All Aluminum Bumper???!!!!???

What do you guys think about it? Here's the thing, I have access to a full 4' X8' sheet of 1/4" heavy aluminum. Do you think it is acceptable to manufacture a front bumper with possibly a 2" receiver also? With the right construction and bracketing and welding would it pass? Would it have the strength for a winch? Please let me know your opinion.

The only thing about aluminum is that is doesn't fatigue like steel does. In other words, Both metal and aluminum can be deformed up to the yield point and will still return to thier natural state. Once past the yield point both metals will reach the "plastic region" in which the metal will no longer return to it's prior state. However, aluminum can break without reaching the yield limit where as steel will not. So lets say aluminum has an ultimate strength of 60,000psi...if you subject the aluminum to a repeated loading and unloading of a stress less than that of the ultimate strength it will eventually still break. Now this is not to say steel won't either, but it will occur for different reasons. That being said, I still don't think you would have any issues building a front bumper out of aluminum...I think it would turn out rather nice.

I seem to remember that there was an aussie company that made them in aluminum, TJM or something like that, looked similar to an ARB only SHINY bling bling...

It will only work if you polish and buff it so its got a nice and shiny appearance :sunshine:


It could work but there would be a huge investement in time and money to do it right. I would stick with steel since its a lot more forgiving than aluminum is.

AARON

I would never use it for anything that will hit rocks or have a recovery point. For a street rig that just needs a little protection, sure.

Aluminum makes great beer cans.

So lets say aluminum has an ultimate strength of 60,000psi

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7075-T6 is only 45ksi tensile

Like MogifiedXJ stated... aluminum has a finite fatigue life. It will fatigue, just a matter of time... Steel on the other hand as a thrushold limit where it will never fatigue if below this limit... the best aluminum's are 45ksi tensile... Even wippy steel is 80ksi... and can hit 350ksi under special conditions...

It ain't real unless it's steel baby.

Aluminum = beer cans

Man, I wish I could score a couple sheets like that! I use heavy aluminum for topping workbenches, it's better than wood, y'ask me...

5-90

robs said:

.. aluminum has a finite fatigue life. It will fatigue, just a matter of time...

.... Even wippy steel is 80ksi...

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FYI, both of those statements are bullshit.

An aluminum bumper would be nice, but you'll have to use thicker material to have the same strength. Even so, you'll have a lighter bumper. The down side is that aluminum is more expensive and fewer people are setup to weld it.

robs said:

7075-T6 is only 45ksi tensile

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Be mindful of the heat treating of what you use (the Tx part). Lots of aluminum will lose strength once it is welded, and the whole structure would need to be re-treated to get your strength back. There are many different flavors of aluminum. Find out what you got... and if it is heat treated. I think it would be cool if you could pull it off, but I think the odds are long.

I remember reading that aluminum needs to be re-heattreated after welding to maintain it's hardness rating.

I also know that it's true that it will fatigue faster than steel and will break before bending under high loads. It seems like it could create a dangerous situation, say if a dring tab snaps off before bending, sending it flying.

I think for the outer bumper, this is a great idea, but I wouldn't make the structure/tow points/hitch with it, why beg for trouble?

mellowvision said:

I also know that it's true that it will fatigue faster than steel and will break before bending under high loads.

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Please explain.

for instance, an aluminum bike frame is far stiffer and hard than a similar steel frame. The constant vibrations of riding and the impacts of bumps and drops fatigue aluminum over time, where steel will bend if pushed too far, but generally keeps it's strength. After years of heavy use, an aluminum bike frame has lost some of it's strength from fatigue and becomes more prone to cracking. There is usually little warning when an aluminum frame is going to crack, often resulting in a catestrophic break, leaving the frame in several parts. it's really unlikely that steel will do this, it will bend first, giving you warning of failure and usually avoiding complete breaks.

in the case of a bumper, it would be far better for it to bend than crack.

JonnyK said:

Please explain.

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See post #2

Perhaps an aluminum fascia over a steel superstructure - I wouldn't trust aluminum for something like that (steel is far more "plastic,") and there's a reason that automakers don't either.

Note that the newer "lightweight" bumpers still use steel for critical or bearing members, they just use polyurethane for the covering. You could do something similar, just using aluminum over steel.

5-90

You guys are full of crap. Aluminum seems to work fine for aircraft structure. If constructed with enough strength margin, there's no reason it won't last forever. The brittleness of either aluminum or steel depends on it's exact composition and manufacturing process. The reason automobile manufacturers don't use it more is because of COST. But you can believe what you want, it doesn't matter to me.

JonnyK said:

you can believe what you want, it doesn't matter to me.

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I think someone really does care.

Try again...

In low-flexion applications, aluminum will do quite well. When it's stressed well below the Yield Limit (the point where plastic deformation will occur and become permanent,) it recovers nicely.

The main reason that aluminum is used in the aero industry is the same that magnesium alloys, titanium alloys, and composite alloys are used - weight. Airplanes are not "lighter than air" and expensive to operate - so anything they can do to lower the weight of the airframe translates into cargo and passengers that can be carried, or less energy used to generate sufficient lift (cf. Bernoulli's Principle.)

Also, airframes are not generally subject to impact stress or point stress - like automotive parts. If an airframe receives an impact, it's usually written off entirely, or major replacement of structural members has to take place. Cheapness of steel is not the only reason it is used in automobiles - it's also cheaper to fix (since you don't need a controlled process to weld it, and it can usually be straightened out after being bent with minimal stress risers - damn near none if you're willing to handle a little extra process.) Steel and alloys can also take a "varaible" heat treat, and that can't be done readily with aluminum.

If you'd really like to get into a discussion of the chemical and molecular properties of engineering materials, I'd be happy to accommodate you - I'm sure I can answer most questions with references to hand. However, suffice it to say, that material cost is not the only reason to use steel (it's easier to work, it's easier to fuse, and it's easier to form - though more difficult to machine, than aluminum) - process control and manufacturing cost also play a HUGE part in it's selection. That's why DeLorean used CRES for his "futuristic, gull-wing" bodies, rather than aluminum - and why Corvette body panels (beyond the unit body and frame) are fibreglas rather than aluminum. I think you'll find that most structural elements are steel as well.

Aluminum finds extensive use on ships (corrosion resistance and a greater displacement ratio than steel) and airframes (less impact stress and lighter mass per unit volume) for the reasons I've gone into - and steel alloys are used in automotive applications for reasons I've referred to as well. I'm not saying that aluminum doesn't have automotive uses (later Ford Rangers, for instance, use aluminum driveshafts...) but if the part is going to be subject to impact stresses or point stresses, you're a lot better off using steel than you are anything else. Aluminum, magnesium, titanium, and all the other "Exotic" materials will do well with uniform stresses, repeating stresses, and stresses below the plastic limit, but take them beyond that point, and you're in trouble.

Also, it's worth noting that pretty much the strongest aluminum alloy in common use - 7075-T6 - approaches (as I recall,) 10L18 steel in strength - and 10L18 is a leaded, free-machining steel that isn't very strong. It's a low-carbon steel, used in low-stress applications (and material in machinery student shops as well...) You can do a LOT better than 10L18 - I think most body panels are 1040 steel, which adds about half again the strength of 10L18/10L20/1018/1020 steels with a slight increase in carbond content (and don't even get me started on comparing steels - we'll be at it this time next month, without even getting into the exotics and "superalloys" of steel...)

5-90

JonnyK said:

You guys are full of crap. Aluminum seems to work fine for aircraft structure. If constructed with enough strength margin, there's no reason it won't last forever. The brittleness of either aluminum or steel depends on it's exact composition and manufacturing process. The reason automobile manufacturers don't use it more is because of COST. But you can believe what you want, it doesn't matter to me.

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You can't really compare airplanes to jeeps...We could make one out of solid steel but I doubt it would fly very well now would it???

You can't just say were all full of crap without provinding a reason why. Materials like steel have a finite fatigue limit in which the material will not fail if it is loaded below this stress, regardless of the number of times it is loaded and unloaded or a torsional stress is applied. However, materials such as aluminum do not show a fatigue limit, and therefor will fail at any stress after a proportionally larger number of cycles. This can be proven by an "S-N" curve if you'd like to see one. The stress and cycles are directly porportional to one another. Period. There are alot of factors that will cause the aluminum to fail sooner or later. You can't really do anything about the grain structure, size, or inherent flaws the material may posses. Take for instance T6 aluminum...it will lose approximately half of it's strength after it has been subjected to welding. In order for the material to return to the T6 rating it must be post-heat treated and then artificially aged. The first step is to heat the material to 920F for a pre-determined amount of time that will allow enough "soak" time at that temp. and then it must be quenched rapidly, it must then be heated to 350F for 10 hours and allowed to air cool. Do you really think he's going to do this to his entire bumper??? NO. He's also wanting to know if a piece of 1/4" plate aluminum will be strong enough for a winch...I wouldn't want to be standing in front of it. I don't even know what grade aluminum he's talking about, but it's proabably not t6. It will also need to be tig welded inorder to obtain proper heat and penetration. You could mig weld it, but for something that's so stucturally crucial I wouldn't do it.

Hahaha... JohnyK you so smart! Enlighten us.. just cause you want something to be true, it don't make it true. You clearly are out of your league here. I'm a metallurgist that's been in the heat treating field since 1982. Think I've got the background.

Couple things still need clarification...

Aluminum is not more rigid than steel. The modulus of elasticity for steel is 30x10^6 in/in/psi. Aluminum is 12x10^6 in/in/psi. So under a given load for identical parts, aluminum will deflect about 3 times as much as steel. Ever wonder why the aluminum flat bed trucks on the interstate have that huge arch in them? That's why.

Just so happens you can do fabrication / manufacture with aluminum such that they behave more rigid by mechanical advantage. The material itself is less rigid.

Metals don't have molecules. They're crystalline.

Compared to steel... Aluminum is butter. Every rock will dig in and try to grab it. Try some AR400 steel if you wanna slide on rocks (someone out there is using this for diff skids, forget who).

Aluminum has it's place. Beer cans is one. Airframes are another. They're light. In airplanes, weight = money.

How many aluminum gears do you see in transmissions or drivetrains? How many aluminum parts you see on earthmovers or bulldozers?

For a Jeep. It ain't real unless it's steel. Build your aluminum bumper. Nice bling. Don't bounce it off rocks, cause it will stick to them anyway.

Hey Robs how does AR400 compare to 300M for torsional and axial loads???

Thanks Jim.