Effect of the fiber orientation on the tensile and flexural behavior of continuous carbon fiber composites made via fused filament fabrication

In recent years, additive manufacturing (AM) of reinforced thermoplastics has seen a rapid growth, garnering increased interest among researchers and developers. What is attractive is the possibility of combining the wide range of composite materials available today with a versatile manufacturing process that also allows flexibility in the design of components. AM is indeed characterized by two pivotal features: direct manufacturing and layer-wise processing [1, 2], it being defined as the process of creating parts by depositing material layer upon layer, starting from digital 3D model data. Among all the AM technologies for fabricating polymer components, fused filament fabrication (FFF, also known as fused deposition modeling—FDM) is widely diffused [3]. FFF is advantageous thanks to its process flexibility, robustness and reliability, low material wastage, and relatively low cost of printers and consumables [4, 5].

In this scenario, 3D printing of thermoplastic-polymer composites is becoming a more and more promising solution for turning AM from a prototyping technology to a fabrication process to be implemented for real-world applications. As also recently pointed out by Fidan et al. [6], the developments in additive technologies have accompanied the evolution of the materials processable, progressively allowing the filling of thermoplastic filaments, such as nylon, polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyether ether ketone (PEEK), with various filler-type reinforcements, such as short fibers including carbon fibers [7‐9], glass fibers [10], carbon nanotubes [11], or even natural fibers [12].

Since 2015, several companies have started to develop systems for the fabrication of composite parts employing the technology commonly named continuous fiber additive manufacturing (CF-AM). This is substantially an evolution of FFF, whereby continuous high-resistance fibers are deposited together with a thermoplastic material acting as the matrix of the composite. This process yields components having remarkable mechanical properties in the direction of the reinforcement fibers [13].

Two systems employing fiber-laydown methods are used today: the first is based on the deposition of fibers utilizing a 6-DOF robot arm (e.g., Continuous Composites, Moi Composites); the second performs the deposition of fiber by means of a standard 3D printer (e.g., Markforged, Anisoprint, Desktop Metal). The present work concentrates on the latter type of systems.

Justo et al. [14] and Blok et al. [15] documented the enhancement of the mechanical properties of components manufactured via CF-AM, respectively, for the tensile and compressive case, and the bending and shear case. In both cases, the specimens were produced by using a MarkOne FDM 3D-printer (Markforged), limiting the analyses to a unidirectional alignment of the fibers. Over time, different authors have focused their attention on many aspects of interest related to the printing criteria/process parameters to adopt in AM of composites. Chacón et al. [16] performed extensive testing of specimens in tensile and three-point bending conditions to assess the effect of build orientation (i.e., “flat” or “on-edge,” with respect to the printing bed), layer thickness, and fiber volume content on the mechanical performance of components made via FFF. The influence of other process parameters such as the build orientation on the impact resistance of 3D-printed composites was investigated by Caminero et al. [17]. In a parallel study, the same authors also examined the interlaminar bonding performance of such materials, relating their behavior to process parameters such as layer thickness and fiber volume fraction [18]. Dickson et al. [19] compared different types of fibers (glass, carbon, and Kevlar), testing tensile and flexural specimens having a unidirectional arrangement of the fibers, by varying the number of built layers. Materials similar to the latter were also evaluated for creep and fatigue by Mohammadizadeh et al. [20]. The flexure behavior of 3D-printed specimens reinforced with continuous glass or carbon fibers, arranged unidirectionally, was then studied by Goh et al. [21]. Araya-Calvo et al. [22] specifically investigated the compressive and flexural response of CF-AM 3D-printed parts, focusing on the effect of factors such as the reinforcement type and its distribution inside the specimen bulk. Yu et al. [23] focused on the influence that the number of fiber layers and concentric Onyx rings has on mechanical properties. Al Abadi et al. [24] performed experimental testing and developed an analytical model to help to predict the elastic properties of CCF components.

The typical fabrication constraints of CF-AM systems imply that the fibers need to be cut at the end of each layer to allow the repositioning of the extrusion head. This constraint, in turn, translates to strongly orthotropic physical properties. Moreover, the fiber orientation with respect to the main loading direction has a profound impact on the final resistance of parts. In practical applications, it is rarely the case that the loads acting on a structure can be predicted with total certainty and provisions are typically made to ensure the structure withstands loads in other directions besides the principal ones. In composite components manufactured with traditional technologies, this issue is solved with mixed layups with up to four different fiber orientations. Stacking layers of fibers with varying orientations achieves the same effect in parts made via CF-AM.

Finding literature studies that comprehensively describe such situations is still difficult. Indeed, to the best of the authors’ knowledge, the only studies that considered such an aspect were those by Mei et al. [25], Pyl et al. [26], and Todoroki et al. [27]. Mei et al. [25] evaluated the tensile properties of mixed-isotropic 3D-printed composites after hot pressing. Pyl et al. [26] carried out studies on the effect of mixed fiber orientations (mainly focused on investigating aspects such as specimen geometry and tab configuration for tensile testing of CCF composites), but, once again, the data are to be considered as preliminary¹. Similar tests were carried out by Todoroki et al. [27] with three fiber orientations, only for the tensile case.

The main results found in the literature are summarized in Table 1. Interestingly, a broad diversity of results can be observed, in the numerical values of the tensile and flexural strengths, and in the testing methodologies and testing approaches. Most tests were carried out with a limited number of samples (generally lower than or equal to three), and no comprehensive study with both tensional and flexural tests has yet been conducted. Moreover, nine out of the twelve works focused on CCF composites available in the literature only consider the case of the reinforcement fibers aligned to the loading direction.

This situation is problematic from a design standpoint because a clear indication of the ultimate strength of the given material is of utmost importance. Furthermore, this type of data is essential for structural simulations. The need to fully characterize the material properties, therefore, led us to conduct this study. More in detail, the current work aims at identifying how the change of fiber direction with respect to the main loading direction affects the mechanical properties of the resulting components, and at characterizing the effect of combining multiple layers of fibers with different orientations. This work concentrates on the use of carbon fiber as the reinforcing material.

In this work, two types of filaments supplied by Markforged® (Watertown, MA, USA) were employed to build the specimens:

Prior to their use, the filaments were stored in a protective dry box, to avoid ambient humidity absorption. For the same reason, before each test, all the specimens were further dried for 1.5 h at 80 °C.

To print the carbon fiber reinforced thermoplastic (CFRT) composite specimens, a Mark Two (Markforged® Inc., Watertown, MA, USA) FFF desktop 3D-printer was used (Fig. 1). This device is equipped with two separate extrusion nozzles, one for Onyx and one for the CCF-reinforced wire. The temperature of the printing heads for the Onyx and the continuous-fiber reinforced filament was set to 265 and 270 °C respectively, whereas the printing bed was non-heated.

The accompanying slicing and pre-processing software, Eiger®, was used to set and adjust the most important printing parameters. Among these, the fiber type, fill mode and fiber orientation, number of layers, number of Onyx rings to generate the outer shell can be cited. The thickness of each layer was set to 0.125 mm for both Onyx and CCF, to make the results comparable between different builds.

The specimen geometries were specifically created via CAD software (SolidWorks 2016, Dassault Systems), exported as a stereolithography file (STL) and imported into Eiger®. Different configurations of test specimens were built to exhaustively characterize the 3D-printed composite parts and, specifically, evaluate the influence of the fiber orientation on their tensile and flexural resistance. Each set of samples was built on subsequent planes along the z-direction, each parallel to the printing bed (“flat” build mode): the CCF layers were laid down horizontally and continuously, with a fill type defined as “Isotropic”² within the Eiger® software. Hence, following the design rationale graphically summarized in Fig. 2, the fiber was at first arranged, layer by layer, according to unidirectional orientations: 0°, 90°, or 45° angle with respect to the longitudinal direction of the specimen. Various combinations of the previous orientations were then considered in order to increase progressively the isotropy of the test specimens. This was done to better characterize the specific contribution of each fiber orientation to the mechanical response of the multilayer material. Specifically, two sets of specimens having [0°/90°]_n and [+45°/-45°]_n sequences (repeated n-times to generate the desired thickness) were printed, and a mixed-isotropic fiber pattern [0°/90°/+45°/-45°]_n, was also produced. Besides, a further set of samples made according to a CCF pattern [0°/+45°/90°/-45°]_n was generated to evaluate whether the change of the lay-down base sequence affects the mechanical performance of the mixed-isotropic material.

The printing criteria followed by Eiger® obliges the user to print every component by adding an outer protective shell made exclusively with the Onyx material (with layers arranged at angles +45° and -45° alternatively with respect to the axial direction), the extent of which is defined as a set-up parameter. In particular, each specimen was built with four Onyx layers on the bottom (“floor”) and four Onyx layers on the top (“roof”). The floor layers avoided any fiber breakage while removing the specimen from the platform, whereas the roof layers were chosen for dimensional accuracy and symmetry. Furthermore, each layer built was made as a combination of two infill arrangements of the material: the infill made of CCF laid following one of the aforesaid sequences; and a concentric infill, consisting of two Onyx rings placed along the specimen perimeter (Fig. 3).

To assess the performance of the AM composite material, tensile and three-point flexural test specimens were built according to the geometries suggested by the related standards, following the testing scheme summarized in Table 2. Each configuration and testing condition was repeated five times (N = 5), to enhance the accuracy of the results. The mean resistance values are reported in the “Results” section, together with the related values of standard deviation.

An Instron 8802 universal testing machine (Instron, Norwood, MA, USA), equipped with a 50 kN load cell, was used to perform all the mechanical assessments.

Tensile testing was carried out on parallelepiped specimens of dimensions 157 mm × 16 mm × 3 mm, in accordance with the indications provided by ASTM D3039 [28]. To obtain a proper CCF/Onyx ratio, the thickness was achieved by superimposing a total of 24 layers, each 0.125 mm thick, arranged as follows: 4 floor layers of Onyx, 16 CCF core layers, and 4 roof layers made of Onyx. These tensile specimens were then tested to failure at a crosshead displacement rate of 2 mm/min, and the results were compared with those obtained from a set of reference specimens having the same geometry and size as the CCF-samples but made only of Onyx. Metallic tabs were bonded at the grip areas to avoid any crushing or shifts of the sample during the tests. Having defined F as the force measured by the load cell and A as the sample cross-sectional area, tensile stress was calculated as σ = F/A.

To determine the flexural properties of the material, specimens having dimensions 153.6 mm × 14 mm × 4 mm were built, following the indications provided by ASTM D7264 [29]. In this case, the thickness was obtained as a superposition of 32 layers—again 0.125 mm in thickness each—comprising 4 floor layers of Onyx, 24 core layers made of CCF, and 4 roof layers of Onyx. A three-point loading test method was carried out (Fig. 4). Thus, according to the aforesaid standard, the radius of both the loading nose and the two support rollers (bending dies) was 5.0 mm, and the specimens were tested guaranteeing a span-to-thickness ratio (L/t) of 32:1 (i.e., a span between the supports equal to 128 mm). The crosshead (loading nose) movement speed was set to 1.0 mm/min.

During bending, the test specimen behaves as a beam simply supported at two points and loaded at the mid-span. Thus, the bending moment increases from the support points to a maximum value at the mid-point: maximum stress occurs along a line at the center of the test specimen/beam, where the flexural stress, σ_f, is calculated with the following simplified equation [30, 31]:

where P is the load (N) corresponding to a deflection δ (mm), L is the length of the support span (mm), w and t are, respectively, width and thickness of the test specimen (mm), for a bending moment M equal to PL/4. However, when L/t exceeds 16:1 (as in this investigation), Eq. (1) has to include additional terms in order to take into account the significant end forces developed and relatively large deflections at the support noses [32]. It follows that, as also suggested by the ASTM D790 [33] standard, σ_f has to be calculated using the following equation:

where δ is the deflection of the centreline of the specimen at the mid-span (mm). The corresponding strain value at the outer surface is calculated as follows:

This study aimed at evaluating the influence of the orientation of continuous carbon fibers (CCF) on the mechanical response of composite materials 3D-printed via FFF technology. In particular, both the tensile and the flexural behavior was evaluated according to the relative test standards, building test specimens having various configurations: three unidirectional laydown patterns of the CCF (0°, +45°, and 90°) were set as references, and further mixed-oriented configurations ([0°/90°], [+45°/-45°], [0°/+45°/90°/-45°], and [0°/+45°/90°/-45°]) were considered to represent the more common building approaches adopted for most composite structures.

The experimental findings highlighted that: