What Is a Shell?

A shell is a curved, thin-walled structure. A quantitative definition will be given in later chapters. Two important degenerate classes of shells are plates (shells which are flat when undeformed) and membranes (shells whose walls offer no resistance to bending). Shells may be made of a single inhomogeneous or anisotropic material or may be made of layers of different materials. The primary function of a shell may be to transfer loads from one of its edges to another, to support a surface load, to provide a covering, to contain a fluid, to please the eye or ear, or a combination of these. Shells occur in nature and as artifacts and include aortic valves, automobile hoods, balloons, beer cans, bellows, bells, bladders, bowls, contact lenses, crab carapaces, diaphragms, domes, ducts, egg coverings, footballs, funnels, inner tubes, light bulb casings, loudspeaker cones, manhole covers, parachutes, peanut hulls, Ping-Pong balls, panels, pipes, pressure vessels, silos, skulls, straws, tents, tires, trumpets, umbrellas, vaults, wine glasses, and woks. The aim of shell theory is to describe the static or dynamic behavior of such structures by equations that involve no more than one or two spatial variables.

Elastic Shells and Nonlinear Behavior

If a shell is prevented from moving as a rigid body, then it is elastic if, upon application and removal of a sufficiently small load, it tends to return to its initial shape.