Structural composite materials are being investigated for use in applications which will require long-term durability in various environments under external loads. Flaws may be created in these composite materials during manufacturing, machining, or use. Adverse use conditions in the presence of seemingly minor flaws may result in significantly reduced mechanical properties and changes in the microstructure and damage mechanisms of composites. E-glass/vinyl ester composite coupons with a single edge notch have been conditioned in water for periods of up to 7,800 hours at room temperature and at an elevated temperature, mostly under a sustained tensile load, to determine the changes in the mechanical properties and damage mechanisms which occur due to the presence of the flaw. The tensile strength has been found to decrease by at least fifteen percent for notched samples when compared with unnotched samples subjected to the same environmental and load conditions. During conditioning, the dominant damage mechanisms have been found to change from transverse matrix microcracking, fiber cracking, and occasional edge delaminations for unnotched specimens to the growth of a significant matrix crack from the notch tip and local tow debonding for notched specimens. Nondestructive techniques have been used to observe the damage in the vicinity of the notch tip.

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