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Aramid fiber reinforced polymer composites have been shown to exhibit impressive mechanical properties, including high strength-to-weight ratio, excellent abrasion resistance, and exceptional ballistic performance. For these reasons, aramid composites have been heavily used in high impact loading environments where ballistic properties are vital. In-situ damage monitoring of aramid composites under dynamic loading conditions typically requires externally bonded sensors, which add bulk and are limited by size and space constraints. To overcome these limitations, this work presents a piezoresistive laser induced graphene (LIG) interface for embedded impact sensing in aramid fiber reinforced composites. Through the monitoring of electrical impedance during ballistic impact, information regarding time and severity of the impact is obtained. The impact velocity correlates with the impedance change of the composites, due to delamination between aramid plies and damage to the LIG interface. The delamination length in Mode I specimens also correlates to changes in electrical impedance of the composite. The interlaminar fracture toughness and areal-density-specific V50 of the LIG aramid composites increased relative to untreated aramid composites. This work demonstrates a methodology to form multifunctional aramid-based composites with a LIG interface that provides both improved toughness and imbedded sensing of impact and damage severity during ballistic impact. 

Structural health monitoring of fiber-reinforced composite materials is of critical importance due to their use in challenging structural applications where low density is required and the designs typically use a low factor of safety. In order to reduce the need for external sensors to monitor composite structures, recent attention has turned to multifunctional materials with integrated sensing capabilities. This work use laser induced graphene (LIG) to create multifunctional structure with embedded piezoresistivity for the simultaneous and in-situ monitoring of both strain and damage in fiberglass-reinforced composites. The LIG layers are integrated during the fabrication process through transfer printing to the surface of the prepreg before being laid up into the ply stack, and are thus located in the interlaminar region of the fiberglass-reinforced composite. The methods used in this work are simple and require no treatment or modification to the commercial fiberglass prepreg prior to LIG transfer printing which is promising for industrial scale use. The performance of the piezoresistive interlayer in monitoring both strain and damage in-situ are demonstrated via three-point bend and tensile testing. Additionally, the interlaminar properties of the fiberglass composites were observed to be largely maintained with the LIG present in the interlaminar region of the composite, while the damping properties were found to be improved. This work therefore introduces a novel multifunctional material with high damping and fully integrated sensing capabilities through a cost-effective and scalable process.