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Embedded fiber Bragg grating (FBG) sensors are attractive for in-situ structural monitoring, especially in fiber reinforced composites. Their implementation in metallic structures is hindered by the thermal limit of the protective coating, typically a polymer material. The purpose of this study is to demonstrate the embedding of FBG sensors into metals with the ultimate objective of using FBG sensors for structural health monitoring of metallic structures. To that end, ultrasonic additive manufacturing (UAM) is utilized. UAM is a solid-state manufacturing process based on ultrasonic metal welding that allows for layered addition of metallic foils without melting. Embedding FBGs through UAM is shown to result in total cross-sectional encapsulation of the sensors within the metal matrix, which encourages uniform strain transfer. Since the UAM process takes place at essentially room temperature, the industry standard acrylate protective coating can be used rather than requiring a new coating applied to the FBGs prior to embedment. Measurements presented in this paper show that UAM-embedded FBG sensors accurately track strain at temperatures higher than 400 C. The data reveals the conditions under which detrimental wavelength hopping takes place due to non-uniformity of the load transferred to the FBG. Further, optical cross-sectioning of the test specimens shows inhibition of the thermal degradation of the protective coating. It is hypothesized that the lack of an atmosphere around the fully-encapsulated FBGs makes it possible to operate the sensors at temperatures well above what has been possible until now. Embedded FBGs were shown to retain their coatings when subjected to a thermal loading that would result in over 50 percent degradation (by volume and mass) in atmospherically exposed fiber.
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Metal-matrix composites embedded with piezoelectric PVDF sensors using ultrasonic additive manufacturing
Metal-matrix composites with active components have been investigated as a way to functionalize metals. As opposed to surface-mounted approaches, smart materials embedded in metals can be effectively shielded against the environment while providing in-situ sensing, health monitoring, actuation, or energy harvesting functions. Typical manufacturing approaches can be problematic, however, in that they may physically damage the smart material or degrade its electromechanical properties. For instance, non-resin-based embedment procedures such as powder metallurgy involve isostatic compression and diffusion bonding, leading to high process temperatures and breakdown of the electromechanical properties of the active component to be embedded. This paper presents the development and characterization of an aluminum-matrix composite embedded with piezoelectric polyvinylidene fluoride (PVDF) sensors using ultrasonic additive manufacturing (UAM). UAM incorporates the principles of solid-state, ultrasonic metal welding and subtractive processes to fabricate metal-matrices with seamlessly embedded smart materials and without thermal loading. As implemented in this study, the UAM process uses as-received, commercial Al 6061 tape foilstock and TE Connectivity PVDF film. In order to increase the mechanical coupling between the sensor and the metal-matrix without the aid of adhesives, the PVDF sensor is embedded with an empirically optimized pre-compression defined by the tape foils welded above the sensor. The specimen is characterized by tensile (d31 mode), bending (d31 mode), and compression tests (d33 mode) to evaluate its functional performance. Within the investigated load range, the specimen exhibits open-circuit sensitivities of 4.6 mV/N under uniaxial tension and 9.7 mV/N under compressive impulse tests with better than 95% linearity and frequency bandwidth of several kilohertz. The technology presented in this study could be applied for load and tactile sensing, impact detection and localization, thermal measurements, energy harvesting, and non-destructive testing applications.
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- Award ID(s):
- 1738723
- PAR ID:
- 10228087
- Editor(s):
- Meyendorf, Norbert G.; Farhangdoust, Saman
- Date Published:
- Journal Name:
- NDE 4.0 and Smart Structures for Industry, Smart Cities, Communication, and Energy
- Page Range / eLocation ID:
- 8
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1117/12.2581845
