Honeycomb Is Not Isotropic!
Honeycomb is used extensively in composite panels to add bending stiffness with very little mass penalty. The panel facesheets carry tensile and compressive loads, and the honeycomb carries transverse stresses. In a highly loaded panel, the transverse stresses may approach the strength of the honeycomb. Thus, it is important to use the correct properties when designing a panel.
This issue comes up in the shop every once in a while, and I am always surprised when someone does not realize that honeycomb transverse shear properties are dependent upon direction. The two in-plane directions on a panel are called the "L" or ribbon direction and the "W" or transverse-to-ribbon direction. The shear modulus and strength in the ribbon direction are roughly twice that in the transverse direction.
These directions may seem difficult to keep track of, but it is really very easy. All you need to know is how honeycomb is manufactured. After that, you will be able to look at a panel and instantly pick out the ribbon direction, and you will have no trouble remembering which direction is the strongest and stiffest.
Honeycomb starts out as flat strips of material, or ribbons. Strips of adhesive are placed on the ribbons in a staggered pattern.
Next, the sheets are stacked together and cured.
Once cured, the blocks are cut to the desired thickness, and then the ribbons are pulled apart or expanded to form honeycomb.
Fully expanded honeycomb forms the familiar hexagon shape. The expansion can be stopped before the hexes are fully formed (underexpanded core) or stopped after the hexes are fully formed (overexpanded or OX core). OX-core cells actually look more like rectangles than I have drawn here.
Normal honeycomb cannot be "bent" very much to form to contours. Underexpanded and overexpanded core can be formed to moderate contours in one direction. For complex, compound contours, Hexcel makes a core called Flex-Core which actually has higher properties when curved than when flat (the shape of the cells is also not hexagonal).
By now, it should be obvious why honeycomb is stronger and stiffer in the ribbon direction: the ribbon consists of continuous lengths of material. The transverse direction, however, is held together by only relatively weak adhesive. If you can get a small piece of Nomex honeycomb, you can easily pull it apart by hand in the transverse direction, but it is difficult to tear in the ribbon direction.
You should also now be able to easily tell the ribbon direction from the transverse direction: the transverse direction is alsways normal to the adhesive strips.
If you are trying to optimize your design, you can design your panels using both the ribbon and transverse properties. I'm always worried, though, that the shop will lay down the honeycomb in the wrong direction. Thus, I usually try to design to the transverse properties. In almost all cases, there will be little or no weight penalty to this approach.