RECOVERING NONISOTHERMAL PHYSICAL AGING SHIFT FACTORS VIA CONTINUOUS TEST DATA - THEORY AND EXPERIMENTAL RESULTS

For isothermal physical aging, a few simple tests to characterize the aging shift factors allow reasonable prediction of the mechanical resp onse. In this paper, a new technique is developed to extract aging shi ft factors from creep data during a nonisothermal history. Previous me thods have generated discrete experimental shift factors by a series o f short-term creep tests, in which the load portion alone is used for evaluation; this is particularly time consuming for nonisothermal hist ories, since many delta points (requiring several tests) may be needed for an adequate characterization of the response. This paper presents a new continuous shift factor (CSF) method, based on the validity of effective time theory, which generates a continuous experimental shift factor curve from a single test. Results are presented for this metho d when applied to a polyimide/carbon fiber composite material tested i n shear under temperature jump conditions; this nonisothermal aging da ta for a polymer matrix composite is shown to exhibit similar response to that of homogeneous polymers. The new CSF technique will be useful in the development of models to predict the shift factor due to coupl ed aging and thermal history.