Characterization, in analogy with composites embedding unidirectional long fibres, of PLA specimens produced via FDM printing process

The present work arises from the idea that the recent and widespread Fused Deposition Modeling (FDM) technology could be used to manufacture small structural elements comprising those for Unmanned Aerial Vehicles (UAVs) of limited dimensions. This technique, also known in the literature as Fused Filament Fabrication (FFF), is based on the consecutive deposition of melted polymeric material through a heated nozzle. The desired shape is accomplished by building parallel surfaces. This described specific technology has an important impact on the mechanical properties of the finished piece because of its process and environment parameters. The easiest path that could be imposed to the nozzle is the rectilinear one, where each band is deposited in parallel with the adjacent ones. In this context, the possibility to predict the mechanical behavior of the finished pieces is fundamental. The resulting pattern of a single layer looks very similar to the one of a composite lamina embedding unidirectional long fibers. The key idea of this study is that each printed layer could be interpreted and analyzed in analogy with a composite lamina embedding a polymeric matrix and unidirectional long fibers. In the FDM printed pieces, the role of the matrix is played by the bonds between the filaments. Therefore, the finished piece could be considered as a multilayered body embedding layers including unidirectional fibers with a certain stacking sequence. The orthotropic mechanical behavior of so printed FDM pieces is, therefore, studied with an extensive experimental campaign in order to determine how to populate the 6X6 matrix containing the 3D elastic coefficients. The adherence between adjacent filaments could change moving from the printing plane to the building direction. For this reason, three tensile tests should be accomplished. The first one by means of the tensile load applied in the filament direction. The second one by means of the tensile load considered in the printing plane and making a right angle with the filament direction. The third one by means of the tensile load pointing in the out-of-plane direction but still making a right angle with the filament. Using these three experimental tests, the Young moduli in the three material directions (E1, E2, E3) and the three Poisson ratios (ν12, ν23, ν31) can be obtained. These terms lead to a complete determination of the top-left block of the 6X6 matrix of 3D elastic coefficients. The latter three elements of the principal diagonal of this matrix could be determined using three shear tests and rectangular specimens with symmetric centrally located v-notches. The first shear test could be accomplished using a specimen with the filaments deposited longitudinally with respect to the specimen. The second shear test could use the filaments deposited perpendicular with respect to the specimen. The latter shear test could be performed with the filaments deposited along the thickness direction of the specimen. Therefore, the three shear moduli of elasticity could be determined (G12, G23, G31). After the described campaign tests and the full determination of all the parameters contained in the 6X6 matrix of 3D elastic coefficients, several numerical and experimental tests will be accomplished on simple structures, such as square plates with different stacking sequences, in order to evaluate the validity of the performed mechanical behavior prediction.