AIRFRAME
of course, but in order to make sense out of the repair procedures we have to take into account the fundamental differences between metals and composites. Let’s use a simple punctured skin in a semi-monocoque
structure as an example. With metals (isotropic materials), we simply follow some old, familiar and long-established “rules” to:
1. Clean up the damaged area. 2. Select a patch material — same alloy and thickness (or one thickness greater).
3. Select the appropriate number, alloy, diameter and length of rivets.
4. Lay out the rivet pattern, observing rules concerning pitch, transverse pitch and edge distance.
5. Install the patch. I left out some detail, but you get the point. Most of the
Figure 1 Strength-to-Weight Ratios
Advanced composites are all about strength-to-weight ratios, or how strong a part is relative to its total weight. When the newest model of an old, familiar airframe makes its debut sporting an all-composite tail boom, one of the first things you might hear is that the new structure is stronger than the metal version. This is a common misconception. Assuming the loads on that tail boom haven’t changed, the new composite tail boom won’t be any stronger than the old one, but it will be lighter and sometimes dramatically so. A significant decrease in weight does a couple of things
for us. First, it shifts the c.g. forward. This is a huge advantage on any aircraft with aft c.g. problems. If you’re like me, you can probably think of several aircraft right off the bat that could benefit from such a modification. Second, it decreases the overall weight and results in an increased useful load, range and a general (but sometimes subtle) increase in overall performance.
One of the major reasons for this substantial decrease in
weight stems from the fact that metals are isotropic materials, meaning that their strength is essentially the same in every direction. On the other hand, advanced composites only exhibit strength in the directions in which fibers run. This means that by orienting the fibers in specific directions, we can engineer the part to be strong along all the axes in which we need strength. Conversely, we reduce or eliminate fibers in directions in which strength is not necessary. The end result is a part that only has strength in the directions it requires to perform its function, thereby saving weight.
Repair of Advanced Composite Structures
As the manufacturing paradigm shifts from aluminum to composite structures, so must our understanding of how these materials are repaired … lest we find ourselves unemployed. From our first class in repair of metal structures, it’s obvious that what we’re trying to do is restore the load path to the damaged area. The same is true for composite structures
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HelicopterMaintenanceMagazine.com October | November 2015
standard practices for sheet metal repairs date back almost 90 years. A good structures mechanic can recite the formulas standing on his or her head. Advanced composite materials have “rules” too. It’s just that most of us aren’t as well versed in these rules as we are in those pertaining to sheet metal. Once we begin to understand these new rules, we begin to understand what we’re being asked to do in a composite repair procedure, and how it differs from what our norms. In a similar repair to a composite skin, we would also start
by removing the damaged material. Not too surprisingly, the end result would look essentially the same as with a metal structure — with gently radiused corners, or preferably an oval or circular shape in order to avoid any stress-risers that might cause cracking later. The major difference here is that the cutting/machining tools used for composite materials are often different than those used on sheet metal. While in both cases we might use a 90-degree die grinder, the “bit” or cutting tool itself would need to be a diamond abrasive router bit (for glass and carbon), or a router bit designed specifically for cutting aramid laminates. Now, since we know that a composite laminate transfers loads based on the orientation of the plies within it, it’s obvious that any patch used to repair that laminate must have
Figure 2
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