Objectives
- Develop a thermally-assisted perforating technique for thermoplastic composites.
- Model the process, including the balance between matrix softening and matrix degradation, and the effect of hole size and distribution on the mechanical properties.
- Model the process of sound attenuation in structures with perforated surfaces.
Project Outline
A current CRP entitled ‘Machining and Mechanical Fastening of Composites’ has successfully developed a new TWI thermally-assisted piercing (TAP) capability. The prototype rig has been used to produce holes in thermoplastic composites, and the technique has been shown to result in reduced detriment to the load-bearing fibres compared with current machining techniques. Test specimens taken from the pierced composites have shown up to 10% improvement in the tension and compression strengths compared to drilled specimens. Although originally developed as a precursor for mechanical fastening, it has become apparent during the project that wider exploitation opportunities are possible, i.e. any application that requires holes to be made in composites. The technique becomes more attractive with decreasing hole size and increasing number of holes (perforation), as this is increasingly difficult to achieve in a fast, cost-effective way, with conventional machining techniques.
This project will develop the current thermally-assisted piercing technique and apply it to the perforation of thermoplastic composites, effectively moving from producing a few large holes to many small holes. Although many of the obstacles remain the same, several new challenges will need to be overcome, not least the problem of rapidly heating a small area or, alternatively, heating larger areas for simultaneous piercing. The second major challenge is to understand the behaviour of a structure comprising multiple small holes within a continuous fibre network, and how this can be exploited to improve performance or reduce weight. Modelling will play an important role in this investigation.
Relevant Industry Sectors
Industry Need
Perforated metal sheets are used across many industries for a number of applications including blast absorption, acoustic damping (on vehicles and buildings), aerodynamic tailoring (ingesting boundary layers) and de-icing surfaces on aircraft. Homogeneous metals are relatively easy to machine into these perforated sheets by milling a large number of holes by laser, water-jet or conventional drilling.
Composite materials have significant advantages over metals in terms of specific properties (strength and stiffness-to-weight ratio). However, their use as perforated panels has yet to expand to a wide range of applications. There are significant difficulties with machining composite materials, eg the cost of tools required to cut the abrasive fibres. Even with the best quality tools, abrasion quickly becomes a significant problem in the drilling of large numbers of holes, causing the hole tolerances to diminish and introducing variability and potential damage into the parts.
State-of-the-art machining techniques (eg laser or abrasive water-jet cutting) may avoid some of the machining problems. Water-jet cutting of composites, however, requires a small initial pilot hole to be drilled in order to eliminate damage, which is not economical for many small holes (perforations). For techniques without this issue (eg laser cutting), there still remains the inherent problem that fibres are cut and removed from the composite when making holes. A greater proportion of the fibres will be cut if a perforated composite is manufactured using this technique, which can significantly affect the mechanical properties. As a result, material must be added to bring the composite back into specification, reducing the potential weight savings initially expected from the use of the composite.