Environmental Effects on Stitched RTM Textile Composites

The effects of temperature and humidity cycling on mechanical properties of AS4/3501-6 quasi-isotropic textile composites were determined. The composites were resin transfer molded from unstitched, Kevlar 29 stitched, and S-2 glass stitched uniweave fabric preforms. Data presented include photomicrographs, compression strengths and compression-compression fatigue results for environmentally cycled and uncycled composites. After manufacture, microcracks developed around the glass and Kevlar stitching. These microcracks did not grow after environmental cycling. The unstitched material developed microcracks only after cycling. 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 affect the fatigue properties of the uniweave materials when the test specimens were dried to their original weights before testing. Temperature cycling at constant 40 percent humidity, resulted in a 5 percent decrease in static compression strength for the unstitched and Kevlar stitched material. Unstitched, glass stitched and Kevlar stitched materials exposed to constant 60°C and 95 percent relative humidity for 80 days and then saturated in 70'C water, lost 17, 7 and 19 percent of their baseline compression strength, respectively. These conditions lowered the fatigue strengths only after saturation. Braided composites including, a stitched 2-D braid, an unstitched 2-D braid and a 3-D braid were also exposed to environmental cycling. The moisture absorption in the AS4/E905L system was lower than the AS4/3501-6 system. Consequently environmental cycling had little effect on the static or fatigue strengths of braided materials.