Damage to composite structures may not be visible following an impact collision. All impacts must be reported immediately. A suitably qualified and licensed engineer needs to inspect the impact area and formally record the inspection and findings in the Aircraft Technical Log.
Failure to detect or report potential damage to an aircraft may result in an aircraft getting airborne in an unairworthy condition.
Techniques for non-destructive testing (NDT)
Due to the anisotropic nature of composites, whose characteristics depend on the direction along which they are considered, the identification of damage requires NDT techniques to be carried out by qualified personnel according to EN4179 standard (under EASA) or in accordance with MIL-STD-410 (under the FAA). These techniques are based on different methodologies.
Optical Inspection - The visual observation of a component, possibly with the use of a lens. Advanced methods include the reflection of a light wave, holographic techniques and interferometry.
Penetrating Liquids. This implies that an inspection can locate open surface discontinuities and take advantage of the ability of some liquids to penetrate by capillarity in the cavities or emerging cracks.
Eddy Currents: If the composite is electrically conductive, another applicable technique is based on the use of eddy currents. A sub-surface control is made with a probe where an alternating current passes through, and it produces eddy currents by electromagnetic induction. This technique allows detecting the presence of defects located below the outer surface of the component under inspection .
Radiography: Through radiography it is possible to have a detection of defects, voids, or the presence of foreign materials up to tens of centimetres in depth - the irregularities are presented with a different density than the material that surrounds them.
Thermography: Thermal imaging with infrared thermo-cameras detect the temperature of the bodies analysed. Defects correspond to changes in temperature. Thermography is aimed to the analysis of thermo-elastic stresses, for improvements in the design of components, and it views and measures the voltages in components under stress, thereby identifying a possible initiation of cracks.
Acoustic techniques: The defects which are present in the material - for example, cracks and/or debonding - are detected by means of ultrasonic signals generated by a suitable probe or by means of acoustic signals generated manually by the operator .