N.A. Warrior

Effect of resin properties and processing parameters on crash energy absorbing composite structures made by RTM

The effects of resin properties and resin processing parameters on the crush behaviour of thermoset composite tubes manufactured using resin transfer moulding are considered. The aims were to quantify the performance of tubes produced over a broad spectrum of conditions and to correlate this performance to the properties of the material. The effects of the mould temperature, post-cure time and resin composition were investigated for random and engineered reinforcement fabrics. Relationships between Youngs moduli and ultimate stresses of the material in tension and compression were established from plaques moulded under similar conditions. Random reinforcement fabrics gave higher specific energy absorption (SEA) levels than engineered fabrics, but were more sensitive to processing conditions. Epoxy absorbed more energy than vinyl ester. Vinyl ester absorbed more than polyester, and additions of vinyl ester resin to unsaturated polyester gave a linear increase in SEA. The ultimate compressive strength of the composite proved the best indicator of performance for the selected materials and processing conditions.

Low-cost carbon-fibre-based automotive body panel systems : a performance and manufacturing cost comparison

Carbon—;fibre—;based composite manufacturing processes have been considered for automotive body panel applications. A full—;scale front wing—fender component was produced using two composite manufacturing processes (a semi—;impregnated (semi—;preg) system and a novel directed fibre preforming—resin transfer moulding process) and compared with an existing stamped steel component for mechanical properties, weight saving, and cost, using a technical—;cost—;modelling procedure. Mechanical testing demonstrates that the carbon fibre composite solutions can provide 40—50 per cent weight saving for an equivalent bending stiffness to steel panels and greatly improved dent resistance. For the part studied, carbon fibre semi—;preg systems offered the lowest cost process up to around 500 parts/annum and directed fibre preforming technologies were cheaper between 500 and 9000 parts/annum. The steel component was seen to be more cost effective at volumes above around 9000 parts/annum.

Net shape spray deposition for compression moulding of discontinuous fibre composites for high performance applications

Abstract Details are presented of a novel carbon/epoxy spray deposition process for producing high performance, net shape charges for low flow compression moulding. The Bentley–Raycell automated carbon composite charge deposition (BRAC3D) process sprays powdered epoxy and chopped carbon bundles onto three-dimensional (3D) tools, offering a fully automated process with no touch labour. It has been demonstrated that fibre volume fractions of up to 54% are achievable for random discontinuous fibre architectures, with low void content (1·6%). This extends the volume fraction range currently offered by liquid moulding/preforming processes and potentially reduces part scrap rate, since the resin flow direction is through thickness rather than in-plane. Results from an experimental programme are presented, which aims to benchmark the BRAC3D material against commercial advanced moulding compounds. Ultimate tensile strength, tensile modulus and Charpy impact values are reported to be 272 MPa, 44·4 GPa and 128 kJ m–2 respectively, for the random fibre architecture at a fibre volume fraction of 54%. This equates to a 99% stiffness retention and a 59% strength retention compared to a continuous fibre, quasi-isotropic counterpart. Observed trends for increasing fibre volume fraction and fibre length have been compared against finite element predictions and an analytical inclusion model. Simulations from a parametric cost model indicate that the BRAC3D process is cost effective for production volumes exceeding 1100 ppa for a structural demonstrator component, compared with prepreg and resin transfer moulding.

Fiber Alignment in Directed Carbon Fiber Preforms — A Feasibility Study:

The directed carbon fiber preforming (DCFP) process has the potential to create discontinuous fiber architectures with significant levels of fiber alignment. This article investigates the achievable levels of alignment and identifies the compromises inherent in the production of aligned preforms. The tensile properties of laminates produced from 6 and 24 K filament counts are compared at three different fiber lengths (28, 58, and 115mm). Experimental characterization indicates that up to 94% of fiber bundles can be aligned within ±10° using the current DCFP alignment method. Consequently, tensile stiffness and strength can be increased by 206 and 234%, respectively, over the random fiber case, as a result of the high concentration of aligned fibers in the loading direction. These alignment properties equate to maximum stiffness and strength retention values of 83 and 31% compared to continuous unidirectional material.

Experimental studies of embroidery for the local reinforcement of composites structures: 1. Stress concentrations

Embroidery techniques were investigated for the manufacture and modification of engineered preforms. Flat 0° fabric specimens were produced by Cornely embroidery and their tensile properties compared to commercial fabric alternatives. Similarly, the effects of through-thickness stitching via Barudan embroidery were assessed for both 0° and quasi-isotropic laminates. Specimens containing a central cut-out were tested by using a variety of in-plane and through thickness fibre architectures to study ways of suppressing damage in the vicinity of stress concentrations. The elastic properties and the accumulation of damage were monitored by measurements of the instantaneous stiffness and the results were compared with conventional, fabric specimens. Cornely embroidery was shown to produce in-plane properties comparable with commercial fabrics. For the hole-in-a plate specimens no significant improvements in performance were achieved for unidirectional laminates but through-thickness stitching effectively doubled the load at which the onset of damage occurred in the quasi-isotropic specimens.

Impact properties of compression moulded commingled E-glass-polypropylene composites

Abstract The impact properties of commingled E-glass-polypropylene composites with varying fibre architectures have been investigated. The fabric structures include balanced and unbalanced twill weaves and a three-dimensional woven fabric. Comparative data for glass mat thermoplastics are included and additional comparisons with crossply continuous filament tape laminates are also made. All the materials were processed into flat panels via non-isothermal compression moulding. First, voidage was correlated with processing parameters to produce well consolidated laminates. Mechanical tests included tensile, flexural, and interlaminar shear. Impact tests include Charpy and falling weight; the latter ranged from 15 J to full penetration and were followed by IR thermography and microscopy to determine the extent of damage areas. The results demonstrate the evolution of damage with increasing impact energies and the mode of impact damage propagation for the different fibre architectures.

Fiber Alignment in Directed Carbon Fiber Preforms - Mechanical Property Prediction

A finite element method is presented for predicting the mechanical performance of discontinuous fiber mesostructures typically produced by directed carbon fiber preforming. High-filament count bundles are modeled using beam elements to enable large representative volume elements to be studied. The beams are attached to a regular grid of 2D continuum elements, which represent the matrix material, using an embedded element technique. The model is validated by comparing simulations with experimental data for random and aligned fiber architectures produced with different tow sizes (6 and 24 K) and fiber lengths (28, 58, and 115 mm). Stiffness and strength predictions are generally within 10% for 6 K preforms, but this error increases up to 40% with increasing tow size because of the assumption that the fiber bundles are circular in cross-section.

Experimental determination and control of prepreg tack for automated manufacture

Abstract The automated tape laying (ATL) process has been examined and found to be sensitive to tack and stiffness properties of the prepreg material being laid. A comparison of existing aerospace and newly developed ATL prepreg tapes has revealed significant differences in tack response to temperature and feedrate. Examination of constituent resin rheology has found that tack, and the two observed failure modes, are somewhat dependent upon viscoelastic stiffness. Observation of temperature and feedrate response revealed a time–temperature superposition relationship. The Williams–Landel–Ferry equation was utilised to make predictions of the temperature response based on the feedrate response. Tack levels were stabilised over the feedrate range by making temperature adjustments. Results from the peel test, where mould conditions at lay-up were recreated, were found transferable to the ATL, where a suitable lay-up feedrate under ambient conditions was predicted.

Development of high value moulding compounds from recycled carbon fibres

Abstract A route for the recovery and reuse of carbon fibres is presented with a summary of technological advances and areas requiring further development. Critical issues in size reduction of recyclate are presented along with results from a study considering comminution of a variety of time expired prepregs. High quality recovered carbon fibres have been incorporated in moulding compounds and preimpregnated composite materials. Fibre alignment is shown to be a critical factor in attaining high mechanical properties and high recovered fibre utilisation. A number of demonstrator materials have been developed and used to manufacture automotive parts which have shown excellent mechanical properties when compared with commercial glass fibre based moulding compounds.

Effect of resin formulation on crash energy absorbing composite structures made by RTM

Abstract Crush experiments have been performed on polyester and vinyl ester composite tubes. The preforms were made of random glass mat and the tubes were produced by resin transfer moulding.The effects ofdifferent processing parameters were investigated. Flat plaques were also produced under similar conditions in order to measure in plane properties of the composite material. The two main objectives of the study were to quantify the effect of industrial manufacturing conditions on the crush performance of composite structures and to correlate the performance to a number of in plane laminate properties. The manufacturing parameters considered are resin related:the mould temperature, post-cure time, and resin composition were varied according to a full factorial experimental plan. In addition to crush experiments, the tensile and compressive moduli and ultimate stresses were determined; the degree of conversion was also measured. The results demonstrate that while relationships between all in plane properties and the crush performance can be observed, the ultimate compressive stress is the most reliable indicator of this performance. The results also show clear advantages associated with the vinyl ester resin, and the many intricacies pertaining to the modelling of the effect of processing parameters on crush performance.