A unit load devise $($ULD$)$ is a container or a pallet for transporting air cargo. The material of

ULD is often aluminum and the dimensions, including those of the bottom plate, are provided for

your variant $($these are standard dimensions for ULD$).$ You are required to design a

symmetrically laminated cross$-$ply composite plate of the bottom of ULD for the load exceeding

that given in standard ULD specifications $($see Table $1).$ The goal is to minimize the weight of

the structure to facilitate its handling at the airports. The load $($maximum gross weight$)$ is

assumed uniformly distributed over the bottom of the ULD. The plate of the bottom is assumed

simply supported along all edges. The material of the plate is specified in your variant. The

properties of the material should be taken from Project $1.$ Note that for ULD, expensive

carbon$/$epoxy is not necessarily the material of choice, i$.$e$.,$ you will see a less expensive

glass$/$epoxy in many variants. The thickness of every layer is equal to $0\mathrm{.}25$ mm$.$ Both the

maximum principal stress and the Tsai$-$Hill strength criteria should be used to determine the

required thickness of the laminate.

Comments:

$1.$ The weight of the ULD structure can be neglected compared to the load carried by ULD.

$2.$ The material is prescribed in your variant. You can vary the number of layers in the $0$o$-$

and $90$o$-$directions, and the thickness of ULD. A more conservative criterion should be

adopted for the final design.

$3.$ Find the principal stresses in each layer of the composite plate to check its strength. The

largest tensile and the largest compressive stresses are acting at the opposite surfaces of

the laminate $($the top surface is in compression and the bottom surface is in tension$).$

Accordingly, you have to only check one outermost $0-$degree layer and one outermost $90-$

degree layer at each surface $(4$ layers total to check$).$ The terms you have to retain in

double Fourier series for deflections and stresses to achieve a good convergence of these

series are with m$=1,3,5$ and n$=1,3,5.$

$4.$ It is quite possible that even as the strength requirement is satisfied in your design,

maximum deflections are too large. Accordingly, if the maximum deflection exceeds

twice the thickness of the plate, increase the thickness until the deflection remains below

twice its value. While large deflections imply that the problem becomes geometrically

nonlinear, using the linear solution is conservative, i$.$e$.,$ your design is safe.

$5.$ As you can observe in Figs. $2$ and $3,$ the bottom plate of ULD is supported by

longitudinal stringers in the cargo deck of the plane. However, designing for multiple

types of cargo planes, we cannot predict the spacing between these stringers as well as

the support of the bottom during handling of ULD outside the plane. Accordingly, we

assume that the bottom plate is supported only by light beams at all $4$ edges and neglect

possible additional supports. This is the rationale for the present design problem:

rectangular plate supported only along its edges.