Ultrasonic bond-testing is a useful means of verifying that layers of material that have been glued together with adhesive are actually bonded together. For structures where thin layers metal or laminate are bonded together a variation known as "resonance" testing can be invaluable. A second variation, "mechanical impedance analysis" (MIA) is useful in detecting defects between the skin and cores cells of sandwich structures
Most large aircraft skins are fabricated with thin layers of aluminum bonded to each other. Bond-testing can be a quick and inexpensive means of finding unbonded areas in these structures both at the manufacturing level and in-service. Helicopter blades and many aircraft structures are made of sandwich structures in aluminum, fiberglass, and carbon/graphite over honeycomb core. Bond testing is a highly portable, cost effective means of verifying he integrity of this type of structure.