2001 Carbon Vs. Kevlar Article

May 25, 2001

To:       Peter D’Anjou, Executive Editor, Sailing WORLD Magazine
From:   Steve Haarstick
Re:       New Carbon Laminates vs. Kevlar Laminates

Dear Peter,

Thanks for your call yesterday requesting some information on the relative durability and strength of the new Carbon Laminates that have been developed this past year for Grand Prix and PHRF racing.

As we discussed briefly, the best cloth choice for any sail is dependent on many conflicting factors, even if we limit the selection to only racing sails. The initial strength of the fabric, as defined by its ability to carry high loads at comparatively low stretch is not the most important factor in choosing the best fabric. Kevlar laminates are extremely low stretch when they are new and unused. I have seen graphs from Dimension Polyant (dp) from 1992 that show a Kevlar sample loosing 80% of its initial strength after 12 months of Ultraviolet (UV) light exposure (graph not available). While this is more than would be encountered in the normal life of a racing sail, it does illustrate one of the significant factors that can degrade the performance of Kevlar with use. Earlier Kevlar laminates with high counts/inch of smaller denier threads, i.e. woven Kevlars and some light weight laminates using 400 denier Kevlar threads were very prone to tears and sail blowouts after a relatively short exposure.

As a result of this significant loss of strength, many cloth manufactures began to increase the denier size in each thread bundle, while reducing the number of bundles (count per inch). The larger diameter bundles exposed less underlying Kevlar fibers for the same amount of surface area, and these laminates showed better resistance to UV Exposure. Lately, there has been efforts made to additionally shield the Kevlar by using UV retardants in the glue, polyester taffetas on the outside of the film laminate, and/or colored films that block out a portion of UV.

Additionally, The loss of strength of any fabric due to flogging, impacting shrouds while tacking, being repeatedly folded and creased, can be surprisingly high, and can vary substantially not only between different types of laminates, but can also vary from one batch of cloth to another batch of the same type of cloth produced by the same cloth manufacturer.

While we don’t have a special UV accelerated tester, we do structural testing on every batch of upwind cloth including laminates before we purchase. Our Instrom tester produces graphs of elongation versus load for four 2” wide strips cut from the cloth sample sent to us for approval, prior to final roll shipment. We also cut and Impact Flutter four more strips and then test them on the Instrom machine. We examine the loss of strength after the impact flutter test, as we feel that this data is much more important to us than just the “new” fabric tests. We believe that the better cloth is one that shows the least loss of strength after our flutter testing. This cloth might not be the lowest stretch when new, but if it retains more of this strength after being impact fluttered, there will be less change of shape, and less chance of overload (stretched past it’s elastic yield point) after a reasonable amount of usage on the boat.

I have enclosed three graphs comparing different weights of the Carbon laminates (GPL 7, GPL 14, & GPL 21) with 3 weights of Kevlar with the “Magna-Shield” Amber film (E09 MS, E140 MS, E180 MS), two weights of regular Kevlar (E06, E22), and finally two weights of carbon reinforced Kevlar laminates (EC 13, EC 18 - now dp’s AC 15 & AC 20 cloth lines respectively).

Each of these graphs have two bars, one for the thread line tests (strength direction), and the second for the strips cut at an angle of 10-degrees off thread line. Graph #1 “LOAD RANGES FOR NEW STRIPS,” is a plot of the load it takes for each 2-inch strip to reach 0.75% elongation for the thread line strip, and 1.0% elongation for the 10-degree off thread line strip. Except for GPL 7 vs. E06, the load ranges for GPL 14 and GPL 21 are somewhat less than the equivalent Kevlar, or carbon reinforced Kevlars. This is most pronounced on the thread line strips, but is also the case, to a lesser degree on the 10-degree strips.

Graph #1 – Load Ranges @ Threadline & 10 Degrees Off for New Strips. (Click To Enlarge)

AFTER our impact fluttering procedure, we tested the strips and plotted the resultant data in Graph #2 “LOAD RANGES FOR IMPACT FLUTTERED STRIPS.” This is a plot of the load it takes to reach 1.0% elongation on the thread line strip and 1.25% on the 10-degree strip. Similarly, the heavier Carbon samples show lesser load ranges compared to the equivalent Kevlar, or Carbon reinforced Kevlars.

Graph #2 – Load Ranges for Impact Fluttered Strips. (Click To Enlarge)

At this point, you might be ready to write off these Carbon laminates as inferior to the existing Kevlars, but the Graph #3 “IMPACT FLUTTER STRETCH AS MULTIPLES OF NEW STRIP STRETCH,” shows a very interesting result. The thread line test of the Carbon laminates show less increase in stretch, (less loss of strength) after impact flutter than any of the other samples tested. While the other thread line samples show 2.2 to 3.8 times the stretch after flutter, the Carbon samples range from a low of 1.4 to a high of 2.3. This is a dramatic improvement versus the Kevlar and Carbon reinforced Kevlar samples. The 10-degree off thread line strips were also somewhat better than the Kevlars, except for the heaviest carbon sample GPL 21.

Graph #3 – Impact Flutter Stretch as Multiples of New Strip Stretch. (Click To Enlarge)

One conclusion that can be drawn is that although the Carbon laminates still have relatively low stretch compared to any non-Aramid laminates (Mylar, Pentex, Vectran), they are not as low stretch as the Kevlar laminates and Carbon reinforced Kevlars. Carbon laminates could prove to retain more of their original strength after impact flutter. Less change of stretch after impact flutter almost always results in less change of sail shape with use on the boat. As long as the load limits of the sail is kept within the slightly lower limits of the Carbon laminates, this could result in sails that show less shape change as the racing season progresses, and for “High Modulus” laminates, this would be an improvement in durability. Time on the water should hopefully confirm this conclusion.

I hope this information adds to the discussion.

Sincerely,

Steve Haarstick
President, Owner, & Designer
Haarstick Sailmakers, Inc.

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