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1 SUMMARY Adhesive bonds have several major advantages relative to mechanical fastenings, including potential savings in weight and in manufacturing costs. A recent test of this technology for structural applications, the U.S. Air Force Primary Adhesively Bonded Structure Technology (PABST) program, demonstrated that under favorable conditions such bonds can be quite reliable. Unfortunately, some unresolved issues remain, particularly the degradation of properties of the adhesive joints under hot and/or wet conditions. This report summarizes the proceedings of a National Materials Advisory Board workshop held to examine the issue of the reliability of adhesive bonds under severe environments. The main conclusion of the workshop was that adhesive technology has advanced sufficiently so that a concerted effort to resolve those issues left by PABST would have a high probability of success. The workshop participants recommended that the federal government spearhead such a major effort. Specific research was recommended in the areas of chemistry, interfaces, and mechanics. Some of the recommendations in the area of chemistry included the search for more processable high-temperature adhesives, the search for new synthetic routes to alleviate the evolution of volatiles, research on improved toughening additives, and research aimed at better room-temperature-curing resins. Some of the recommendations in the area of interfaces (interphases) included research to clarify the general role of adherend topography, search for new adherend surface preparation techniques, research directed at elucidating the requirements for a strong bond, and research on the mechanisms of interphase degradation with time, emphasizing the effects of water, mechanical stress, temperature, and adhesive composition. Some of the recommendations in the area of mechanics included additional research in the area of nondestructive evaluation (NDE), the
search for mechanical test methods that yield results more directly interpretable in terms of material properties and interphase character- istics, and, in particular, research to determine the relative influences of viscoelasticity (creep), cyclic fatigue, and static strength in severe environments and to characterize interactions between fatigue and creep. To facilitate interlaboratory comparison of results, it was also recommended that standard reference materials be developed and adopted. Although most of the recommendations were presented under the three separate headings of chemistry, interfaces, and mechanics, the workshop participants stressed that in fact these three areas are closely coupled and the research program must recognize that an adhesive bond is a complex system, requiring an interdisciplinary approach for its optimization.