A significant safety concern preventing extensive use of composite materials for large polymer composite structures is the ability to detect, locate, and quantify damages that occur at one or several locations in large polymer composite structures. Real-time health monitoring of large polymer composite structures improves their performance, durability, and reliability while minimizing the life cycle cost. In this article, we present a new, practical, and real-time structural health monitoring technique for detecting, locating, and quantifying damages in large polymer composite structures made of carbon fibers and carbon nanotube networks. In this technique, electrically conductive epoxy resin was prepared by dispersing multiwalled carbon nanotubes into epoxy matrix. This modified epoxy matrix was then incorporated with long carbon fibers to make large composite plates. Two sets of grid points made from silver-epoxy paste were mounted on the surface of the large plates. The first set was used to apply the constant electric current, and the second set was utilized to measure the electric potential. The electric potentials across the second set of grid points on the undamaged plate were measured and used as a reference set. Two different damages were created by drilling holes and by applying impact loading on the large plates. It is found that the electric potential between the contact points surrounding the damage changes. The significant change in electric potential corresponds to the damage location in the plates. As such, drilled holes, impact damages, and barely visible impact damages are detected, located, and quantified.
In this work, we have developed a novel, practical and real-time structural health monitoring (SHM) technique to detect, locate and quantify damage that occurs at one or more locations in large polymer composite structures (LPCSs) made of electrically non-conductive fibers and carbon nanotube networks. Our technique exploits the piezoresistive effect of multiwalled carbon nanotubes (MWCNTs) in epoxy resin. The electrically conductive epoxy resin was used to prepare glass fiber reinforced composite plates. The plates were marked with grid points where electrically conductive silver-epoxy pastes were deposited. The electrical resistances between the grid points were measured and used as a reference set. Two new concepts are introduced. One is uniformity of MWCNT distribution which gives rise to uniformity in electrical conductivity. The second is maximum sensitivity to change in electrical resistance due to the occurrence of damage. These issues are demonstrated as criteria to determine the optimal quantity of MWCNTs. This optimal quantity is used to assure damage detectability at any region in the large plates. Drilled holes and impact testing were conducted to simulate damage. The damage causes the electrical resistance between the contact points surrounding the damage to increase. This increase is used to detect, locate and quantify damage.
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