Summary of Key Concepts
Packaging includes any technology designed to protect, contain, display, dispense or transport something. Every package is an example of a structure. A structure is anything designed to keep objects in place, by resisting the loads that would try to shift them. Some structures are large, complex, and expensive, such as bridges, towers and buildings. However, most structures are fairly simple, cheap and easy to find. These include bags, boxes, furniture, shoes, ladders, wall fixtures, and items of all kinds that have been glued, taped, stitched, stapled, sewn or woven.
Most of the time, structures are taken for granted. Only when a structure stops doing its job - when it fails - do most people think about how the structure is supposed to work. A structure fails when it experiences loads (or forces) that are beyond its capacity. The loads that bear upon a structure fall into three main categories:
- tension forces, which pull on a structure, trying to make it longer;
- compression forces, which push parts of a structure together, tending to make it shorter and fatter;
- shear forces, which shift opposite sides of a structure in opposing directions.
Different kinds of structures are designed to resist these three types of loads. Tension structures are often suspended from a point higher up, and designed to resist gravity. String, rope, wire, cable and chain are strong in tension, so they are often used for this purpose. Hammocks, light fixtures, clotheslines and belts are all examples of tension structures. Compression structures typically rest on the ground or floor. They resist loads by "refusing" to be squashed appreciably. Some examples are the legs of a table, chair, or person; boxes supporting other boxes; canes, tires, and tree trunks. Points of attachment are generally designed to withstand shear. If one thing is not attached to another securely, they may shift in opposite directions, and the shear forces could win. A broken shoulder bag strap, which has pulled away from the bag, is an example of shear failure. Other examples occur when one side of something is moving and the other side is stationary, as with a broken CD case hinge, a torn-out shoelace, and the sheared- off side-view mirror of a car.
Most loads are not pure examples of tension, compression or shear, but involve some combination of the three. For example, a shelf works as a beam, supported at both ends, and subjected to a load somewhere in the middle. Because of the way it is supported and loaded, the top surface of a beam is compressed, while the bottom surface tends to get longer, and is therefore loaded in tension. For this reason, a broken craft stick has short broken fibers on the side where it was pressed, and much longer jagged fibers on the opposite side, which was in tension. Shear failure occurs here too, because these jagged fibers have to slip out of the natural adhesive that was holding them to the rest of the wood.
When a structure fails, it is not generally hard to determine whether it failed in tension, compression or shear. This analysis can be the basis for redesigning the structure to make it stronger, so it will be able to resist a larger load. To illustrate this point, we will discuss some of the failure modes of shopping bags. In each case, the type of failure suggests a remedy for strengthening the bag.
Flimsy plastic shopping bags usually fail when one of the handles tears apart. Because it was due to pulling forces, this is an example of tension failure. When a handle of a paper shopping bag pulls out of its holder, it is a case of shear failure. Sometimes, the bottom of a paper bag gives way. The bottom of a bag functions as a beam. While its top surface is in compression, the bottom surface is subjected to tension. In some places, the paper might tear, which is an example of tension failure. In other spots, the glue might stop holding the two flaps together. Because the glue is supposed to keep the flaps from sliding in opposite directions, the weakening of the glue is an example of shear failure.
