At the core of sandwich composites
A novel interlayer between the skin and foam core in sandwich structures could provide composite components with an inherent self-healing mechanism.
Inventors at the University of Sheffield, UK, believe the material complements research conducted by other scientists into self healing resins and systems for skin materials.
Mechanical damage of such composites causes skin fracture and disbond and core crushing. The work at Sheffield focuses on filling the void left by core crushing and on reattaching the skin to the core. This reinstates the backface support to prevent buckling during compression.
Peter Bailey, of the University’s Composite Systems Innovation Centre, explains, ‘Currently to repair these composites, you drill a big hole through the structure, take out the core and patch it up. By drilling through the outside skin, you have to replace a larger area than the original damage. This takes time, money and effort’.
The interlayer (the material’s exact composition is confidential) may be co-cured with the prepreg or sprayed in situ, substituting conventional adhesives. It expands when heated to a level above the part’s service temperature, filling the compressed core area.
The concept has been demonstrated producing samples made from glass fibre-reinforced epoxy skin with a polymethacrylimide foam core material, and a 0.5mm interlayer. The pieces were subjected to impact energy of up to seven joules, and on heat activation the composite recovered 85% of its original strength. Similar performance was obtained with a wet laid polyester skin. The potential for repeatedly self-healing the same spot is to be explored.
Bailey says, ‘We look to repair minor damage – if you have major damage, the skin is perforated and the mechanical benefits of repair become negligible’.
The demonstrator was cured at 115ºC with an estimated service temperature of 80-100ºC and healing at about 190ºC. ‘In principle, you can tune the healing temperature to between 90 and over 200ºC,’ he notes. ‘You would not want it to expand during curing, so you would tune your blend according to what the skin, application and curing temperature are.’
Furthermore, incorporating the layer is said to have limited impact on the overall mechanical properties of the sandwich structure. The team at Sheffield suggests the material in fact improves the composite’s impact resistance. Bailey explains, ‘In a foam core sandwich, you have a stiff skin and a weaker foam core, and one of the issues is that the bond between them is easily broken because of this mismatch in properties. Introducing a thin layer that is less compliant than the foam but less rigid than the skin gives you a half way step between the two’.
The researchers are investigating the system using carbon fibre reinforced epoxy skins and other foam materials, and hope to extend the work to honeycomb cores.
‘Industry collaboration is the next stage,’ Bailey adds. ‘It is an application facing idea. We need someone to give us a brief and we can focus on optimising the material for that purpose.’
While an external heat source is required to activate healing, Bailey says there is potential for automated self-healing by linking it with Dr Simon Hayes’ research at the University on self-sensing.