Abstract
Piezoresistance based damage sensing of flax/epoxy composites under quasi-static mixed mode fracture loading conditions was conducted. Three-dimensional electrically conductive network was developed in these composites by embedding multi-wall carbon nanotubes and micro carbon fibers (CF). Alkali and silane treatments were performed to make flax plain weave fabrics hydrophobic for improved service life. Novel four circumferential probe method was employed along with high resolution Kiethley instrumentation for electrical measurements. Mixed mode open notch flexure specimen configuration was employed to determine the fracture initiation energy and electrical resistance associated with both crack initiation and propagation. The effect of two CF lengths (150μm and 350μm) at four CF fiber areal densities (500:500:2000 fibers/mm2) on electrical and mixed mode fracture response was investigated. For all configurations, mixed mode fracture toughness increased compared to control specimens without CF. All 150μm CF configurations tended to exhibit greater electrical response than 350μm CF configurations. A modified innovative split tensile Hopkinson pressure bar (SHTB) was designed using AutoDesk Inventor Professional, machined entirely with UMassD machine shop, and validated with 6061 aluminum specimens. This design uses a U-shaped striker bar as a projectile and offers several advantages over classical STHB designs such as 1.) longer stress pulse duration with respect to U-shaped striker bar length, 2.) ability for a wide range of pulse shaping methods, 3.) easy access to loading bar and striker bar, 4.) ability to fire with low air pressures, and 5.) efficient manual reloading.