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North American Aviation, Inc., has been in the adhesive bonding field for the past eight years. During this time, the company's Torrance California Facility has produced over 150,000 adhesive-bonded aluminum sandwich structures. The quality of these honeycomb assemblies has been controlled by conscientious application of rigid in-process control covering cleaning, etching, priming, and bonding. Nondestructive inspection of the end product has been limited to sonic methods, generally outgrowths of the original coin-tapping principle. By variations in audible response, this sonic inspection will detect gross voids but will not indicate substandard bonds. Unfortunately, this lack of effective nondestructive inspection of the end product has discouraged the full utilization of this valuable sandwich construction. Conventional structures are still being used despite the obvious weight penalty. One of the reasons for this is the difficulty of convincing designers that bond quality can be reliably checked and guaranteed by tapping the outside of a sandwich structure. With added confidence in the quality of bonds, the 150,000 or more parts produced by the company could have easily been 500,000 or even a million, considering the large number of F-86 and F-100 aircraft delivered. Being acutely aware of the need for an effective system of nondestructive inspection, the company's quality control laboratory has been evaluating all available nondestructive testing techniques for the past four or five years—the STUB meter, variations of the STUB meter (Coindoscope), thermally sensitive dye coatings, frost patterns, and vibrating sand patterns. The final decision was in favor of two dissimilar yet complementary ultrasonic techniques.
Kammerer, Calvin C.
Research Engineer, North American Aviation, Inc., Los Angeles, Calif.