Carbon Fiber

[bc.gold 3,403]

Would like to share my interest in carbon fiber, let your imagination be your guide.

 

 

[wh500special 2,184]

That is a really nice looking part.  
 

Carbon fiber is a very versatile material with some great physical properties.  And it can be really attractive when finished properly. 
 

The first 20 years of my career mostly revolved around making parts from carbon fiber and fiberglass using various processes, but I’ve never been particularly good at making hand layups like shown in the video look nice.   Some of the things people can do really qualifies as artistry, but a lot of what they do really doesn’t take advantage of the material’s physical potential.  Layups like that usually are fairly heavy on resin and are full of micro-level porosity that hampers strength. 

but gosh they look great. 
 

The last time I bought 400 gram/meter^2 2x2 twill like he shows there it was about $25/kg for the fabric.  Aerospace grade stuff is a lot more expensive because it has to meet higher standards so the wing doesn’t fall off the airplane.  Until recently resin was really cheap comparatively with a good epoxy going for about $2.50/lb but it’s about doubled in the last year.  
 

Carbon fiber is one of the few things I encounter with any regularity that even in the US is measured in metric units. 
 

By weight, a good CF composite is about 60% fiber.  Glass runs about 70% by weight since it’s more dense.  Either way, it’s good to target a 50-60% fiber by volume range for a structural part.  Hand layups with woven fiber like that shown could be maybe 40% by volume. 
 

Glass isn’t much less strong than carbon, but it is nowhere near as stiff.  Stiffness is where carbon really shines.  Those who fish can vouch for the whippiness of a glass fishing rod compared to a graphite rod.  Glass is actually pretty incredible stuff too and offers great cost and weight performance when done well.  Stretches like crazy which gives it great impact resistance. 
 

Cost really holds back widespread use of truly engineered carbon fiber and fiberglass in everyday products.  Unless the fibers are chopped short - which really kills the strength of the cured part - processing is slow and wasteful.  Trimming the cloth to fit the mold scraps a lot of material and avoiding wrinkles, folds, and darts does too.  That’s why we still see a lot of sprayup in boats, bathtubs, and RV parts.   Fiber volumes in these sprayup parts are probably in the 35% range which is why it’s hard to consider them very structural.  That’s why fiberglass boats, for example, tend to be pretty thick and heavy.  
 

There’s a bit of irony in that I’ve spent my career in composites but don’t really see myself ever having a fiberglass boat.  Huh. 
 

For decades the composite industry has been saying more widespread use is just around the corner, but it’s been an unrealized goal.  The holy grail is to get acceptance in the auto industry, but it’s hard to beat the bang for the buck afforded by steel and aluminum that can be stamped, formed, sheared, machined, and welded so much more easily.  The composite processes that are somewhat fast and automated don’t really lend themselves to oddly shaped parts.  
 

Composite parts play a huge role in the background of things we use every day but the structural ones are rarely front and center. 
 

nice video. 
 

Steve 

[8ntruck 7,007]

Around 1980, the wheel company I had just started working for was experimenting with composite wheels.  As a young engineer, I was tapped a couple of times to help the lead engineer on that project lay up the charges to go into the heated compression mold.  We called that serving on the itch bergrade.  We would use utility knives to cut XMC and SMC to size and stack them up in the proper order.  The development work was on the resin to stand up to brake heat and still be able to maintain lug nut torque.

 

The failure mode on these wheels was interesting.  Rather than growing a definite crack like steel or aluminum wheels, the composite wheels would gradually lose stiffness and kind of turn to mush.  In fact we were able to use one wheel to complete radial, rotary, and impact tests required for qualification.  Normally, each one of those tests would destroy a steel wheel.

 

That product only made it into limited production.  It was used on the Shelby GLH Omni/Horizion for one or two model years.

[wh500special 2,184]

45 minutes ago, 8ntruck said:

 

 

The failure mode on these wheels was interesting.  Rather than growing a definite crack like steel or aluminum wheels, the composite wheels would gradually lose stiffness and kind of turn to mush.  In fact we were able to use one wheel to complete radial, rotary, and impact tests required for qualification.  Normally, each one of those tests would destroy a steel wheel.

Interesting!

 

I guess that eventual degradation and failure was a function of the resin gradually microcracking then those

would propagate. 
 

Sheet Molding Compound is heavy with chopped fiber, but usually pretty long lengths.  Still more load going through the resin phase than if it was continuous fiber.  But those don’t mold well. 
 

49 minutes ago, 8ntruck said:

  It was used on the Shelby GLH Omni/Horizion for one or two model years.

Goes

Like 

Hell!

[8ntruck 7,007]

The cracks  (micro?) would propagate until stopped by a fiber  running roughly perpendicular to the crack.  We used randomly oriented fiber in part of the wheel and oriented strand in another part of the wheel - tailored to the expected loads.

[Handy Don 12,232]

Thnks @8ntruck and @wh500special.

One daughter is a engineer specializing in composites. She and I have quite interesting conversations but one eye-opener for me was her explanation that some composites are far from inert. In particular, that direct contact between certain fibers/resins and certain metals causes corrosion to the point of fastener deterioration and failure.

Apparently aircraft operators consider this quite awkward for composite parts essential to airworthiness--hence yet another restriction for design and fabrication.

[8ntruck 7,007]

 A similar lesson was learned early in the SR 71 program.  They had some failures that were traced back too steel contamination on titanium parts from the steel wrenches used during maintenance.  This was solved by using titanium tools.

 

I studied and applied fatigue theory throughout my career in the wheel industry.  Steel wheels are loaded in a cyclic manner, creating an inverionment ideal for growing fatigue cracks.  Additionally, wheels are a safety related item on a vehicle, so it is essential that the design is right before production starts, and that adiquate testing is done to insure each production batch is good.

 

With that background, I tend to notice cyclic loading in machines around me.  One day my boss and I were flying somewhere.  We had seats over the wing, the boss in the window seat.  I noticed him idly looking out the window as we were flying through mild turblance.  He gave a somewhat shocked look when I asked him if he was counting fatigue cycles on the wing.  Then I reminded him that aluminum has a finite fatigue life.

Edited May 27, 2022 by 8ntruck

[wh500special 2,184]

Carbon fiber really does like to talk to metals. But the extent of how much you worry about it has to do with how critical your application is and how long you want your fastener to last before it returns to its elements.  Hardware on a fishing rod?  Who cares.  Wing spar on an F22?  That’s a different kettle of fish.  

Despite exhausting study, there’s a lot to corrosion engineering that remains magic. We know you can’t mix certain metals, that you have to provide or prevent certain electrical conditions, and that you have to protect some materials with specific coatings.  But there are still so many cases of things that shouldn’t work but do that there is still a cloud of mystery hanging over things.  
 

But surface contamination is a real thing.  A few years ago when the F150 aluminum body went mainstream Ford was quick to sink tremendous resources into training body shops not to use their steel tools on aluminum parts lest they impact them with iron contamination that would lead to corrosion under the repair area and paint failure.  I wonder if those lessons have stuck. 
 

fatigue analysis is out of my swim lane.  I don’t remember much of that topic. 
 

steve

 

Edited May 28, 2022 by wh500special