Are There Similarities Between Quasi-Static Indentation and Low-Velocity Impact Tests for Flax-Fiber Composites?

Flax-fiber composites are increasingly used as a replacement for classical synthetic composite materials. Flax fibers’ good energy absorption properties represent a promising alternative in structures susceptible to low-velocity impact (LVI) damage. However, this type of dynamic loading is complex, expensive to perform, and still not fully understood. A simpler way to tackle this problem involves investigating quasi-static indentation (QSI) tests, but until now, very few researchers have explored this alternative regarding natural fiber composites. The aim of this paper is thus to provide a preliminary comparison between both types of loading to facilitate the analysis and modeling of flax fabric laminates submitted to a low-velocity impact. Six layers of a flax 2/2 twill fabric were used as reinforcement for epoxy laminates made through vacuum infusion with a stacking sequence of [03]s. Samples were then submitted to instrumented LVI and QSI tests at comparable energy levels. Load-displacement curves were analyzed, and damage was investigated using visual inspection and acoustic emission. The results show good analogies between both testing methods in all the stages of damage development, as long as the energy level remains low compared to the energy of failure. At 15 J, the QSI load-displacement data and energy distribution between the absorbed and recovered categories diverge from LVI. The energies and modes of failure also differ: a perforation appears at approximately 22 J for LVI, whereas a rupture at approximately 31 J for QSI. These preliminary observations suggest that QSI monitoring can provide characteristic indications on damage evolution during an LVI test up to impact energies below a threshold estimated at 70% of the perforation energy. Above this threshold, an extensive study is required.