PSI - Issue 74

Marta Kianicová et al. / Procedia Structural Integrity 74 (2025) 38–43 Mara Kianicová / St ru ctur al Integrity Procedia 00 (2025) 000 – 000

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1. Introduction Advanced 3D metallic porous structures produced by additive manufacturing, known as scaffolds, find applications in biomedicine as the bone implants, as reported by Ryan et al. (2006), Han et al. (2019) or Putra et al. (2024). Among various AM techniques, sinter-based methods that incorporate powder sintering as a final processing step, such as direct ink writing (DIW), provide control over the filament (fiber) porosity. The previous research by Slámečka et al. (2023) has shown that the titanium scaffolds produced by DIW with higher (open 14 %) porosity of filaments (microporosity) exhibited superior fatigue strength in cyclic compression tests, comparable to Ti lattices with dense filaments fab ricated using powder bed fusion AM methods, and markedly better than the fatigue strength of DIW scaffolds with smaller (closed 5 %) microporosity. Slámečka et al. (2023) attributed this improvement to fatigue crack growth shielding mechanisms (decrease in the local stress intensity factor) caused by crack deflection and branching induced by interactions between micropores and the advancing crack front and leading to a retardation of the crack growth rate. This is reflected by a higher roughness of the fracture surface, which leads to an extension of the crack path and further increase in the resistance to fatigue fracture. A two-scale finite element model of scaffolds published by Sl ámečka et al. (2024) revealed a negligible length of the crac k initiation stag e, thus confirming the importance of the crack growth initiation stage. Fo r the quantitative description of shielding processes and crack path length some parameters of fracture surface roughness are important, as shown by Pokluda et al. (2004) and Pokluda and Šandera (2010). Given the limited number of findings and the complexity of fracture process of filaments in scaffolds, it was useful to find out whether the higher fatigue strength observed in DIW Ti -scaffolds with porous fibers can be reproduced by cyclic three-point bending (CTPB) tests of individual filaments, prepared under the same sintering conditions and with similar microporosities (porous 14% and compact 6%). This study presents the results of confocal laser scanning microscopy (CLSM) investigations of surface roughness parameters of fracture surfaces of filaments fractured in CTPB experiments. They will serve for quantitative assessment of shielding effects and for interpretation of the dependence of fatigue resistance of Ti-scaffolds on their microporosity. 2. Experimental samples and methods Filament fabrication followed the DIW protocol detailed in Slámečka et al. (2023). Briefly , a Ti ink prepared from commercially pure, argon‑ atomized titanium powder (20– 63 µm, ASTM Grade 1) and a 10 wt.% bovine‑gelatin binder was printed layer‑by‑layer into straight green fibers. After drying, the fibers were debound for 12 h at 350 °C before argon-sintering at 1300 °C for 3 h to obtain porous fibers and at 1400 °C for 10 h to obtain compact fibers. In both cases, the resulting fibers were approximately 1.7 mm in diameter and 32 mm in length . The fracture surface roughness was examined using the scanning electron microscope Tescan Mira3, Czech Republic . The fracture surface morphology and roughness of broken filaments was studied by the confocal laser microscope Olympus LEXT™ OLS5100, Japan. The software OLS5100 - Analysis application, version 3.1.1.296, was employed for the analysis of obtained roughness data. The objective LEXT 20× was utilized for images and the objective LEXT 100× was employed for roughness analysis to capture fine details of the surface topology. Each sample was subjected to 7 measurements on small surfaces of size 4500 µm 2 spread over the fatigue fracture surface and the average value with the standard deviation was computed. Experiments were performed up to one million loading cycles or until significant loss of stiffness. Stress‑controlled cyclic three‑point‑bending (CTPB) tests were carried out at a constant amplitude with a stress ratio of R = 0.1 and a frequency of 2 Hz. The fixture used cylindrical supports 10 mm in diameter at a 20 mm span. Specimens were tested in the fatigue live range from 10² to 10⁶ cycles, encompassing both low‑ and high‑cycle fatigue regions. 3. Results of roughness measurements The first part of this section is devoted to overview images illustrating macromorphology and height topology of fatigue fracture surfaces of compact and porous filaments. The dependence of selected roughness parameters of fracture surfaces on fatigue life of both kinds of fibers is presented in the second part.