Stitching vs. a Toughened Matrix: Compression Strength Effects

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Abstract
The compression strength of a stitched and a toughened matrix graphite/epoxy composite was determined and compared to a baseline unstitched untoughened composite. Two different layups with a variety of test lengths were tested under both ambient and hot/wet conditions. At longer gage lengths where failure was due to global buckling, no significant difference in strength was found between the different materials. For shorter specimens, a 30% reduction in strength due to stitching was found for both layups, presumably due to an increase in fiber misalignment. An observed increasing strength with decreasing gage length was found for all materials and was explained using statistics and a random distribution of flaws (misaligned fibers). The toughened material showed a small increase in strength over the baseline material, presumably due to the compensating effects of a more compliant matrix and straighter fibers. A hot/wet environment reduced the strength of the baseline and stitched material by 30% and the toughened material by 20%. Video images of the failing specimen supported the theory of a random distribution of flaws controlling composite failure.