Optical strain measurement technique for estimating degradation of the properties of carbon fiber reinforced polymer composites under cyclic loading

Abstract

The goal of the study is the fatigue process in polyimide-based composites reinforced with short carbon fibers. Parameters of mechanical hysteresis loops such as the loop area, secant and dynamic moduli were used in the study. Hysteresis loops were constructed using the developed hardware and software system based on the optical method of strain measurements using a digital image correlation (DIC) technique. Methods for calculating the moduli and the parameters of mechanical hysteresis loops is considered. The results of their evaluation and the experimental data on the fatigue behavior of polyimide-based composites reinforced with short carbon fibers are presented. It is shown that an important quantitative measure of the differences in the fatigue behavior of the studied composites is the hysteresis induced energy loss. For a composite with carbonized fibers, the energy loss level per cycle is 35 kJ/m3, whereas for a composite with graphitized fibers it is 34% lower (23 kJ/m3). At the same time, the fatigue durability of the latter is - 40 times lower. A decrease both in the secant modulus (up to 11%) and the dynamic modulus (up to 3.5%) was observed in cyclic tests. However, the reduction was twice as much in a composite with carbonized fibers possessing a longer durability. Thus, the DIC-based estimation of mechanical hysteresis loops by the parameters of the secant and dynamic moduli, as well as the loop area can be successfully used to interpret the difference in the fatigue characteristics at the stage of scattered damage accumulation, whereas an unambiguous prediction of the residual life appeared impossible. The problem requires further systematic studying using approaches of the fracture mechanics.