Shearographic Inspection of a DeHavilland DHC-7
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Shearographic Inspection of a DeHavilland DHC-7

Filetype[PDF-964.79 KB]

  • English

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    • Edition:
      Technical note; Aug 1992
    • NTL Classification:
      NTL-AVIATION-AVIATION;NTL-AVIATION-Aviation Safety/Airworthiness;
    • Abstract:
      Under a Cooperative Research and Development Agreement between the Volpe National Transportation Systems Center (VNTSC) and Henson Aviation, Inc., operator of US Air Express, a shearographic demonstration inspection of the fuselage of a DeHavill and DHC-7 aircraft was performed at a US Air repair station at Norfolk, VA, on August 8,1992. The inspection compared the effectiveness of shearography with currently mandated methods in detecting disbands in the fuselage. Adhesive bonding is utilized in modern aircraft fuselages, frequently in combination with rivets. As aircraft age, bond failure becomes a major problem, since it may promote fatigue cracking, moisture intrusion, and subsequent corrosion. Any of these events may cause cabin pressure loss and, sometimes, catastrophic fuselage failure. The shearographic method of detecting disbonds depends on the deformation of the aircraft skin under mechanical stimulus. When illuminated by coherent light, the phase relationship and intensity of the light reflected from any two points of the skin changes as a result of this deformation. Surface changes down to 0.00025 millimeter can be detected and displayed as a real-time image of the field of view. Comparison of successive images as the deformation changes permits interpretation of the condition of abond. In addition, other selected areas of the aircraft suspected to contain disbands were inspected. No disbands were found either by shearography or confirming ultrasound readings. Shearography was clearly able to identify the presence of waffle doublers wherever drawings of the aircraft indicated that they should exist. The demonstration indicated potential advantages of shearography overcurrently used inspection techniques, namely, improved coverage of bonded areas ofthe fuselage and reduced inspection time of the aircraft.
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