The effect of sustained load and hygrothermal environments on Mode I delamination behavior of T300/934 unidirectional graphite/epoxy laminates has been studied. Several double cantilever beam specimens were subjected to constant load in presence of various hygrothermal conditions either for short duration (a few minutes to a few hours) or for long duration (several months). The pre-loaded specimens were then tested to determine delamination behavior. It is indicated that the crack growth does not occur as a result of exposure to load and environment. On the contrary, an increase in fracture toughness around the crack tip is observed for pre-loaded specimens and the increase is seen to depend on exposure duration and environment. In general, the loaded specimens exposed to water show a significant increase in fracture toughness. The failed specimens were examined by SEM and topographical features are noted. The crack tip exposed to high stresses in presence of environment show distinguishable features.

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