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A combined experimental and analytical investigation of an adhesively bonded composite joint was conducted to characterize both the static and fatigue debond growth mechanism under Mode I and Mixed-Mode I and II loadings. Two bonded systems were studied: graphite/epoxy adherends bonded with EC 3445 and FM-300 adhesives. For each bonded system, two specimen types were tested: (1) a double-cantilever-beam specimen for Mode I loading and (2) a cracked-lap-shear specimen for Mixed-Mode I and II loading. In all specimens tested, failure occurred in the form of debond growth. Debonding always occurred in a cohesive manner with EC 3445 adhesive. The FM-300 adhesive debonded in a cohesive manner under Mixed-Mode I and II loading, but in a cohesive, adhesive, or combined cohesive and adhesive manner under Mode I loading. Total strain-energy release rate appeared to be the driving parameter for debond growth under static and fatigue loadings.

Abstract. Hot sections in turbine engines are subjected to large variations in temperatures and mechanical/thermal loadings. As such, accurate predictions of fatigue crack growth must account for many physical phenomena: temperature dependent crack growth behavior, load interaction history effects, time at temperature effects, temperature history effects, and the effects of stress/temperature/time on the materials. Through extensive experimental work on superalloys, a very definite "temperature history effect" on the resulting crack growth behavior has been identified and modeled. This work also identified a Temperature Affected Zone (TAZ) that occurs ahead of the crack tip and affects subsequent crack growth rates. The size of the TAZ is dependent on temperature, hold time, and stress state. Measurements of the TAZ were made under various conditions. The changes that occur in this TAZ are a combination of oxidation and material microstructure evolution. Various simplified "hot section" engine spectra (changing temperatures and stress levels) were tested to determine resulting crack growth behavior. Correlation between the experiments and model predictions were good and generally conservative.