JEC COMPOSITES MAGAZINE - Issue #50 - July/August 2009 - 52

KNOW-HOW Ductile prepregs Formable aligned-fibre composite materials The challenge facing structural composites is to industrialise manufacturing. The need for new, more compliant materials that can be processed rapidly and cheaply is now recognised as an essential step in realising this transformation. Perform discontinuous fibre prepregs are being developed to meet this need. Theoretical studies on the rheological behaviour of highly concentrated suspensions of aligned fibres [5] subsequently revealed that the axial elongational viscosity η11, which determines the axial deformation stress, is a complex function of the matrix viscosity η, the fibre volume fraction f and the fibre aspect ratio (l/d): η11 = (η.f)/2.(κ-1)(l/d)2 where κ is a function of the fibre volume fraction. With fibre diameters in the range of 0.005 to 0.025 mm and a normal spread of fibre lengths generated by stretch-breaking of 25-150 mm, individual fibre aspect ratios can be as high as 30,000, with a mean value in the region of 15,000. Since η11 is proportional to (l/d)2, it is hardly surprising that these materials proved difficult to form. ROGER FORD, MANAGING DIRECTOR BOB GRIFFITHS, BUSINESS DEVELOPMENT DIRECTOR INTEGRATED MATERIALS TECHNOLOGY LTD (IMT) o be useful, materials need to be shaped. This can be by addition or removal of material, but more frequently it involves deformation, which is less wasteful. By definition, this requires material flow, which for existing aligned-fibre composites involves the following five flow processes: • Resin percolation • Transverse flow • Inter-ply shear or slip • Inter-ply rotation • Intra-ply shear T However, vital flow along the fibre axes is prevented in materials that contain continuous reinforcement. As a result, successful forming of more complex shapes can be a slow and difficult process. To overcome this elongational flow problem, it is clearly necessary to use materials with aligned discontinuous fibres. However, achieving the required balance between the mechanical and deformation properties of such aligned discontinuous fibre systems, based on chopped [1, 2] or stretch-broken fibres [3, 4], has proved to be more difficult than expected. The problem with chopped fibres was that fibre lengths were limited to ~3 mm by the method of alignment , which depended on convergent flow in a viscous fluid. The products were easy to form, but the mechanical properties, in particular impact properties, were inadequate. In the case of stretch-broken tows with a fibre length spread from 25-150 mm, the mechanical properties were acceptable, but the materials were found to have very high peak yield stresses during forming that resisted the initiation of axial flow and led to serious problems in controlling the forming process. The Perform process To reduce the axial deformation stress to a level that will ease forming, the aspect ratios of the micro-structural elements involved in deformation must be substantially reduced. This clearly will not be achieved if deformation continues to depend on shearing between individual fibres. The Perform process [6, 7] was developed to overcome this difficulty. It uses laser micro-perforation to convert standard UD prepregs to discontinuous form by drilling a pattern of small holes in a single UD ply. The process calls for precision, speed and a small hole size. Laser drilling was chosen because of its capability to meet these needs. Also, as a non-contact process, it offered prolonged operation with consistent hole size. The hole densities used, which are determined by the required fibre length, are typically between 105 and 106 holes/m2. The associated void levels introduced by the drilling are in the 0.05-0.15% range, which is not enough to cause laminate problems. In practice, they increase the ease of gas evacuation during lamination before being filled by matrix flow. 52 jec composites magazine / No50 July-August 2009

JEC COMPOSITES MAGAZINE - Issue #50 - July/August 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #50 - July/August 2009