Emory, you'd be rich man if you could find a way to double the tensile strength of any of these carbon fibers. Tom's example built with a higher modulus, higher strength carbon fiber is actually better than what we're dealing with now since generally speaking, the fiber strength drops as you move to higher and higher modulus materials. So not only are the structures lighter, but the materials are less strong and breaking strains get smaller, so they're more brittle.

"Graphite is a great material for rods but it is really not very tough or durable and that is to a large degree because it does not have a very high tensile strength"

I agree on the lack of toughness...... but I would argue that its not necessarily the lack of a higher tensile strength in carbon fiber that limits the toughness, but more related to the breaking strain. For, example, the strength of S glass (665ksi) is within the range of many of the carbon fibers.... but the strain to break is greater by factor nearly 3X, which lead to a significant difference in the brittleness as compared to the carbon fiber/composite as you pointed out. In other words, you can have two materials with the same strength, but behave much differently due to the elongations that you can achieve at failure.

So, given the limitations of the current materials I have to agree with Tom's points about design and construction. And that's also why I think there's still some potential for differentiation in the market...St. Croix's IPC and ART would be one example.


mark

Mark,
If you look at the modulus of elasticity curve and the area under the curve, the strain energy or toughness, the only way to increase the area under the curve is to either lower the modulus of elasticity which would increase the area under the curve by increasing the horizontal dimention or increase the tensile strength, ultimate strength or the point where the elasticity becomes non-linear or becomes plastic which would increase the area by increasing the vertical dimention of the curve.
It is not just the increased tensile strength that would have a big impact on the durability of rods but also the increased strain energy that would result.
Yes, I am sure that you are correct that there are a number of very smart people, smarter than I am, who are working on increasing the tensile strength of carbon fibers. I sure wish them well because it is my belief that the biggest weakness of graphite as a material for rods is that it is not tough enough, and that a very high percentage of graphite rods end up broken as a result, particularly those built from the higher modulus fibers.

Tom,
I do not disagree with you about the design of the blank being important but I think that the trade offs for the blank designer are very limited. The stiffness of a blank goes up at the fourth power of the diameter, double the diameter and the stiffness increases sixteen times. This means that when the designer decreases the diameter in an attempt to make the blank more durable he/she must rapidly increase the wall thickness to maintain the stiffness, which also rapidly increases weight. In other words, stiffness going up at the fourth power also means that the load on the rod is being carried primarily by the outer portion of the blank, the material closest to the surface, and will drop off at the fourth power as you get closer to the center of the blank. In other words, as you get closer and closer to the center of the blank the load that the added material is carrying drops off at the fourth power so it rapidly gets to the point that it is doing little more than adding weight.

It is also interesting that the stiffness increasing at the fourth power and therefore the majority of the load on the rod being on the material near the surface plus the fact that carbon fibers are relatively soft is why it is so easy to break a rod by hitting it against something. All one has to do is damage the outer most layer of fiber and the rod has been very significantly weakened and will probably later break at the point of the impact.

I have often wondered why blank designers do not reverse the prepreg on the mandrel so that the glass scrim is on the surface rather than the graphite being on the surface. The glass has roughly one fourth the modulus of the graphite so it is roughly four times as tough and it is also much harder than graphite. This would have the effect of dropping the modulus of the blank a little but I am sure that there is something that I am over looking because I am sure that this has occured to blank designers and has probably been tryed numerous times.

Yes, good discussion and I completely agree that HM with toughness is a key factor. Interestingly, of all the fibers, IM9 stands out among the others in this regard. Cost/lb is always a factor though.

And on the consideration of the area under the stress strain curve, that's exactly what I mean, and sometimes the strength of the material alone doesn’t tell the whole story. The results can get even more complicated. In the case of S-glass, the breaking strain can out distance that of the epoxy and polyester matrix resins and in the composite, the stress-strain curve is not linear, so the results are much less predictive.

There may well be some advances in fibers to come along the way, but you might also see more efforts in the development of better matrix resins and also ways of getting increases in fiber to resin ratios. I'm sure the fiber densities are nowhere near optimum in a rod blank based on the process that is used. There are much higher performance composites available than what is being used in the sporting goods industry but one of the limitations to the blank manufacturer would be the higher process temps. It'll be interesting to see where it goes from here.

mark

Mark,
I completely agree. The last time that I looked at this the scrim, resin and finish content was higher than the graphite fiber content and in effect dropped the modulus of the blank to less than half of the modulus of the graphite fiber. The scrim is doing several things including adding to hoop strength but it seems that the lower resin content that could be achieved the better.
I have wondered about your other point a number of times. I am sure that you are right, the resin must be high strain or at least higher than any of the fibers so that it does not give when the fibers are stretched or compressed.
Yes, a brochure that I have from Cape Composites, that is actually several years old now, shows a number of graphite fibers that have high tensile strengths than IM6, IM7 or IM8. Im9 is higher at 920 KSI than anything other than T1000G which is slightly higher at 924 KSI. I do not know why these are not being used unless it is as you suggest a function of their price.

Another piece of this whole issue that bugs me is the fact that on many blanks, particularly longer and faster action and those made from a single piece of prepreg, the graphite fibers actually spiral up the blank rather then running straight up the blank. I realize that to get the fibers running straight up the blank could be a problem but I think that blank manufacturers should attempt to get the fibers as straight as possible especially toward the tip of the blank because the effective modulus drops with this angle. For example, if the fiber is at an angle of 45 degrees the effective modulus of the fibers is cutt in half.

Mark,
I have a question that you may very well know the answer to. I have looked for the answer to this but have only found very limited information. Looking at the test fixture in the site from your post reminded me.
The tensile strength of a material drops as the applied force is applied more and more rapidly. In the case of carbon fiber how rapidly must the applied force be applied and how much does the tensile strength drop? In the context of graphite blanks the question is, can a fishermanapply enough force rapidly enough for the tensile strength to be significantly lowered?

Emory,

That is an excellent question!.... and also why I think it can be confusing when manufacturers mix the idea of breaking strain and strain rate. The rate of the applied force is the strain, or deformation rate and a lot of people don't think about the rate dependence of many of these materials. Carbon fibers don't exhibit much, if any strain rate dependence although glass fibers, carbon fiber composites, polymers and metals do exhibit rate dependent behavior. Generally speaking, the modulus and strength of most materials actually tends to increase as a function of increasing strain rate....it's a positive dependence. The breaking strains, or elongations tend to decrease with deformation rate, so the materials tend to become more brittle at higher rates. I'll see if I can find you a couple of good references.

In polymers ( read as matrix resins or polymer fibers) the strain rate dependence is closely tied to the relaxation time of the molecule. If you do a search on the “Deborah number”, “time-temperature superposition” etc you get some interesting reading. The main idea is that time (rate) and temperature are equivalent with high temp properties relating to long times (slow rates) and low temp is comparable to high rates (impact) again, where materials can tend to become brittle. It would be interesting to measure the strain rate of a hook-set, but typically I don’t think you’ll see a large effect on the composite materials for anything but that.



mark

Mark,
Thanks, I thought if anyone would know you would. I will look on the web for information on the Deborah number. It should be very, very interesting. To tell you the truth, at this point, the concept of time-temperature superposition or rate and temperature equivalence is not one that I am familiar with at all.

Emory, here’s a link to a search on the strain rate dependence that produced a number of good hits:

[www.google.com]

This one sums it up best: The split Hopkinson method they refer to is actually a type of impact test:

[www.grc.nasa.gov]

Here's an easy way to see the effect of strain rate dependence. Take some silly putty and ask yourself, is it a liquid or a solid? Answer, it depends on the time frame of the experiment. Make a ball of it, set it on the counter, then come back a day later and you'll see that it flows and has liquid-like behavior. Next, shape it into a ball again and bounce it (higher strain rate), and it behaves like an elastic solid. Lastly, take it in your hands and pull it apart as rapidly as you can and it will fracture like a brittle solid. The Silly Putty has a relaxation time that's dependent on the material (PDMS). So, as the time frame of the deformation approaches and exceeds the relaxation time of the molecules, they can't relax fast enough and in the limit, the energy goes into breaking bonds. The ratio of the relaxation time of the material over the experimental time frame is called the Deborah number.

TTS is also a very facinating subject. There were three fellows, Williams, Landel and Ferry, that developed the so called WLF equations which allow the extrapolation of the time-temperature data to orders of magnitude beyond the experimental time frame. The utility is that you can create what are called modulus frequency "mastercurves" that allow you to predict the material properties at exceedingly short (>10E10 Hz.) or very long time frames (months to years) in an experiment that only takes hours.

mark

Mark,
Thank you very much. I am going to enjoy this reading and if I am not careful I am going to learn something.

Mark, Emory,
I hope you're happy now that you've given me a monster headache!!!!!

Mike

Thanks guys,
I've learned more about the mechanics and science of rods on this forum in the last 6 months than in the past 45 years of fishing and buying tacle! Heck, I actually though I was fairly well schooled, especially w/ fly rods. Boy was I wrong, I was like one of the people who came into Andy's shop. Man, I love this forum & Rodmaker Mag. I just can't wait to get a real education in High Point this coming February! I'm set, have the better half's blessing and a reservation at the Radisson> I'll see you all there!

Snooker Pete