An Evaluation of the Compression Response of High-Performance Prepregs for AFP Applications

In this study, the compaction response of unidirectional out-of-an autoclave (OOA) prepreg tape has been evaluated experimentally at 25,degrees 45 degrees and 65 degrees C. Instantaneous and incremental displacement compaction experiments have been carried out in a universal testing machine until a compressive strain of 0.42 +/- 0.02 was achieved. The fiber-bed stress was determined by transposing the results of displacement-controlled experiments onto data from corresponding load-controlled experiments. The material was then modelled as a viscoelastic material using a third order exponential decay function, based on a generalized Maxwell model. This procedure enabled the extraction of the relaxation time constants and the peak stress. The results highlight a linear relationship between the operating temperature and the load/stress relaxation response of the prepreg/tape, suggesting that if the fiber placement procedure is performed at 65 degrees C, the effect of roller stiffness on tape/fiber laying is negligible. The maximum contribution of fiber stress to the overall relaxation is approximately 6% at 45 degrees C and 65 degrees C, indicating the formability to be matrix dependent. A minimum spring-back is observed in specimens that were tested under incremental displacement conditions. The difference in thickness of the as-laid laminate and the cured laminate at 45 degrees C and 65 degrees C were similar. The linear stress relaxation model has been shown to be capable of predicting the maximum stress accurately using the viscosity values of the dashpots and the moduli of the springs.