The integrity of composite structures gradually degrades due to the onset of damage such as matrix cracking, fiber/matrix debonding, and delamination. Over the last two decades, great strides have been made in structural health monitoring (SHM) community using various sensing techniques such as acoustic emission, eddy current, strain gages, etc., to diagnose damage in aerospace, mechanical and civil infrastructures. Embedded sensing offers the prospects of providing for real-time, in-service monitoring of damage were weight savings is a major factor in Aerospace Industry. In this present work, magnetostrictive particles such as Terfenol-D were embedded in a composite structure, along with multiple SHM techniques, to capture the damage in an IM7-carbon fiber reinforced polymer composite system undergoing fatigue loading. As the internal stress state increases, the change in the magnetization flux intensity was captured using a non-contact magnetic field sensor. A damage diagnosis system was established along with an acoustic emissions technique to further validate the damage captured by the embedded system. The goal of this project is to identify the change in the mechanical and magnetic property within a composite material during the evolution of damage. Several characterization techniques were used to determine interfacial fiber-matrix interactions which will provide for a more comprehensive understanding of the composite interfaces.

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.

Poly(urethane-urea) (PUU) has been infused into ultrahigh volume fraction carbon nanotube (CNT) forests using a heat-curable polymer formula. Polymer nanocomposites with carbon nanotube volume-fractions of 1%, 5%, 10%, 20%, and 30% were fabricated by overcoming densification and infusion obstacles. These polymer nanocomposites were nanoindented quasi-statically and dynamically to discern process-structure-(mechanical) property relations of polymerizing PUU in such densely-packed CNT forests. A 100× increase in indentation modulus has been observed, which is attributed not only to CNT reinforcement of the matrix, but also to molecular interactions in the matrix itself. Quasi-static elastic moduli ranging from 10 MPa–4.5 GPa have been recorded. Storage modulus for all materials is found to track well at loadings of 200 Hz, with little effect observed from increasing CNT volume fraction.