Whitney Reynolds

Acousto-elastic measurements and baseline-free assessment of bolted joints using guided waves in space structures

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.

Elastic Spiral Folding for Flat Membrane Apertures

A folding concept has been developed which allows a precision membrane aperture to be elastically stowed with a very high compaction ratio without creasing. The lack of creasing is extremely important when the membrane is used in RF or diffractive optics applications because of its effect on the relative placement of surface features. This concept utilizes the spiral wrapped folding approach with a reduced thickness membrane in the folding hinge lines and through-hole strain relief at the intersecting folds. In application, it is intended that this packaging method will be used in an architecture where it is tensioned discretely from the outer edge. These developments allow for high compaction ratios previously only available through folding methods which caused plastic deformations in the membrane. I. Introduction ransmissive diffractive optics systems offer an attractive solution to imaging due to their out of plane deformation tolerance being an order of magnitude higher than reflective systems depending on the ratio of focal length and aperture diameter. In-plane tolerances are not relaxed over traditional optical apertures. The relaxed tolerance requirements has led to significant interest in using diffractive lenses as primary apertures of space-based telescopes in the 10-30m range 1-4 . The main challenges to using deployable membrane diffractive optics are the thermal dimensional stability requirement for accurate focusing, the high focal length required for the system, focusing errors caused by in-plane distortion from creasing the membrane during stowing, and the method for accurately tensioning the membrane in order to pull out those wrinkles. The first two challenges require materials and system architecture solutions respectively, which are not addressed in the current work. This work will address how to package the membrane without creasing such that the tension required to hold the membrane flat is minimal. While the highly precise requirements of the optical system are discussed in this paper, the folding method is equally applicable to planar RF apertures.

Highly Compact Wrapped-Gore Deployable Reflector

Deployable space structures have long been investigated as a means for packaging the desired solution within the confines of the launch environment. In this study, the design of a highly compact reflector that can be stowed in a 1U CubeSat is addressed. The ideal diameter of the finished reflector is 1 meter. In order to accomplish this objective, a folding method is developed and design constraints are derived from the packaging requirement. A design method is presented in which the deployed shape is divided into gores and flattened for manufacturing. Constraints on the shape of the gores are imposed in order to produce a flat configuration with desirable stowing characteristics. Two iterations of the design are presented with deployed diameters of 0.8 and 0.5m and final stowed diameters of 16 and 14cm respectively.

Advanced Folding Approaches for Deployable Spacecraft Payloads

Radio communications apertures for spacecraft have long been implemented using deployable architectures in order to fit within the allowable launch vehicle volume. Apertures for optics missions have traditionally not been segmented because of the tight requirements on the deployed surface. By the nature of the problem, larger apertures are generally better, but complicate orbital delivery. While there are several reflectors commercially available, high packing ratios come at very high cost due to the extremely complex nature of the designs. Researchers at the Space Vehicles Directorate have been investigating ways to enable high packing ratios while reducing the design, integration, and testing complexity of deployable systems, thereby driving down cost and enabling greater mission capabilities. Recent advances in flexible composites have opened up the possibilities of packaging apertures using either distributed or concentrated strain. This paper offers an overview of recent work done to enable lower complexity deployable apertures. Several origami-inspired designs are presented including a flat spiral folding membrane, a parabolic antenna reflector, and a phased array structure.Copyright

Wave Propagation in Rib-Stiffened Structures: Modeling and Experiments

This work focuses on the analysis of wave propagation in rib-stiffened structures as it is related to Structural Health Monitoring (SHM) methods. Current satellite validation tests involve numerous procedures to qualify the satellite for the vibrations expected during launch, and for exposure to the space environment. SHM methods are being considered in an effort to truncate the number and duration of tests required for satellite checkout. The most promising of these SHM methods uses an active wave-based method in which an actuator propagates a Lamb wave through the structure, which is then received by a sensor. The received waves are evaluated over time to detect structural changes. Thus far, 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. Complex isogrid reinforcements within the satellite panel significantly affect any conclusions that can be made about the arriving waves. For this purpose, an experimental and numerical study of wave propagation within rib-reinforced plates has been undertaken. Wave propagation was modeled using finite element software. These results were analyzed for an understanding of dispersion within the structure, particularly how the group velocity and mode conversion are affected by the rib interaction. Experiments were carried out to validate the model and gain further insight into the wave propagation phenomena in the structure. The analysis indicates that mode conversion plays a significant role in the first wave arrival, although this can be accounted for through proper frequency selection, and signal analysis. A range of excitation frequencies which are most appropriate for the structure are presented.

Active Loose Bolt Detection in a Complex Satellite Structure

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.

Experimental Study of Impact Modulation for Quantifying Loose Bolt Torque in On-Demand Satellites

The development of on-demand satellites has created the need for methods that quickly assess the structural integrity of the system. In particular, because of rapid testing and assembly, workmanship errors such as loose bolts within the satellite structure are a concern. Current methods of loose bolt detection, which often rely on baseline data or analytical models, are not practical due to the variable geometry of the satellites and the time constraints on testing. The method of impact modulation is a nonlinear vibration method which is quick to perform and is robust to changes in the geometry of the structure. This paper presents the results of applying impact modulation to a simplified structure used to model a single bolted joint within a satellite. Results show that impact modulation can distinguish changes in torque within the bolted joint. In addition, several examples of how the underlying linear characteristics of the system affect impact modulation results are given.Copyright

Structural Health Monitoring as an Enabler for Responsive Satellites: An Update

The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RVSV) is developing Structural Health Monitoring (SHM) technologies in support of the Responsive Space (RS) initiative with plans for future capabilities on orbit to assist in overall awareness. Such technologies will significantly reduce the amount of time and effort required to assess a satellite’s structural surety without increasing system level risk associated with changed testing. Furthermore, successful implementation of multifunctional sensor capabilities may lead to savings in size, weight, and power (SWAP) allowing more options for technical performance. Although SHM development efforts abound, RS drives unique requirements on the development of these SHM systems; the biggest difference being that deviation from maintenance requires a technology driver. This paper describes several potential niches for SHM technology development efforts by AFRL, aimed at solving those technical issues unique to responsive space, as well as how an ideal SHM system could be implemented within various other processes including the potential for on-orbit performance.

Wave propagation in isogrid structures

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.

Computational Setup of Structural Health Monitoring for Real-Time Thermal Verification

Research at the AFRL Space Vehicles Directorate is being conducted to reduce schedule times for assembly, integration, and test, to make satellite-based capabilities more responsive to user needs. Structural Health Monitoring has been pursued as a means for validating workmanship and has been proven on PnPSat-1. Embedded ultrasonic piezoelectric wafer active sensors (PWAS) have been utilized with local and global inspection techniques, developed both in house and by collaborating universities, to detect structural changes that may occur during assembly, integration, and test. Specific attention has focused on interface qualification. It is now reasonable to believe that evaluation of interfaces through the use of such sensors can also be used to indirectly qualify the structure thermally and that tedious thermal-vacuum testing may be truncated or eliminated altogether. This paper focuses on the computational development of extracting thermal properties from ultrasonic transmission records. Methods are validated on simple bolted lap-joint cantilever beams.