Monitoring impact damage in composites with large area sensing skins

The effect of low energy impacts can seriously impair the operational life span of composites in the field. These low-energy impacts can induce a permanent loss in the toughness of the composite without any visible indication of the material’s compromise. The detection of this damage utilizing nondestructive inspection requires dense measurements over much of the surface and has been traditionally achieved by removing the part from service for advanced imaging techniques. While these methods can accurately diagnose the damage inflicted internally by the impacts, they accrue non-trivial opportunity costs while the structure is inspected. To enable the capabilities of in-service monitoring of the composite, the novel soft elastomeric capacitor was investigated as a sensing solution. The sensor is made of three layers comprised of a styrene-ethylene-butylene-styrene (SEBS) matrix, a commercially available elastomer. These layers consist of a titania filled center layer that forms the dielectric of the capacitor and two highly conductive outer layers doped with carbon black. This simple formation allows for a capacitor that has extremely robust mechanical properties. The soft elastomeric capacitor functions by taking up deformations on the surface of the composite that is transduced into a measurable change in capacitance. This study provides an electro-mechanical model for impact damage and experimentally investigates the efficacy of these sensors for use in damage detection given their promising characteristics; that being that the sensor geometry can be arbitrarily large allowing for much fewer sensors than traditional sensor networks employed for this task at a much lower cost than installing traditional in-situ sensing solutions. To investigate these properties a set of impact trials were undertaken on a drop tower using small samples of glass fiber reinforced plate, of random orient and short fiber, with a soft elastomeric capacitor mounted directly opposite the impact site. The impactor head was only allowed one contact with the sample before being intercepted. The testing range for the samples ranged from well below the yield strength of the glass fiber reinforced plate to the ultimate strength of the plate. Experimental results reported a square root relation between the impact energy given to the plate when inducing plastic deformations and the sensor’s measured change in capacitance.