High-temperature oxidation, aging and creep in carbon-fiber reinforced polyimide composites during thermal fatigue

Concerns about oxidation, chemical and physical aging, and residual stresses have been noted in carbon-fiber reinforced polyimide composites when exposed to an elevated temperature environment. Among other important considerations, issues relating to thermal fatigue are recognized to be critical barriers for long-term reliable performance of high-temperature composite materials in various advanced applications. It is therefore crucial to understand the thermal-fatigue behavior of the composite.;The main objective of this study is to predict long-term thermal and mechanical behavior, and service life of polyimide composites subjected to isothermal and nonisothermal (thermal cycling) environments, based on short-term tests. In this research, a combined experimental and theoretical study of high-temperature oxidation, aging and mechanical creep of polyimide composites is carried out.;A viscoelastic constitutive model with support of the free-volume theory is developed to predict high-temperature isothermal and nonisothermal creep coupled with oxidation, chemical and physical aging, mechanical loading and associated damages. A thermal-fatigue furnace with quartz-rod extensometer is developed for strain measurements during elevated nonisothermal history. A systematic study of isothermal and nonisothermal oxidation, aging and creep experiments is conducted to verify the model.;The present study suggests that the contribution of chemical aging is more dominant than physical aging in governing the aging and creep properties of polyimide composites. In the thermal fatigue study, the nonisothermal history is approximated by a finite number of isothermal steps and an integration over the temperature profile. The behaviors of thermal-fatigue aging and creep properties are predicted by short-term isothermal experiments and excellent agreement is achieved. However, as the testing time or thermal cycles become longer or larger, the experimental results begin to deviate substantially from the predictions due to the oxidation, creep and residual stress induced damages. Using the damage mechanics concept of effective stress, the agreement of long-term predictions with experimental results can be further improved.;Mechanisms of oxidation, physical aging and chemical aging for polyimide composites are identified. The interactions/coupling among aging, mechanical creep, thermal fatigue and associated damage are investigated. The results indicate that cyclic thermal history and their interactions may induce further property degradation.