TWI Technical Literature Review 25265
By Chris Worrall
There is a clear industrial need for development of perforated materials. Perforated metal sheets are used across many industries for a number of applications including blast protection, acoustic damping, aerodynamic tailoring and aircraft anti-icing. Homogeneous metals are relatively easy to machine into these perforated sheets by milling many holes by laser, waterjet, punching or conventional drilling.
Composite materials can offer significant specific property advantages over metals; high stiffness and strength combined with low weight. Their use as perforated panels, however, is yet to be applied in many applications. There are significant difficulties with machining composite materials; the cost of tools to cut the abrasive fibres, and reduction in structural efficiency when fibres are cut. Even with the best quality tools, abrasion quickly becomes a significant problem when drilling large numbers of holes and the tolerances of the holes diminishes, which introduces variability and damage into the parts. Using state-of-the-art machining techniques, such as laser or abrasive waterjet cutting, can reduce the machining problems. Waterjet cutting of composites, however, will require a small initial pilot hole to be drilled in order to eliminate damage, and this is not economical for many small holes (perforations). For those techniques without this issue there is still the inherent problem that fibres are cut and removed from the composite when making the holes. A greater proportion of the fibres will be cut if a perforated composite is manufactured using this technique, which can significantly reduce the panel’s mechanical properties, and as a result additional material must be added, reducing the potential weight savings expected from using the composite.
Cutting a hole in any material reduces its strength, and composites are no exception. A typical carbon fibre composite can suffer tensile strength reduction of up to 35% through inclusion of a single drilled hole (Poon, 1991), although this figure can depend on the composite lay-up. To address this, a recently completed TWI Core Research Project (CRP) successfully developed a new TWI capability to make holes in thermoplastic composites using a novel Thermally-Assisted Piercing (TAP) technique (Brown and Worrall, 2015). The technique offers a process where holes can be machined in thermoplastic composites with reduced detriment to the load bearing fibres (when compared with current machining techniques). Coupons containing holes made using the TAP process exhibited an improvement in openhole tensile strength of up to 10% compared to coupons with holes made using a conventional drilling/reaming process. Although originally developed as a precursor to mechanical fastening, it became apparent during the project that wider exploitation opportunities were possible; any application that requires holes to be made in composites. The technique is likely to become more attractive economically when the hole size reduces and the number of holes increased (perforation) as this is increasingly difficult to achieve in a fast, cost-effective way with conventional machining techniques.
This literature review examines several current and potential applications where perforated composites could be exploited to offer improved performance over similar metal-based structures; anti-icing, sound attenuation, blast protection and joining. Materials of interest are both thermoplastic and thermoset composites; thermoplastics for their potential to be perforated after manufacture, and thermosets for the ease of fibre displacement during perforation before the matrix resin has fully cured. The review also covers simulation of both the process of perforating the composite and the performance of the perforated structure. Finally, a review of competing perforating technologies is included to offer insight into when the Thermally-Assisted Piercing process is most likely to be adopted as a perforating technique for composite structures.