Controlling thermal deformation through the use of lattice structures

Aerospace vehicles, both aircraft and spacecraft, experience significant changes in temperature during the regular course of operation. Often the structures of these vehicles include joined components that are composed of... [ view full abstract ]

Aerospace vehicles, both aircraft and spacecraft, experience significant changes in temperature during the regular course of operation. Often the structures of these vehicles include joined components that are composed of different materials. When temperatures change, this typically results in stresses due to the mismatch of coefficients of thermal expansion. This paper describes a type of lattice structure that mitigates or eliminates thermal mismatch stresses through a combination of geometry and material. The lattices are composed of non-identical bimaterial cells. Specifically, the two materials should have a large ratio of coefficient of thermal expansion (CTE). Each cell consists of a skewed hexagon of one material surrounding an irregular triangle of the other material. In general, each lattice cell has three tailorable CTEs arranged as a triangle and hence the lattice can be strongly anisotropic. Depending on how the two materials are arranged, the net CTE of the lattice can be artificially increased, decreased or tailored to provide varying CTE throughout the lattice. If all the joints between the parts of each cell, neighbouring cells, and the lattice and the substrates are pinned, the whole structure will be free of thermal stresses. This paper explains the design process by which lattices with desirable thermal expansion properties can be designed. As examples, the design of one-row lattices, two-row lattices, angled lattices, and non-planar lattice (which can be used to connect concentric circular rings) are described. One-row and two-row titanium-aluminum lattices are of particular interest because titanium and aluminum are the most common materials in the aerospace structures. Without some optimization, the lattices are typically very compliant; this paper discusses a method to improve the lattice cell design to increase the lattice stiffness. A general algorithm for lattice optimization in terms of structural efficiency is elaborated and illustrated by several examples of lattices including a polygonal ring connector to prevent distortion in cylindrical optical components. While the lattice designs are intended specifically for reduction in thermal mismatch, the concepts can also be used for the design of thermally-driven actuators such as thermal switches and valves. Also, the lattices can be used to control the maximum total deflection of two adjoining parts of an aerospace vehicle.