The interlaminar interface of a carbon fiber epoxy-matrix composite as an impact sensor

The interlaminar interface of a crossply carbon fiber epoxy-matrix composite was found to be an impact sensor, with sensitivity to impact from 0.8 mJ to 5 J (though no indentation was observed) and to multiple (up to 35) impacts at the same energy. However, the sensing of a small impact after a large one was not effective if the small impact was much lower in energy than the large one. The higher the impact energy, the lower was the contact electrical resistivity of the interlaminar interface after impact. Above 1 J, the resistance tended to increase with partial or complete reversibility upon impact, though this effect was minor compared to the irreversible resistance decrease mentioned above.