Determination of the feasibility of using flax fiber reinforced with shear thickening fluid for ballistic applications

Soft body armor has been part of human attire since the Stone Age, providing confidence and security to humans as they developed their activities. This gear has evolved through the centuries, incorporating new technologies that have reduced its weight while increasing its protection level. Nowadays, it can protect against threats from handguns and firearms, such as 9 mm and .44 MAG cartridges. Over the last decade, the development of materials to enhance the performance of body armor has focused on incorporating shear-thickening fluids. Their rheology enables them to behave like a solid under stress and revert to their initial state when the stress is removed. These characteristics, along with their excellent adherence to natural fibers, have motivated this study. This study aims to determine the feasibility of applying flax fiber reinforced with shear thickening fluid for developing ballistic panels with enough resistance to support handgun threats. The response in terms of maximum supported force and elongation of plain flax fiber when reinforced with different commonly used shear thickening fluids was studied through tensile testing. Additionally, the fracture mechanism was observed using scanning electron microscopy, revealing different behaviors depending on the amount and composition of STF. Then, after identifying the best STF for impregnating flax fiber and subjecting it to ballistic impacts, panels made of four layers of both neat and reinforced fabric were tested to compare their energy dissipation capacity. The results showed positive outcomes in terms of energy absorbed per layer; however, this increase in energy absorption capacity came with a significant weight increase, reducing the specific energy absorption per layer, which is not ideal for ballistic applications. Finally, various panels with increasing layers—starting with four and growing by four until reaching 24 layers—were subjected to ballistic impacts. During these tests, the energy dissipated by each panel was measured, allowing the development of a function that predicts roughly 68 layers are needed to stop a 9 mm full-metal-jacket threat. Since this number exceeds practical limits for soft body armor, it was concluded that although shear thickening fluid enhances impact resistance per layer of flax fiber panels, this improvement is insufficient to reach handgun protection levels, because the added weight from reinforcement limits practicality.

https://orcid.org/0000-0003-2744-2346