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3 RECOMMENDATIONS A broad recommendation emerged from the workshop discussions to the effect that, in view of the scientific and technological progress* made since the completion of the PABST program (see p. 9), it is advisable to mount an interdisciplinary effort to go the rest of the way and establish the suitability of adhesive bonding for a range of military and commercial applications requiring high reliability in severe environments. This interdisciplinary effort should involve material science, surface science, mechanics, and process engineering. It should recognize that adhesive bonding must be studied as a complete system, and it should emphasize the specific areas identified in the specific recommendations listed in the sequel. If this is done, the potential benefits are great, the probability of success is high, and the time for a major, concerted effort in this area is now. More specific recommendations are listed below under the subheadings previously mentioned. Background for these recommendations is given in Chapters 8 through 10 of this report. CHEMISTRY o Synthetic work should be performed to investigate polymer-forming reactions that eliminate evolution of volatiles and provide moisture-resistant polymers with a favorable combination of adhesive properties. This pertains to adhesives for use at all temperature ranges. o Studies on model high temperature polymers should be conducted to generate fundamental information on the relationships among structure, cure, physical properties, and mechanical properties. *Significant progress has been made in new materials (polyimides, polyimide sulfones, polyphenylquinoxalines, etc.), in understanding the mechanism of adhesion failure, and in mechanical modeling of adhesive joints.
Basic synthetic work should be directed toward elimination of tenaciously held and toxic solvents, especially for high-temperature polymers. Other known high-temperature polymers should be evaluated as adhesives for short-term, very-high-temperature service 538 to 760°C (1000 to 1400°F). Research should be carried out to improve the processability of high-temperature 288 to 371°C (550 to 700°F) adhesives while retaining good adhesive properties at the temperature of use. New synthetic routes should be investigated to circumvent the need to use toxic monomers. Research should be directed toward toughening the adhesives based on addition-type polyimides, acetylene-terminated phenylquinoxalines, aryloxy-s-triazines (formed from cyanates), and polysulfones for use in the medium-high temperature range 177 to 232°C (350 to 450°F). This may require the systhesis of new toughening agents (e.g., high-temperature elastomers) and investigation of toughening mechanisms at medium-high temperatures. Research should be directed toward investigating crosslinking (curing) mechanisms to obtain fast curing adhesives with no volatile products with a view to optimize curing conditions for adhesive joints. Research should be performed to develop adhesive systems (epoxy replacements, such as acetylene-terminated polymers, polyimides, and aryloxy-s-triazines) that alleviate the shortcomings of conventional epoxy adhesives, for use from room temperature to moderately elevated temperatures [say, to 93°C (200°F)]. Research should be performed to develop room-temperature-curing resins with shorter cure time, longer pot time, and better mechanical properties than current room-temperature-curing systems. The desired requirements for low-temperature (below normal room temperature) adhesives should be defined, and priorities for low- temperature applications should be established. Research is encouraged on the exploration of the utility of novel polymers as adhesives, including ordered polymers. INTERPHASES o Further research should be carried out to clarify the general role of adherend topography on bond performance and to optimize it for specific adherends. For example, studies should be made of anodized surfaces to determine the best combination of oxide surface chemistry for a given application. Particular attention should be given to the surface preparation of steel, aluminum, and titanium in view of the importance of these to the armed forces.
o Alternative adherend surface-preparation techniques should be inves- tigated. For example, can nonaqueous methods be used to anodize and create a mechanically stable and corrosion-resistant surface? Do conversion coatings offer a viable alternative to anodization for titanium and aluminum? Can currently used anodization methods be applied successfully to the new generation of structural materials, such as metal matrix composites and powder metal products? o Fundamental research should be directed at quantifying the effect of the adherend on the adhesive and of the adhesive on the adherend. The chemical reactions occurring between polymers and metals at high temper- ature and their role in the durability of adhesive bonds should be identified. o Research should be directed at elucidating the requirements for a strong bond. For example, are chemical forces required or are physical forces sufficient? Are there adequate theoretical concepts to predict adherend- adhesive interaction? What is the role of adhesion promoters and coupling agents? o The micromechanics of the interphase should be investigated, taking into account the effects of the presence of materials of radically different composition at distances of only hundreds of nanometers. o Techniques should be investigated that might provide in a nondestructive manner a detailed physical and chemical characterization of the inter- phase. In situ probes may be particularly useful in this connection. o Mechanisms of interphase degradation with time should be investigated, with emphasis on the effects of water, mechanical stress, temperature, and adhesive composition. Emphasis should be placed on the approaches discussed on pages 23 and 24. Particular attention should be given to the mechanisms of both water transport and destruction of interfacial bonds through the action of water. Research should also be carried out on the effect of stress on water transport and, in general, on the interaction of the various factors that promote bond degradation. o Research should be carried out on the effects of additives and impurities on adhesive bond durability. MECHANICS o Efforts should be made to develop NDE approaches, combining testing and analysis, that would allow identification of good or bad bonds in the absence of obvious physical defects, both as manufactured and in service. o Improved nondestructive testing (NDT) techniques should be developed, including automated NDT and techniques suitable for field testing under realistic conditions matching the intended use of the adhesive joint.
8 o Improved quality-control methods for manufacturing should be developed. o Procedures, to be conducted in conjunction with stress analysis and failure mechanism studies, should be developed to determine critical flaws in critical locations. o Mechanical test methods should be developed that yield results more directly interpretable in terms of material properties and interphase characteristics and that are suitable to investigate durability under a variety of loading and environmental conditions. It is particularly important to have input from both the experimentalists and the analysts in the development of these test methods. o Mechanical test methods should be developed that require only small quantities of adhesives for the evaluation of novel, experimental materials, which initially may be available only in small amounts. o Models should be developed to describe the stress-strain relation throughout an adhesive joint as a function of load, time, temperature, and humidity, with specific attention to the interphase region. o The curing process should be investigated and mathematically modeled, with emphasis on the development of residual stresses and defects. o Orthotropic material analyses should be made of adhesive joints, especially to account for the performance of scrim cloth and various reinforcements. o Finite element analysis (FEA) methods need to be improved to allow handling geometrically nonlinear systems, plasticity, viscoelasticity, three-dimensional conditions, thermal effects, and moisture absorption and migration. o Failure mechanisms need to be characterized and a data base developed for several frequently used structural adhesives subject to severe environments. o A long-term durability data base should be developed and utilized to verify accelerated characterization test methods. o Relative influences of viscoelasticity (creep), cyclic fatigue, and static strength in severe environments should be determined and the interactions between fatigue and creep characterized. What is needed is a fundamentally sound but relatively simple nonlinear three-dimensional viscoelastic-plastic constitutive model for large deformations. Damage development should be a part of the model, and it should be capable of accelerated life (durability) predictions under various severe environments of stress, temperature, moisture, aging, etc.
