A Continuous Test Data Method to Determine a Reference Curve and Shift Rate for Isothermal Physical Aging

The mechanical response of a material undergoing isothermal physical aging is governed by a reference curve (typically time-dependent modulus or compliance) and a shift rate (which describes the rate of aging). In the standard analysis approach, these parameters are determined from a series of load/unload creep or relaxation tests, in which only the load portion data is used for property evaluation. In this paper, a new continuous test data (CTD) method is presented to determine these parameters using both the load and unload data from such a test series. This is accomplished by employing effective time theory to predict the full load/unload data set, with the reference curve and shift rate obtained as those which best fit the experimental data. It is demonstrated that the reference curve obtained via the standard approach is not capable of predicting the full load/unload data set, while the CTD method using a Prony series fits this same data very well. Several concerns regarding the CTD method are addressed, including the differences in the CTD and standard method shift rate, the effect of vertical shifting or material non-linearity on the results, and application of the method to other loading scenarios. Results are presented for long-term and variable stress tests. The data used in the study was obtained from polyimide resin and polyimide/carbon fiber composite specimens at several temperatures below T_g.