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Print 33 comment(s) - last by Keeir.. on Jan 10 at 12:47 AM

CFRP will also make the BMW i3 more lightweight with extra space

BMW said the i3 will have lower repair and insurance costs thanks to the use of carbon fiber reinforced plastic (CFRP). 
 
According to a new report from Autoblog Green, the BMW i3's use of CFRP will make repairs easier because most bumps at a traffic light or a parking lot will only damage exterior plastic parts, which are easily replaced. But for bigger collisions that actually do affect the carbon fiber, the damage stays local to that one spot. 
 
"When we evaluated carbon fiber, we started doing the safety, crash and repair concepts right from the beginning because just deciding on carbon fiber and then, when we're done, looking at [the details] would be a huge risk," said Manuel Sattig, communications manager for BMW i.
 
"Carbon fiber is, of course, a new material. Our dealers need to be trained for that specific repair system. But, if you look at the i3, if the car has a small bit of damage, someone hits you at a traffic light or bumps into you in a parking garage, you don't hit carbon fiber, you mostly damage the exterior plastic parts. They can very easily be replaced because you click out the damaged part and replace it with a new one. If you have a stronger accident, then, of course, carbon fiber will be damaged. The interesting thing is that carbon fiber is not deforming, so the damage only happens locally and it breaks only at that specific area."


Sattig noted that BMW has already talked the U.S. National Highway Traffic Safety Administration (NHTSA) and the insurance industry about the use of the new material, and they've decided to go with a "very low" insurance system. 

CFRP will also make the BMW i3 more lightweight with extra space. 

BMW officially announced the all-electric i3 back in July, giving the new vehicle a price tag of $41,350 USD. 

The i3 will feature a 22-kilowatt, 450-pound lithium ion battery, which will provide power to a rear-mounted electric motor. The i3 packs 170 horsepower and 184 pound-feet of torque, allowing the single-gear i3 to accelerate from 0-30 miles per hour in 3.5 seconds and 0-60 mph in about 7.0 seconds. Its top speed is limited to 93 MPH. 
 
For those concerned about range, the i3 has an electric range of 80-100 miles, and the battery can be charged with a standard system in about three hours.

In October, it was reported that BMW may boost production of the i3 EV due to early demand. At that time, the automaker said customers reserved over 8,000 i3s ahead of the official launch in Europe. 
 
BMW has plans to sell 10,000 i3 units next year and previously announced that it would adjust build capacity according to market demand.

The i3 will go on sale in the U.S. during the second quarter of 2014.

Source: Autoblog Green



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RE: Deforming
By theapparition on 12/13/2013 10:35:05 AM , Rating: 4
Wow, so much misinformation. Not particuarly you, just replying to your post.

First off, let the Apple marketing esque begin. There is no magic to CFRP. It's just plastic with a carbon fiber filler. People have been using this for decades. When you design plastic parts, you can add various fillers to give the parts strength. Fiberglass is common, but both carbon fiber and stainless steel fibers are also used per the application. For example, in the automotive world, there is the misconception that the Corvette is fiberglass. While older models did used hand laid fiberglass, new models use plastic molded parts with fiberglass fill. No magic here, BMW is just using carbon fiber instead. Some benefits to that approach and some drawbacks. But make no mistake, the body panels are just plastic and would be replaced just as any other plastic parts would be.

The other issue is structural members, where they seem to be using actual Carbon Fiber. Thats a completely different animal. Carbon fiber is awesome, but comes with lots of issues. First off, its really carbon fiber "composite". Composite means using two or more materials as one. Pure carbon fiber is either unidirectional or woven and isnt much different than a bed sheet. The matrix is an epoxy like "glue" that gives carbon fiber its compressive strength. Carbon fiber by itself is very strong in tensile strength, but horrible in compressive. Think of a string. You can pull it very hard, but collapes instantly. The epoxy gives the composite its compressive strength. So any comparisons to steel need to be accordingly. Tensile strength can exceed steel, but compressive strength is much weeker.

In a failure mode where the carbon fibers have been damaged, you basically cannot repair it. You have to replace the entire part. Once the fibers are broken, no way to mend them. With metal, you can weld. No such repair with CF. Thats simplistic, but realistic considering how Joe's Autobody would be doing the repair. And before people jump in talking about repairing fiberglass, while the mechanism is the same, your not using fiberglass in a structural situation. Sure, you can add a CF patch, but its almost like taping a structural member together. Not going to happen. Structural damage will require replacing the part.

So what BMW is saying in this press release is that major structural damage will require significant repair, but they will use a lot of plastic body parts to minimize the cost for most minor body damage claims. Ignore the CFRP and other hype.


RE: Deforming
By Keeir on 12/13/2013 2:02:13 PM , Rating: 1
quote:
The epoxy gives the composite its compressive strength


No.

For compression loading there are two things to consider.

1.) Compression Yielding, a material related property
2.) Compression Stability, a material and geometry related property

Carbon Filaments are very resistance to compression yielding. In the sizes typically used in CFRP, they have no/little compression stability. The epoxy is very weak in 1, but it allows carbon fibers to be loadshare/stablized each other. It changes the geometry of the situation, especially when plys are orientated in different directions such that the large minority of not oriented axially to the applied compression loading. It does this by transfer of shear loading. Epoxy in and of itself is typically quite weak, and a sheet of epoxy itself would be very weak in both 1.) and 2.) sitations.

Epoxy allows carbon fiber filaments to act as a (more) unifed sheet and develop significant 2.) to get closer to the theortical limit of the material as expressed by 1.). In and of itself, the epoxy add very little compression capability.

quote:
Tensile strength can exceed steel, but compressive strength is much weeker.


Strength and stiffness are two seperate issues. In the event of a "crash" event, the most important thing is to ensure that the acceleration applied to the occupants is low. Acceleration is change in velocity over the time period. Materials with high stiffness values (Young's modulus) shorten the crash duration increasing the velocity. This is the foundation of crumple zones. In place of finding/using low stiffness materials (which would create many secondary issues), automotive engineers use materials that deflect and deform by design, significant increasing the duration of a crash and reducing the elastic feedback of the material. The i3, if not properly designed, could work similiar to a billards ball. The structure could be very stiff and highly elastic, which would result is more change of velocity and shorter durations. Having worked with large scale composites, a CFRP part will typically deflect very little before complete failure. To resist the bending loads that likely drive the initial requirements for i3 structure would probably result in structural members that also resist axial compression from crash situations.

quote:
In a failure mode where the carbon fibers have been damaged, you basically cannot repair it.


Interesting. This is false.

http://www.mansbergeraircraft.com/Aircraft%20Compo...

A simple example. Even in the event of complete failure/severage, composite repairs can occur. It will likely not be economical, but there will be areas of the i3 that are not designed for economical part replacement either.

quote:
Sure, you can add a CF patch, but its almost like taping a structural member together.


Which is essentially all that epoxy is right? CFRP are essentially taped together fibers.


RE: Deforming
By Samus on 12/13/2013 10:32:13 PM , Rating: 2
Carbon fiber absorbs and distributed energy without deforming. Metal compresses through this process, where carbon fiber returns to its original shape (depending on epoxy.)


RE: Deforming
By theapparition on 12/14/2013 2:39:58 AM , Rating: 1
Sigh, you use a lot of textbook replies but obviously no experience.

Example 1, you still refer to CFRP in structural loading talking about deflection. CFRP is just plastic, so either you mispoke, or still lack the fundamental understanding. CFRP is NO different than what's being used on modern car body panels. CFRP is NOT Carbon Fiber. True carbon fiber panels get strenth from strand weave.

Example 2. Saying CF is just taped together by epoxy matrix anyway. The fibers are what gives carbon fiber its strength, and severing them would lead to a repaired part with the same strength as just epoxy matrix.

Example 3. I said quite explicitly that I kept the discussion simple. You went out of your way to get detailed, but still lack some of the basic fundamentals from experience. You think the average body shop will have the capabilities to repair? Yes, composite engineering is complex, and the dailytech readership only needs the cliffs. While the filament weave helps, the matrix still takes most compression loading.

So I ask you this. Have you ever done any design with composites?


RE: Deforming
By Keeir on 1/10/2014 12:47:07 AM , Rating: 2
Your suffering from some misconceptions. http://www.youtube.com/watch?v=gt1k3BLN7pw&list=PL...

With the i3, BMW is using CFRP from the entire body frame. They are using continuous fiber tape/sheets for many of the parts currently. Since multiple layers are required, they are using a epoxy type resin. BMW is not using "flake" filled molded parts for body structure . Sure for the body panels, and even some structural components, but a substantial portion of the underlying frame is composed of oriented plys.

In this situation, it is entirely possible to repair them. These type of structures have been used in aviation for years with repairs. It takes less than 3 days to be trained on the process. Its very simple. Maybe not quite as simple as welding steel, but still relatively simple. Taper sand damage. Lay in some adhesive epoxy sheet. Lay in some sheets/tape with some more epoxy. Cover with vacuum bag. Heat. Simple.

quote:
While the filament weave helps, the matrix still takes most compression loading.


Hmmm. So, how do you explain a nearly 20 fold increase in compression strength from a typical un-reinforced epoxy matrix to one filled with multi-orientated plys/tape runs? Yes, your right, in the flake filled situation, your stability will be limited by matrix. In an oriented sheet lay-up, the epoxy is not a matrix so much as a shear transfer/binding agent.

quote:
So I ask you this. Have you ever done any design with composites?


I'll ask you something in return. How many oriented ply layup panels have you had repaired and tested to failure? I've done several. You can completely repair oriented ply carbon fiber reinforced plastic composites. Simply scarf them. http://www.crc-acs.com.au/index.php/contact-us/12-...


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