Pressurized ring test method for the evaluation of environmental conditions on composite strength and stiffness

A pressurized ring test method developed at Alliant Techsystems has been used extensively to characterize the hoop strength and stiffness of filament wound composite rocket motor cases. Recently the hardware necessary to condition and test 20-in. rings at elevated temperatures was developed. Composite rings were moisture conditioned and tested under various environmental conditions. The ring tests showed moderate decrease in strength, up to 8% and 13% for pressure based and hoop-strain based, respectively, for the wet/71degreesC (160degreesF), wet/room temperature (RT) and dry/121degreesC (250degreesF) test conditions. For the wet/121degreesC test condition, the decrease was more significant: -28% based on pressure and -33% based on hoop strain. Even the moderate decrease in strength, is significantly higher than the strength knockdown factor predicted based on micro mechanical models that use the change in resin stiffness at elevated temperature and moisture.