Bonded Waffle Doubler

Bonded Waffle Doubler (or Doppler) is a constructive measure in aircraft construction, which serves to prevent crack propagation in the outer skin of an aircraft.
A Waffle Doubler is a kind of aluminum sheet mesh that attaches to the inside of the aircraft skin ( between skin and stringer and frame ). If a crack occurs in the outer skin, it continues to propagate until it runs on the Doubler. There, the crack can no longer grow and is deflected. This facilitates its discovery in a visual inspection.