Abstract
The fatigue performance of additively manufactured auxetic meta-biomaterials made from commercially pure titanium has been studied only recently. While certain assumptions have been made regarding the mechanisms underlying their fatigue failure, the exact mechanisms are not researched yet. Here, we studied the mechanisms of crack formation and propagation in cyclically loaded auxetic meta-biomaterials. Twelve different designs were subjected to compression-compression fatigue testing while performing full-field strain measurement using digital image correlation (DIC). The fatigue tests were stopped at different points before complete specimen failure to study the evolution of damage in the micro-architecture of the specimens using micro-computed tomography (micro-CT). Furthermore, finite element models were made to study the presence of stress concentrations. Structural weak spots were found in the inverted nodes and the vertical struts located along the outer rim of the specimens, matching the maximum principal strain concentrations and fracture sites in the DIC and micro-CT data. Cracks were often found to originate from internal void spaces or from sites susceptible to mode-I cracking. Many specimens maintained their structural integrity and exhibited no signs of rapid strain accumulation despite the presence of substantial crack growth. This observation underlines the importance of such microscale studies to identify accumulated damage that otherwise goes unnoticed. The potential release of powder particles from damaged lattices could elicit a foreign body response, adversely affecting the implant success. Finding the right failure criterion, therefore, requires more data than only those pertaining to macroscopic measurements and should always include damage assessment at the microscale. Statement of significanceThe negative Poisson's ratio of auxetic meta-biomaterials makes them expand laterally in response to axial tension. This extraordinary property has great potential in the field of orthopedics, where it could enhance bone-implant contact. The fatigue performance of additively manufactured auxetic meta-biomaterials has only recently been studied and was found to be superior to many other bending- and stretch-dominated micro-architectures. In this study, we go beyond these macroscopic measurements and focus on the crack initiation and propagation. Full-field strain measurements and 3D imaging are used to paint a detailed picture of the mechanisms underlying fatigue. Using these data, specific aspects of the design and/or printing process can be targeted to improve the performance of auxetic meta-biomaterials in load-bearing applications.
Highlights
Architected materials have attracted much attention recently
The mechanisms underlying crack initiation and propagation in twelve different types of additive manufacturing (AM) auxetic metabiomaterials subjected to compression-compression fatigue testing were studied
Geometrical variations in the re-entrant hexagonal honeycomb resulted in four different designs (A-D), while each of them was manufactured in three different relative density classes (RD ≈ 5%, ≈ 25%, ≈ 45%)
Summary
Architected materials have attracted much attention recently. With the advances in additive manufacturing (AM), it has become possible to manufacture these complex, micro-architected three-dimensional structures. We focus on a specific class of meta-biomaterials that expands laterally in response to axial stretch These materials are collectively referred to as auxetic meta-biomaterials, of which first proof dates back to the 1870s [4]. With the lateral application of an auxetic meta-biomaterial, the implant-bone contact could be improved, thereby enhancing bony ingrowth, strengthening the mechanical fixation of the implant, and decreasing the risk of aseptic loosening [3,8]. The latter is associated with the mechanical failure of the implant-bone interface and is the most prevalent failure mode in total hip replacements [9,10]
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