Stress relaxation behavior of carbon fiber-epoxy prepreg composites during and after cure
This study presents the experimental results of time-temperature dependent viscoelastic behavior and cure kinetics of two commercial carbon fiber/epoxy prepregs, IM7/977-2 unidirectional tape (UD) and IM7/977-2 plain weave fabric (PW). An in-depth study on the oscillatory and transient rheological behavior of prepreg composites is conducted using a dynamic mechanical analyzer (DMA) and the study of cure kinetics is conducted using a differential scanning calorimeter (DSC). A novel experimental methodology is proposed in this study to describe the stress relaxation behavior of prepreg composites during cure. Time- and cure-dependent stress relaxation behavior of prepreg composites is studied in three in-plane directions: 0, 45, and 90 degrees of the laminates. Several other factors, such as the mismatch of fiber orientation of adjoining plies, stacking sequence of the laminate, and relative position of the plies with respect to the neutral axis are considered to study their effects on the stress relaxation behavior during cure. The stress relaxation behavior of cured composites is obtained by utilizing the time-temperature superposition (TTS) principle. TTS study on UD laminates (in 0, 45, and 90 degree fiber directions) is conducted at four distinct cure states, whereas the PW laminate is studied in the same fiber directions but at a fully cured state only. Experimental results are used to predict the life cycle of the composite products by generating the stress relaxation master curves for different combinations of fiber orientation and cure states. The study shows that the relaxation modulus during cure is case dependent, which leads to the assumption that the relaxation of process-induced stresses during cure varies for different conditions. Moreover, when comparing the performance of UD and PW prepregs, it shows that the PW laminate relaxes more residual stress than the UD laminate during cure. At cured condition, the PW material shows better long-term mechanical performance than UD material.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering.