Accelerated tests of environmental degradation in composite materials

Combined moisture cycling and thermal cycling environments are known to create damage, such as cracking of material near exposed surfaces and edges, in many new materials intended for high-temperature service. Material models suggest the damage mechanisms; these models are used to design tests to (1) isolate the effects of different damage mechanisms, and (2) accelerate them. Tests are carried out in the combined environment and in its individual components: time at moisture, moisture cycling, time at temperature, and thermal cycling. Accelerated moisture cycling and thermal cycling tests are also designed and carried out. Results are presented which suggest that, for the IM7/PETI-5 and IM7/PIXA-M materials tested, time at moisture is the most important cause of microcracking damage, with thermal cycling playing some role. Moisture cycling plays a role in the distribution of damage. The models are used to reduce laminate-specific microcracking data to general design data in the form of fracture toughness versus cycle (G(Ie)(N)) curves.