The detection of sub-surface discontinuities is based on low-frequency inspection at operating frequency between 100 Hz to 10 kHz. It is used for detection of corrosion and fatigue cracks in the second and third layers hidden under the sheet metal. A typical application is the detection of fatigue cracks in fasteners under the sheet metal, such as the areas of stringers, ribs, spars, areas of windows near the fuselage, etc.
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