Enhancing the bearing strength of woven carbon fibre thermoplastic composites through additive manufacturing
Introduction
The additive manufacturing (AM) of composites is an area of growing interest, with studies on short fibre/particle [1], [2], [3], [4], [5] and continuous fibre [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17] composites among those reported in recent years. An example of a short fibre study is provided by Ning et al. [3], they examined the use of chopped carbon fibre as an additive in an ABS filament, it was reported to yield an increase in tensile strength and stiffness versus unreinforced equivalents. The content of fibres in these filaments was limited to approximately 10%, as at higher levels the increase in porosity resulted in a reduction in tensile strength, toughness, yield strength and ductility. As a step toward further increased mechanical performance, studies have examined the use of continuous fibre reinforcement within a polymer printed part. An example of a study of continuous fibre is that of Matsuzaki et al. [10], they carried-out studies on a custom in-nozzle impregnation system, this facilitated the printing of Jute or Carbon fibre in a Polylactic acid (PLA) matrix. This study demonstrated a 4-fold increase in tensile strength with the use of carbon fibre over the PLA only specimen. Whilst earlier studies, such as Matsuzaki et al, were typically carried out on custom built printing systems, later studies have focussed on the Markforged line of composite printers. Markforged released the first commercial composite AM system in 2014 utilising continuous carbon, Kevlar and glass fibre in a PA6 (Nylon) matrix. This system has been evaluated by a number of authors [8], [9], [11], [12], [13].
Tailored fibre placement (TFP) and fibre steering are techniques for the placement of fibres with the use of sewing or tape systems. Sewing based systems have been utilised in attempts to increase composite performance, and typically follow a path for fibre determined by Finite element models [18], [19], [20]. These systems utilise an un-sized bundle of mono-filaments, which are fixed to a fabric backing by a thread, guided by a sewing needle. These preformed fibre rows are then placed into a mould, vacuum bagged and impregnated with an appropriate resin. Fibre bundles placed via this method are often unevenly loaded by the fixation threads, this has been reported by Uhlig et al. and resulted in changes in volume fraction where threads located [20]. Another method for guiding/placing fibres is that of Tape laying systems. These are typically larger in scale and are used extensively in the aerospace industry for laying of large, relatively flat areas of fibre i.e. aircraft wings [21]. In both technologies mentioned above, unidirectional fibre layers are deposited therefore layers must be deposited at 0 and 90-degree angles to achieve higher mechanical isotropy.
Woven fibre fabrics constructed on a loom are the most utilised form of reinforcing materials used in small-medium scale applications. These fabrics are typically formed from thousands of fibre tows threaded together on a loom to form a semi-rigid sheet, this is then cut to size and placed into a mould or forming plate to be impregnated with an appropriate resin or polymer. As the fabrics consist of two or more fibre layers interlocked together, the resulting composites exhibit more isotropic behaviour than obtained for unidirectional composites (mentioned above). As these composite sheets are made to generic sizes, a subsequent machining or water jet cutting processing step is required in order to obtain the required shape and size [22], [23]. As an example, aerospace applications often require machining of composites to facilitate their integration into assemblies of both metal and composite materials (wing boxes, shrouds, nacelles or panelling). Whilst machining ensures more accurate final part dimensions, mechanical machining can result in damage of the reinforcing fibres and/or matrix material [18], this often leads to the use of bulking to increase mechanical performance, increasing weight. A method that has proven successful in limiting fibre damage around a hole to be placed in a composite, is the use of a large heated spike [24], [25]. This spike is driven into the composite laminate and the heating action helps to part the fibre bundles within the matrix. A difficulty however can be the presence of exposed damaged fibres around the hole, as well as fibre path deformation, as the matrix material is burnt off, reducing the potential for load distribution. In this paper the use of additive manufacturing (AM) is investigated as a means of addressing this problem by printing woven multilaminate composites with selective fibre placement for optimisation of internal fibre orientations, particularly around major stress risers such as a notch/hole.
In our previous work the AM technique for producing woven fibre laminates was successfully demonstrated [26]. These structures contained engineered notches where fibres are routed around a region to leave an opening, this results in a continuous fibre structure around the notch. It has been demonstrated that this process retains 93% of the unnotched structures tensile performance. This same process is utilised in this study to produce multilaminate structures through stacking of the aforementioned laminates. In this study, the performance of these AM engineered notched structures is compared with machined equivalents for fastening applications. Through the use of bearing response testing, the printed structures load bearing capacity in both metal-composite (Double-shear) and composite-composite (Single-shear) bolted joints were assessed.
Section snippets
Carbon fibre and polymer filament
Carbon fibre filament was sourced from Markforged CA. These filaments (0.35 mm diameter) consist of two materials, a fibre bundle (reinforcement) and an impregnated polymer (matrix). The fibre bundle consists of approximately 1000, continuous 10 μm diameter monofilaments, this provides the main load bearing component within the filament. The polymer matrix material is a proprietary Nylon blend developed by Markforged. The filament is provided on 150 cm3 spools and stored in a dry box prior to
Specimen preparation
ASTM D5961 Bearing response testing was utilised to assess the AM multi-laminate structures response to bolted fastening. Both Single-Shear (Procedure B) and Double-shear testing (Procedure A) was employed from this standard, with ‘Tailor Woven’ holes and ‘Drilled’ holes bearing strengths compared.
‘Drilled’ test specimens were evaluated as a comparison with the ‘Tailor Woven’ composites. The latter were obtained by drilling the composite using a diamond coated drill-bit to a target diameter
Bearing response testing (ASTM D5961): A and B
As detailed in the previous section the mechanical performance of the composites was assessed using both single and double shear bearing response. This was in order to evaluate the structures response to in-hole applied loads, typically experienced in fastened composite assemblies. The ASTM standard involves the use of two test methods, firstly to assess the materials structure in response to fastening (A), and secondly to assess a specific composite-composite fastener joint configuration (B).
Conclusions
Through an additive manufacturing technique, woven multilaminate carbon fibre reinforced composite structures have been produced for the first time. This printing technique has demonstrated its ability to auto layup woven multilaminate structures containing apertures/holes without the need for post-machining. These fibre structures were continuous in nature, with no break in fibre continuity around the hole perimeter. When tested according to the approach detailed in ASTM D5961 which is the
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to legal and technical reasons.
Acknowledgements
Supported by Irish Manufacturing Research and by SFI through the I-Form Advanced Manufacturing Research Centre 16/RC/3872.
Funding for procurement of the X-Ray μCT scanner was provided through a Science Foundation Ireland Infrastructure Award (16/RI/3747) and is directed by Dr. Saoirse Tracy.
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