Development of coaxial structured fiber sensor for composite cure strain and structural health monitoring

Abstract

As the greenhouse gas emission regulations are recently accelerated due to the carbon neutrality, all industries, including automobiles, aviation, and marine, based on the internal combustion engine propulsion system, are trying to respond to the regulation by converting to an electric propulsion system based on secondary batteries. However, there is a limit to improving the operating distance due to the low energy density of the secondary battery, which is an obstacle to the complete conversion to an electric propulsion system. In order to travel longer distances, more batteries must be loaded, but there is a trade-off relationship that reduces efficiency due to increased weight. Therefore, it is necessary to load more batteries by reducing the weight of the structural components. Fiber-reinforced composite materials are widely applied to light-weight structures that need to support high loads due to their excellent specific strength and stiffness. In the past, it was used limitedly in special fields such as aerospace, but the applications based on lightweight structural materials are explosively increasing in response to the recent demand for conversion to electric propulsion systems. The demand for sensors is rapidly increasing, particularly for the development of manufacturing processes for high-quality composite materials and structural health monitoring (SHM). These sensors are crucial in predicting defects and damages within composite materials. Currently, Fiber Bragg Grating (FBG) sensors are used for cure strain monitoring during the manufacturing process and SHM of composite structures. However, they have several limitations, such as having a diameter 10 times greater than that of reinforcing fibers, low mechanical properties, and low impact resistance. This results in a degradation of the overall mechanical properties of the composite structure. To address these limitations, nanocomposite-based sensors with a high degree of design freedom such as electrical and mechanical properties and measurement characteristics can be an alternative, but most previous studies have applied them to applications such as bio, wearable and healthcare. For this purpose, research and development are focused on improving flexibility and strain sensitivity, thus there is a limit to apply to the fields of cure monitoring and SHM of composite materials, which must have high mechanical properties. In this dissertation, a novel fiber sensor was developed to overcome the limitations of existing sensors and apply it to the manufacturing and structural health monitoring of composite materials. Four perspectives were used to design the fiber sensor, including load-bearing capacity, improvement of measurement characteristics such as linearity, repeatability, and noise reduction, minimization of measurement errors through improved creep resistance, and the ability to measure both temperature and strain. To achieve this, a coaxial structure was adopted to provide load-bearing capacity and measurement characteristics simultaneously, and a core fiber based on ultra-high molecular weight polyethylene with high stiffness and strength was selected, and the mechanical properties of the fiber were greatly improved through optimization of its manufacturing and post-processing conditions. In addition, a microwave-assisted cross-linking method was developed to suppress creep deformation, which causes significant measurement errors in polymer-based sensors, and greatly improved the load-bearing capacity and creep resistance of the core fiber. A dip-coating method was applied to give the manufactured core fiber the ability to measure temperature and strain. A multilayer structure with conductive layers of different concentrations and a polyurethane layer was designed to improve the noise level and linearity of the measurement results from the perspective of strain measurement. The structure of the strain measurement layer was designed to significantly improve the linearity of the measurement results while maintaining a low level of noise. The characteristics of each layer were compared and analyzed. A laser processing method was developed to form a perforation area on the outermost polyurethane layer without compromising the measurement characteristics of the strain measurement layer, to enable temperature measurement through thermoelectric voltage changes which is generated from junction point between strain sensing layer and conductive polymer layer. The effects of processing conditions on the laser processing were analyzed, and the measurement characteristics of the conductive polymer layer for temperature measurement were analyzed and compared under various temperature and load conditions. Finally, the developed fiber sensor was used to monitor the curing strain and structural health of composite materials during the manufacturing process, demonstrating its applicability.