Development of an integrated repair and monitoring methodology for fatigue-damaged metallic structures based on carbon nanotube composites
Date
2019
Authors
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Publisher
University of Delaware
Abstract
All structures exposed to cyclic loading, such as aerospace structures, civil infrastructure, automobiles, and vibrating machinery, are susceptible to deterioration by fatigue cracking. For example, steel bridges are particularly susceptible to fatigue cracking as increased traffic demands often exceed their design load capabilities, and structural members often have fatigue-sensitive details. According to ASCE's 2017 Infrastructure Report Card, one out of eleven bridges in the United States are structurally deficient. Corrosion and fatigue are two common reasons for this classification. Given the significant budgetary constraints, maintaining these deteriorated bridges is a major challenge. This dissertation research focuses on developing a methodology for sustainable rehabilitation of fatigue-damaged structures. For repairing fatigue-damaged structures, adhesively bonded fiber reinforced polymer (FRP) composite materials have been proven to be an excellent candidate. However, the brittle debonding failure mechanism and the challenge to monitor debonding are major obstacles to deploy this repair technique in the field reliably. Also, the FRP repair scheme usually covers the damage, preventing visual inspection. In this work, an integrated repair and monitoring scheme by combining FRP with carbon nanotube (CNT)-based sensing fabrics were developed, evaluated, and implemented in the field to mitigate these challenges. ☐ A thin, flexible CNT-based sensor was developed in this research for fatigue crack monitoring using a nerve-like conductive network. The sensing layer, composed of a random mat of aramid fibers coated with carbon nanotubes, offers tremendous application flexibility for integration of sensing capabilities in structures. First, laboratory scale experimental tests were conducted to evaluate the performance of the CNT sensor to monitor fatigue cracks. The experimental tests were designed to simulate various real-life fatigue crack growth scenarios. It was found that the CNT-based sensing layer can effectively and continuously monitor the fatigue crack. Second, a technique was established to embed the CNT-based sensing layer in the adhesive layer of a metal/composite repair scheme. Experimental tests were conducted to investigate the influence of the embedded sensor on the adhesive bond strength. These coupon level experimental tests demonstrate that embedding the sensing layer in the adhesive bond does not reduce the bond strength of an adhesive joint and the sensing layer can monitor the bond integrity in situ. Experimental tests were conducted to evaluate the capabilities of the integrated repair and monitoring schemes. Experiments examining fatigue crack propagation in structural steel with a composite repair and integrated bond line sensing increased the fatigue life by 380% to over 500%, depending on the configuration. The sensing layer was able to monitor deformation and crack propagation in real-time. Finally, this innovative integrated repair and monitoring scheme has been deployed in the field to evaluate the performance in a real-life scenario. Data in the field is streamed over the cellular network and can be monitored remotely. ☐ The integrated repair and monitoring schemes are inexpensive and have tremendous flexibility to be applied in different applications. Given the significant budgetary constraints of infrastructure owners, this solution has the potential to allow for the rehabilitation and repair of damaged civil infrastructure while simultaneously adding structural monitoring capability. Moreover, current fatigue crack inspections on highway bridges rely heavily on visual inspection with inspection frequencies that can range from a few months for fracture-critical damage to a couple of years. This CNT-based fatigue crack sensor is an accurate and reliable solution allowing bridge inspectors to assess fatigue crack activity during regular inspections. The sensor could also be connected to a wireless sensor network for continuous monitoring, allowing a bridge owner to assess cracks remotely at any time, as demonstrated in the field tests. Thus, asset management decisions can be made more efficiently.
Description
Keywords
Carbon nanotube composites, Metallic structures, Fatigue damage