AIRFRAME Figure 4
From here we can properly mix any amount of resin because we know that 70 percent of the total weight is Part A and 30 percent is Part B.
Fiber/Resin Ratios
For optimal performance, composite laminates should contain a specifi c percentage of resin by weight. We call this the fi ber/ resin ratio. For example, a common aerospace fi berglass fabric (style 7781) performs best if approximately 60 percent of
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the weight of the laminate is fi berglass and the remaining 40 percent is the epoxy resin matrix. This fi ber/resin ratio would be expressed as 60:40. Having signifi cantly more resin than this does not make the laminate stronger — it only makes it heavier and stiff er. Having signifi cantly less resin than desired causes some of the laminate’s physical properties to fall off quickly, most notably its compressive strength. Because carbon fi ber weighs less than glass, desirable fi ber/resin ratios tend to be closer to 45:55. For aramid fabrics, which are lighter still, you’re looking for about 40:60. Simple enough, right? Here’s the problem: if you mix up a batch of resin equaling 40 percent of the fi nished laminate, you’ll quickly fi nd out that it just isn’t enough to wet-out the repair material suffi ciently. Generally speaking, you’ll fi nd that the amount of resin you need is at least half again the weight of the fabric. Expressed as a fi ber/resin ratio, that’s 40:60. So, using fi berglass as an example again, how do we get from 40:60 to the desired 60:40 as the resin cures? The answer is as much art as it is science. Vacuum bagging has been used to apply pressure to repairs of all sorts for decades. A simple vacuum bag like the one seen in Figure 4 actually performs two tasks for us in a repair. First, the use of a vacuum bag allows for the extraction of air and volatiles from the resin. Second, it provides uniform pressure to the patch as it cures. The physics involved are fairly simple. By drawing the air out from inside the bag, atmospheric pressure pushes down on the outside of the bag. Atmospheric pressure is about 14.7 psi (22 in/hg) at sea level, so you can see how we can develop substantial, uniform pressure on our repair area using lightweight materials. Between the vacuum bag and your patch, there will be
a bleeder schedule and a breather. The bleeder schedule is a series of materials applied to the patch in such a way that they serve as a conduit allowing gasses to escape, while also wicking away the resin in excess of our desired 40 percent (Figure 5). The fi rst of these materials, called the release layer, is always made from a material that will not stick to the resin being used, thus the entirety can be removed after the cure is complete. The breather often covers the entire repair area under the vacuum bag. The breather is simply a woven or non-woven material that helps evacuate the air from the bag and distribute the vacuum pressure evenly. In a perfect world, the repair procedures would clearly
defi ne how much resin to start with, and specify a bleeder schedule that would consistently leave you with the desired amount of resin in your repair. Unfortunately, some repair manuals off er precious little guidance on bleeder schedules, and those that do sometimes yield questionable results. In these cases, I’d recommend contacting the OEM before proceeding unless your repair station already has procedures in place authorized for use on the part in question.
Learn the “Rules”
There at least as many “rules” or standard practices concerning composite repair as there are for sheet metal repairs — certainly too many to list here. Hopefully I’ve given you a glimpse of some of them and how they correspond (or not) to the standard practices for sheet metal repairs with which we’re
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HelicopterMaintenanceMagazine.com October | November 2015
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