ZnO Piezoelectric Nanowires for Use in a Self-Powered Structural Health Monitoring Device for Fiber-Reinforced Composites Uploading Attachment Instructions

Composite materials have significantly contributed to advances in state-of-the-art aircraft and spacecraft resulting in lighter, more flexible structures which have pushed the boundaries of performance and efficiency while maintaining a high standard of safety. However, they are still susceptible to multiple modes of failure including fiber-matrix debonding, delamination, matrix cracking, and fiber failure which are capable of leading to the overall failure of the component or structure. For this reason increasing attention is being given to in situ damage detection and structural health monitoring (SHM), which typically use piezoelectric sensors and actuators to gather real-time structural damage characteristics without requiring the structure to be taken out of service and without compromising its integrity. SHM thus has potential to significantly reduce maintenance costs and increase safety and reliability, however, a major road block to the adoption of SHM technology by industry is the requirement that sensors and actuators be externally bonded to the structure under inspection, adding mass and bulk and imposing constraints in size and space as well as surface state requirements. The current state-of-the-art SHM sensors are unacceptable to the modern demand for lightweight and reliable structures indicating a need for self-sensing multifunctional materials which are capable of efficiently performing the tasks of multiple subsystem components (i.e. power storage devices or sensors) without adding mass or bulk. A fully integrated in situ sensor capable of providing on demand damage information would be game changing in embedded SHM methodology. Zinc oxide (ZnO) nanowires have recently gained attention in multifunctional composites as multiple researchers have shown that nanowires provide a tailorable fiber-matrix interface while meeting the multi-industry demand for increased tensile strength and modulus, interlaminar shear strength, and damping characteristics. In addition to their appealing structural benefits, ZnO nanowires also possess both piezoelectric and semiconducting properties enabling actuation and sensing potential as well as the ability to harvest small amounts of power from ambient structural vibrations. The objective of this work is to research the use of multifunctional composites containing ZnO nanowires in SHM and in-situ damage detection to enable monitoring of real structures. It is anticipated that the nanowires are capable of providing information regarding the time of damage occurrence, damage location, and damage severity such that they can be used as a fully integrated replacement for externally bonded SHM sensors. In this work, ZnO nanowires are fully integrated in fiber-reinforced composites by conformally growing the nanowires onto woven aramid fabrics (Kevlar®) using a scalable hydrothermal growth process. Initially, a seeding layer of ZnO nanoparticles is deposited onto the fabric surface using a colloidal suspension method, after which the fabric with nanoparticles is placed into an aqueous solution in a convection oven for a low temperature hydrothermal growth. After thoroughly rinsing and drying the fabric, the resulting nanowires are imaged using a scanning electron microscope (SEM). The Kevlar® fabric with ZnO is then placed between layers of carbon fiber electrodes using vacuum assisted resin transfer molding (VARTM). The outer layers of carbon fiber function as both conductive electrodes as well as reinforcing layers in the final composite. After cutting the completed composite plates into test coupons, wire leads are directly attached to the outer carbon fiber electrodes using a combination of conductive silver paint and epoxy for voltage measurements across the sample thickness during all testing. The completed test specimens are then utilized for in situ damage detection during mechanical loading using both three-point bend and tensile test setups during which the voltage across the thickness of the test specimen is passively measured. As damage to the composite occurs the integrated piezoelectric nanowires are strained resulting in charge separation which can be measured as a voltage emission due to the direct piezoelectric effect. Due to the passive nature of the measurement, any charge measured is also a positive input to the system and can be used to simultaneously harvest energy during damage detection. In comparison to alternative in situ damage detection techniques, this method carries the added benefit of a completely distributed sensing material and conductive electrodes covering the entirety of the composite sample as well as the elimination of any reliance on initial measurements or input current requirements. In addition to the previously described in situ damage detection method, additional work investigating the ability of the integrated nanowires to detect damage which has previously occurred is also completed. Externally applied vibrations generated using a piezo-actuator with a proof mass are used to excite the test specimen at a single frequency. The voltage response of the sample as a result of the direct piezoelectric effect is then measured across the test specimen thickness. After collecting an initial baseline response signal as a reference, controlled and localized damage in the form of cracking and debonding is applied to the test specimen and the same external excitation is applied to the sample. Since the sensing material is present throughout the sample, damage to the sample also results in damage to the sensor itself thus leading to a decreased amplitude response when excited with the same external source as the baseline measurement. Thus changes in amplitude are used to correlate increased damage extent. Each method described in this work utilizes a multifunctional interphase of ZnO nanowires as a fully distributed and integrated sensing material. Since both the sensing constituent as well as the electrodes are comprised of the structural material itself, and as such are fully distributed through the plane of the composite, the resulting functional material is ideally suited for detecting minor damage to complicated structural components. Additionally, the nanowire interphase referenced in this work has been shown in previous research to increase the mechanical properties of the composite while also being used to scavenge small amounts of power from ambient structural vibrations. This work builds on that multifunctionality in demonstrating the nanowires’ ability to self-sense damage during various types of mechanical loading thus further advancing the state-of-the-art in multifunctional materials.