Environmental effects on stitched RTM composites

The effects of temperature and humidity cycling on mechanical properties of AS4/3501-6 quasi-isotropic composites prepared from unstitched, Kevlar 29 stitched, and S-2 glass stitched uniweave fabric were determined. Data presented include compression strengths and compression-compression fatigue results for environmentally cycled and uncycled composites. Ten1perature cycling ranged from 60°C to -55°C. The relative humidity varied between 95 percent at the high temperature and a percent at the low temperature. Microcracks which were found predominantly around individual stitches were recorded using photomicrographs taken at the end of each cycling period. The glass stitched and Kevlar stitched laminates showed significant microcracking before cycling. The unstitched uniweave material developed microcracks only after cycling. The glass stitched material had lower baseline compression strength than the unstitched and Kevlar stitched materials. Temperature and humidity cycling reduced the static compression strength of the unstitched and Kevlar stitched uniweave materials nearly 10 percent. Under the same conditions the glass stitched uniweave material lost 3 percent of its baseline strength. Combined temperature and humidity cycling did not effect the fatigue properties of the uniweave materials when the test specimens were dried to their original weights before testing.