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Old 01-08-2015, 06:19 PM   #1
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Default Carbon Fiber Chassis Life Expectancy

Does carbon fiber fatigue or weaken over time? Should carbon fiber chassis' be replaced over a x-period of race time?

Most of us know carbon fiber chassis' are an important factor in chassis setup and handling. When components on our vehicles are visibly worn out or broken, we replace them. Should we consider doing the same with the main chassis? The only downside is we can't see or test to know whether or not the chassis needs renewing.

What are your thoughts?

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Old 01-08-2015, 06:35 PM   #2
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Having experienced this first hand I would say yes, and I probably don't use a chassis for more then 6 months,

With my 14 Yokomo, someone crashed and ended on the wrong side of the track on the main straight just as I was there and I hit them at full speed head on. Broke both top and bottom desk. (and front bumper mount and body) But after I replaced both the car was much faster, and it wasn't just me, other people also commented on how much quicker the car was. I still bring my 14 out occasionally, its still the best car I have ever owned, I don't want to sell it.

I race usually once a week on tarmac so my cars generally get a workout.
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Old 01-08-2015, 07:05 PM   #3
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Fatigue is such an unpredictable aspect of material use, and the fatigue life of carbon fiber is so long it's probably not worth talking about. If your car was faster after changing your chassis, GREAT! It probably wasn't because the old chassis was fatigued and the new one isn't.

If it's not broken, run the chassis until it breaks. I've run the same one for years.
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Old 01-08-2015, 07:12 PM   #4
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CF of today is produced pretty good with the advances in resins, laminates, and heated pressure presses.

DesertRat is right, in onroad its minimal. If its offroad, and a heavy car, with minimal chassis rigidity, then yes, a CF chassis could fatigue over time from the higher stresses and cause microfractures.

Chassis is a wear item for me, but mainly when it gets thin from scraping. I do replace it in heavy crashes (happened once for me), its never the same after it has fractured (even if not easily visible.)
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Old 01-08-2015, 07:13 PM   #5
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Surely the constant flex motion of a chassis would over time start to break the epoxy bonds of the carbon strands at a very microscopic level ??
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Old 01-08-2015, 07:21 PM   #6
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Quote:
Originally Posted by evochick View Post
very microscopic level ??
It absolutely does. (I went to school for this shit and I'll be damned if I don't use it somewhere! ) BUT, it will take so long and so many hard hits you'll wear out the car and every other component long before it gets to where it will be noticeable in the flex of the car. Also, the part of a touring car that experiences the most flex and strain per unit area in both normal suspension operation and crashes is the top deck. The chassis is nearly impervious to fatigue failure.
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Old 01-08-2015, 07:27 PM   #7
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So what your saying it was more likely my fatigued top deck that made the difference
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Old 01-08-2015, 07:41 PM   #8
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Quote:
Originally Posted by evochick View Post
So what your saying it was more likely my fatigued top deck that made the difference
I won't say your original chassis wasn't damaged in some way, but I don't think it was normal fatigue that caused that. Fatigue is something you see in an airplane that is just strong (AKA heavy) enough to carry its own weight and takes off, lands, and flies through turbulence four times a day for 35 years. That's fatigue.
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Old 01-09-2015, 11:04 PM   #9
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Had a friend back in the days of T-bar 1/12TH, who worried about the fatigue and consistency of the fiberglass in the T-bar. So, he built a strain gauge. Clamp one end of the T-bar firmly, attach a cross bar to the other end and hang a known weight off the end of the bar. Measure the deflection (twist). He found an amazing range of stiffness in AE T-bars. I'm sure you could devise something similar for any graphite part if you're really worried about it. Most of us buy a new car before the old one would fatigue much, unless you hit a lot of stuff.
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Old 01-10-2015, 11:38 AM   #10
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Well think about this
I have a 1986 Chevy C4 Verte. (auto cross) it has carbon composite leaf Springs (1 in front, 1 in rear) they have gotten softer but are preferred over new ones. Some say it's not much different tho after 100k miles and 30yrs....
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Old 01-10-2015, 11:59 AM   #11
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Internal delaminations are common in composite chassis. I doubt any hobbiest would have the type of equipment needed to accurately measure the size and depth of any indication of internal delaminations though. In aerospace, composite parts are tested with different forms of ultrasonic scanning to see if there are voids, delaminations, or foreign materials within a composite laminate. They can even say the approximate size and depth of the indication from the scanning surface.

The old trick is to take something hard like a quarter and do a "tap" test. Just tap around the carbon plate and see if you notice the sound of the tap change. Generally a good section will be a nice and sharp tap sound, while bad sections might be more of a "thud" sound haha.

Causes for our chassis to weaken are usually the resin mechanical properties deteriorating from UV radiation. Its probably the most common way composite laminates start to fail over a long period of time. Usually you can tell if it has UV damage from a yellowing appearance. There are special coatings to help protect from UV. Think of it as sunscreen for composites .

Other reason for decreased mechanical properties is just our normal mishaps on the track. Those impacts are always something to consider when dealing with composite chassis.

In the end, I replace my main chassis plate before any really important event. If you notice your car is becoming inconsistent and suddenly seems to tweak a lot easier, a lot of times it can be the main chassis plate (or a bent bulkhead/motor mount). Just something to check into from time to time.

Hope that helps!

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Old 01-11-2015, 10:47 AM   #12
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I have also noticed variations in carbon plate thickness. I am sure that effects the stiffness. I had a run of chassis that were better than others of the same design. I assume that it was due to the difference in carbon.

While its not fatigue changes, there is variability in the base material.
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Old 01-11-2015, 11:25 AM   #13
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I agree with Desert Rat and wish I had the certainty of other posters whose statements go unverified or unreferenced.

There are a myriad resins in use and an almost infinite number of lay ups (ply books) in use, so the first thing to say is that one cannot simplify that down to one rule for all cars and all their parts made from CFRP. Some chassis' will be better than others simply because they have better layups and/or better resins. That can vary form batch to batch for the same part from the same manufacturer.

In addition to not knowing the materials or their construction, CFRP layups may be designed to have more strength in one direction than another. You can't tell that by looking at the top layer - the only one you can see - you need to know the ply book and the proportion of carbon-fibre to resin.

One of the biggest issues with CFRP, and composite materials in general, is the inability to define a fatigue endurance limit. While steel and many other structural metals and alloys do have estimated fatigue endurance limits, the complex failure modes of composites mean that the fatigue failure properties of CFRP are difficult to predict and design for. The default position of all designers because of this is to over-engineer leading to the long life Desert Rat describes.

The explanation for Evochick's change of car handling is just as likely to be that the new chassis properties were different from the old one (stiffer, change of strength axis, change of resin and fibre density, etc.) as it is that the old one was damaged.

For all practical purposes consider the life of a CFRP part in an RC car to be infinite under normal use. If you do hit it against something, then it might accelerate a local failure... or it might not. For you to know that if it is not obvious on the outside you will need X-ray vision!

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Old 01-11-2015, 09:49 PM   #14
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Maybe 2 years ago i read a report from Radio race car mag, in the uk, saying one of the disadvantages of T-Bars was the delamination of the fibreglass (frequent replacement reqd), not sure if this is true for carbon fibre also, but a tc chassis probably doesn't flex as much as a pro 12 t-bar
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