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carbon vs fiberglass

24K views 26 replies 14 participants last post by  flatline16 
#1 ·
We all know that carbon fiber is ultimately lighter than fiberglass, especially "dry carbon," but what I want to know is how much lighter is it in comparrison with cost?

Before I get silly replacing things with carbon I just wanted to see what kind of averages the two have in weight savings, especially over stock pieces on the mkII deuce. Basically so I dont go over board on price.
 
#5 ·
www.fibreglast.com

Is a good site for information as well as pricing. You can see that carbon is quite a bit more expensive than fiberglass. If you're on a budget, you're better off with fiberglass. The weight difference will be negligible and you'll save a ton of money. They have so-called "black fiberglass" that is woven to look like the 2x2 twill carbon weave but I don't know how similar it looks in real life. If you need to design a structural part then of course you'll need the strength of the carbon fiber, but then you probably wouldn't be asking us for advice about it.
 
#6 ·
Carbon ever so slightly lighter and maybe 10% stronger unless you use advanced layup methods. It will not be any more heat-resistant than glass unless you cook it. It is also about ten times as expensive, which is why the boy racer tuner companies just use one or two ply of carbon for looks with glass for the structure. Carbon doesn't become useful or anywhere close to being worth the price until you start to make structural things -- i.e. monocoques (not hoods). Using just a few layers of carbon with a core of balsa or aluminum honeycomb or any of a dozen things will make an incredibly stiff structure -- this is how race cars are made. I actually got a chance to hold a prototype section of the wing skin for a Boeing 787 recently -- about 100 ply of carbon with no core, almost an inch thick. It felt as hard and stiff as granite in my hands, but was lighter than aluminum. Unfortunately there's no real reason to make anything out of carbon for street cars, except for looks. It should also be noted that replacing plastic parts with CF copies does not reduce the weight at all, just makes it more expensive.
 
#7 ·
Good things to know, thanks gents. The idea is a streetable mr2 with plenty of punch for my weekend warrior stints at autox events etc. Carbon is sweet looking, and I had been looking into it, but the amount of things I would of replaced in carbon would easily have run me in the $3500 range. Style is fine and dandy, but a good paint job on frp ought to go a long ways and save me a good bit of coin in the process that could be well spent other places

All in all, helpful thread :smile:
 
#8 ·
Before we all just go by what we hear, let me explain. When they say strength, they are measuring TENSILE strength. This means pulling length wise on the fiber. They usually test in bundles. Each fiber (fiberglass and carbon) is about 6-10 micrometers in width. Based on a horizontal strength tests there is no difference. What makes the difference in the type of resin used and the process used to create the part.

Lets say you have 1 carbon square and 1 fiberglass square with an ester resin. You use force on the square to try to break it. It will take about the same force to snap the square wether it is carbon or fiberglass. Alot of the companies use ester resins for their carbon products so there is not much difference besides the look. They even use frp shells for the hoods and then skinned with carbon. I laugh everytime someone tells be they bought a carbon hood, I simply say no what you bought is a frp shell with a carbon OVERLAY. The race inspired companies (buschur is a good example) will typically use a polysulfide terminated resin which is alot stronger than ester resin. It all depends on the resin used and process. Not much difference in the fiber besides tensile strength.
 
#9 ·
flatline16 said:
Lets say you have 1 carbon square and 1 fiberglass square with an ester resin. You use force on the square to try to break it. It will take about the same force to snap the square wether it is carbon or fiberglass. Alot of the companies use ester resins for their carbon products so there is not much difference besides the look. They even use frp shells for the hoods and then skinned with carbon. I laugh everytime someone tells be they bought a carbon hood, I simply say no what you bought is a frp shell with a carbon OVERLAY. The race inspired companies (buschur is a good example) will typically use a polysulfide terminated resin which is alot stronger than ester resin. It all depends on the resin used and process. Not much difference in the fiber besides tensile strength.
This is why structural composite design is so tricky. If you make a part with all the fibers running the same direction you will have strength in the length direction on the order of 10x what you have accross the fibers. This is why composite parts typically have many plies running in different directions. Yes, the strength of the resin comes into play, but it is so low that if you want strength in a given direction you put more plies in that direction, not try to up the resin strength and hope it holds together.
 
#10 ·
Alex W said:
This is why structural composite design is so tricky. If you make a part with all the fibers running the same direction you will have strength in the length direction on the order of 10x what you have accross the fibers. This is why composite parts typically have many plies running in different directions. Yes, the strength of the resin comes into play, but it is so low that if you want strength in a given direction you put more plies in that direction, not try to up the resin strength and hope it holds together.
The first part of your reply doesnt make too much sense. Yes when a structural part is built such as something on an airplane, the weaves are alternated with each ply.

Like I said about the square you can compare that analogy to ANY carbon part produced for cars. Ie a fender. If I kick your a fiberglass or carbon fender, it will take the same amount of energy to break them. If I try to pull the fender in half then it is a different story. People always assume carbon fiber is stronger, when it really depends on how it is used. Resin plays a bigger roll than you think. Its all about chemistry. Resins a specifically designed for certain compostites and OZ ratios.
I have done research on it in a lab...
 
#11 ·
why not just stick to metal? composites are nasty to work with, cost way too much, are more apt to fail entirely when hit, and they delaminate. they are lighter but is saving a few pounds worth the money and effort? but if i had to pick i'd go with kevlar. its still light, fairly flexible, and not as much of a pain in the ass to work compared to carbon fiber
 
#12 ·
im not going to mold it myself, so no headache. And the few pounds saved IS the entire point of switching. Its more about cost vs performance between the two to me. If the performance is farely close but one costs half as much then its a done decision. FRP for the win with some paint on it and call it good enough. I dont have the money to pay people to custom make carbon fiber pieces that are ultra light, dont have the experience to do it myself either.
 
#13 ·
Eversor said:
im not going to mold it myself, so no headache. And the few pounds saved IS the entire point of switching. Its more about cost vs performance between the two to me. If the performance is farely close but one costs half as much then its a done decision. FRP for the win with some paint on it and call it good enough. I dont have the money to pay people to custom make carbon fiber pieces that are ultra light, dont have the experience to do it myself either.
Just to let you know frp is heavier than an ester based resin with matte.
 
#17 · (Edited)
russell89 said:
why not just stick to metal? composites are nasty to work with, cost way too much, are more apt to fail entirely when hit, and they delaminate. they are lighter but is saving a few pounds worth the money and effort? but if i had to pick i'd go with kevlar. its still light, fairly flexible, and not as much of a pain in the ass to work compared to carbon fiber
I'm not sure what you mean. There are tons of advantages to composites, but you need to understand what you are doing to get those advantages. Plastics can be shaped much more precisely than metal, and with less effort. You construct a mold, and then can bang out parts. With metal you need a press and fancy die to push against to get sheet metal with the appropriate shape. Even then you to attach some sort of skeleton which you have to build into it. If a metal panel is broken, fixing it can be a real pain in the butt, but much simpler when repairing composites. Properly made composites are impervious to chemicals, oxidation, water, fuel, oil, etc. The main thing that kills them (at least epoxy based composites) are heat and UV, which is completely different from metal.

You can also get unidirectional rovings for specific applications, say a driveshaft, and this drastically decreases the weight. A lot of production driveshafts are composite these days.

As for picking kevlar... Fiberglass, carbon fiber, kevlar, each have their own unique properties and are useful for different things. They have different modulus, tensile strength, shear strength, impact resistance, abrasion resistance etc. That said, these are usually difference per unit weight or volume. The tensile strength of fiberglass for instance is lower than carbon fiber given the same cross-section of glass, but if you make the cross-sectional area higher, glass can be significantly higher. You should pick based on a specific application.

Generally speaking, Kevlar has better abrasion resistance, but otherwise fiberglass does pretty darn well compared to Kevlar and carbon fiber. It isn't as good per unit weight or volume, but its almost as good and MUCH cheaper.

flatline16 said:
to let you know frp is heavier than an ester based resin with matte.
Say what? FRP = Fiber Reinforced Plastic. Chopped strand mat in ester based resin is a form of FRP as is woven glass, which is what I assume you are calling FRP. That said, chopped strand mat is about as bad as you can get in terms of strength/weight ratio. It is only used in cheap applications where weight is a non-issue (like people building "carbon fiber hoods" for sale to morons on eBay who want the CF look and don't mind the fact that is 2-3x heavier than a straight glass hood that was made with proper techniques and materials).

The reason that multiple layers of woven glass are used is because you may need more strength than one layer can provide. You could use "heavier" glass, but heavier glass is often harder to work with, especially if there is any significant curvature or you have to work against gravity. It is used to improve off-axis performance, but you don't really need more than 2 layers to do that. With two pieces of glass oriented 45 degrees apart from one another, the maximum error angle is 22.5 degrees. That means your worst case scenario is you are 7% off axis (shear). Three layers brings it down to 2%. I've seen aircraft wings made with 2 layers of glass on top of a layer of foam on top of a single layer. And that was it.
 
#18 ·
flatline16 said:
If you make a part with all the fibers running the same direction you will have strength in the length direction on the order of 10x what you have accross the fibers. This is why composite parts typically have many plies running in different directions.

I was referring to what you were talking about with carbon and fiberglass having similar strength, and pointing out that it depends on the direction of force application relative to fiber orientation. A uni-axial layup only gives you strength in one direction. You will have somewhere in the area of 10x as much strength along the fiber axis as you have accross the fibers. Yes, accross the fibers a fiberglass part will have similar strength to a carbon fiber part. But thats now how you design composite parts. Designed properly a composite part can be strong in all directions, and then what you were saying about the different materials having the same strength (when the force is applied in the "wrong" direction) isn't valid anymore.
 
#19 ·
flatline16 said:
The first part of your reply doesnt make too much sense. Yes when a structural part is built such as something on an airplane, the weaves are alternated with each ply.

Like I said about the square you can compare that analogy to ANY carbon part produced for cars. Ie a fender. If I kick your a fiberglass or carbon fender, it will take the same amount of energy to break them. If I try to pull the fender in half then it is a different story. People always assume carbon fiber is stronger, when it really depends on how it is used. Resin plays a bigger roll than you think. Its all about chemistry. Resins a specifically designed for certain compostites and OZ ratios.
I have done research on it in a lab...
What work have you done with composites? What sort of testing and what kind of lab?
You sound like you think you know a lot but your posts make you sound fairly uneducated on the subject.
As Alex said the fiber makeup and orientation hugely effects strength attributes of a part.
As hpmaxim said all different fibers have different strength attributes which effect the end characteristics of the part.
They have different modulus, tensile strength, shear strength, impact resistance, abrasion resistance etc. That said, these are usually difference per unit weight or volume
Right now I am working in Thermoplastics and I can tell you for sure that FG, CF and Kevlar in the same matrix will all respond very differently to stress tests as will fiber orientation.
 
#20 ·
Polymer chemistry. Mixing resins specificially engineered to compliment each type of composite. I know that I hold a wealth of information on this subject. You people are talking about aero space... Show me a company which orients fabric in alternating array of patterns that builds car parts. Have you looked at the a single fiber under a microscope or done any measurements ect? The strength tests are not done with single fibers however in bunches or bundles.Have you done these measurements or are you going by what everyone else says? Do you know how each of those syntheic fibers are produced and the chemistry behind them? Until you have looked for yourself and done tests I take what you say lightly. Yes alternating the patterns can create greater strength than others, thats a given and that is delt with in aero engineering. I dont see one person in the thread arguring that statement including myself. How do you think a typical car part is built? If test the amount of force to crack or break a fg, carbon and kevlar sheet or part it will take the same amount to break each of them given they are produced through the same process, same resin used and same weave pattern is used.

He asked most strength for the money. Its clearly fiberglass. CAR PARTS for your everyday consumer are all produced the same; all frp or frp shell with kevlar or carbon single ply OVERLAY. Im not going to post any more on the topic unless you have some scientific data to back what your saying on this topic, not in the aerospace field. We are not taking about alternating weaves and building planes but simply car parts.
 
#21 ·
yoshimitsuspeed said:
Right now I am working in Thermoplastics and I can tell you for sure that FG, CF and Kevlar in the same matrix will all respond very differently to stress tests as will fiber orientation.
raw or infused? If infused with what? Generally companies use polyester or ester so tests should be conducted with that as the controlled variable as well as the fiber orientation to a chosen axis. Stress meaning what? Tensile strength? I have already stated that they will respond differently. A strength test across the x axis of the fiber will be imperceptible, which is what Im arguing.
 
#22 ·
95% of the work we do involves unidirectional tape so everything we do relies heavily on fiber orientation. We use many different plastics infused with whatever fiber most closely suits our needs.

http://fiberforge.com/thermoplastic-composites/thermoplastic-composites.php

We do work in the automotive field and many others and like I said fiber orientation is always paramount in anything we make.
http://fiberforge.com/applications-and-markets/applications-and-markets.php

I don't do much work in the lab and don't have any numbers to give you. Even if they are easy enough to come by I don't care enough to make the effort. I do know I have personally tested both by hand and lab equipment and seen very different results between say CF and FG in the same layup, thickness, and matrix. They flex different, they fail different and withstand varying forces different.
I worked with thermoset before thermoplastics and the same theories apply to both.
 
#24 ·
GinX said:
Anybody seen Myth Buster's episode on Carbon Fiber? They wrapped a whole house with carbon fiber. And it reinforced the whole house from tearing down from winds and debris equal to a category 5 hurricane.
I haven't but mythbusters is about as scientific as measuring the speed of dog farts with a tape measure so I never invest a lot in their results.

Besides as we are in a thread topic called carbon vs fiberglass I'll contest that they would have gotten very similar results using fiberglass. Beyond that the proper material would have been kevlar or a hybrid combination or possibly some of the more advanced fibers that are replacing kevlar like Dynema.
 
#25 ·
flatline16 said:
Polymer chemistry. Mixing resins specificially engineered to compliment each type of composite. I know that I hold a wealth of information on this subject. You people are talking about aero space... Show me a company which orients fabric in alternating array of patterns that builds car parts. Have you looked at the a single fiber under a microscope or done any measurements ect? The strength tests are not done with single fibers however in bunches or bundles.Have you done these measurements or are you going by what everyone else says? Do you know how each of those syntheic fibers are produced and the chemistry behind them? Until you have looked for yourself and done tests I take what you say lightly. Yes alternating the patterns can create greater strength than others, thats a given and that is delt with in aero engineering. I dont see one person in the thread arguring that statement including myself. How do you think a typical car part is built? If test the amount of force to crack or break a fg, carbon and kevlar sheet or part it will take the same amount to break each of them given they are produced through the same process, same resin used and same weave pattern is used.

He asked most strength for the money. Its clearly fiberglass. CAR PARTS for your everyday consumer are all produced the same; all frp or frp shell with kevlar or carbon single ply OVERLAY. Im not going to post any more on the topic unless you have some scientific data to back what your saying on this topic, not in the aerospace field. We are not taking about alternating weaves and building planes but simply car parts.

As far as structural purposes, carbon, glass, and Kevlar can have similar mechanical properties depending on the resin system but have benefits the other lack. Carbon is the strongest and stiffest but lacks impact and abrasion resistance. Glass has superior strain at failure and is good for impact resistance and absorbing energy but it's not as stiff. Kevlar is also good at impact resistance and energy absorption but has far better abrasion resistance than both carbon and glass and is used on the bottom of chassis for skid plates.

Even playing with plies can get you better impact resistance. An asymmetric layup (more plies on one side of core) will change perimeter shear properties.

flaunting around that you know the exact chemical tracer that is used to make XYZ fiber and the chemical recipe for ABC resin means nothing...

All we engineers need to design a structural part is material properties. Mechanical properties for just carbon has a very broad range. You can get low modulus in the 10GPa range all the way up to uni's that are in the 300Gpa area. Same goes for tensile strength as well as the strain at which they fail. The resin system will change your interlaminer shear strength and laminate shear strength as well as the Tg. With a low Tg, heat becomes a problem very quickly. Simply swapping out one fiber for another can drastically change how the structure performs.

You seem to have a lot of lab knowledge as far as the chemistry into making strands of composites and testing them but absolutely none when it comes to structural composite and engineering.

Source: Engineers that designed numerous monocoque race cars. I've also designed two composite monocoques for my universities FSAE team.
 
#26 ·
yoshimitsuspeed said:
I haven't but mythbusters is about as scientific as measuring the speed of dog farts with a tape measure
... and then they blow it up, whatever it is, they blow it up.

Because explosions make science fun! :rolleyes:
 
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