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

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 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.