INTEGRATION AND INTERFACIAL ENGINEERING OF STRAIN SENSORS IN ADDITIVELY MANUFACTURED POLYMERIC STRUCTURES

Additive Manufacturing (AM) (more popularly referred to as 3D printing) provides a novel solution to create multifunctional designs and parts with geometries and properties, which were impossible to manufacture by traditional processes. Polymer fused filament fabrication (FFF) technology as a process category of AM is currently the most popular on the market. The structure of the 3D printed neat thermoplastic polymers (TPs) and continuous fiber-reinforced thermoplastic composites (CFRTPCs) manufactured by this technology bears interfaces at different scales being the cause of non-optimal mechanical properties. Although some work has been carried out to optimize the interfacial bonding and so the structure, yet still requires further improvement. AM also opens up the possibility for embedding functional elements such as fiber Bragg grating (FBG) sensors to carry out structural health monitoring (SHM). The technology landscape of CFRTPCs created by FFF is still in an early stage of development. Further, there is also still limited work reported on interfacial engineering investigations between the polymeric matrix and the embedded FBG sensor polymer jacket for health monitoring.
This dissertation studied the influence of the interfacial bonding quality on the strain transfer between the 3D printed polylactic acid (PLA) matrix and the polyimide (PI) jacketed FBG strain sensor compared to a reference strain measurement which was the digital image correlation (DIC) method. The bonding quality between the PLA matrix and the PI jacket was discussed based on the most tangible parameters which were the intrinsic adhesion and porosity. Developing bonding methodologies for improving the interfacial adhesion was therefore the first priority and were tested on PI films and PLA plates as model materials. These bonding methodologies made use of cleaning, plasma activation, roughness modification and/or the use of polydopamine (PDA) nanocoating, cyanoacrylate (CAC) glue and dichloromethane (DCM). The solvent welding by DCM and adhesive bonding with CAC and
PDA as the bonding methodologies providing the highest adhesion were then implemented for the integration of a PI jacketed FBG strain sensor in PLA FFF printed structure. The main results revealed that an interfacial adhesion between the embedded FBG sensor jacket and 3D printed matrix has been necessary but not sufficient to have a good interfacial bonding since the pores’ volume in the interfacial region also contributed. Indeed, the interfacial porosity fraction which must be minimized when embedding the sensor was primarily responsible for the sensor’s strain measurement accuracy of the actual strain in the FFF printed structure. By considering the percentage of strain in the 3D printed specimen transferred to the embedded FBG strain sensor, the deposition of a PDA nanocoating onto the sensor or the addition of a CAC adhesive at the PLA-PI interface with no channel employment were identified as the best method to embed an FBG strain sensor into 3D printed PLA. The use of CAC adhesive was regarded as the most reliable method based on its lower standard deviation value for strain transfer when compared to PDA. The PDA treatment of the sensor, however, was conducted prior to printing, thus not affecting the practical aspects and with a water-based solution. Nonetheless, when selecting a methodology, the practical aspects linked to the sensor embedding in the 3D printed polymer components can also be considered based on the specific needs, and production type. To illustrate, the proper introduction of a sensor into non-linear paths in complex geometries can be facilitated by a channel. That being the case, the combination of a channel and a CAC adhesive could be considered to best suit.
The original findings presented in this thesis explain the link between the matrix-sensor effective strain transfer and interfacial properties and demonstrate that the reliability of an embedded FBG sensor’s measurement depends on its good bonding with the surrounding matrix. The results presented shed light on the understanding of the importance of interfacial engineering in FFF built structures with FBG sensors, which is a fundamental step in this technology towards the development of more efficient smart polymer structures.