TWI Industrial Member Report Summary 1032/2013
By Nick Brown and Chris Worrall
Using current methods, continuous fibres within composites are cut to produce holes for fastening. This can reduce the performance of the structure. Thermal piercing is a new technique to produce holes in thermoplastic composites by displacing the fibres around a hole rather than cutting them. This research programme was focussed on establishing an initial mechanical performance assessment of thermally pierced holes.
Conventional drilling, laser machining and abrasive water-jet cutting are among the current techniques that can be used to produce holes in composites by cutting and removing material. Since the fibres in a composite are the primary contributors to the overall strength, cutting them will further reduce the mechanical properties when machining holes. Displacing continuous fibres around a hole, rather than cutting them, allows fibres to remain continuous along the length of the composite and transfer loads away from highly stressed areas. This means composite components requiring holes for fastening will be able to tolerate larger loads and can therefore be reduced in thickness to save significant weight when applied to large numbers of holes.
- Assessment of relevant and related literature, in addition to preliminary experiments, has identified the key variables associated with the thermal piercing process.
- Instrumented piercing experiments have highlighted the key stages of the piercing process and how these can vary with the spike geometry used for piercing.
- Despite a reduction in pin bearing strength when compared to drilled holes, the failure of the thermally pierced holes is more progressive and energy absorbing than that of drilled holes. This could prove valuable for applications where non-catastrophic failure is a critical requirement.
- The thermal piercing process requires further improvement and optimisation before providing holes with comparable strength properties to conventionally drilled holes.
- The technology is still at low TRL, but could be considered as a technique for forming holes in the next 5-10 years.