Hygrothermal stability of laminated CFRP composite mirrors

摘要

This paper is intended to address accuracy issues associated with hygrothermal stability of ultra-lightweight composite mirror structures. Hygrothermal stability of a mirror is ultimately defined as its optical performance when subjected to temperature or moisture variations. Stability is dictated by a combination of material behavior and geometric configuration. Ideally, an isotropic mateiral could be used that is lightweight, has high stiffness, and has no response to temperature or moisture variances. This type of material would therfore be independent of geometry. Quasi-isotropic (QI) laminated CFRP composite materials offer most of these characteristics, but are transversely isotropic with near zero hygrothermal response in the plane of the laminate and a relatively high response through the thickness. Typically, mirrors made from laminated material consist of a thin curved shell supported by an array of ribs. Interference problems arise at the rib/shell interface resulting in a "print-through" effect by the ribs. Also, adhesive used to bond the ribs to the shell pull the shell causing additional "print-through". Additional sources of instabilities result from material variances, processing, and assembly. Theses multiple sources of instabilities superimpose onto each other resulting in the structures overall hygrothermal stability.