Civil Engineering ETDs

Ran Fu

Publication Date

9-3-2010

Abstract

Precision astronomic structures such as telescopes often require structural materials that possess ultra low coefficient of thermal expansion (CTE) and coefficient of moisture expansion (CME) so that strict dimensional stability requirements can be met. Composite materials such as carbon fiber reinforced epoxy (CFRE) composites can meet those requirements because of their nearly-zero CTE and potentially low CME. To employ these composite materials for telescope structural design, it is necessary to first develop practical and economical methods to determine their CTE and CME. Although many previous studies have discussed CTE and CME measurements of various materials, none of them suitably serve the purpose of measuring environmentally-induced deformations of in situ telescope structural support members in terms of feasibility, repeatability and economy. Two metrology techniques, suitable for measuring small deformations of large in situ telescope structural support members, have been developed for determining the CTE and CME of carbon fiber reinforced epoxy (CFRE) telescope components. Both are relative (rather than absolute) techniques, measuring the axial deformation of the telescope component with respect to a known reference standard. Two techniques are described: the single-mirror optical lever (SMOL) and the double-mirror optical lever (DMOL). In the first method, the temperature of the test component is varied while the reference standard is maintained at constant temperature, while in the second method the temperatures of the test component and the reference standard are varied together by changing the ambient temperature surrounding both. Quantitative CTE and CME results are reported for CFRE rods. Also, error analysis including random errors and systematic errors are discussed for each measurement. The magnitude of the error reflects the accuracy and reliability of the techniques. The DMOL technique is shown to be a significant improvement upon the first and also more practical and economical than comparable techniques reported in the literature. We believe this DMOL method is superior to metrology methods for large telescope components published to date.

Keywords

Telescopes--Materials--Testing, Expansion of solids--Measurement, Fibrous composites--Testing, Epoxy compounds--Testing, Structural analysis (Engineering)

Sponsors

Air Force Research Laboratory (AFRL)

Document Type

Thesis

Language

English

Degree Name

Civil Engineering

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Maji, Arup

Second Committee Member

McGraw, John

Third Committee Member

Murphey, Thomas

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