled a characterization based on the integral J. The critical values of this parameter do not show relevant variations either between the materials processed by SLS or by IM, or between the different orientations tested. Nevertheless, it is worth mentioning that the SLS specimens tested with the load applied parallel to the layered structure showed the best crack propagation resistance and the IM samples presented the highest energy at crack growth initiation.
In relation to the fatigue behavior, the S-N curves and the fatigue limits have been determined using plain specimens. Additionally, fracture mechanics specimens (compact tension configuration) have been employed to obtain crack propagation curves and the threshold values of the crack driving force parameter proposed in this thesis, ∆√G. All this information has been employed for the construction of the Kitawaga-Takahashi diagrams, extended to finite life and unstable failure cases.
The general trend has been that the worst mechanical and fatigue behavior occurred in the SLS specimens tested with the load applied perpendicularly to the layered structure. The defects induced during processing together with the weak interlayer strength accounted for this behaviour.
Finally, the dominant mechanisms of failure have been identified, which in most cases are nucleation, coalescence and growth of crazes, which usually have their origin in internal defects associated with the manufacturing process or in the crystalline phase itself. In some cases, ductile tearing has been also detected.
As an overall conclusion, the results obtained in this thesis indicate that the fatigue behaviour of PA-12 manufactured by SLS is quite similar to that of PA-12 manufactured by IM, a good starting point to incorporate parts processed by SLS in structural applications.
