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APPENDIX III OVERVIEW OF ADHESION SCIENCE PROGRAM REVIEW/WORKSHOP AT VIRGINIA POLYTECHNIC INSTITUTE AND STATE UNIVERSITY APRIL 30-MAY 2, 1984 A Program Review/Workshop of the Center for Adhesion Science was held at Virginia Polytechnic Institute and State University immediately preceding the NMAB workshop on Reliability of Adhesive Bonds Under Severe Environments. The purpose of a brief review of the former was to bring to the latter, in as concise a form as possible, a summary of the discussions held at VPI that would be germane to the topic of the NMAB workshop. The workshop was organized so that first-day presentations were by VPI faculty, while the presentations during the next day and one-half were by government scientists or their contractors. The latter speakers were determined by suggestions obtained from the NASA-DOD Committee on Adhesion. The presentations by the five principal investigators at VPI, H. F. Brinson, D. W. Dwight, J. N. Reddy, T. C. Ward, and J. P. Wightman, emphasized the interaction of chemistry, materials, and mechanics while at the same time indicating the nature of their own contributions. Professor Ward spoke of the type of polymers currently under study to be best suited for high performance under adverse conditions. These adhesives are likely to be "morphologically complex materials" with either two phases or one phase with partial mixing of several components and are also likely to be thermoplastics rather than thermosets. He suggested that a fundamental difficulty with all systems cured under high temperature and pressure is the occurrence of residual stresses. Obviously, methods to quantify the magnitude of residual stresses are needed. Both solid state nuclear magnetic resonance (NMR) and dielectric measurements were shown to provide data of a more molecular nature and to be complimentary to the usual rheovibron information. Inverse gas chromatography was used to provide information about the diffusion of small molecules and/or thermodynamic interactions with cross-linked systems. Professor Wightman discussed his efforts for the characterization of titanium 6-4, lithium/aluminum, and 7075 aluminum adherends pretreated by 47
48 different methods and subsequently bonded with both modified epoxy and polysulfone adhesives. The Ph.D. program of one of his students, J. Filbey, is related to the reproducibility of the surface of pretreated titanium 6-4 adherends. Emphasis is placed on determining the properties of the surface oxide layer, including crystalUnity, topography, thickness, surface area/porosity, composition energy, acidity, and environmental stability. In this program Ms. Filbey interacts strongly with faculty and students in mechanical engineering (roughness measurements), mechanics (stress analysis and testing), and materials [fractography using scanning electron or scanning transmission electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES)]. Professor Brinson discussed two new methods aimed at measuring the deformation state inside and on the surface of the adhesive layer respectively. The first utilizes embedded strain gauges that have not been used previously, and the second uses moire1 interferometry to visualize and measure deformations on the surface of the adhesive layer. The latter work was performed by D. Post under NASA-Langley funding. It was suggested that utilizing both procedures together with a Krieger gauge (American Cyanamid) might produce understanding heretofore not possible. Professor Brinson also gave experimental results to indicate that viscoelasticity of neat epoxy resins and bonded joints is a significant problem for long-term durability assessments. Apparently, viscoelastic deformations are nonlinear for even small loads and ambient temperatures over a long time scale and are intensified by high temperature, moisture content, and high stress levels. Future analyses for durability predictions must of necessity include these as well as time-dependent cumulative damage and failure concepts. Professor Brinson and Professor Reddy gave results of finite element analyses of stress distributions in single, thick adherend and crack lap geometries. Results on single lap specimens for shear and peel stresses were shown to be in agreement with a Goland and Reissner solution. Axial stresses were not the same. E. G. Henneke reviewed various NDT, NDI, and NDE methods currently in use at VPI for composite materials. The same techniques of C-scan, acoustic emission, X-ray radiography, thermography, and neutron radiography are useful for adhesively bonded joints. An important feature outlined was that NDE represents a total systems approach to flaw detection, quality control, flaw tolerance and damage growth, stress analysis, and lifetime prediction and is not limited to merely detecting flaws using a specific technique. Also, bond quality assessment methods are needed. Surface modification and fractography was discussed by D. W. Dwight as a means to understand why good or bad bonds occur and why they fail. Studies on siloxane-containing block copolymers and their homopolymers were studied using XPS and transmission electron microscopy (TEM). The kinetics and mechanisms of anodized oxide layer growth on titanium 6-4 were found by potentiostatic and galvanostatic techniques. XPS was used to determine the chemical composition and bonding of the oxide and residual contaminants. STEM was used to follow oxide topography development.
49 Syntheses of polymeric adhesives were discussed by J. E. McGrath, including anionic polymerization, step growth or condensation poly- merization, and block and graft copolymerization. He reviewed work to develop styrene, diene, and star-block polymers and their hydrogenated derivatives, polyCarylene ether sulfone imide) copolymers. On the other hand, G. L. Wilkes reviewed his efforts to use siloxane modifiers for epoxies, including functionally terminated polymethylsiloxane and its statistical copolymers, with methyltrifluoropropyl (TFP) or diphenyl siloxane. Improved fracture toughness was demonstrated, and wear rate was shown to be dramatically reduced. L. H. Peebles, Jr., addressed the "ilities" of adhesives and indicated that for the Navy these include reliability, processability, durability, and repairability. He suggested that a transdisciplinary approach involving the wide spectrum from chemistry to mechanical engineering is a necessity. As an example, he cited the rubber-to-metal bond in a sonar transducer, which must survive environmental exposures such as seawater, temperature fluctuations, pH, dissolved oxygen, and rapidly varying 200-psi pressures over 10^ lifetime cycles, among a number of other factors that naturally occur in the life of the transducer. The scientific question posed was, What type of meaningful test (and analysis) schemes can be used to rank various systems during production, in-service damage, and repair with regard to the subject "ilities"? These questions can only be addressed by a broad multidisciplinary approach to the problem of adhesion. D. Mulville and S. R. Brown spoke of the Naval Air Systems Command's adhesive bonding problems. The former discussed the need for cumulative damage concepts as well as NOT methods for composite bonded structures. The latter reviewed work at the Naval Air Development Center to assess joint durability under adverse environmental conditions using various surface treatments (alkaline peroxide, Pasa Jell, and silanes) on primed titanium adherends bonded with different types of adhesives. S. E. Wentworth of the Army Materials and Mechanics Research Center gave an overview of U.S. Army programs on adhesion while R. A. Everett, Jr. of the U.S. Army Structures Laboratory, Aviation Systems Command (AVSCOM) discussed failure modes in realistic bonded metal-to-composite joints. Projects such as the all-composite bonded helicopter program (ACAP) and adhesive bonding for lightweight bridging were mentioned. In all cases the potential weight savings were large. The realistic bonded metal-to- composite joint was composed of three large-scale test panels of titanium bonded to graphite/epoxy adherends with a high-strength structural adhesive. Notably, static strengths were 30 percent more than predicted, and failure under both static and fatigue loading was always in the graphite/epoxy adherends and not in the adhesive layer. E. Becker, J. S. Thornton, and W. L. Hufferd discussed and gave applications of the VISTA (Viscoelastic Stress Analysis of Adhesively Bonded Joints) finite-element code developed with Air Force funding. Professor Becker presented the details of the code (plane strain, nonlinear viscoelastic, time-stepping methodology, small deformation, etc.). Dr. Thornton gave information on using VISTA for the Navy sonar transducer problem, and Dr. Hufferd gave an analysis of crack lap shear geometries.
50 Sheldon Mostovoy presented a finite element analysis of two adhesive test specimens, the modif ied-zero-K-gradient (MZKG) specimen and the crack lap-shear specimen. The first is primarily mode III, while the latter is primarily modes I and II. Information on all three modes is often necessary to properly interpret fractures of adhesively bonded structures. For his analysis Dr. Mostovoy used both a SUPERSAP and an ABAQUS finite element code. An assessment of the two was given. E. J. Ripling discussed a new adhesive scarf joint specimen for mixed mode I-III fracture. Fracture was found to change from center-of-bond to the interface when load changed from monotonically increasing to fatigue. The NASA-Langley combined analytical and experimental investigation of adhesively bonded composite double cantilever beam and crack lap shear specimens was presented by W. S. Johnson. The specimens were fatigue- loaded, and in all cases failure occurred by debond growth. The total strain energy release rate, Gj, appeared to be the governing parameter, as opposed to either Gi or Using his previously developed explanation of damage growth in tapered double cantilever beam adhesively bonded specimens, Don Hunston of the National Bureau of Standards gave an explanation of why toughened epoxies are not as effective in increasing composite delamination tolerance as they are in increasing adhesive fracture toughness. The study graphically showed the similarities and differences between the problems of adhesive debonding and composite delamination. L. H. Lee of Xerox spoke on the new directions for adhesive chemistry. D. K. McNamara of Martin Marietta discussed surface preparation for steel adherends and John Venables of the same organization covered similar topics elsewhere in the proceedings. Improved hydrothermal stability of adhesive bonds using silane primers was addressed by F. J. Boerio. Reflection-absorption infrared spectroscopy (RAIR) and XPS gave evidence that silane is absorbed into the oxide layer at low pH values to form siloxane polymers. At higher pH levels, 2024 aluminum is etched and alumino-siloxane polymers containing copper ions are formed. The silane and the copolymer can form interpenetrating networks during lamination of the copolymer onto the primed substrate. T. L. St. Clair of NASA-Langley emphasized the need for high- temperature structural adhesives for aerospace applications. One area focused on was the development of an adhesive that could be useful for supersonic aircraft where skin temperatures may reach 177Â°C (350Â°F) and the lifetime should exceed 50,000 hours. He described a LARC-deve loped thermoplastic polyimide and a siloxane-containing polyimide. Durability studies of the former were conducted for over 20,000 hours at 232Â°C (450Â°F) with thermoplastic polyimide bonded titanium 6-4 adherends, using both Pasa Jell and "Boeing anodizing" surface treatments.