Recent years have seen an increased interest in exploring outer space for space tourism or for unmanned or manned planetary explorations. The captivated interests among various stakeholders to employ advanced technologies to meet the requirements of these missions have necessitated the use of newly developed asset monitoring systems to ensure robustness and mission reliability. Although, Non-Destructive Testing (NDT) methods provide sufficient information about the state of the structure at the time of inspection, the need for continuously monitoring the health of the structure throughout the mission has asserted the use of Structure Health Monitoring (SHM) technologies to increase the levels of safety and thereby, reducing the overall mission costs. However, since the implementation of SHM technologies for space missions can be affected by several factors including, environmental conditions, measurement reliability and unavailability of adequate standards, additional considerations on its employability must be reconsidered. This article demonstrates a structured approach to compare the capabilities of some of the most promising SHM technologies in consideration of these influential factors. Additionally, remarks on the feasibility of employing these SHM technologies and the role they could play in such critical missions would be elaborated. Introduction Over the past few decades, several stakeholders in the space exploration sector have thoroughly understood and implemented strategies for monitoring mission critical structures to ensure improved levels of mission safety, reliability & affordability [1, 2]. Prominent stakeholders including ESA and NASA have therefore incorporated Structural Health Monitoring (SHM) sensors and systems for on-board condition monitoring, fault detection and to generate prescriptive recovery actions. In addition, in order to pave the way towards having reusable space assets, reducing the vehicle downtime, operating costs and the maintenance costs, these stakeholders are also investigating approaches to understand and predict the lifetime of a mission critical structure. SHM was born from the conjunction of several techniques that share a common basis with Non-Destructive Testing (NDT). In fact, by permanently installing and integrating some of the NDT sensor technologies onto the structure of interest, they could be converted into SHM techniques. Some of the most commonly used SHM technologies include, conventional strain gauges, fiber optic sensors and acoustic sensing techniques, among many others [3]. Since the fundamental basis of each of these technologies are different, their performance is highly dependent on the use-case under consideration. For monitoring the health of space structures, one such performance hindering parameter is the ambient space condition itself (ambient temperatures vacuum, cosmic radiation and electromagnetic emissions) [4]. Therefore, when selecting appropriate SHM techniques for such application, the capability of the sensors must be fully Structural Health Monitoring Materials Research Forum LLC Materials Research Proceedings 18 (2021) 343-351 https://doi.org/10.21741/9781644901311-42 344 understood and their performance must be optimized to compensate for the influences of the space environment. Even though, several studies investigate the performance of SHM technologies for monitoring the assets in a space environment, the majority of the research have looked into the performance of an individual SHM technology [1, 2, 5, 6]. Furthermore, since the application of SHM for space assets is a relatively new innovation, additional developments, especially for standardizing the behavior of such sensors and systems are quintessential. This would require the creation of standards to define the minimum requirements for a SHM system, which is nonexistent. In this context, this paper investigates into the most commonly used SHM technologies and compares their performance based on a set of requirements. These requirements have been derived from several factors including, environmental conditions, measurement reliability and the technology maturity. In addition, since the system requirements for space applications can be comparable with (to an extent) the ones defined for fixed wing aircrafts, we have used the SAE ARP 6461 standards for our study [7]. This standard provides, “Guidelines for implementation of structural health monitoring on fixed wing aircrafts”, which could be relevant for applications including (not limited by), helicopters, space crafts and launchers. Since it is impossible to compare the performance of all of the available SHM systems for space asset monitoring, we have limited our scope to identify and compare some of the most commonly used and promising sensor technologies. The work done in this article is a follow-up to our previous work, wherein we had evaluated the performance of fiber optic sensors, Piezoelectric Wafer Active Sensors (PWAS), Acoustic Emission (AE) sensors and conventional strain gauges in meeting the requirements posed by the space sector [8]. Upon studying the capabilities of each of these sensor technologies and its alignment with the requirement list for monitoring operation parameters & damage parameters, fiber optic sensors were determined to be the most promising. Although, PWAS (e.g. SMART sensors from AcellentTM) were observed to be not so capable in monitoring the operational parameters, their ability in monitoring damage parameters, especially in the case of composite structures, provide a promising solution for the future of monitoring futuristic space structures. Therefore, the analysis and evaluation presented in this paper compares the capability of the fiber optic sensors with two additional promising sensor technologies for monitoring the health of aerospace structures, namely, Comparative Vacuum Monitoring (CVM) and Surface Acoustic Waves (SAW). The next few sections would detail the capabilities of each of these sensor technologies which would be followed by the definition of the set of requirements. The aim of the analysis is to determine one (or more) promising technology (or technologies), which can be envisaged to revolutionize the space industry in the near future by setting very high standards for safety, reliability and robustness. Optical Fiber Sensors The use of sensors based on fiber optic technology for strain sensing, vibration monitoring, temperature measurements etc., have gained momentum due to its higher sensitivity & form factor. In essence, a fiber optic sensor fundamentally consists of an optical source that is optically aligned with a single mode fiber optic cable. The relationship between the optical properties of the signal (light) transmitted through the optical fiber and the measurement parameter is used to measure the structural condition & performance in real-time. Depending on the sensing mechanism, fiber optic sensors can be classified into three broad categories, namely, single point sensors, multiplexed sensors and distributed sensors [9, 10]. Single point sensing technology consists of one small, durable and highly accurate measurement device connected to a high-bandwidth fiber optic cable. Fiber Bragg Grating (FBG), Structural Health Monitoring Materials Research Forum LLC Materials Research Proceedings 18 (2021) 343-351 https://doi.org/10.21741/9781644901311-42 345 which is one of the most commonly used fiber optic sensing system, is an example of this category. In essence, a FBG sensing unit is manufactured by modifying a single-mode optical fiber using a UV laser. The resultant germanium-doped microstructures creates a periodic variation in the refractive index altering the optical properties of the coherent light source passing through the optical fiber. The periodic variations in refractive index, also called Bragg gratings inherently reflect a very narrow wavelength, while transmitting all the other wavelengths through the optical fiber. The wavelength band reflected by the Bragg gratings are strongly dependent on the grating period and thus any external perturbations that affect the same can be correlated with the shift in the reflected wavelength band. Using an instrument, which is known as an interrogator (i.e. the data acquisition device), the shift in the wavelength is recorded. From an application standpoint, these sensors are multiplexed and located at strategic locations along the fiber to create a series of connected units for a quasi-distributed measurement [9]. A distributed fiber optic sensor on the other hand relates the changes in scattered light along the entire length of the optical fiber. The change in scattered light is used to determine the local variation of physical quantities (strains or temperature) [10]. In this case, the entire fiber acts as the sensor element. Depending on whether the mechanism of scattering the light within an optical fiber is elastic or inelastic, the sensing technique can be classified, Rayleigh, Brillouin and Raman scattering techniques. Whilst Rayleigh scattering is a physical phenomenon caused due to non-propagating density fluctuations (scattered power is proportional to the input power), Brillouin and Raman scattering result from inelastic physical phenomenon causing large degrees of frequency shifts. Considering the advantages of each of the techniques in the measurement of physical quantities, Rayleigh and Brillouin scattering are being widely investigated for strain measurement applications and Raman scattering is being studied for temperature measurements. Fiber optic sensing technologies have seen widespread applications for monitoring the operational parameters (strains & temperatures) in several industries, including, infrastructure, aeronautical, automotive and for mainstream industrial processes (not limited to) [9, 10]. To ensure the accuracy, repeatability and stability of the measurements made by the
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