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Super Strong Metal Foam Discovered
MikeChino writes to tell us that a North Carolina State University researcher has discovered what appears to be the strongest metal foam yet, capable of compressing up to 80% of its original size under load and still retain the original shape. The hope is that this amazing material could be used in cars, body armor, or even buildings to absorb the shock from earthquakes. "Metal foam is exactly what you might think – a cellular structure made from metal with tiny pockets of space inside. What makes Rabiei’s metal foam better than others is that she’s been able to make the tiny pockets of space more uniform. And that apparently is what gives it the strength as well as elasticity it needs in order to compress as much as it does without deformation. Many tests are being performed in the laboratory to determine its strength, but so far Rabiei says that the spongy material has 'a much higher strength-to-density ratio than any metal foam that has ever been reported.' Calculations also predict that in car accidents, when two pieces of her composite metal foam are inserted 'behind the bumper of a car traveling at 28 mph, the impact would feel the same to passengers as an impact traveling at only 5 mph.'"
http://www.rexresearch.com/rabiei/rabiei.htm
The foam is made by filling a mold with hollow steel spheres and then filling the gaps with molten aluminum. VERY scalable.
I wonder how it would fair if, instead of using molten aluminum to fill the gaps, you coated the steel spheres with aluminum (or other binder that melts at a temp lower than the spheres would start to collapse at) and sintering it into a solid block. More air gaps means it's lighter, but still very uniform.
=Smidge=
You'd rather have a big hunk of metal than an airbag? Don't let the "foam" fool you: slamming your face into a block of it at 35mph would only be a little better than running face first into a brick wall at the same speed.
It's squishy and springy...for metal. But it's not what you'd call soft.
ad logicam Claiming a proposition is false because it was presented as the conclusion of a fallacious argument.
Two youtube videos about the material:
http://www.youtube.com/watch?v=mI5ZzfOlbKA - earlier video
http://www.youtube.com/watch?v=wfFcs25KmMc - one week old video
Shows among other things compression tests of the material.
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TFA is a poor re-blog of the original article here, which has this video, where you actually hear how it is made: Hollow steel balls are pored into a from, (and presumably agitated to settle them in a uniform matrix), then aluminum is pored over them to fix them there. So yes, should scale up well.
The foam is made by filling a mold with hollow steel spheres and then filling the gaps with molten aluminum. VERY scalable.
Well, yeah, if you assume hollow steel spheres are "off the shelf". Kind of like saying starships are very scalable, you just make them with warp drives, problem solved.
I have cast aluminum and have had porosity problems. Basically some gasses dissolve better in hot aluminum and bubble out as it cools. Preventing porosity in castings is very old technology. I always assumed metal foams did the opposite of preventing porosity, and tried to supersaturate molten metal with hydrogen or argon or something under pressure and then froze it at a rate that grew the bubbles to just the right size. Metallurgists have no problem doing all kinds of complicated heat treatments and all kinds of weird alloys, so I figured the limitation was dissolving enough "whatever" in the metal to make it work.
"Science flies us to the moon. Religion flies us into buildings." - Victor Stenger
I read it as his desire to use this foam as a replacement for the bumper and crumple zones. It would turn the existing crumple zones into something in the car's frame and bumper system that would absorb a great deal more of the impact and, therefore, largely eliminate the need for airbags.
I'm not sure I'm buying it, though. Airbags are an "also need" feature, and cannot be replaced wholly by a better crumple zone.
The problem lies in the elasticity and the distance. If you hit a brick wall doing 65MPH and your crumple zone is too squishy, it will continue crumpling up until you are included in the crumple zone. In other words, you're dead.
Make it too hard, and the car will stop more quickly than your flesh can handle. The airbag is a crude but effective way of allowing a relatively stiff crumple zone that can manage to keep your passenger cabin intact during a VERY major impact, and still accommodate your body's need to decelerate as gradually as possible. If you hit a brick wall doing 65MPH, the crumple zone decelerates the car from 65MPH - 0MPH in the distance represented by the zone (usually a few feet at best), and materials aren't going to improve on that a whole hell of a lot. You are still going from 65MPH-0MPH in just a few feet. That's a SERIOUS amount of deceleration.
The airbag is what takes your head and torso and slows them down as gently and slowly as possible, leveraging the deceleration already provided by the crumple zone and making the best use of it to keep your brains from splashing around in your noggin, and/or snapping your neck. Which is not to say the airbag is gentle or slow at all, far from it, just more gentle and slower than making your dainty neck bones absorb all of the force as your torso is stopped by the seatbelt and your several pounds of head really wants to keep going to make Newton happy.
Could be worse, though. You could be wearing no seatbelt at all and expect your chest and head to absorb all of the speed when they impact the steering wheel and windshield respectively. That always ends colorfully, particularly in shades of red and grey.
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Do you have physics to back you up? No, I didn't think so.
Take a new Toyota Tacoma. Assume weight savings in replacing bumper with foam metal is used elsewhere so you have the same mass vehicle. A Tacoma weighs approximately 4000 pounds, which is approximately 1800 kg.
Kinetic energy is given by:
e=0.5*m*v^2
m = mass
v = velocity (or speed for our purposes).
The kinetic energy of a Tacoma moving at 28 miles per hour is approximately 141 kJ.
The kinetic energy of a Tacoma moving at 5 miles per hour is approximately 4.5 kJ.
That is, the foam bumper only has to absorb 31 times as much energy as the solid bumper to perform to the quoted standard.
See quote below, which is from here: http://www.rexresearch.com/rabiei/rabiei.htm
We see they estimate a factor of 80 improvement of energy absorption over the foam metal's equivalent bulk material. They don't say, but let's assume (reasonably) that they are talking about linear compression. Let's assume for a second that the stock bumper is made of a block of solid steel that doesn't absorb any energy. It's not, and it does, obviously.
If their estimate is correct, and a foam bumper of the same size will absorb 80 times as much energy as its solid counterpart, then the passenger in the 28 mph impact would feel 1-2 kJ of energy instead of ~140 kJ of energy. Obviously the bumpers are not solid metal, and they already have some energy absorption capabilities built into them.
Based on the factor of 31 between the kinetic energies of the vehicle at different speeds, I think their claim is the opposite of bullshit. It's reasonable.