Assessment of residual composite properties as influenced by thermal mechanical aging
In this study, two quasi-isotropic carbon fiber-reinforced polyimide material systems, IM7/K3B and IM7/PETI-5, were thermally aged at 163°C for up to 3000 hours under a static compressive load. The future goal of this study is to determine the effect of 10,000 hours (currently underway) and possibly seven years of aging on these materials. The compressive load was applied using steel fixtures supplied by The Boeing Company. Back-to-back gages on the aging panels monitored compressive strain as well as bending strain. Following aging at 1500 and 3000 hours, ambient residual compression properties were assessed using the IITRI compression test method. Unexposed specimens were also tested to obtain baseline properties for comparison. These results showed that 3000 hours of stressed isothermal aging did not significantly affect the compressive properties of the two composite systems. A slight increase in scatter of the residual strength numbers was observed. Statistical evaluation of the results was used to compute design allowables and quantify changes in scatter.
Differential scanning calorimetry (DSC) was used to monitor any changes in glass transition temperature as a result of the aging. Only small changes were observed in the DSC scans. Dynamic thermogravimetric analysis (TGA) tests were used to compare weight loss rate versus temperature results for the different age materials. Results show different rates of degradation occurring in the different K3B polymer aging histories. In addition to establishing the effect of isothermal aging under load on the compressive moduli and strength of two polymer-matrix composites, this experiment links increasing scatter in residual strength measurements to changing thermogravimetric results.