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 response. In this paper, a new technique is developed to extract aging shift factors from creep data during a nonisothermal history. Previous methods have generated discrete experimental shift factors by a series of short-term creep tests, in which the load portion alone is used for evaluation; this is particularly time consuming for nonisothermal histories, since many data 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 method when applied to a polyimide/carbon fiber composite material tested in shear under temperature jump conditions; this nonisothermal aging data 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 coupled aging and thermal history.