Tension-compression asymmetric mechanical behaviour of lattice cellular structures produced by selective laser melting

Abstract

Additive manufacturing is an evolving technology for fabricating porous structures used in a broad array of applications, ranging from the aerospace industry to biomedical engineering. Porous titanium alloy (Ti6Al4V) structures play a major role in biomedical implants and are preferred over conventional solid implants because their properties can be tailored to obtain the stiffness required to avoid stress shielding and improve osteointegration. The mechanical properties of these structures are dependent on unit cell topology and overall porosity. In the present work, three open cellular configurations were studied, namely regular (square), irregular (skewed square) and fully random structures, at three different porosity levels. The samples were manufactured using the selective laser melting of spherical Ti6Al4V powder. The deviations of manufactured samples from as designed were assessed using morphological characterisations and porosity analyses. The mechanical characterisations of the samples included monotonic and cyclic tensile tests, along with conventional compression tests under monotonic and cyclic conditions. The results from the study indicate a clear deviation of thickness values from as-designed values. The effect of inclination of the strut with respect to the loading axis has been studied in compression samples. The off-axis loading in compression led to the asymmetry in the Young's modulus in compression and tension. These led to finite element modelling of structures in the elastic regime and its validation using Gibson–Ashby model for cellular structures.

Correlation between as‐designed and as‐built Young's modulus of cubic regular, cubic irregular, and trabecular cellular materials

“…Twelve different specimens were obtained for each cell topology by varying their strut thickness (50, 100, and 200 μm) and pore size (700, 1000, 1500, and 2000 μm) values. The designed porosity was determined using the as‐designed Computer Aided Design (CAD) model and the as‐built porosity was calculated from the manufactured specimens using the methodology described in our previous work 17 …”

“…All the specimens were subjected to compression and tensile tests under monotonic and cyclic loading conditions. Compression test was carried out on cylindrical specimens according to ISO 13314:2011, 18 while tensile tests were carried out on specifically designed specimens with elongated cellular part attached to solid ends that act as grips for clamping into the testing machine 7,17 . Instron 8516 universal testing machine was used; the test was carried out at room temperature with a constant crosshead speed of 1 mm/min; and data sampling was carried out at a frequency of 1 kHz.…”

“…The displacement under compression loading was measured using an Linear Variable Differential Transformer (LVDT) and in tension, an extensometer was used. The cyclic test is carried out between 20% and 70% of the yield load obtained from the monotonic test as discussed in our previous work 17 . Since this study focuses to obtain a relationship between the as‐designed and as‐built Young's modulus, the unloading Young's modulus of the cyclic test is considered.…”

Tensile Properties of Four Types of ABS Lattice Structures—A Comparative Study

“…The aim of the work of Raghavendra et al [21] was to examine three configurations of structures, namely regular (square), irregular (oblique square), and complete random structures on three different porosity levels. Samples were made using the selective laser melting of Ti6Al4V powder.…”