362 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju recoverable [98].Although these formulations are an important aspect of the overall methodology to model the performance of HTPMCs,a com- prehensive review of such is beyond the scope of the current discussion. The discussion here is limited to modeling the thermooxidative aging of HTPMCs and the role of the various constituents.Although hygrothermal degradation,which is a significant concern for HTPMCs,is not speci- fically addressed here,the methodology described herein can be applied to modeling hygrothermal degradation behavior.As with oxidation,hygro- thermal degradation involves transport and chemical reactions of the polymer with the diffused media.The primary challenge for modeling the coupling of oxidative and hygrothermal degradation of HTPMCs is an understanding of the numerous degradation mechanisms. Several major aging mechanisms that lead to weight loss and damage growth and,hence,degradation of the performance of the polymer resin, fibers,and their composite can be identified.They are: Physical aging.The thermodynamically reversible volumetric response due to slow evolution toward thermodynamic equilibrium is identified as physical aging.The decreased molecular mobility and free-volume reduction lead to strain and damage development in the material. Chemical aging.The nonreversible volumetric response due to chain scission reactions and/or additional crosslinking,hydrolysis,deploy- merization,and plasticization is classified as chemical aging.A dominant chemical aging process for HTPMCs is thermooxidative aging-which is the nonreversible surface diffusion response and chemical changes occurring during oxidation of a polymer (hence a modality of chemical aging).The oxidative aging may lead to either the reduction in molecular weight as a result of chemical bond breakage and loss in weight from outgassing of low molecular weight gaseous species,or chain scission and formation of dangling chains in polymer networks. Mechanical stress-induced aging.Mechanical and thermal fatigue loading cause micromechanical damage growth within the material. The damage evolution,in turn,exacerbates the physical aging and thermooxidative response of the material.This aging mechanism may be the least understood and least modeled by researchers. The capability to predict the performance of HTPMCs for both primary and secondary structural applications is elusive.The highly coupled physi- cal,chemical,and mechanical response of these materials to the extreme hygrothermal environments provides formidable challenges to the com- posite mechanics community.Polymers (in particular amorphous polymers) have been studied from the standpoint of physical aging [98],chemical aging [66],and strain-dependent aging [118].Although models to predictrecoverable [98]. Although these formulations are an important aspect of the overall methodology to model the performance of HTPMCs, a com￾prehensive review of such is beyond the scope of the current discussion. The discussion here is limited to modeling the thermooxidative aging of HTPMCs and the role of the various constituents. Although hygrothermal degradation, which is a significant concern for HTPMCs, is not speci￾fically addressed here, the methodology described herein can be applied to modeling hygrothermal degradation behavior. As with oxidation, hygro￾thermal degradation involves transport and chemical reactions of the polymer with the diffused media. The primary challenge for modeling the coupling of oxidative and hygrothermal degradation of HTPMCs is an understanding of the numerous degradation mechanisms. Several major aging mechanisms that lead to weight loss and damage growth and, hence, degradation of the performance of the polymer resin, fibers, and their composite can be identified. They are: – Physical aging. The thermodynamically reversible volumetric response due to slow evolution toward thermodynamic equilibrium is identified as physical aging. The decreased molecular mobility and free-volume reduction lead to strain and damage development in the material. – Chemical aging. The nonreversible volumetric response due to chain scission reactions and/or additional crosslinking, hydrolysis, deploy￾merization, and plasticization is classified as chemical aging. A dominant chemical aging process for HTPMCs is thermooxidative aging – which is the nonreversible surface diffusion response and chemical changes occurring during oxidation of a polymer (hence a modality of chemical aging). The oxidative aging may lead to either the reduction in molecular weight as a result of chemical bond breakage and loss in weight from outgassing of low molecular weight gaseous species, or chain scission and formation of dangling chains in polymer networks. – Mechanical stress-induced aging. Mechanical and thermal fatigue loading cause micromechanical damage growth within the material. The damage evolution, in turn, exacerbates the physical aging and thermooxidative response of the material. This aging mechanism may be the least understood and least modeled by researchers. The capability to predict the performance of HTPMCs for both primary and secondary structural applications is elusive. The highly coupled physi￾cal, chemical, and mechanical response of these materials to the extreme hygrothermal environments provides formidable challenges to the com￾posite mechanics community. Polymers (in particular amorphous polymers) have been studied from the standpoint of physical aging [98], chemical aging [66], and strain-dependent aging [118]. Although models to predict 362 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju