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The tensile, fatigue, creep, and stress-rupture behavior of unidirectional, unidirectional off-axis, and cross-plied composites of 25 volume percent boron filaments in a matrix of 6061 aluminum has been investigated. The tensile investigation has shown the composites to be of reasonably high quality. The fatigue strength of boron-reinforced 6061 aluminum was observed to be superior to those of boron-reinforced 1100 and 2024 aluminum composites. The failure mechanism is discussed in terms of buildup of stress concentration in the matrix to stress levels capable of fracturing proximate filaments. The effect of off-axis loading on the fatigue behavior was found to be similar to that observed on their tensile properties. Cross plying has changed the fracture mode from predominately matrix shear failure to failure involving filament fracture. The excellent creep and stress-rupture properties of boron filaments were utilized fully in the unidirectional composites when tested in the reinforcement direction. The off-axis creep and stress-rupture behavior of the unidirectional composite was characterized by a rapid reduction of properties with increasing off-axis orientation. Cross plying changed the mode of creep failure for many composite configurations from shear to tensile creep and improved the off-axis stress-rupture properties compared to corresponding unidirectional composites. In general, the creep behavior of cross-plied composites was characterized by a large degree of filament rotation towards the load axis.

The tensile behavior, erosion/impact resistance, mechanical fatigue strength, thermal fatigue behavior, and creep strength of filamentary reinforced aluminum and titanium alloy matrix composites were briefly compared. Parallel to the filament direction, aluminum matrix composites are slightly stronger than titanium matrix composites up to a temperature of about 600°F (314°C). Titanium matrix composites, however, have shown significant off-axis strength advantage even at room temperature. B/SiC-Ti has shown a five-fold advantage in transverse strength over B-A1 at a test temperature of 500°F (260°C). The erosion rate of fiber-reinforced composites was found to be controlled by the matrix until the filaments became exposed. The titanium matrix, having an order-of-magnitude stronger matrix yield strength, exhibited greatly improved ballistic impact resistance, when compared to the aluminum matrix system. The low-cycle fatigue strength of titanium composite was superior to that of comparable aluminum composite. On the other hand the high-cycle fatigue strength in the 0 deg orientation for the titanium composite was significantly lower than that of a comparable aluminum composite. The resistance to thermal fatigue damage, when measured by tensile strength degradation, delamination, and dimensional distortion, was in favor of titanium composites. The transverse creep strength of B/SiC at 800°F (427°C) was shown to be ten times better than that of B/SiC-A1 at 575°F (302°C).

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