System Integration of Flexible and Multifunctional Thin Film Sensors for Structural Health Monitoring

Abstract

Greater information is needed on the state of civil infrastructure to ensure public safety and cost-efficient management. Lack of infrastructure investment and foreseeable funding challenges mandate a more intelligent approach to future maintenance of infrastructure systems. Much of the technology currently utilized to assess structural performance is based on discrete sensors. While such sensors can provide valuable data, they can lack sufficient resolution to accurately identify damage through inverse methods. Alternatively, technologies have shown promise for distributed, direct damage detection with flexible thin film and multifunctional polymer-nanocomposite materials. However, challenges remain as significant past work has focused on material optimization as opposed to sensing systems for damage detection. This dissertation offers novel methods for direct and distributed strain sensing by providing a fabrication methodology for broadly enabling thin film sensing technologies in structural health monitoring (SHM) applications. This fabrication methodology is presented initially as a set of materials and processes which are illustrated in analog circuit primitive forms including flexible, thin film capacitors, resistors, and inductors. Three sensing systems addressing specific SHM challenges are developed from this base of components and processes as specific illustrations of the broader fabrication approach. The first system developed is a fully integrated strain sensing system designed to enable the use of multifunctional materials in sensing applications. This is achieved through the development of an optimized fabrication approach applicable to many multifunctional materials. A layer-by-layer (LbL) deposited nanocomposite is incorporated with a lithography process to produce a sensing system. To illustrate the process, a strain sensing platform consisting of a nanocomposite film within an amplified Wheatstone bridge circuit is presented. The study reveals the material process is highly repeatable to produce fully integrated strain sensors with high linearity and sensitivity. The thin film strain sensors are robust and are capable of high strain measurements beyond 3,000 μϵ. The second system developed is an array of resistive distributed strain sensors and an associated algorithm to provide an alternative to electrical impedance tomography for spatial strain sensing. An LbL deposited polymer composite thin film is utilized as the piezoresistive sensing material. An inverse algorithm is presented and utilized for determining the resistance of array elements by electrically stimulating boundary nodes. Two polymer nanocomposite arrays are strain tested under cyclic loading. Both arrays functioned as networks of strain sensors confirming the viability of the approach and computational benefits for SHM. The third system developed is a thin film wireless threshold strain sensor for measuring strain in implanted and embedded applications. The wireless sensing system is comprised of two thin film, inductor-capacitor circuits, one of which included a fuse element. The sensor is fabricated on polyimide with metal layers used to pattern inductive antennas and a strain sensitive parallel plate capacitor. A titanium thin film fuse is designed to fail, or have a large resistance increase, when a strain threshold is exceeded. Three prototype systems are interrogated wirelessly while under increasing tensile strain. One of two sensor resonant peaks disappear at a strain threshold as designed, validating the sensing approach and thin film form for use in SHM systems. The fuse approach provides a platform for various systems and sensing elements. The reference peak remains intact and is used for continuous real-time strain sensing with a sensitivity of 0.5 and a noise floor below 50 microstrain.