Why is carbon fiber safer than steel for the cars?

[Shinyoung Noh]

Not just for carbon fiber vs steel,
I went to a seminar and heard that the team was researching how to make the steel sheets used for car making to be 'stronger', so that, a car can be safer using their material.

Then, when I asked what does it actually mean when they say their improved steel is 'stronger' than normal steel, and their answer was higher modulus.

As far as I know, strength, young's modulus, and toughness are different things.

strength means it can tolerate force more before plastic deformation,
Young's modulus or stiffness means it bends lesser for the same force,
Toughness means more energy to deform it,

Then, about how the skin material of a car should be to make it safer for collisions,
Isn't it toughness that should be higher?
more energy to deform = consume energy from the collision and lesser shock to a driver ?

Anyhow, we can see Ferraris literally scatteringly break when an accident happens.
Then,
1) Why does higher strength mean more safety to the driver if it breaks anyway ?
2) or in case when force from the collision is smaller than the capable strength?

 

[Windadct]

Safety in a collision is all about managing energy. Absorbing it, dissipating it, deflecting it - etc - making the steel stronger gives the designer more tools to use when designing the whole vehicle. The passenger compartment you generally do not want to be deformed in any way, all of the rest of the car you may design to absorb or deflect energy. You may still want high strength materials in these areas so you can engineer where the energy goes - for example crumple zones that crumple outward - and not towards the inhabitants.

Also though stronger steel would allow less steel to be used for the same effective design, so reduced weight is also favorable.

 

[CWatters]

+1 If you have a stronger material you can use less of it reducing cost. Stronger steel might allow thinner an lighter panels. Weight saving reduces fuel consumption.

 

[mastermechanic]

I think steel has more ductility but it is not a requirement for automobiles. Strength is the key factor and strength per unit weight is much more in carbonfiber that's why performance car manufacturers prefer it.

Actually, during a collision you need a fissionable material in order to distribute the energy and momentum. If you choose too strong material the only thing that absorbes the whole energy would be you.

 

[DaveC426913]

The passenger compartment you generally do not want to be deformed in any way, all of the rest of the car you may design to absorb or deflect energy.

Riffing off this, it is possible that the corollary is true. That you want deformation in the expendable parts of the car.
I don't know from cars, but those Ferraris may be dissipating collision energy by disintegrating.

 

[Ketch22]

Not just for carbon fiber vs steel,
so that, a car can be safer using their material.

My current field of work is in emergency services. In that respect we work to evaluate injuries with respect to three separate collisions. The 'first" collision is the vehicle encountering another object. This is the impact that initiates the chain. The "second" collision is the body encountering the restraints. This is the impact that has the greatest potential for determining the peak force and time duration of the impact impulse. The "third" or final collision is the internal organs encountering the restrained body. This is the impact that has the greatest potential for injury.

Composite materials are relatively finite in their action. We can adjust a minor amount but the affect on the second collision is relatively fixed. With materials such as steel or aluminum the affects can be widely varied. Remember that we are looking at an Impulse. If one can change the same force from a .01 second period to a .05 second period the peak applied rate is significantly changed, possibly even from a unsurvivable level to a survivable one.

Human bone structures start to break at an applied force of 12 Nm. Human organs start to tear and fail at close to 6 Nm. Even a small change in materials or design can make a huge difference if it crosses one of these threshold values.

This can be a very convoluted challenge. The interactions can be very complex. The shorter answer is that anything that can redirect the force or dissipate the force in such a way that human occupants are injured at a lower level or rate is considered "safer." Right now this seems to be easier with metals than with composites.

 

[Baluncore]

Making the vehicle heavier increases the time it takes to decelerate and so reduces the G to the occupants.
A head-on collision with a tree, should shear the tree. That requires greater vehicle mass.
A head-on collision with a heavier vehicle will hurt you more than them as you go backwards.
Who are you trying to protect here?
There seems to be an inverse relationship between safety and fuel economy due to vehicle mass.

 

[DaveC426913]

Making the vehicle heavier increases the time it takes to decelerate and so reduces the G to the occupants.
A head-on collision with a tree, should shear the tree. That requires greater vehicle mass.
A head-on collision with a heavier vehicle will hurt you more than them as you go backwards.
Who are you trying to protect here?
There seems to be an inverse relationship between safety and fuel economy due to vehicle mass.

Indeed. A few years ago, I saw a video of one of those tiny Smart Cars in a crash test.
It survived quite well.
But upon reflection, I wasn't so sure that was a good thing.
The less massive the car, the relatively more massive the occupants as part of the whole, to-wit:
A 150lb person in a 3000lb car is 5%.
A 150lb person in a 1500lb car is 10%.

Does that not mean that the car will stop faster, but the occupants won't? i.e. a less massive car is less able to absorb the impact instead of the passengers doing so.

 

[berkeman]

Does that not mean that the car will stop faster, but the occupants won't? i.e. a less massive car is less able to absorb the impact instead of the passengers doing so.

Two words: Airbags!

 

[Ketch22]

Does that not mean that the car will stop faster, but the occupants won't? i.e. a less massive car is less able to absorb the impact instead of the passengers doing so.

This is an excellent question. It is one that involves thinking outside of our perceived limits.
First a little side note for the engineering types among us. There is no "absorbing" of force, it can be dissipated, redirected, or translated but not absorbed. I am sure there are other ways it is utilized that are commonly thought of as absorption but these are the ones I am currently working with and remember.

Second, is the concept that safer does not mean slower or faster. It is in this respect a referral to higher likelihood of human survival or lower likelihood. Given that there is two known parameters of 12 kiloNewtons of force or 6 kiloNewtons, what and by how much do we cross these limits? This is the first conceptual box we need to open. Let's for example stay in round numbers. If a typical Human organ starts to break down or be damaged at 6 Kn of force this gives us a parameter. There is good research that shows this to be a valid number. It is very hard to define the Impulse strength of the defined force. A 6 Kn total force applied over a .01 second period and translated through redirection or heating has the same amount of force as a 6 Kn force applied over 0.1 seconds. It has effectively done the same amount of work, however, by changing the time it has done a lower Hp of work by a factor of 10. Most of the current research is focusing on Human "safety" and not on the actual numbers which are hard to define as the research is done with cadavers. There is also a challenge as the test subjects are "preserved" which changes the physical characteristics.

Third is again a known. A 1,000 Kg vehicle traveling at 50 m/s has 1,250,000 Joules of kinetic energy. If changed to a 3,000 Kg vehicle traveling at 50 m/s it has 3,750,000 Joules. There will be a 3-1 ratio directly proportional to the weight difference. However, for the occupant, even if they are changing direction of travel, does this involve an application of force greater or less than threshold for injury. Once again this is not easy to test and is masked by how well our participants (cadavers) survive.

Best available science says that even if restricted by a severe mismatch, extending the time of impulse application, the survivability is increased (safer). A reduced impulse time usually leads to reduced survivability.

We are back to what is actually safer (depends on the speed involved as that affects the kinetic energy and the proportional relationship weight as this has an observable but reduced effect) and how do you define safer?

 

[Baluncore]

Think of it not as a car, but as packaging.

An inner capsule with restraints and bubble wrap, supported and surrounded by sacrificial energy absorbing materials that succeed by progressive failure.

The inner capsule must take significant energy to distort, but a door should still open. The inner capsule will be accompanied by a fuel tank. If the fuel tank burns will that destroy a composite capsule or a steel firewall?

One question is how much energy it takes to crush the polymerised resin between the fibres, as opposed to that needed to crumple sheet steel structures. I am concerned that composites preserve too much resin and so fail along restricted lines. That is a good argument for spherical or doubly curved surfaces that consume more energy as the damage progressively spreads over a larger area.

There is a packaging parallel in the early use of plastic design. That is the design of the Morrison shelter for WWII. They were flat pack and much lighter weight than other designs, so could be assembled inside an existing building. Over half a million were distributed because the material cost was so much lower than a heavier rigid construction. The building took the brunt of the damage, while significant distortion of the cage was expected to occur when needed.
https://en.wikipedia.org/wiki/Air_raid_shelter#Morrison_shelter
There were many stories by survivors, because they survived with few injuries and were able to get on with their lives. Like so much of engineering technology today, plastic design and crumple zones got their start in that war.

Only after most Morrison shelters were installed did information of effectivness start to accumulate. Today we have sufficient cars involved in accidents to get good feedback for optimum design of the many different safety features.

 

[DaveC426913]

Second, is the concept that safer does not mean slower or faster. It is in this respect a referral to higher likelihood of human survival or lower likelihood. Given that there is two known parameters of 12 kiloNewtons of force or 6 kiloNewtons, what and by how much do we cross these limits?
-snipped-

I wonder if this is over-analyzing?

All other factors being equal, surely a car weighing half as much will deliver correspondingly more energy to the occupant?

If we hypothesized a car made of magical material that was as strong as a regular car, but it only weighted 150lbs, then the car would essentially stop instantly (since it has so little inertia (momentum?) I always get those confused). The passenger would not benefit from a car that continued forward.

 

[DaveC426913]

Two words: Airbags!

All other factors being equal.

i.e. the more massive car has airbags too.

 

[berkeman]

All other factors being equal.

i.e. the more massive car has airbags too.

Two more words: Crumplezones!