Evans, Anthony David
(2018)
Hybrid carbon fibre architectures for high performance, high volume applications.
PhD thesis, University of Nottingham.
Abstract
The interest in compression moulded discontinuous carbon fibre compounds is growing for semi-structural automotive applications, with target Takt times of less than 5 minutes. The main advantage of these compounds, such as sheet moulding compounds (SMC), is that they produce complex 3D geometries by using in-mould flow during compression. This results in short cycle times by eliminating preforming and infusion stages, generating low wastage by using net-weight charges and keeping costs low by minimising touch labour.
An automated manufacturing process has been developed to produce low cost carbon fibre / epoxy moulding compounds. Directed fibre compounding (DFC) uses a robot arm to direct the deposition of chopped carbon fibre bundles simultaneously with a liquid epoxy via liquid resin spray (LRS). The resulting compound can be tailored in terms of areal density, fibre length (15-75mm) and fibre volume fraction (up to 55%). The resin is chemically thickened by B-staging to prevent fibre-matrix separation, which negates using fillers that limit the fibre content of more traditional moulding compounds. The DFC material has similar tensile properties to more expensive prepreg derived carbon fibre moulding compounds.
This thesis aims to understand the effect of co-compression moulding discontinuous and continuous to form hybrid fibre architecture, offering greater mechanical properties in local regions to overcome the strength limitations of the discontinuous fibre architectures. This also enables the manufacture of complex 3D geometries with patches of local reinforcement to enhance the properties in high stress state regions without increasing the Takt time.
Mechanical testing and microstructural analysis, demonstrate that DFC produced with 100% mould coverage, 25mm long fibre bundles and 85 bar in-mould pressure can produce planar isotropic material with tensile modulus and strength values of 36.3GPa ±3.3GPa and 323MPa ±27MPa, respectively. These properties increase when the initial mould coverage area is reduced to 50% (47.6GPa ±5.7GPa and 480MPa ±58MPa), as fibres align in the direction of flow. Void content is also significantly reduced (to ~0.3%) as the charge flow increases, as the trapped air is forced out of the open flash gap, enabling lower mould pressures to be effectively used.
Adding 20% (by vol.) of unidirectional (UD) fibre to the discontinuous material increases the tensile strength and stiffness by 110% and 60% respectively, when the UD fibres are aligned in the primary loading direction. The tensile properties followed a linear relationship as the ratio of the volume of UD carbon fibre to random DFC increases. The effect of different ply layups in the transition zone from the continuous fibre plies to the discontinuous fibre material is also investigated by determining the relationships between architectural features and strain distributions (in-plane and through thickness) surrounding these joints under axial load. Results agree with both joint and ply-drop aerospace design guidelines, indicating that the continuous-discontinuous joint design requires stepping or tapering of the continuous fibre plies to achieve an interface angle of less than 7° in order to minimise the peel stress.
The interface between the continuous and discontinuous fibres is characterised by a double cantilevered beam test (mode-I separation). Fracture toughness values indicate that the discontinuous architecture improves the overall fracture toughness of the system by up to 23%. There was also a 37% improvement in the crack initiation load compared to the UD material, due to discontinuous ‘fibre bridging’ which suppresses crack development. The hybrid interfaces experience a mixture of DFC or UD failure mechanisms during mode-I separation. Therefore, although the crack initiation load increased by 15% at the hybrid interface due to fibre bridging, in comparison to UD separation as a result of fibre bridging, sudden failure of the bridged fibres momentarily increases rate of crack growth through resin rich regions at the interface and resulted in an overall reduction (11%) in mode-I fracture toughness.
A final study was conducted to understand the damage tolerance of the hybrid fibre architecture. Drop weight impact and compression after impact (CAI) testing was performed to determine the influence of the laminate layup sequence, in particular the through thickness position of the continuous plies with respects to the position of the discontinuous architecture. This was performed to determine if the multi-directional material would provide higher damage tolerance to the highly stressed load bearing UD plies, by establishing the size of the damage zones and the retained compressive properties post-impact. The DFC material retained over 73% of its compressive strength following an impact energy of 6.7J/mm. This is high in comparison to a cross-ply UD material, which only retained 32% of the compressive strength post-impact. Hybridising the fibre architecture increased strength retention of the UD material to 39-50% depending on through thickness position of the continuous plies and the thickness of the ply stack.
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