Here’s some rather interesting information on bullet-proof armor called Dragon Skin that uses electrical current generated by bullet hits to identify the size of bullets and perhaps more.
Smart armor being developed by scientists and engineers at U.S. Army Tank Automotive Research, Development and Engineering Center in Michigan can not only predict its own failure, but also identify the size of bullets shot at it and even generate electrical power upon impact.
Piezoeletric sensors are used for measuring the voltage generated by the bullets:
Each plate of armor, whether its wrapped around a soldier’s body or a vehicle’s chassis, has two piezeoelectric sensors attached to it.
An electric current flows into one sensor and turns it into mechanical energy in the form of a tiny vibration that ripples through the armor plate. The other piezoelectric device takes that mechanical vibration and turns it back into electrical energy.
Also see this video demonstrating Dragon Skin’s capabilities such as handling AK-47’s 7.62mm rounds with ease: