Temperature Insensitive Curved Bimetal Elements

Space structures are subjected to appreciable temperature variations when they enter and leave direct solar radiation zones. These variations cause thermal strains and related structural distortions. Such deformations are often a nuisance and much engineering e ort, such as active control methods, is currently being invested in reducing the deformations and alleviating their e ects. This paper will present a novel structural concept which can reduce thermal distortions and in many cases eliminate them. The idea hinges on curved bimetal elements where the coe cient of thermal expansion of the outer layer is larger than the coe cient of the inner layer. When the element is heated the element expands axially and the curvature increases due to the fact that the outer layer tends to elongate more than the inner one. The combined action of these e ects often cause reduced apparent expansivity and in some optimally designed cases the apparent expansivity can be reduced to zero. The paper develops a general theory of bimaterial curved 3D elements under thermal loading. From here two cases are investigated, a planar bimaterial curved element and a bimaterial helix. It is shown that under a particular choice of the design parameters the chord-length of the 2D circular element is insensitive to a uniform temperature variation. In the case of 3D curved elements the paper shows that one can design bimaterial helices for which the axial distance of any two point on the helix is independent of temperature. The theory is illustrated by numerical examples.