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This study critiques the use of both rubber particles and short-glass fibers for the improvement of polymer fracture toughness ( K ). Although dry neat nylon is brittle with only a moderate K , value (4.2 M P a g ) , additions of either second phase produce rising K,-curves and associated high K , values (8.1 M P a 6 for rubber-toughened nylon, and 10.0 M P a G for 17 vol% glass-fiber neat nylon). In the rubber-modified resin, the high K, value is associated with extensive plastic blunting at the crack tip. In the fiber-reinforced neat resin, K, is improved due to a combination of fiber-bridging and increased strength, the latter being associated with additional load carrying capacity of the fibers. When both rubber and fibers are added, however, no further increase in K, is noted ( K , = 9.3 M P a 6 for 17 vol% glass-fiber rubber-modified nylon). The extent of ductile blunting in the rubber + fiber resin is not as great as in the rubber-only resin. Furthermore, the fracture strength of the rubber + fiber resin is not as high as the fiber-only resin.The net result is a balance of properties for the rubber-toughened composite.