An experimental study is performed to investigate the electro-mechanical response of three-dimensionally conductive multi-functional glass fiber/epoxy laminated composites under quasi-static tensile loading. To generate a threedimensional conductive network within the composites, multi-wall carbon nanotubes are embedded within the epoxy matrix and carbon fibers are reinforced between the glass fiber laminates using an electro-flocking technique. A combination of shear mixing and ultrasonication is employed to disperse carbon nanotubes inside the epoxy matrix. A vacuum infusion process is used to fabricate the laminated composites of two different carbon fiber lengths (150 mm and 350 mm) and four different carbon fiber densities (500, 1000, 1500, 2000 fibers/mm 2). A four circumferential probe technique is employed to measure the in-situ electrical resistance of composites under tensile load. Although composites of both carbon fiber lengths showed significant decrease of sheet resistance under no mechanical load conditions, composites of 350 mm long carbon fibers showed the lowest resistivity of 10 X/sq. Unlike the resistance values, composites of 350 mm carbon fibers showed a significant decrease in Young's modulus compared to 150 mm counterparts. For the electro-mechanical response, composites containing carbon fibers of 150 mm long demonstrated a maximum value of percentage change in resistance. These results were then compared to both 350 mm and no added carbon fibers under quasi-static tensile loading.

A detailed experimental study is performed for piezo resistance damage sensing on conductive glass fiber/epoxy composites under mode-I fracture conditions. The conductive composites are fabricated by homogeneously dispersing carbon nanotubes (CNTs) within the epoxy matrix and electro-flocking short carbon fibers onto the laminates along with a vacuum infusion process. A parametric study is done on the in-situ damage sensing properties by varying the carbon fiber lengths (150 µm and 350 µm) and the carbon fiber areal densities (500, 1000, 1500, and 2000 fibers/mm2). The change in resistance is captured with a four-point probe measuring methodology by measuring the resistance through the thickness of the composite. The crack initiation toughness value of the composites containing carbon fibers showed improvement over control composites. Composites containing 350 µm length carbon fibers and 2000 fiber/mm2 not only showed the best crack initiation toughness but also provided sensitive network for detecting crack growth.

Electrical and shear behaviour of electrically conductive glass fibre/epoxy composites is studied under interlaminar shear loading. A well-connected network is developed by dispersing carbon nanotubes in the matrix and reinforcing micro carbon fibres between the glass laminates. The effect of carbon fibre length and their densities on the electrical and shear behaviour of the composite is investigated. Although interlaminar shear strength was increased by 20% with addition of carbon fibres, they failed to bridge the delamination between the laminates. For all composite types, there is no change in resistance during elastic deformation due to the formation of new contacts between the CNTs. However, during the non-linear deformation, the carbon fibres debonding and micro-crack coalescence increased resistance steadily for all cases. The composites of shorter carbon fibres showed a higher slope in the resistance change and a maximum peak resistance change compared to that of longer carbon fibres.

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Early-stage damage detection could provide better reliability and performance and a longer lifetime of materials while reducing maintenance time of a variety of structures and systems. We investigate the early-stage damage formation and damage evolution in advanced multi-functional laminated aerospace composites embedded with a very small amount of carbon nanotubes (CNTs) in the matrix material and short carbon fibers along the Z-direction to reinforce the interlaminar interfaces. The three-dimensional (3-D) conductive network formed by the CNTs and the flocked carbon fibers allows for sensitive in-situ damage detection in materials in addition to providing improved mechanical properties such as superior fracture toughness for damage tolerance. We optimize several parameters such as fiber length, diameter, and density to generate an effective 3-D electrical conductive network, and characterize the responses of these composites under mechanical loading to investigate damage formation and evolution, advancing science and technology towards superior damage-tolerant and zero-maintenance structural materials.