For cost, I am thinking of sourcing some custom carbon fiber tube from outside of North America. I want to be able to request carbon fiber of a minimum acceptable quality.
What is considered the industry standard strength for carbon fiber, and how or rather in what kind of units is it expressed?
About 90% of the carbon used is T700 or similar quality, see the Toray website for T700 specs.
The rest is either T300 lower tech, mostly used for wovens, or high tech stuff like M46J
The 700 in T700 refers to 700Kpsi of tensile strength right? I bought carbon from Soller composites (Northeast US) and the carbon they sell is rated as ‘aerospace certified’ 640Kpsi strength. It’s woven in 2x2, how does that compare to other woven fabrics?
Thanks…and hope this isn’t considered a thread hijack!
Not at all. I’d be interested in the answer to your question as well as broadening the scope of the discussion to include anything that pertains to carbon fiber specs.
For an industry as specialized and important as carbon fiber, it is surprising that there is not a more rigid and universal grading system other than regular, intermediate, and high modulus. I mean even roller skate bearings (which are less critical than say airplane components) are rated with the ABEC scale, which has precise values. My casual understanding at this point is there is no fixed boundary between regular, intermediate, and high modulus.
I am also curious as to how carbon fiber has changed over the years. I wonder if old stock is considered substandard, and whether dry carbon fiber degrades with age. These are important questions if one is buying carbon fiber from a discount source, where the stock could be old.
Your safest bet is to work with the tube manufacturer to pick a fiber and then request fiber lot numbers and certificates with each shipment of tubes. Alternatively, you can specify a minimum tensile strength and modulus for the fiber itself and request that they provide the fiber certificates showing these values are met. The tensile values of T700S from Toray are a good reference point, tensile strength is typically listed in ksi or MPa and tensile modulus is listed in Msi or GPa.
Requiring this type of information may increase the cost due to the extra paperwork.
The problem is that everything changes everything. For a steel bearing, you have a specific steel grade, and a lube grade. You can fine tune that with each alloy, so you have a larger scale of standards. But that is also only a scale for bearings, nothing else. It is the end part.
Composites however, you have to take in to account the fiber, the areal weight, the weave, and sizing, the resin, the thickness, the loading, etc. Each singular component can be changed for each specific purpose. That is why most manufactures will give strength data for a uni-directional fabric, and they will then break down the numbers for the fiber, (or in case of pre-preg, fiber and resin), and then you have to recalculate for the specific fiber, AND part.
Also, for your original post, since a carbon tube can be built any number of ways, there is no spec for a tube. A PVC or HDPE tube uses one material, and that has a spec. Change the thickness or diameter, specs change, but in the end, material is the same. Composites is not meant for specific parts that can be mass produced. you have to know exactly what YOU want and what YOUR end product needs, and then you can spec out that tube for that part.
So for quality, there has to be testing done for consistency on the manufactures end of things. They have to spend the money to be able to produce a consistent part, and have it tested, and certified. Most companies will not do that unless the end user requires, and will pay for it. A plastic extruded PVC pipe will not change specs from one producer to another, using the same resin. But you can easily screw up composites.
Everything in this industry is an estimated guess. There’s too many variables.
The only way you’re gonna know is to prototype (using softare that costs thousands) and send it for testing. Even then, nothing is certain until it goes through quality control. We’re working with a very large customer at the moment. Their scrap rate is 80%.
I wouldn’t say T300 is “lower” tech per se, just “older” tech. Most T300 is readily used on secondary structures for airplanes, but you won’t find any T700S there…it was developed specifically for industrial use only. “Aerospace quality” generally means the fiber was made on lines that are very tightly monitored and none of the settings/raw materials are allowed to change (all laid out in what is called a PCD). Fibers like T700S are produced on lines or line set-ups that do not have as much scrutiny.
There used to be an organization called SACMA (Suppliers of Advanced Composite materials Association) that used to provide test methods and standards in the industry, but it has been dissolved now for several years (not really sure why). You can still find some of their test methods (SRM) being used, but most of the time the fiber producers use their own in house test methods to determine tensile strength, modulus, size amount, etc.
In terms of carbon fiber shelf life, generally its around 1-2 years, but it really depends on how it is stored. Dry carbon always has a reactive size (sizing) on it that can cure/harden when exposed to high temperature, UV, and humidity. If kept in dryer, cooler environments carbon fiber can be useful for up to several years. If stored improperly, it will get very stiff and “krinkly”.
I think this was replaced by ASTM standards. Producers won’t just make up their own methods to test things…there are standards, be it ASTM, Boeing, or SAE. I’m sure many of them overlap the data, but slightly change things, like fixtures, or amount of data collected, etc.
ASTM does have a great collection of test standards/methods and reporting procedures for composites, but many of the big carbon fiber producers have developed their own test standards especially when it comes to tensile properties, linear density, and others mainly dealing with the fiber properties. Most of the methods are close to ASTM and ISO versions, but can differ.
As an example, Toray’s method for testing of tensile modulus (TY-030B-01) differs from the analogous ASTM method (I think its D4018) in terms of over what strain range the modulus is measured. Since impregnated carbon fiber is slightly strain hardening (or stiffening is a better term), there can be a good deal of difference between the Toray and ASTM method results.
As for SACMA, you’ll still see the compression strength test floating around (SRM1 I believe), as it has provided fairly consistent results before. I think some of the newer methods will gain favor as they are more readily used (like the combined loading compression method).