(Dr. Steven Johnson, advisor)
"Characterization of Polymer Matrix Composites and Adhesively Bonded Joints in a Cryotank Environment"
Previous research in various technologies has focused on the effect of cryogenic temperatures on the constitutive response and tensile strength of polymer matrix composite materials. These previous findings note an increase in tensile stiffness and strength for common polymer matrix composites at cryogenic temperatures. In contrast, research into fracture mechanics characterization of these materials in cryogenic environments is limited, and this work efforts not only an evaluation of the materials system studied, but also proposed an important viewpoint when selecting polymer matrix composites and adhesively bonded joints for use in cryotank environments.
In this work, adhesively bonded joints with composite adherends are evaluated both experimentally and analytically in environments similar to those encountered in cryotanks. Adherends are of unidirectional graphite-bismaleimide (IM7/5250-4) construction, bonded with AF-191M film adhesive. Monotonic mode I adhesive fracture toughness tests are performed at liquid nitrogen temperatures (-196°C), room temperature (26°C) and elevated temperature (121°C) to determine temperature effects on critical strain energy release rate. Experimental results exhibit reduced adhesive fracture toughness at temperatures other than room temperature. Cold temperature is noted to have less of an effect than tests performed at elevated temperature.
The experimental tests performed in this research are modeled analytically using FRANC-2D finite element software. Mechanistic effects of residual thermal stresses and their role in affecting strain energy release rate are evaluated. Adhesively bonded joints of dissimilar adherends (composite and metallic) are analyzed in this fashion to display criticality of residual thermal stresses in design of adhesively bonded joints in cryotank applications.