It is a general term for the sheet structure which is surrounded by the inner outer surfaces whose thickness t is far less than the small curvature radius r of the middle surface the plane size.
At present, there are mainly the following categories in the market: plastic shell, aluminum shell, sheet metal shell, stainless steel shell, etc. Thin shell refers to shell with T / R less than 0.05.
The shell resists the external load mainly by the middle plane stress distributed uniformly along the thickness, rather than the bending stress varying along the thickness. This internal force characteristic of shell makes it make full use of material strength more than plate, so it has greater bearing capacity. In water conservancy engineering, shell is widely used, such as double curved flat shell gate, arch dam so on.
The theory of shell belongs to the category of applied elasticity. It needs to introduce new hypothesis besides the basic hypothesis of elasticity. It includes thin shell theory medium thick shell theory. Thin shell theory is a classical theory in shell. It is based on the assumption of straight normals. These assumptions are as follows: ① the positive strain perpendicular to the middle plane is small can be ignored. ② The normal of the middle plane is kept as a straight line, the right angle between the normal of the middle plane its vertical line segment is kept unchanged, that is to say, the shear strain of the two directions is zero. ③ The normal stress on the section parallel to the middle plane (i.e. extrusion stress) is far less than that on the vertical plane, so its effect on deformation can be ignored.