Assessment of the effects of the addition of continuous fiber filaments in PA 6/short fiber 3D-printed components using interrupted in-situ x-ray CT tensile testing

Fused Filament Fabrication (FFF) is an additive manufacturing (AM) method that has proven to have great potential for producing components of high geometrical complexity. Among its unique features is the combination of neat short fiber-thermoplastic filaments with continuous prepreg filaments for creating hybrid composite structures that are characterized by comparably increased mechanical performance. Nevertheless, some series of manufacturing defects may be introduced in the final product, such as pores, fiber misalignment and waviness. These defects are mainly related to errors attributed to the printing process. In the present work, an experimental campaign is presented, where samples made of Onyx® (short carbon fiber/PA6) and continuous carbon fiber are subjected to tensile tests. The neat Onyx® material was considered as reference while continuous fiber samples containing 2, 4, 6 reinforcing layers out of a constant total layer number of 16 were examined. Representative samples were subjected to micro–X-Ray Computed Tomography (XCT) measurements before and during the test execution, implementing the interrupted in-situ technique. From the results obtained, the additional fiber filaments appear to favor the introduction of defects that provoke local failure damage. The tensile strength is not linearly increased while moving from 0 to 4 continuous fiber layers while the 6 layer-reinforced samples delivered lower tensile strength compared to those of 4. The failure mode is also identified from the initiation until the final specimen rupture. The present analysis may be considered as a step further towards the understanding of the FFF process output when combining short and long carbon/glass fibers as well as the actual outcome of the increase of the number of continuous fiber layers that aim to reinforce the material.