Ultrasonic nondestructive evaluation techniques and the effects of fiber architecture on mechanical performance in multi-directionally reinforced textile composites

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Hale, Richard
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Daniel O. Adams David K. Hsu
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Aerospace Engineering

Constant load stress rupture tests were performed on alloy 908. The test matrix used varied oxygen concentration, applied load, temperature, and percent cold work. The effect of modifying surface residual stresses on intergranular oxidation and cracking was examined using constant strain C-ring tests;The mechanism for high temperature intergranular fracture in alloy 908 is stress assisted intergranular oxidation cracking. A direct correlation between percent intergranular fracture and oxygen concentration was observed. This result was comparable to the oxidation assisted, intergranular fracture behavior of alloy 718. Internal oxidation, for temperatures at or near 650∘ C and under an applied tensile stress, manifests itself as intergranular oxidation in alloy 908. Intergranular oxidation penetrates to the same depth as predicted for internal oxidation. The concentration of Cr incorporated into the intergranular oxide increases with increasing test duration. This result is consistent with internal oxidation theory. The depth of intergranular oxidation is controlled by both the oxygen partial pressure and the Cr concentration of the alloy. These factors influence the growth of intergranular Cr2O3 and NiCr2O4 precipitates that serve to block the diffusion path for oxygen. Analogous to internal (matrix) oxidation, a transition from intergranular to external oxidation in alloy 908 occurs when the partial pressure of oxygen at the surface is below 7.6x10-4 Torr;The activation energy for intergranular oxidation in air was determined to be 202 kJ/mole for this alloy. At low (<195 ppm) oxygen concentrations, the activation energy for intergranular oxidation increased to 500 kJ/mole. This higher value reflects the oxygen concentration dependence observed for intergranular oxidation. The dependence on alloy Cr concentration is similar to that observed for intergranular stress corrosion cracking in aqueous environments;An oxygen concentration threshold based on zero percent intergranular fracture is a better indicator of the potential for intergranular fracture during heat treatment than one based on time to rupture. An oxygen partial pressure below 1x10-4 Torr is recommended for heat treatment of alloy 908 under residual or applied stresses. Shot peening appears to be relatively simple solution to the stress accelerated grain boundary oxidation problem. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)

Sun Jan 01 00:00:00 UTC 1995