January 8th, 2001

What the *%@# is Modulus?
By James Castwell


First off, I want to thank one of our readers for the email; it read something like this: "I am new to fly fishing and do not understand what 'modulus' is. Can you help me?"

Well sir, I am not new to fly fishing but I felt I could not properly answer the question. (I did answer his email, I told him to please watch this column for the answer.) This has been a fun task. I called several rod makers and told them I was writing this and why and asked them, if they wished, to give me their definition of what modulus is.

I did consider calling a few 'fly-shops' for their ideas, but felt that may lead more to the problem than the solution. Here are, in no particular order, the responses.

When you called and asked me to write about modulus in one or two paragraphs, I wondered if it was possible? Modulus, for history buffs, was coined by Thomas Young (cerca 1810), an English physician and physicist. He gave the word energy its scientific import, and Young's modulus, a constant in mathematical equation explaining elasticity was born. For design engineers, this meant that at low tensile stresses, there is a linear or proportional relationship between stress and strain (known as Hooke's Law) and the slope of the strait line is (you guessed it) modulus of elasticity. In the 21st century, the above equations are critical for advanced composites which utilize a combination of resins and fibers, customarily carbon/graphite, kevlar, or fiberglass with an epoxy resin.

The fibers provide the high stiffness, while the surrounding polymer resin matrix holds the structure together allowing infinite design capability. In general, carbon composites are two thirds the weight of aluminum, and two and a half times as stiff.

All this means that graphite composites as we know them have a high tolerance to bending. The higher the modulus, the higher its tolerance. Graphite is resistant to fatigue, and can handle harsh temperature fluctuations. Perfect for the walk-in fisherman--right? To you fishermen out there: a rod touting a higher modulus rating will be stiffer or bend less than a rod with a lower modulus rating. Be careful though. Remember that these composites have an infinite design capacity.

That means marketing has unlimited means with which to present modulus. Since there are no industry standards that I am aware of, there is no way to compare apples to apples, i.e. the modulus of say IM6 to one manufacturer will not be the same as the modulus of IM6 used by another manufacturer.

Of greatest importance in my opinion, is the design of the mandrel used to manufacture the blank and the quality assurance of the end product based on the composite being used. Rather than talk modulus, I like to talk performance with my customers. Under what conditions are you fishing? What are you fishing for? What are the characteristics of your ideal rod in this given situation. Now I know what the customer wants. Forget modulus, you don't need to be a physicist or a design engineer. I can help them select the rod based on performance required. We can discuss power relative to stiffness of the rod desired, the action which is relative to the taper design. I use G.Loomis blanks, not because of my knowledge of modulus. I use G.Loomis blanks because I like the overall rod performance . . .power, accuracy in casting, weight, and sensitivity . . . look at the tool required for the job. ~ Mike Mattox, D&E RODS

The full definition of modulus of elasticity is a straight forward engineering term and, although lengthy, can be found in any engineering text and perhaps even the Webster's unabridged. It relates to the measurement of stiffness to weight ratio of any material. However, it is not independently related to the performance of laminates. Percent of resin (epoxy) in the blend with fiber (graphite) implies the fiber volume.

This feature (amount of graphite Vs/ resin in a laminate cross section), among other things is far more important than fiber modulus. Advertised modulus ratings are only a rating of the raw fiber. IM6 is merely a trade name for Hercules fiber of (not especially high) modulus.

IM6 is about 36 million modulus. There are many other brands of fibers in this range. However, most makers of high performance rods now employ the use of fiber with higher modulus.

The modulus of a laminate (fiber plus bonding resin) is always less than the original fiber (since it has been "diluted" by the resin component). This modulus "drop" is, more or less, varying with the percent and especially the resin used. This feature far outweighs fiber modulus alone. Our fiber, our resin type and percent mix is a recipe we prescribe for our use. As you can understand, it is therefore very proprietary and closely guarded. ~ Trevor Bross, T&T Rods

In general terms it is a measure of stiffness. The less a material moves with an applied load, the stiffer the material. The higher the modulus value, the "stiffer" the material. ~ Kevin Thompson, Sage Rods

Modulus is the measurement of resistance to strain expressed as a ratio. In context of fishing rod design, it describes the stiffness of the fiber used to construct the rod. It does not by itself determine rod stiffness, weight or strength. High modulus materials can be used to make soft rods, and conversely, low modulus material can be designed into making stiff rods. Several other important measurements of rod material to consider include fiber strength, elongation, compression rate, resin content percentage and resin toughness, scrim type and weight. The goal of many rod makers is to optimize these variables, achieve the desired action (combined taper and stiffness), high strength at low weight and reasonable cost. ~ Steve Rajef, G. Loomis

Technically stated modulus of elasticity is the ratio of unit stress to unit strain within the proportional limit of a material in tension or compression. Otherwise known as Young's Modulus. Where unit stress is force per unit area, usually expressed in pounds per square inch. It can be either tensile (pulling), compressive, or shear stress. And unit strain is the amount by which a dimension of a body changes when the body is subjected to a load. Divided by the original value of the dimension.

To an angler, modulus is stated as a value of the raw material (the carbon fibers only) used in manufacture of a rod, and can best be described as a materials ability to resist bending. Higher modulus rods that are manufactured correctly can produce lighter, sensitive rods that quickly want to return to the straight unbent state. Lighter because less material is needed to attain the same flexural strength of lower modulus graphite. Lower modulus rods typically tend to be slower to return and slightly heavier since more material is needed to attain the same level of flex (although I have cast some low modulus rods that felt light and quick). Although many anglers have gone to the higher modulus graphite, there are still other that like the feel and action of the lower modulus rods as they tend to be a bit slower and facilitate slower casting strokes.

Although modulus has become a buzz word in the rod industry there are many other issues that are just as important, and maybe more so, than the modulus of the material. Tapers (distribution of the graphite along the length of the rod), wall thickness, ferrules, scrim or no scrim, epoxy resins, manufacturing techniques, and more need to be considered in order to come up with a good casting rod, regardless of the modulus of elasticity. All the manufacturers continue to seek out higher modulus fibers to use in rod manufacturing, but in doing so must also seek out new resins and techniques to manufacture using these fibers. The search goes on. ~ Jim Lapage, Orvis

    Modulus of elasticity - Bulk

    Modulus of elasticity - Compressive

    Modulus of elasticity - Flexural

    Modulus of elasticity - Tensile

Bulk Modulus: The stiffness or flexibility of rubber, resin, or other relatively soft materials. Usually expressed in units of lb/in2".

Modulus of elasticity: The stiffness or resistance to deformation of high performance materials when stressed in their elastic range. Expressed in units of lb/in2". Also referred to as "Young's Modulus." ~ John Brazelton, Redington

I am neither an acknowledged nor professed expert in the field of graphite.  I am not an engineer, a materials mechanic or a chemist.  But, I have been involved in the fishing rod industry for almost 25 years as a private label rod maker.  I have been involved almost exclusively in the manufacture of fly rods for the past 10 years.  I have long questioned what modulus means.  As I understand the term, it is simply a designation of any material's capacity to withstand deflection, or bending without breaking.

Most any material can be tested in this fashion, but we are talking about materials used in the construction of rod blanks so we shouldn't muddy up an already cloudy subject by talking about iron bars or copper wire or whatever.  The less a material will bend before it begins to fail, the higher the modulus.  All materials used in rod making can be measured and assigned a modulus rating.  Cane has a modulus rating, fiberglass has a modulus rating, and the various "generations" of graphite have their modulus ratings.

Each new generation of graphite has had a higher modulus rating than the generations that came before.  This, to me says that if a blank builder trades material generation for generation, his blanks are becoming stiffer and more brittle with each new generation.  This has obviously not been the case.  So some adjustment for the newer materials has to have been made.  I believe that in most if not all cases, materials have been blended to arrive at a functional blank.  I haven't heard of anyone that advertises a 100% highest modulus blank.

In experiments, I've achieved no significant reduction in weight or improvement of performance in a blank by substituting high modulus graphite for standard graphite.  In fact, in some cases, higher modulus graphite does not perform as well.  In cases where a great deal of flex is a desirable characteristic, lower modulus graphite greatly enhances the blank's performance and extends it's useful life. ~ Ray Bauer, Global dorbeR Group 

So now you know, right? ~ James Castwell

Addendum: There is more information on modulus on our Bulletin Board too!

Till next week, remember . . .

Keepest Thynne Baakast Upeth

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