As space exploration becomes more commercialized, there is a greater demand for innovative technologies for space and planetary exploration and satellite missions. Assessing structural integrity is critical for ensuring the safety and reliability of space structures.
Image Credit: PopTika/Shutterstock.com
Harsh space environments, such as radiation and temperature variations, exacerbate material degradation and wear. In addition, micrometeorite hits are constantly risky and can cause significant structural damage.
Challenge in Space
The number of satellites in Low Earth Orbit (LEO) is increasing, raising the possibility of collisions caused by space debris. The ESA believes there are approximately one million particles larger than one centimeter in orbit, but only about 30,000 have been cataloged and tracked, such as by the Space Surveillance Network.
Collisions with these particles and hits from micrometeorites can cause structural damage, impair structural elements and electronic subsystems, and, in the worst-case scenario, terminate the mission.
Goal: In-orbit Evaluation
Conventional non-destructive testing procedures used on Earth have limited application in space. As a result, an in-orbit assessment of the condition of mission-critical structures and systems is required to track both the deterioration process and the occurrence of single events such as impact damage.
Structural monitoring systems have demonstrated their technological maturity as a non-destructive testing tool in a wide range of industrial and terrestrial applications.
The next logical step is to transform and qualify existing monitoring approaches and systems for use in space, which is being carried out as part of the SeRANIS project via the "Structural Event Monitoring" flight experiment for vibration and ultrasound-based structural monitoring.
Integrating a prototype on the SeRANIS research satellite Athene-1 will provide vibration data from low-Earth orbit to increase Space Situational Awareness (SSA) and support future research activities.
Test Campaign
The reliable detection of small particle collisions and impacts is hampered by structural background noise emitted by other subsystems on the satellite platform. To maximize the monitoring system's event detection rate, algorithmic detectors must be trained to recognize the expected vibration pattern.
For this purpose, tests were conducted at the Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut (EMI) in Freiburg to examine Hypervelocity Impacts (HVI). The impact campaign was carried out with a two-stage light-gas gun (SLGG), which can accelerate microparticles to speeds of up to 7 km/s.
The particles were placed in a sabot, which was accelerated to the desired launch velocity by igniting a black powder charge and compressing the gas.
A particle cloud of spherical glass beads (100-200 μm) impacted a target sample at approximately 2.5 km/s (refer to Figures 1 and 2). This consisted of a photovoltaic element supplied by Airbus Defense and Space GmbH perpendicular to the target chamber's firing axis.
Figure 1. Front of the photovoltaic element aligned in the target chamber. The impact pulse is recorded by vibrometers on the left-hand side of the element and by piezoceramic acceleration and ultrasonic sensors on the right-hand side. Image Credit: Courtesy of the authors
Figure 2. Back of the photovoltaic element aligned in the bombardment chamber. Deflection mirrors guide the measuring beams of the vibrometers, which are guided through the viewing windows, onto the target sample. Image Credit: Courtesy of the authors
The target chamber was emptied to 80 millibars to accelerate the small particles to hypervelocity. When the particle-loaded sabot entered the target chamber, friction from the residual atmosphere caused the sabot to separate, isolating segments of the particle cloud and hitting the photovoltaic element in a targeted manner.
The target sample was fitted with piezoceramic acceleration and ultrasonic sensors, both of which will be employed in space.
Overcoming The Performance Limits of Ultrasonic Sensors: The Use of Polytec Laser Vibrometers and Signal Processors
The mechanical construction of acceleration and ultrasonic sensors limits their detectable frequency range. Additional elements, such as sensor inertia and physical attachment to the carrier structure, influence measurement quality, as high-frequency signals may not always be adequately collected.
Four laser vibrometers from Polytec GmbH were used to ensure that the information content of the measurement findings remained intact. These took simultaneous measurements of the impact signal at four places spread across the front and back of the target sample.
The vibrometer's laser beams were oriented to specific measuring spots for the impact test using deflection mirrors.
Laser Doppler vibrometers employ a frequency shift of light proportional to the surface speed of the measuring surface. This approach has no fundamental constraints for measuring frequency, which allows it to precisely catch even high-frequency movements of several megahertz.
Laser vibrometers allow for the measurement of transient signals that are considerably outside the measuring range of conventional sensors.
For the impact campaign, it is also preferable that the measurement on the test specimen surface is performed contactless using a laser: On the one hand, there is no mass influence on the test specimen, ensuring great consistency of the measurement data; on the other hand, measurements are easily collected through the target chamber's viewing windows.
Figure 3. Perforated photovoltaic cell through several individual particles. Image Credit: Courtesy of the authors
Figure 4. Perforated photovoltaic cell through a dense particle cloud. Image Credit: Courtesy of the authors
The experiments focus on detecting and analyzing so-called acoustic emissions propagating through the structure as elastic ultrasonic waves. These emissions, known as Lamb waves, are a good sign of high-speed impacts induced by microparticles.
Existing structural damage and degrading effects present themselves in identical ways. Therefore, structure-borne sound emissions allow us to draw inferences about a component's health.
Figure 5. Processing of the measurement signals using the Polytec signal processor and comparison with the sensor signals. The powerful and flexible analysis software enables the measurement settings to be optimized in the test laboratory. Image Credit: Courtesy of the authors
One challenge is that the signal characteristics of the hits and the needed measurement settings are unknown at the start of the tests and must be identified and adjusted between experiments.
This is accomplished by employing the Polytec signal processor's powerful and versatile analysis software.
To improve the measurement settings properly, the impact signals can be compared to the recorded sensor signals using software operations such as frequency transformation, filtering, and numerical differentiation/integration.
Figure 6. Perforated photovoltaic cell and detailed image of the impact of the microparticles. Image Credit: Courtesy of the authors
Outlook
Several complex physical processes accompany the impact-induced wave pulse: The impact site emits electromagnetic emissions in the visible, infrared, and microwave spectrums; plasma and secondary particle clouds erupt from the crater (see video sequence); and damage ranges from microcracks to total cell perforation.
Predicting damage caused by micrometeorites and minimizing the consequences
Individual images of the impact process of a dense particle cloud. Plasma and secondary particle clouds are expelled from the crater, the photovoltaic element is completely perforated. Video Credit: Polytec
Understanding the induced vibration and structure-borne sound patterns is necessary to detect the occurrence of such impact events properly. The research contributes considerably to the creation of unique detection algorithms, which are at the heart of the SeRANIS flight experiment, "Structural Event Monitoring."
The results of the bombardment tests will then be used to validate appropriate sensor and hardware components in the following step.
The detection and classification algorithms based on vibration and ultrasonic signals will be trained to determine their appropriateness in real-world settings. The experiment's prototype will be evaluated in low Earth orbit for at least two years.
This information has been sourced, reviewed and adapted from materials provided by Polytec.
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