I have a question about tensile modulus. Being relatively new in this topic I hope someone with more experience can help me.

The tensile modulus of a finished blank isn’t the same as the tensile modulus of the carbon fibre in the blank, I suspect the difference to be quite big.

For example IM7 graphite have a tensile modulus of around 44 million PSI but how much is the tensile modulus of an average blank made of IM7?

Of course, this may vary between different manufacturers and depending on fibre content and resins used. If no one knows for sure I am grateful to anyone giving me a qualified guess on how big the difference might be in average. Is it 2 or 3 or even 4 times?

It's going to vary based on a number of varibles. First, the modulus figure given may or may not be for the prepeg used in the blank - it may be for the fiber itself. Next, the particular design (thin-wall, large diamater versus thicker-wall, smaller diameter) will play a large role.

I think the answer that most would give you is that the modulus of the finished structure, if you want to phrase it that way, is going to be about half that of the figure given for the fiber used.

By the way, the modulus of elasticity and tensile strength are two different things.

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A cross section through a blank reveals fibers in a matrix of resin. The resin has a very low modulus compared to graphite. The effective modulus of the finished blank then is the modulus of the carbon fibers times the fraction of the cross section area of the blank material that is graphite. It will be less than that of fibers themselves. I expect that a rod designer could play tunes with that by varying the proportion of graphite fibers and resin at different points in a blank. The temptations of advertising being what they are, I am pretty sure what is advertised is the modulus of the fibers, not the blank. I believe tensile modulus is a correct term, referring to modulus of elasticity measured in tension. It could also be measured in compression. There is also a shear modulus relating to elastic deformation at right angles to an applied force.

Mike

Thank you Tom,

Half the figure seems reasonable. A blank made of IM7 fibre will then have a tensile modulus, or modulus of elasticy(not tensile strength), of around 22 million PSI.

I know the design of the blank is very important for it's performance but I can't see that it effects the modulus, the modulus being a material property?

For the sake of the argument. If you design one blank from a finished structure with a modulus of elacticy of 20 million PSI and then design another blank with a modulus of 10 million PSI with a bending resistance (W, cubic inch) which exactly compensates the difference in modulus along the entire blank(twice the bending resistance?), will you get the same performance given that the weight and cross section area are exactly the same for both blanks? (The only difference beeing diameter and wall thickness)

Or is it more complicated than that?

There have been many arguments over whether or not the structure itself has a "modulus." In any case, I understand what you're asking and it has to do with the relationship between the number that is advertised and what you actually have there in a finished blank. In most cases, the number that is advertised is for the fiber itself, not even necessarily the prepeg (fiber and resin combined) and certainly not the finished blank or rod.

There are other ways to look at this as well. One reason for using a higher modulus fiber is to build a blank is more efficient - has the same stiffness or power at less weight. But even that isn't a given. All things are not equal. In order to offset some of the natural problems with reduced "toughness" in the higher modulus blanks, some designers use a very heavy-wall, small diameter design which can actually lead to a blank which is less efficient and heavier for the same stiffness than one made from a lower modulus fiber.

I've always thought that a better way to compare/advertise blanks would be on the basis of stiffness/power to weight of the final structure. Anything else isn't likely to really tell you that much about what you're looking at there in the catalogs.

I understand the term "tensile modulus" but had assumed (perhaps wrongly) that you might have been asking about general tensile strength as well.

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I agree with you, some kind of standardisation would help a lot.

The blank is such a crucial part and also a great deal of the total cost for graphite rods. I would appreciate if I could have a fair idea of a new blank before ordering it. I also think it affects the degree of craftsmanship in a pretty high extent not knowing the actual composition and what effect it will have on the finished rod.

If there were ways to customize the rod blank to each demand, by a standard module based system were you put together different blank sections or that there were ways to make, or specify, the blank yourself without having to start an industry behind it I think the rod building would come into a whole new dimension. Maybe it’s hoping for to much?

If the manufacturers would adopt the Common Cents ERN power ratings, and then add back the blank weights (most of them pulled this info in the past few years) we could arrive at a pretty good guess and comparison of blank to blank in terms of overall efficiency. Resonant frequency, or the more easily done and understood Common Cents Frequency would be another great indicator of what we're looking at, but for obvious reasons many manufacturers would rather that you not be able to compare their blanks to those of a competitor.

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