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The use of adhesives in the fabrication of metal composite structures is a technique that has become very useful in many fields of design and manufacturing, particularly those related to aircraft. The utilization of adhesive materials has made it possible to satisfy design requirements that would be extremely difficult or impossible to satisfy by the use of more conventional fastening methods. Many factors affect the quality of adhesive bonds, and several types of destructive and non-destructive tests have been proposed and used in efforts to measure bond quality. Destructive tests on adhesive-bonded samples are widely used; they determine bond quality by destroying the bond, thus making the assembly unusable. As a result, the evaluation of usable bonds is based on statistical and process control variables, which is quite satisfactory in some applications. There are, however, many applications in which a direct indication of bond strength in usable assemblies is necessary. Situations in which an adhesive bond contributes to the structural integrity of aircraft exemplify these applications. The need for a non-destructive method of bond evaluation has thus arisen, and only with the development of such a method will designers be able to exploit fully the adhesive fastening technique. The adhesives used in such applications are complex organic materials. The application of the adhesive requires extreme cleanliness and rigid process control. Most of the adhesives attain their strength at the conclusion of a “cure” at elevated temperature. The most common types of construction for aircraft employ the adhesive to fasten two or more pieces of metal together in a lap joint or laminate and in the construction of “sandwich,” which consists of a core of aluminum honeycomb or other material, covered with facings or skins, the entire sandwich being adhesive-bonded. Metals and plastics are the usual materials. Figure 1 shows examples of lap joint and sandwich construction. The state of knowledge of the adhesive bond does not appear to be sufficiently advanced to allow a direct analytic approach to the problem of devising a nondestructive test technique, although publications indicate that in some circumstances, at least, the service properties of an adhesive bond can be predicted from ultrasonic measurements of the complex elastic modulus in simple geometric configurations. The complex elastic modulus is one that includes the effects of viscosity in describing the stress-strain relationship in situations of vibratory forcing. The conversion of mechanical to thermal energy (loss) is thus included in the system description.
Arnold, James S.
Physicist, Stanford Research Inst., Menlo Park, Calif.