Preliminary evaluation of the use of elevated pressure to accelerate thermo-oxidative aging in composites

A study was conducted to evaluate the use of elevated pressure to accelerat e the rate of thermo-oxidative degradation in polymeric composite materials . The motivation for this work was based on earlier studies by Tsotsis [1,2 ], Kerr and Haskins [3] and by Ciutacu, et al. [4] which demonstrated that higher pressures of air or oxygen tended to increase the rate of degradatio n of polymeric composites. Moreover, accelerated aging methods are needed t o evaluate materials which are to be used under long-term exposure to eleva ted temperature in oxidative environments such as the High-Speed Civil Tran sport (HSCT). The methodology proposed herein augments elevated temperature aging with elevated pressure to accelerate the rate of thermo-oxidative de gradation. Temperatures below those used by previous workers were employed in order to avoid anomalous non-linear effects which can occur near the gla ss-transition temperature of polymeric materials. In the present work, Hexc el AS4/3501-6 [+/-45 degrees](2s) shear and [+45 degrees/0 degrees/-45 degr ees/90 degrees](2s) quasi-isotropic Hexcel IM7/8552 open hole compression s pecimens were aged at 121 degrees C (250 degrees F) at pressures of 0.101, 0.345, 1.03, and 1.72 MPa (14.7, 50, 150, and 250 psi). Three pressure vess els were designed and built to contain the prescribed pressures. These vess els were placed inside an oven and used an air bleed to recycle the air ins ide the vessels. For the open hole testing, a test method was derived from Boeing BSS 7260 to reduce the material required for each specimen. Specimen s were tested as-made and after 1000 h exposure to each of the pressures. R esults show up to a 20% reduction in tensile shear strength for the [+/-45 degrees](2s) specimens but show no decrease in open hole compression streng th though weight losses and visible discoloration of the specimens are pres ent in both materials. Differences are attributed to the different lay-ups used as well as the different materials systems of the two specimen types. (C) 1999 Elsevier Science Ltd. All rights reserved.