Flow characteristics of carbon fibre moulding compounds

This paper presents the development of a low-cost carbon fibre moulding compound using an automated spray deposition process. Directed Fibre Compounding (DFC) is used to produce charge packs directly from low cost carbon fibre tows and liquid epoxy resin. A range of material and process related parameters have been studied to understand their influence on the level of macroscopic charge flow, in an attempt to produce a carbon fibre moulding compound with similar flow characteristics to conventional glass fibre SMCs. Charge packs covering just 40% of the mould can be effectively used to process DFC, without detrimentally affecting void content, fibre distribution and mechanical properties. Tensile stiffness and strength values of 36GPa and 320MPa are reported for isotropic materials (100% charge coverage), which increase to 46GPa and 408MPa with flow induced alignment (50% charge coverage) at 50% fibre volume fraction

Structural optimisation of random discontinuous fibre composites: Part 1 – Methodology

This paper presents a finite element model to optimise the fibre architecture of components manufactured from discontinuous fibre composites. An optimality criterion method has been developed to maximise global component stiffness, by determining optimum distributions for local section thickness and preform areal mass. The model is demonstrated by optimising the bending performance of a flat plate with three holes. Results are presented from a sensitivity study to highlight the level of compromise in stiffness optimisation caused by manufacturing constraints associated with the fibre deposition method, such as the scale of component features relative to the fibre length

The influence of inter-ply friction during double-diaphragm forming of biaxial NCFs

Inter-ply friction plays an important role in the formation of defects in automated preforming of complex components, as relative sliding between plies can generate local compressive stresses in the direction of the primary yarns, leading to out-of-plane wrinkling and other defects such as fibre buckling and bridging.This work presents a novel characterisation method for measuring the coefficient of friction at fabric-fabric interfaces, using a set up that can generate inter-ply slip conditions representative of those experienced during double diaphragm forming. Results for a pillar stitched, biaxial NCF indicate the sensitivity of the coefficient of friction to the level of applied compaction pressure generated by the diaphragm. Friction values are observed to be typically 100% higher than those recorded using a conventional sled test (ASTM D1894). Additionally, this behaviour is dependent on the relative fibre orientation at the inter-ply interface, which affects the nesting/compaction behaviour of the plies. This has been confirmed using optical micrographs and through-thickness compaction tests.The significance of these orientation and pressure dependencies has been studied using a modified hemisphere forming rig to control the degree of inter-ply slip. Results from the test indicate that punch force is dependent on the fibre angles at the inter-ply interface, and that variation in the local friction behaviour can affect the formability of a component and induce unwanted defects

Double diaphragm forming simulation using a global-to-local modelling strategy for detailed defect detection in large structures

A global-to-local modelling strategy is presented based on a sub-modelling approach to predict the formation of macroscale defects in bi-axial non-crimp fabric (NCF) during double diaphragm forming (DDF). A full-scale global simulation is initially performed using a coarse membrane element mesh (5 mm edge length) to locate areas containing potential defects. Refined local simulations are subsequently performed using high a fidelity shell-element mesh (1 mm edge length) to explicitly predict the shape of forming induced defects in these areas, using boundary conditions derived from the global simulation. The methodology is validated by forming a fabric blank over a generic geometry comprising local changes in cross-sectional shape, in order to invoke forming induced defects in a controlled manner. The defective areas predicted by the simulation agree well with the locations observed from the forming experiments, including the shape and length of surface visible defects such as fabric wrinkling and bridging. The CPU time for this two-stage approach is shown to be approximately 13% compared to the CPU time required for the high fidelity full-scale model for the same geometry

Simulating the effect of fabric bending stiffness on the wrinkling behaviour of biaxial fabrics during preforming

A macroscopic finite element model has been established to investigate the forming-induced wrinkling behaviour for bi-axial fabrics. Results indicate that using a linear bending model with a constant bending stiffness produces unrealistic wrinkle patterns in the fabric plies. A non-linear bending model produces more accurate forming induced wrinkle patterns compared to experimental data, since the bending stiffness parameter is varied as a function of the applied forming load to account for the onset of fibre buckling. Areas of high in-plane shear are more likely to induce out-of-plane wrinkles, indicating a positive correlation between wrinkling onset and shear deformation. A new methodology has been developed to quantitatively evaluate the severity of fabric wrinkles based on the FE simulation results. The distance between the surface of the preform and the mould tool is used to locate areas with out-of-plane defects, using the principal curvature to isolate wrinkles from areas of fabric bridging (poor conformity)

Structural optimisation of random discontinuous fibre composites: Part 2 - case study

This is the second paper in a two part series presenting the development of a stiffness optimisation algorithm to intelligently optimise the fibre architecture of discontinuous fibre composites. A Multi-Criteria Decision Making (MCDM) strategy is used to select parameters associated with the fibre architecture, to produce components that satisfy stiffness, cost and mass criteria. The model has been successfully demonstrated using an automotive spare wheel well geometry, which shows that a highly optimised discontinuous fibre composite solution can compete against a continuous fabric counterpart in terms of specific stiffness, whilst presenting an opportunity for significant cost reduction. This could potentially lead to the application of composite materials into new areas where cost has previously been prohibitive

Compression moulding of composites with hybrid fibre architectures

Advanced Sheet Moulding Compounds (ASMC) and unidirectional (UD) prepregs have been co-compression moulded to form a hybrid composite material. In-mould flow influences the UD fibre architecture in two ways. When UD fibres are aligned transversely to the ASMC flow direction, shearing occurs which causes local changes in fibre volume fraction and fibre waviness. When the UD fibres are aligned with the ASMC flow direction, ply migration takes place. In general, the composite stiffness follows a rule of mixtures relationship, with the stiffness proportional to the UD fibre content. A grid analysis method has been developed to quantify distortion in the UD plies. Staging the resin to 50% cure was shown to reduce ply distortion during moulding, whilst maintaining suitable inter-laminar shear strength. Adding an interfacial prepreg ply between the reinforcing UD fibres and the ASMC charge successfully prevented distortion in the UD fibres, avoiding shear thinning and fibre migration

Optimisation of local in-plane constraining forces in double diaphragm forming

Rigid blocks (risers) were introduced in the double diaphragm forming (DDF) process to change the local in-plane strain distribution in the diaphragms, aimed at reducing wrinkling defects in the production of fabric preforms. A two-step optimisation method was developed to determine the position and dimension of each riser. In Step I, optimisation of the riser position was conducted using a simplified finite element (FE) model coupled with a genetic algorithm (GA). The height of each riser was optimised in Step II using a detailed FE model with the optimised riser positions from Step I. For demonstration, a hemisphere preform was manufactured by DDF using the optimum riser arrangement established by the optimisation routine. Results indicate that the optimum riser pattern (shape and position relative to the component boundary) can dramatically improve the preform quality through reduction of out-of-plane wrinkles, validating the feasibility of the two-step routine

Heterogeneity of discontinuous carbon fibre composites: damage initiation captured by Digital Image Correlation

This paper aims to identify architectural features which lead to damage initiation and failure in discontinuous carbon fibre composites formed from randomly orientated bundles. A novel multi-camera digital image correlation system was used to simultaneously view strain fields from opposing surfaces of coupons, in order to map progression of failure. The highest strain concentrations were found to occur when the ends of fibre bundles aligned in the direction of loading coincided with underlying transverse bundles. The failure plane was observed to grow between a number of strain concentrations at critical features, coalescing sites of damage to create the final fracture surface. Although potential failure sites can be detected at low global strains in the form of strain concentrations, the strain field observed at low applied loads cannot be extrapolated to reliably predict final failure