Current satellite transportation sensors can provide a binary indication of the
acceleration or shock that a satellite has experienced during the shipping process
but do little to identify if significant structural change has occurred in the satellite
and where it may be located. When a sensor indicates that the satellite has
experienced shock during transit, an extensive testing process begins to evaluate the
satellite functionality. If errors occur during the functional checkout, extensive
physical inspection of the structure follows. In this work an alternate method for
inspecting satellites for structural defects after shipping is presented. Electro-
Mechanical Impedance measurements are used as an indication of the structural
state. In partnership with the Air Force Research Laboratory University
Nanosatellite Program, Cornell's CUSat mass model was instrumented with
piezoelectric transducers and tested under several structural damage scenarios. A
method for detecting and locating changes in the structure using EMI data is
presented.
A methodology based on Lamb wave analysis and time-frequency signal processing has been developed for
damage detection and structural health monitoring of composite structures. Because the Lamb wave signals
are complex in nature, robust signal processing techniques are required to extract damage features. In this
paper, Lamb wave mode conversion is used to detect the damage in composite structures. Matching pursuit decomposition algorithm is used to represent each Lamb wave mode in the time-frequency domain. Results from numerical Lamb wave propagation simulations and experiments using orthotropic composite plate structures are presented. The capability of the proposed algorithm is demonstrated by detecting seeded delaminations in the composite plate samples. The advantages of the methodology include accurate time-frequency resolution, robustness to noise, high computational efficiency and ease of post-processing.
KEYWORDS: Sensors, Wave propagation, Structural health monitoring, Satellites, Signal attenuation, Actuators, Finite element methods, Data modeling, Analytical research, Nondestructive evaluation
This work focuses on an analysis of wave propagation in isogrid structures as it
relates to Structural Health Monitoring (SHM) methods. Assembly, integration,
and testing (AI&T) of satellite structures in preparation for launch includes
significant time for testing and reworking any issues that may arise. SHM methods
are being investigated as a means to validate the structure during assembly and
truncate the number of tests needed to qualify the structure for the launch
environment. The most promising of these SHM methods uses an active wave-based
method in which an actuator propagates a Lamb wave through the structure; the
Lamb wave is then received by a sensor and evaluated over time to detect structural
changes. To date this method has proven effective in locating structural defects in a
complex satellite panel; however, the attributes associated with the first wave
arrival change significantly as the wave travels through ribs and joining features.
Previous studies have been conducted in simplified ribbed structures, giving initial
insight into the complex wave propagation phenomena. In this work, the study has
been extended numerically to the isogrid plate case. Wave propagation was
modeled using commercial finite element analysis software. The results of the
analyses offer further insight into the complexities of wave propagation in isogrid
structures.
This work focuses on the detection, localization, and quantification of damage in the form
of loose bolts on an isogrid satellite structure. In the process of rapid satellite development
and deployment, it is necessary to quickly complete several levels of validation tests.
Structural Health Monitoring methods are being investigated as a means for reducing the
number of validation tests required. This method for detecting loose bolts enables quick
confirmation of proper assembly, and verification that structural fasteners are still intact
after validation testing. Within this testing framework, feature selection is presented as well
as a localization methodology. Quantification of fastener torque is also developed. Locating
damage in an isogrid structure is complicated by the directionally dependent dispersion
characteristics caused by a propagating wave passing through ribs and holes. For this
reason, an actuation frequency with the best first wave arrival clarity is selected. A
methodology is presented in which a time map is constructed for each actuator-sensor pair which establishes times of flight for each location on the sample. Differences in time between healthy and damaged sensor signals are then extracted and used to create a map of possible damage locations. These resulting solution maps are merged yielding a final damage position. Fastener torque is correlated to a damage parameter, and the loose bolt position is calculated within 3 cm.
Integrity of bolted joints is critical for successful deployment and operation of space structures. Conventional
structural qualification tests span weeks if not months and inhibit rapid launch of space systems. Recent developments
in the embedded ultrasonic acousto-elastic method offer fast diagnosis of bolted joints and opportunities for locating
the fault. However, in current acousto-elastic measurement procedures, a baseline representing the healthy condition
of the joint is necessary. To mitigate a requirement of the baseline, a new methodology based on relative amplitude
and phase measurements is developed. The approach has been validated on laboratory specimens, and modifications
were suggested for applications in realistic structures. The paper discusses principles of the baseline-free acoustoelastic
method, its practical realization, and respective advantages and disadvantages. Comparison of baseline and
baseline-free approaches is presented showing the utility of the recently proposed methodology. Fundamentals of the
acousto-elastic response were studied in experiments involving guided wave propagation in a thin plate under tension.
The results indicate a difference between acousto-elastic responses collected using sensors oriented parallel and perpendicular to the applied stress. It is suggested that this effect may be used to infer stress orientation in the sample. Practical issues related to acousto-elastic measurements in realistic complex structures are discussed, damage diagnosis algorithms are presented, and potential extensions of the acousto-elastic technique are proposed.
One of the key issues in enabling fast and reliable deployment of space systems is structural qualification before launch.
The current qualification process is rather long and may span many months. It is envisioned that structural health
monitoring (SHM) could assist with verification of structural assembly during pre-launch procedures and enable
diagnosis of satellite components. The proposed satellite SHM system utilizes active sensors to launch and receive
elastic waves carrying elasticity information about the structural material. Sensor signatures are analyzed for variation of
the elastic behavior due to damage. Satellite structural components typically feature complex geometries involving isogrids
and bolted joints. Simple representations of complex structures are studied first, followed by investigation of elastic
wave propagation in a realistic satellite panel. The sensor network approach is utilized to detect and locate structural
damage. The acousto-elastic method is implemented for diagnosis of bolted joints in the satellite panel. Sensitivity of the
method is explored for various damage scenarios and a practical damage detection algorithm is suggested. It is shown
that the acousto-elastic method allows for assessment of the structural integrity of complex structural elements with
bolted joints.
KEYWORDS: Satellites, Structural health monitoring, Space operations, Data modeling, Visual process modeling, Sensors, Satellite communications, 3D displays, Defense and security, Aluminum
The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing Structural Health Monitoring
(SHM) technologies in support of the Department of Defense's Operationally Responsive Space (ORS) initiative. Such
technologies will significantly reduce the amount of time and effort required to assess a satellite's structural surety.
Although SHM development efforts abound, ORS drives unique requirements on the development of these SHM
systems. This paper describes several technology development efforts, aimed at solving those technical issues unique to
an ORS-focused SHM system, as well as how the SHM system could be implemented within the structural verification
process of a Responsive satellite.
Responsive space satellites must be assembled and tested in extremely short times. Integrity of structural joints is one of
the major concerns during satellite assembly and qualification processes. A structural health monitoring (SHM)
approach based on nonlinear ultrasonics is suggested for rapid diagnostics of structural connectors and joints. Embedded
piezoelectric sensors are utilized to enable propagation of elastic waves through bolted aluminum panels. Signal
parameters indicative of the nonlinear behavior are extracted from the received waveforms and are used for assessment
of structural integrity. Experimental studies reveal variation of the nonlinear response of the joint due to applied
structural loads. These changes are explored as diagnostic features of the method. We discuss theoretical aspects of the
nonlinear wave propagation through joints and provide experimental data showing feasibility of the embedded nonlinear
ultrasonics method for monitoring of structural integrity.
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