The three-dimensional (3D) printing technology is a manufacturing technique for fabricating a wide range of structures and complex geometries from 3D model data. The fused deposition modelling is a common additive manufacturing (AM) technique in this industry, using filaments of raw material to produce the final product, while Polylactic acid (PLA) is considered to be one of the most convenient polymers for use in this kind of fabrication. The PLA components produced by this technique are usually required to maintain good mechanical properties in several applications, especially when they are manufactured with complex geometries resulting in high stress concentration. Therefore, it is recommended to investigate the strength of AM PLA components under different kinds of loading.
The Theory of Critical Distances (TCD) is the name that has been given to a group of design methodologies that are considered highly precise and reliable tools for predicting the static strength of brittle notched materials. The TCD represents an ideal method for optimising the mechanical properties of 3D printed PLA parts used in sensitive applications like tissue engineering. In this respect, this study used the TCD to predict the strength of a large number of AM PLA components, tested under both tensile and bending loading and containing different geometrical features. Two groups of specimens were tested in this experimentally based study. The first group was solid AM PLA with an infill ratio of 100%. The influence of several printing parameters on the strength of plain specimens was investigated.
The TCD’s validity as a method for determining static strength of notched PLA specimens was checked with different notch shapes and root radii, under tension and bending loading. The TCD was found to be highly accurate in estimating the static strength of notched AM PLA solid specimens, with its use returning estimates falling mainly within an error interval of ±20%.
The second group of specimens was for AM PLA plain and notched porous specimens manufactured with variable infill levels. A novel approach combining conventional TCD with the equivalent homogenised material concept was formulated to perform a static assessment of plain/notched objects of PLA when this polymer is additively manufactured with different infill levels. The key idea was that the internal net structure resulting from the 3D-printing process could be modelled by treating the material as a continuum, homogenous and isotropic, thus allowing the internal voids to be considered in terms of the change in their mechanical/strength properties. This idea was initially applied by addressing this problem in a Kitagawa-Takahashi setting via the Theory of Critical Distances, for plain porous specimens. Subsequently, the approach was extended to the static strength assessment of notched porous components of 3D-printed PLA. The results showed that the TCD applied alongside the equivalent homogenised material concept was able to model successfully the static strength of plain AM PLA materials, as well as notched materials, fabricated with variable infill levels. Again, predictions fell mainly within an error interval of ±20%
