| Autor: |
Gabriel J. Garde, W. J. Sterling, Leyland G. Young |
| Rok vydání: |
2007 |
| Předmět: |
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| Zdroj: |
AIAA Balloon Systems Conference. |
| DOI: |
10.2514/6.2007-2607 |
| Popis: |
This paper presents a study of thin-film strain measurements using photogrammetry. The experimental work covered in this paper involves polyethylene thin-film of 0.038 mm thickness which is primarily used in NASA super pressure balloons. Accurate strain measurement of thin-films requires a non-contacting measurement source; photogrammetry offers pseudo non-contacting measurements with retroreflective targets. Recent years have seen an increase in engineering applications using the photogrammetry software PhotoModeler ® for 3-D measurements. The objective of the work discussed in this paper was to select a numerical approach to compute the biaxial strains from the PhotoModeler ® 3-D spatial coordinates and empirically validate the accuracy of the strain measurements at different temperature regimes. Surface fitting, spline fitting, and linear approximations are the numerical approaches examined to compute the curvilinear distance between points of doubly curved surfaces. The study encompasses the selection of the surface fitting method as the best approach for any arbitrary doubly curved surface. The accuracy validation of the strain measuring approach includes the measurement of the coefficients of thermal expansion (CTE) for the polyethylene film and comparing the values with an independent source. Strain measurements at lower temperatures require the film to be inside of a thermal chamber and pictures taken from the outside through a plexi-glass door. As a corollary to the CTE measurements, the taking of photographs through the plexi-glass door of the thermal chamber was shown to have negligible effects on strain measurements. The overall accuracy of the photogrammetric strain measuring process is demonstrated with the comparison of measured biaxial creep data and measurements from conventional Vernier scales and finite element analysis results. In conclusion, the surface fitting approach taken to extract biaxial strains from the PhotoModeler ® 3-D spatial points is capable of |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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