Fatigue crack propagation in additively manufactured porous biomaterials

Abstract

Additively manufactured porous titanium implants, in addition to preserving the excellent biocompatible properties of titanium, have very small stiffness values comparable to those of natural bones. Although usually loaded in compression, biomedical implants can also be under tensional, shear, and bending loads which leads to crack initiation and propagation in their critical points. In this study, the static and fatigue crack propagation in additively manufactured porous biomaterials with porosities between 66% and 84% is investigated using compact-tension (CT) samples. The samples were made using selective laser melting from Ti-6Al-4V and were loaded in tension (in static study) and tension-tension (in fatigue study) loadings. The results showed that displacement accumulation diagram obtained for different CT samples under cyclic loading had several similarities with the corresponding diagrams obtained for cylindrical samples under compression-compression cyclic loadings (in particular, it showed a two-stage behavior). For a load level equaling 50% of the yield load, both the CT specimens studied here and the cylindrical samples we had tested under compression-compression cyclic loading elsewhere exhibited similar fatigue lives of around 10 cycles. The test results also showed that for the same load level of 0.5F, the lower density porous structures demonstrate relatively longer lives than the higher-density ones. This is because the high bending stresses in high-density porous structures gives rise to local Mode-I crack opening in the rough external surface of the struts which leads to quicker formation and propagation of the cracks. Under both the static and cyclic loading, all the samples showed crack pathways which were not parallel to but made 45 angles with respect to the notch direction. This is due to the fact that in the rhombic dodecahedron unit cell, the weakest struts are located in 45 direction with respect to the notch direction.

Fatigue and quasi‐static mechanical behavior of bio‐degradable porous biomaterials based on magnesium alloys

“…In recent years, some attempts have been made to manufacture regular periodic structures based on magnesium or its alloys . Manufacturing of these structures using additive manufacturing techniques has more challenges compared to other biocompatible materials such as titanium, Nitinol, tantalum, and steel due to the high reactivity of magnesium and its alloys with oxygen and their significant flammability at high temperatures . Future developments in safe additive manufacturing techniques could assist in manufacturing Mg alloy porous structures with the desired topology that satisfies many factors such as low stiffness (close to bone), less surface area exposed to body fluids, and so forth.…”

Fatigue of Cellular Structures – a Review

“…bone implants [14] to [16]. Porous titanium implants, in addition to preserving the excellent biocompatible mechanical properties of titanium, have very low stiffness values, which are comparable to those of natural bones [17]. Some typical regular cellular structures are shown in Fig.…”

“…Most of the research works in this field have been focused on experimental testing [15], [16], [19] to [24] or numerical simulations [25] and [26]. In [20], [27] and [28], the authors investigated the influence of the cell's shape (see Fig.…”

Additively manufactured porous metallic biomaterials

“…30 Finally, there is growing evidence indicating that sheet-based unit cells particularly those based on TPMS result in fatigue strengths that are 2-3 times higher than those achieved through beam-based unit cells (for the same base material). 35 Despite a growing interest in the fatigue behavior of AM porous biomaterials including some studies that try to shed light on the mechanisms of fatigue crack propagation, 116,117 the exact mechanisms relating the topological design and the effects of the AM process to crack propagation at the microscale and the S-N curves at the macroscale remain largely elusive. More systematic research is therefore required to better understand the mechanistic aspects.…”