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INTERESTS AND ACTIVITIES OF THE DEPARTMENT OF DEFENSE AND THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION An adhesive joint must be considered as a system consisting of adherend, metal oxide, primer, adhesive, and other additives. The properties of the joint depend on all parts acting in concert, because any part of the joint can act as a weak boundary. A multidisciplinary approach is necessary to design, fabricate, and test the joint to attain the required reliability and durability necessary for joining of structural components. The sizes of interest within an adhesive joint vary over seven orders of magnitude, from atomic distances of 10~^ micrometers, through the inter- phase region, latex toughener particles, primer thickness, scrim fabric thickness, surface roughness, voids, and flaws to bondline thicknesses of 1CH micrometers. Until recently it has not been possible to examine the microstructures in these size ranges. Indeed, it is these size ranges, from the interphase to the matrix microstructure, that control the mechanical properties, toughness, fatigue, and durability of the joint. The USAF PABST program (Potter 1980) demonstrated that primary adhesively bonded structures could be fabricated. The test component was a portion of the fuselage of the YC-15 aircraft. After 76,230 pressure cycles, equivalent to four lifetimes of fatigue, no fatigue cracks were initiated in the structure as a result of any of the bonding operations. There were no failures attributed to poor surface preparation of the aluminum alloy. There was not one instance of disbonding initiated during the test. Furthermore, in spite of the existence of 844 initial bond flaws, there was no significant flaw propagation after the four design lives of durability testing. However, because of uncertainties of surface preparation, fabrication, and nondestructive testing, in addition to the degradation of properties under hot and/or wet conditions, this procedure has not been accepted by the U.S. military services for joining structural aluminum components, in spite of the success of the project and the joining of structural parts by adhesive bonding in European-built commercial aircraft. Large portions of nonstructural parts are joined by adhesive bonding in both military and commercial aircraft manufactured in the United States.
10 The U.S. Army has an important investigation under way at Bell and at Sikorsky to design an all-composite airframe for a helicopter. Through the use of adhesively bonded composites, it is expected to save some 17 percent in airframe manufacturing costs and a 22 percent airframe weight reduction. This program highlights the need to form reliable composite-to-composite and composite-to-metal joints. The major source of concern is the required 30-year durability of adhesive joints under conditions of high humidity, high temperature, and sustained loads. Adhesive joints, with few exceptions, do not retain significant strength when subjected to these combined harsh environments. Moisture can enter the joint through the adherend (if it is an organic composite), through the edges of the adhesive, or through cracks and flaws. The moisture lowers the glass transition temperature (Tg), can displace bonds from the adherend interface, and can convert strong metal oxides into bulkier, weaker hydroxides. The effects of moisture on an adhesive joint can be minimized by lowering the permeability of the adhesive, by improving the interfacial bonding, and by using a corrosion inhibitor. There is a critical need for improved methods of nondestructive inspection. Flaws located near an edge are difficult to detect. One must be able to distinguish between a subcritical and a critical-sized flaw located in a critical area, to detect whether the adherend surface has been properly prepared, and to determine how well "stuck" the joint is (i.e., the quality of the bonded joint). It is also necessary to develop methods for field determination of whether a damaged joint can continue in service, must be repaired, or must be replaced. Adhesive bonding will be a generally accepted method of joining structural members only after analytical models have been developed that can predict the long-term life of a joint following moisture intrusion, matrix aging, bond displacement, stress relaxation, and damage accumulation. The needs and interests of DOD and NASA involve the disciplines of mechanics, materials science, and chemistry to achieve the requisite long-term adhesive joints. Mechanical engineering approaches are required to design test methods that relate the experimental observations to the material properties of the components and that are translatable from test specimens to the design of structural components. Of particular need are simple test methods that require a minimum of sample for rapid screening of experimental materials and procedures, yet provide some material properties. Material and surface science approaches are required to characterize the various methods of adherend preparation, to determine the mechanisms of joint failure, and to quantify the composition and properties of the interphase region. The remaining discussion here of the DOD and NASA needs concentrates on the required properties of adhesives. A wish list of desired properties includes low-temperature and low-pressure cure procedures that can be used in high-temperature applications, the repair of complex parts in the field, the capability of high production rates, and long shelf life of components.
11 "Low temperature cure" is a misnomer. The desire is to effect adhesive cure without recourse to ovens or heat blankets. The cure temperature within the joint has to be high, however, to attain a high Tg. Typically, for a thermoset, Tg is some 30°C (85°F) above the cure temperature. High bondline temperatures can be obtained by chemical reaction or by high-energy irradiation*. It is very difficult to maintain cold-storage conditions in the field, and hence the need for a long shelf life for the components. An adhesive system must be processable, the conditions of processing cannot destroy the surface-preparation treatment (a possibility with high-temperature processing requirements), it must be tough, have resistance to a variety of chemicals (fuels, lubricants, de-icers, paint strippers, chemical agents, etc.), be able to withstand temperature excursions within the design limit (high temperature or cryogenic temperature), and be durable and reliable. Any new adhesive system must be designed to minimize adhesive debonding, maximize processability, maximize toughness, and involve many disciplines in the development, fabrication, and certification processes. A large number of high-temperature adhesive candidates have been developed in DOD, NASA, and university laboratories but have not been fully evaluated. These include but are not limited to LARC-13 and LARC-TPI (U.S. Polymeric), Nadimide terminated polysulfone, polyimide sulfone, partially fluorinated polyimides, polyquinoxalines, and polyphenyIquinoxalines. A limited number of high-temperature adhesives and resins are available commercially. These include FM34B (American Cyanamid), Thermid 600 (U.S. Polymeric), Skybond 708 (Monsanto), Isoimide (National Starch), PBI (Acurex), PEEK (ICI), Torlon (Amoco), and Ryton (Phillips). These potential candidates have a number of drawbacks. They are expensive, many require high processing temperatures and pressures, and some thermoplastics can only be used in thin sections because thick sections cool slowly and create large weak crystalline regions. There is no such item as a generic adhesive. The various steels have different alloy compositions and heat treatments; if grit-blasting is to be avoided as a surface-preparative technique, special physical or chemical techniques have to be developed; the acid or base character of the steel surface will be highly variable; and a variety of conversion coatings are available. Thus each specific joint must be separately tailored for the specified materials and the environmental stresses to which it will be exposed. The present high-temperature adhesives are generally brittle. New means of improving the toughness must be found. Perhaps the failure mechanism can be modified to allow the joint to craze or form many microcracks without developing a catastrophic macrocrack. Alternatively, it may be possible to develop a thixotropic adhesive, one that will be stiff under normal cyclic fatigue conditions yet will be elastic or plastic under abnormal impact conditions. *This subject is discussed in National Materials Advisory Board Report NMAB-412 entitled High-Performance Low-Energy-Curing Resins.
12 The original high-temperature organic materials were absolutely nonprocessable. Processability has been achieved by incorporation of "flexibilizers" or by end-capping oligomers with reactive groups. Unfortunately, these various groups tend to be less stable than the aromatic backbone. Improvements in high-temperature adhesives may be possible by invoking some creative chemistry to look for new synthetic routes to intermediates, new toughening agents or systems, new coupling agents or systems, and new addition-type monomers that can produce thermally stable polymers. In conclusion, DOD and NASA have a large, growing number of critical opportunities to utilize adhesive joining technology in advanced systems. The reliability of adhesive bonds in a variety of severe environments is the issue that must be resolved if these opportunities are not to be missed. Recent advances in a variety of analytical techniques and methodologies together with new approaches and insights in polymer chemistry have put the solutions within reach. The recommendations summarized in Chapter 3 of this report point the way for DOD and NASA to achieve the desired goals. REFERENCES Potter, D. L. 1980. Primarily Adhesively Bonded Structure Technology (PABST), Service Report AD B 55632. Alexandria, Virginia: Defense Technical Information Service.
