Browsing by Author "Parvatareddy, Hari"

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    Durability of Polyimide Adhesives and Their Bonded Joints for High Temperature Applications
    Parvatareddy, Hari (Virginia Tech, 1997-11-20)
    The objective of this study was to evaluate and develop an understanding of durability of an adhesive bonded system, for application in a future high speed civil transport (HSCT) aircraft structure. The system under study was comprised of Ti-6Al-4V metal adherends and a thermosetting polyimide adhesive, designated as FM-5, supplied by Cytec Engineered Materials, Inc. An approach based on fracture mechanics was employed to assess Ti-6Al-4V/FM-5 bond durability. Initially, wedge tests were utilized to find a durable surface pretreatment for the titanium adherends. Based on an extensive screening study, chromic acid anodization (CAA) was chosen as the standard pretreament for this research project. Double cantilever beam specimens (DCB) were then made and aged at 150° C, 177° C, and 204° C in three different environments; ambient atmospheric air (14.7 psia), and reduced air pressures of 2 psi air (13.8 KPa) and 0.2 psi air (1.38 KPa). Joints were aged for up to 18 months (including several intermediate aging times) in the above environments. The strain energy release rate (G) of the adhesive joints was monitored as a function of exposure time in the different environments. A 40% drop in fracture toughness was noted over the 18 month period, with the greatest degradation observed in samples aged at 204° C in ambient atmospheric air pressure. The loss in adhesive bond performance with time was attibutable to a combination of physical and chemical aging phenomena in the FM-5 resin, and possible degradation of the metal-adhesive interface(s). Several mechanical and material tests, performed on the bonded joints and neat FM-5 resin specimens, confirmed the above statement. It was also noted that physical aging could be "erased" by thermal rejuvenation, partially restoring the toughness of the FM-5 adhesive material. The FM-5 adhesive material displayed good chemical resistance towards organic solvents and other aircraft fluids such as jet fuel and hydraulic fluid. The results from the FM-5 adhesive and its bonded joints were compared and contrasted with VT Ultem and REGULUS polyimide adhesives. The FM-5 adhesive showed the best performance among the three adhesive systems. The effect of mode-mixity on the fracture toughness of the Ti-6Al-4V/FM-5 adhesive bonded system was also evaluated. DCB tests in conjunction with end-notched flexure (ENF) and mixed-mode flexure (MMF) tests, were used to fracture the bonded joints under pure mode I, pure mode II, and a combination of mode I and II loadings. The results showed that the mode I fracture toughness was twice as large as the mode II toughness. This was a rather surprising find, in sharp contrast to what several researchers have observed in the past. Our current understanding is that the crack path selection during the failure process plays a significant role in explaining this anomalous behavior. Finally, failure envelopes were generated for the titanium/FM-5 bonded system, both prior to and following thermal aging. These envelopes could serve as useful tools for engineers designing with Ti-6Al-4V/FM-5 bonds.
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    The effect of environmental aging/exposure on the durability of high performance polymeric composites
    Parvatareddy, Hari (Virginia Tech, 1994)
    High-performance polymeric composites are currently being considered as state-of-the-art material systems for future supersonic aircraft and space structures. However, the long-term durability and environmental stability of these materials continue to be under question. Continued application of these composites in aerospace structures is contingent upon the long-term durability of these material systems. Polymeric materials have been known to undergo both physical as well as chemical aging. The aging time, temperature, and environment play a significant role in affecting the physical and chemical aging behavior in the polymers. Currently, there is a dearth of information on the combined effects of physical and chemical aging in polymer-based composites. This study describes the effect of sub-Tg environmental aging on the mechanical properties of two high-performance polymeric composite systems. The effect of chemical degradation on the durability of the material systems is discussed. Further, the effect of environmental stress cracking (ESC) behavior of high-performance composite materials in the presence of organic solvents is investigated and the implications of ESC on durability are studied. Also included in this thesis is a study of the physical aging characteristics of the composites, via measurement of the viscoelastic (creep) properties. Accelerated characterization techniques were employed to predict long-term physical aging behavior. Fiberite 954-2 (a thermoplastic toughened cyanate ester resin) and its graphite-reinforced composites, and Fiberite ITX (a semicrystalline thermoplastic resin) and its graphite fiber-reinforced composites (IM8/ITX) were used in the study. These material systems were under consideration for usage in high-speed civil transport (HSCT) aircraft. This aircraft is expected to have an operating temperature of around 150°C (based on a 2.4 Mach number), an operating pressure at service altitude of 2 psi (0.136 atm), and a flight life in the excess of 60,000 hours at the above-mentioned conditions. The aging of the specimens was carried out for periods of up to 9 months at temperatures between 140°C to 200°C in three different environments; an inert nitrogen environment, an environment with a reduced air pressure of 2 psi (0.136 atm), and ambient atmospheric air. The results from stress-strain, flexure, and micro-indentation tests indicated a substantial reduction in material properties with aging in the different environments. The bending strength, strain to failure, and hardness values of the two composite systems decreased by as much as 20-50%. Tensile modulus on the other hand showed an increase of 20% after 6 months of aging in air, indicating apparent embrittlement with aging. Chemical degradation/damage was also monitored by penetrant enhanced x- radiography, scanning electron microscopy (SEM), and scanning acoustic microscopy (SAM). The chemical aging/degradation was seen to be sensitive to the oxygen partial pressure in the aging environment. The greater the amount of oxygen in the aging environment, the more the loss in the material properties. The glass transition temperatures (T​​g) of the two material systems were sensitive to both the aging environment and aging time. The Tg of both systems increased over long aging times as seen from dynamic mechanical analysis (DMA) measurements. However, increased oxygen concentrations appear to reduce the Tg. Changes in the Tg of both material systems were a complex behavior attributable to the varying oxygen concentrations in the aging environments, and the combined occurrence of physical aging, degradation, etc. in the materials. The chemical degradation in the composites appears to be via an oxidation mechanism and the micro-indentation results further indicate diffusion-controlled oxidation. Weight changes of samples (neat resin and composites) were also monitored over the entire period of the study and these showed a sensitivity to the oxygen concentration in the aging environment. The greater the oxygen in the environment, the greater the weight loss in the specimens, indicating an oxidation phenomenon. DMA and tensile creep were performed to study the interaction of creep and physical aging in these material systems. Long-term creep predictions of the composites were made using Time-Temperature Superposition (TTSP) and Effective Time Theory (ETT) techniques. The IM8/954-2 composites behaved in an anomalous fashion at times. This may be attributable to the blended nature of the 954-2 resin system, possible post-curing and phase separation of the resin, and thermal decomposition at elevated temperatures. The solvent testing of composites based on thermoplastic polymers revealed susceptibility to ESC. Bending strength losses up to 30% were seen from flexure tests on unidirectional composites. It was also seen that residual stresses in cross-ply laminates were sufficient to trigger ESC after exposure to common organic solvents for an hour. The damage/failure modes were captured by SEM micrographs.