Efficient accelerated assessment of fatigue life in Kevlar, Flax, and Kevlar/Flax hybrid composites

Abstract

This paper presents experimental investigations on the fatigue behavior of Flax, Kevlar, and Kevlar/Flax-reinforced epoxy specimens under accelerated step loading. The samples consisted of a 12-ply-Flax core in a sandwich structure surrounded by a two-layer Kevlar skin, and they were tested under tension–tension progressive cyclic loading. The fatigue strength of the hybrid and its constituents were predicted using thermography and energy dissipation methods with a reduced amount of testing. The results for the Flax/epoxy, Kevlar/epoxy, and Kevlar/Flax/epoxy composites showed similar plateauing for both temperature and energy dissipation at around 6000 cycles. It was observed that Flax, Kevlar, and their hybrid exhibited bilinear behavior based on the stabilized temperature and stabilized dissipated energy over the tested stress levels with the fatigue strength of 47%, 53%, and 47%, respectively, concurring with results reported in the literature. The crack density at 100× magnification increased with increasing load level in all cases, with Kevlar having the lowest and Flax having the highest. Furthermore, the Kevlar/Flax hybrid displayed better performance than pure Flax, with lower crack density and improved crack initiation and propagation within the composite. Thus, the combined use of both methods to accelerate testing is recommended because it resulted in good and reliable predictions of high-cycle fatigue strength.