Shearographic Inspection of a DeHavilland DHC-7
FEDERAL AVIATION ADMINISTRATION TECHNICAL CENTER ATLANTIC CITY NJ
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Under a Cooperative Research and Development Agreement between the Volpe National Transportation Systems Center VNTSC and Henson Aviation, Inc., operator of USAir Express, a shearographic demonstration inspection of the fuselage of a DeHavilland DHC-7 aircraft was performed at a USAir repair station at Norfolk, VA, on August 8, 1992. The inspection compared the effectiveness of shearography with currently mandated methods in detecting disbonds 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 a bond. In addition, other selected areas of the aircraft suspected to contain disbonds were inspected. No disbonds 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. Nondestructive evaluation shearography, Inspection aging aircraft, Aircraft inspection.
- Test Facilities, Equipment and Methods