Mechanisms of fatigue crack initiation and propagation in auxetic meta-biomaterials.

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 SIGNIFICANCE: The 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.

Mechanical performance of auxetic meta-biomaterials.

The innovative design of orthopedic implants could play an important role in the development of life-lasting implants, by improving both primary and secondary implant fixations. The concept of meta-biomaterials aims to achieve a unique combination of mechanical, mass transport, and biological properties through optimized topological design of additively manufactured (AM) porous biomaterials. In this study, we primarily focused on a specific class of meta-biomaterials, namely auxetic meta-biomaterials. Their extraordinary behavior of lateral expansion in response to axial tension could potentially improve implant-bone contact in certain orthopedic applications. In this work, a multitude of auxetic meta-biomaterials were rationally designed and printed from Ti-6Al-4V using a commercially available laser powder bed fusion process called selective laser melting. The re-entrant hexagonal honeycomb unit cell was used as a starting point, which was then parametrically tuned to obtain a variety of mechanical and morphological properties. In this two-step study, the morphology and quasi-static properties of the developed meta-biomaterials were assessed using mechanical experiments accompanied with full-field strain measurements using digital image correlation. In addition, all our designs were computationally modelled using the finite element method. Our results showed the limits of the AM processes for the production of auxetic meta-biomaterials in terms of which values of the design parameters (e.g., re-entrant angle, relative density, and aspect ratio) could be successfully manufactured. We also found that the AM process itself imparts significant influence on the morphological and mechanical properties of the resulting auxetic meta-biomaterials. This further highlights the importance of experimental studies to determine the actual mechanical properties of such metamaterials. The elastic modulus and strength of many of our designs fell within the range of those reported for both trabecular and cortical bone. Unprecedented properties like these could be used to simultaneously address the different challenges faced in the mechanical design of orthopedic implants.