The relationship between percent retention of flexural strength or modulus and time is linearized by taking the natural logarithm of the time in hours based on Eq. (8) and were plotted it in Figs. 8 and 9 respectively. It is noted that the natural logarithm of the time at zero hour could not be calculated mathematically, accordingly, the natural logarithm of ‘‘0.1 h” was assumed to be initial point for the fitting curve. The fitting equations at different temperatures, aging environments, and specimens directions are also presented in Figs. 8 and 9. After establishing the relationship between strength or modulus retention and time, both flexural strength and modulus were predicted using Eq. (8). As is shown in Figs. 3 and 4, the predicted flexural strength and modulus agreed well with experimental results, except at the end of aging period and the final hygrothermal aging failure period between 1560.0 and 2070.0 h as stated earlier at a temperature of 80 C, where experimental results are smaller than predicted value. This indi- cates that Phillips model could not accurately predict variation of laminate’s mechanical behavior at the end of hygrothermal aging time closer to final hygrothermal aging failure. A comprehensive model which may consider all three hygrothermal aging period in Ref. [10] will be further studied in the future.
The Arrhenius prediction model [32] is commonly used as a life prediction model in accelerated testing. The model is derived from the Arrhenius reaction rate equation proposed by Arrhenius in 1887 [33]. The Arrhenius reaction rate equation for a phenomenon under consideration is as follows: