Abstract
A detailed experimental study is performed for piezo resistance damage sensing in (a) glass fiber/epoxy composites under mode-I fracture conditions and (b) glass fiber/carbon fiber epoxy hybrid composites under tensile and flexure conditions. The novel 3D conductive network for fracture conditions is generated by homogeneously dispersing multi-walled carbon nanotubes (MWCNTs) within the epoxy matrix and electro-flocking short carbon fibers onto the glass fiber laminates along with a vacuum infusion process. The conductive network for hybrid composites is obtained from the carbon fibers within the laminates along with electro-flocking short carbon fibers onto the laminates along with a vacuum infusion process. The electrical response is captured via a four-point probe measurement methodology under mechanical loads. A parametric study is performed on the in-situ damage sensing for modeI fracture loading by varying the carbon fiber lengths (150 μm and 350 μm) and the carbon fiber areal densities (500, 1000, 1500, 2000 fibers/mm2). The crack initiation toughness of composites containing Z-axis reinforcement showed significant improvement over composites without short carbon fibers. Composites containing 350 μm length and 1500fibers/mm2 showed an improvement in crack initiation toughness (23% increase over the control)while also providing the most sensitive damage sensing network, while composites containing 350 μm length and 2000 fibers/mm2 showed the best crack initiation toughness (106% increase over the control) and was the only parameter to show steady crack growth throughout. Again, a parametric study is performed on the in-situ damage sensing capabilities for tensile and flexure loadings by varying hybridization methods (inter-laminar and intra-laminar) and by inclusion of short carbon fibers between the laminates (150 μmlength and 1000 fibers/mm2). Intra-laminar hybrid composites with glass fibers along the loading directions showed the highest initial resistance value (1183 Ω) and ultimate tensile strength (402 MPa) due to carbon fibers being located only in the weft direction. Intra-ply composites with a [±45/∓45] layup orientation was found to have the lowest ultimate tensile strength (56.8 MPa) but the highest elongation at break (9.7%). Each composite type under tensile and flexural loading conditions showed two distinct zones of damage propagation. Intra-laminar composites under tensile conditions and intralaminar composites with [±45/∓45] orientation under tensile and flexural conditions showed three distinct zones of damage propagation where the third zone reached the composites maximum possible change in resistance.