Interesting. Might be very useful in large composite transport aircraft. Might be expensive for little guys. Widespread use in airliners and spacecraft would bring the costs way down, just like some other aircraft technology like glass panels.
Zlin, I think it is, has a sealed tubular wing spar that has a bit of pressure in it, with pressure sensors that alert the pilot if pressure is lost, implying possible cracking. That would be the primitive, analog equivalent of smart fibers, but it would also be a pain if sealing was difficult to maintain. Might spend more money keeping it working than on all the rest of the airplane's maintenance, and I think the smart fibers might present the same sort of hassles. The more complicated we make things, the more it costs to make and care for it.
Yak 55 has a pressurized main wing spar. If it leaks, a warning appears in the cockpit.
Helicopters have also used a similar Blade Inspection Method. I once got stranded in New Jersey when our Sikorsky Sea King got a BIM warning light. We had to wait a week while they trucked a replacement down from Nova Scotia.