PSI - Issue 42

A. Chiocca et al. / Procedia Structural Integrity 42 (2022) 799–805 A. Chiocca et al. / Structural Integrity Procedia 00 (2019) 000–000

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failure have been identified by di ff erent colored dashed lines depending on the specimen post-processing conditions. Two main outcomes are observable from the results of Figure 6. The first one is that by increasing the test frequency the fatigue endurance increases for both the Coated and As-Printed specimens, as also observed by Eftekhari and Fatemi (2016). This first behavior can be explained by the time in which the specimen remains subjected to a condition of maximum stress on equal displacements. At lower frequencies, indeed, the material is subjected for a greater time at higher stress values (Kocjan et al. (2022)). This can be stated if the specimen maintains the same temperature during testing, condition that was respected for the reported tests. The specimen surface temperature, indeed, was monitored using a thermal-imaging camera and only a maximum variation of 0 . 8 ◦ C was detected between specimens tested at the two di ff erent frequencies. The second outcome shows how the coating post-processing (i.e., for the same test frequency) causes an increase in the fatigue resistance. Since one of the main fatigue drivers is surface roughness, the surface smoothening provided by the thin film of coating may be considered one of the factor in the enhancement of the component fatigue endurance. The fatigue test results have been summarized in Figure 7 by means of a displacement amplitude vs. number of cycles to failure diagram. The 50% probability-of-failure curves and the colored bands identifying the bounds of 10% and 90% probability-of-failure were provided besides the experimental data. The results evidence how the curve slope remains unchanged under the same test frequency for both Coated and As-Printed specimens while the fatigue strength shifts, with an increase for Coated specimens. On the contrary, the curve slope varies considerably from k = 7 . 2 to k = 3 . 6 as the test frequency varies from f = 3 Hz to f = 20 Hz. It is worth noting that another factor that may be relevant in di ff erentiating the behavior between As-Printed and Coated specimens can be the increased cross-sectional area of the struts after the coating process. However, as accurate measurements are not yet available, it is not possible to determine the extent of this e ff ect. This aspect will be covered as part of future works. In this paper, static and fatigue strength data of coated and as printed PLA-FCC lattice structures were presented. A specific specimen geometry was introduced to enable the application of both tensile and compressive loads. The steps necessary to carry out the coating process were illustrated and discussed. Finally, static and fatigue results were reported, the latter at two di ff erent testing frequencies. The data evidenced how the presence of a surface coating increased both the static and fatigue strength characteristics of the specimen. Statically, a 21% increment in maximum strength occurred, while the maximum elongation decreased by 9% in the presence of the coating. During fatigue tests, the coating process enhanced the fatigue strength for both test frequencies by 14% and 18% for f = 3 Hz and f = 20 Hz, respectively. Clearly, more specimens should be tested to achieve higher results consistency. However, these preliminary results are interesting and encouraging. Further steps should consider the study of the coating thickness variation between the di ff erent specimens and the evaluation of numerical models to gain more insights into the fracture behavior of the component. 4. Conclusion

Acknowledgement

This paper is supported by the Ministry of University and Research (MUR) as part of the PON 2014-2020 “Re search and Innovation” resources – Green Action - DM MUR 1062 / 2021 - Title of the Research: Development and conversion of automotive devices towards sustainability: the decarbonisation of vehicles and new uses of thermo hydraulic systems.

References

Barone, S., Neri, P., Paoli, A., Razionale, A.V., Tamburrino, F., 2022. E ff ects of Coating Post-processing on the Compressive Properties of Strut and-Node-Based FDM Lattice Structures, in: Lecture Notes in Mechanical Engineering, Springer Science and Business Media Deutschland GmbH. pp. 442–450. doi: 10.1007/978-3-030-91234-5_45 . Chohan, J.S., Singh, R., 2017. Pre and post processing techniques to improve surface characteristics of FDM parts: A state of art review and future applications. doi: 10.1108/RPJ-05-2015-0059 .