Jack Leifer

Evaluation of Shear Compliant Borders for Wrinkle Reduction in Thin Film Membrane Structures

Many ultra-lightweight (Gossamer) space structure designs under consideration for future missions (e.g. solar sails, sun shields, reflectors) are comprised of thin film, flat membrane panels. Such structures must be maintained in a state of biaxial tension in order to suppress wrinkle formation. One such support method under consideration involves the incorporation of integrated, shear compliant borders along the top and bottom edges of the membrane. In this paper, finite element modeling is used to predict the effectiveness of various shear-compliant support geometries. The wrinkle configurations predicted (e.g. amplitude, angle) compare favorably with experimental surface measurements, made using photogrammetry, of membranes incorporating shear compliant borders.

Experimental and Numerical Correlation of Gravity Sag in Solar Sail Quality Membranes

family, due to their potential to provide propellantless propulsion. They are comprised of ultrathin membrane panels that, to date, have proven very difficult to experimentally characterize and numerically model, due to their reflectivity and flexibility and the effects of gravity sag and air damping. Numerical models must be correlated with experimental measurements of subscale solar sails as the first step in verifying that the models can be scaled up to represent full-sized solar sails. In this paper, the surface shapes of two horizontally-supported 25-m-thick aluminized Kapton membranes were measured to a 1.0-mm resolution using photogrammetry. Simple numerical models were developed and their output matched the corresponding experimental data in all cases with less than 33% error. This correlation between the numerical predictions and the experimental data is in line with similar resultsobtainedbyotherswhousedmorecomplexnumericalmodelstopredictthegravity-induced sagoflargersail panels. The results indicate that the largest source of discrepancy between predicted and measured data is membraneslack.Itispostulatedthatincorporationofgeometricslackintothenumericalmodelswouldlikelyreduce this discrepancy.

Measurement of In-Plane Motion of Thin-Film Structures Using Videogrammetry

the shaker. The presence of modally induced in-plane film deformation was confirmed by tracking the change in distance between points F1 and F2. The standard deviation of the value of the measured distance between these two points was found to be about 57 m. This value was well above the noise floor for this measurement, 11 m, experimentallydeterminedbycalculatingthestandarddeviationofthemeasureddistancebetweenpointsR1andR2 on the aluminum film holder, which was considered to be rigid and hence was not expected to undergo in-plane deformation.

Simplified Computational Models for Shear Compliant Borders in Solar Sails

Shear compliant regions have been demonstrated to reduce both the amplitude and extent of structural wrinkles on the surface of thin-film (Gossamer) membranes under simultaneous tension and shear. Rather than modeling the full geometric details of the thermoformed strips that comprise a shear compliant region, two simplified approaches have been developed for efficiently incorporating them into existing finite element models. One approach replaces the three-dimensional geometry of the thermoformed region with alternating flat strips of high E/ν~0.3 and low E/ν~0 materials. The other approach uses a properly aligned uniaxial orthotropic material to model the behavior of the shear compliant region. Both approaches were simulated in finite element software, and yielded similar results that (at least qualitatively) reflected the behavior of existing test articles containing thermoformed shear compliant regions.

Zero- and One-g Comparison of Surface Profile in Single-Curved Parabolic Membrane

This experiment was designed to quantify the effects of gravity and boundary support conditions on a scale model of an orbiting, singly curved parabolic thin-film membrane antenna. A 1-m-scale model of the parabolic antenna and support system was constructed and tested on NASAs KC-135A Weightless Wonder microgravity aircraft. A fabric-backed membrane (76.2-μm Nylon, 12.7-pm Mylar, 12.7-μm adhesive, 0.1-pm Al) was placed in the test fixture designed for this work and tensioned using edge clamps that maintained the desired parabolic profile at the membrane boundaries. Targets for tracking the full-field surface deflection were provided by about 7000 3-mm dots stenciled on to the membrane surface. The membrane surface configuration was monitored by four digital cameras mounted in the test enclosure. Using photogrammetry, the high-resolution digital images taken in-flight (at 0-g conditions) and on the ground (at 1-g) were processed, and the three-dimensional location of each target visible in at least three images was calculated. It was found that under the same support conditions both structural wrinkling and sag in microgravity were significantly less pronounced than at 1-g.

Measuring and Modeling the Dynamics of Stiffened Thin-Film Polyimide Panels

Stiff, ultralightweight thermal-formed polyimide panels are examples of next-generation space structures that address some of the issues of membrane-dominated ultralightweight structures while maintaining their low mass and low stowage volume characteristics. The research presented here involved dynamically characterizing and modeling two of these panels, one 0.0625 m 2 with a mass of 38 g and the other 0.1875 m 2 with a mass of 81 g, to develop validated computer models that can be used to determine the effects of changing manufacturing parameters and scalability. Modal testing using an impact hammer and accelerometer extracted the first four structural natural frequencies, the first occurring at 71.9 Hz. These data were replicated by simple, coarsely meshed shell element finite element models that are significantly smaller than previous finite element models of similar structures.

Gravity-Induced Wrinkling in Subscale, Singly Curved Parabolic Gossamer Membrane

†‡ In this study, close-range photogrammetry was used to reconstruct the three-dimensional surface contour of a parabolic membrane as a function of local gravity. One of the most persistent challenges involving the deployment of full-scale gossamer structures is the development of predictive numerical models for full-scale, zero-g behavior, which can be verified using sub-scale models at one-g. One step in the development and verification of such models is simply obtaining suitable data at zero-g and one-g that can be used for such purposes. This paper describes a half-scale 76.2-µm thick parabolic membrane, fabricated from Kapton HN, which was flown aboard NASA’s KC-135A and used to obtain surfaceprofile-versus-gravity data. Surface measurement was made using a five-camera photogrammetry system, which allowed non-contact measurement of the lower two-thirds of the membrane to be made. Using this method, it was found that membrane surface wrinkle configuration at zero-g changed as the membrane was accelerated to 1.8-g, but then returned to its initial configuration once weightless conditions were restored. Measurement resolution over the extent of the membrane varied between approximately 0.0001 – 0.001 m, which was not as good as that typically obtained using close-range photogrammetry. This discrepancy was likely due to the non-ideal camera positions and orientations necessitated by the space restrictions aboard the KC-135A.

The Incredible Shrinking Dewlap: Signal Size, Skin Elasticity, and Mechanical Design in the Green Anole Lizard (Anolis Carolinensis)

Abstract The expression of male secondary sexual traits can be dynamic, changing size, shape, color, or structure over the course of different seasons. However, the factors underlying such changes are poorly understood. In male Anolis carolinensis lizards, a morphological secondary sexual signal called the dewlap changes size seasonally within individuals. Here, we test the hypothesis that seasonal changes in male dewlap size are driven by increased use and extension of the dewlap in spring and summer, when males are breeding, relative to the winter and fall. We captured male green anole lizards prior to the onset of breeding and constrained the dewlap in half of them such that it could not be extended. We then measured dewlap area in the spring, summer, and winter, and dewlap skin and belly skin elasticity in summer and winter. Dewlaps in unconstrained males increase in area from spring to summer and then shrink in the winter, whereas the dewlaps of constrained males consistently shrink from spring to winter. Dewlap skin is significantly more elastic than belly skin, and skin overall is more elastic in the summer relative to winter. These results show that seasonal changes in dewlap size are a function of skin elasticity and display frequency, and suggest that the mechanical properties of signaling structures can have important implications for signal evolution and design.

Experimental Characterization and Modeling of Dynamic Behavior of Semi-Rigid Thin Film Polyimide Panels

Stiff, ultra-lightweight thermal-formed polyimide panels considered here are examples of next generation gossamer structures that resolve some of the technology barriers of previous, membrane-dominated gossamer designs while maintaining their low mass and low stowage volume characteristics. The research involved dynamically characterizing and modeling several of these panels to develop validated computer models which can be used to determine the effects of changing manufacturing parameters and scalability. Modal testing extracted the first four structural natural frequencies, the first occurring at 71.9 Hz. These data were replicated by simple, coarsely-meshed shell element finite element models that are significantly smaller than previous finite element models of similar structures. Overall, the research contributes to the total knowledge base of gossamer technologies, advances stiff panel-based structures toward space qualification, and demonstrates their potential for use in apertures and other spacecraft.

Global Static Testing and Model Validation of Stiffened Thin-Film Polyimide Panels

Stiff, thermal-formed polyimide panels are examples of next generation space structures that address some of the issues of membrane-dominated ultra-lightweight structures while maintaining their low mass and low stowage volume characteristics. The research presented here involved statically characterizing and modeling two of these panels both 0.0625 m, one with a mass of 38 g and one with a mass of 25 g – to develop validated computer models which can be used to determine the effects of changing manufacturing parameters and scalability. Static bending tests showed linear full-panel behavior over a range of applied loads for two different polyimide film thicknesses. These data were replicated by simple, coarsely-meshed shell element finite element models that are substantially smaller than three-dimensional models that faithfully represent the detailed internal geometry of the stiffened panels.