Why is steel stronger for tanks but not for airplanes?

[wnissen]

This may be a bit off-topic, but I was always under the impression that aluminum was stronger than steel, for a given weight. Thus why aluminum bike frames are preferred to cro-moly steel, airplanes are 80% aluminum, even car engine blocks are aluminum instead of steel, which was practically unheard of until the 1990s.

And yet a steel tank of equivalent capacity weighs less than an aluminum one and is smaller. What's going on with the material properties here? Is this tensile strength vs compressive or shear? If anyone can enlighten me, I'd appreciate it. Thanks!

 

[JamesBon92007]

This may be a bit off-topic, but I was always under the impression that aluminum was stronger than steel, for a given weight. Thus why aluminum bike frames are preferred to cro-moly steel, airplanes are 80% aluminum, even car engine blocks are aluminum instead of steel, which was practically unheard of until the 1990s.

And yet a steel tank of equivalent capacity weighs less than an aluminum one and is smaller. What's going on with the material properties here? Is this tensile strength vs compressive or shear? If anyone can enlighten me, I'd appreciate it. Thanks!

I sure don't know why, but I do know that if you compare a steel 72 to an AL80 the aluminum tank is larger overall (more displacement), has thicker walls, and weighs more and doesn't hold a whole lot more air, specifically if you fill the 72 +10% and the AL80 to it's 3000 psi rating. One guess on my part would be what they call "sustained load." It seems that, over time, the steel holds up better if kept filled to the maximum pressure but the aluminum (at least some of the alloys) has had some failures.

 

[tbone1004]

many car engine blocks have gone back to steel fwiw.

it all has to do with the type and quantity of forces.
Diesel engines are typically no more than 25:1 compression ratio and I am unaware of an aluminum block diesel *I'm sure there is one, just don't know about it*. Gas engines don't really go above 17:1. That compression ratio is easily converted to pressure since the "1" is 1 atmosphere, 1 bar, or 14.7psi. So 25:1 is roughly 370psi. That's a LONG way from 3000psi in most aluminum tanks. Aluminum is quite soft so you need a bunch of it to be able to resist bending forces which is why aluminum tanks are about 3x as thick as steel tanks. Roughly offset by the density of steel being ~2.5x that of aluminum, but the aluminum tanks also have flat bottoms which adds a considerably amount of weight to them.

If we want the best comparison out there, we can look at the Luxfer AL72, against the Faber LP85. Same diameter, same length, one aluminum flat bottom, one steel round bottom.
Both 7" long, both 26" tall. AL72 weighs 28.4lbs, the LP85 is 31lbs. Due to the difference in the wall thickness, there is a difference in water capacity, the LP72 holds about 10 liters of water where the LP85 holds 13 liters of water. Pretty significant increase in volume which explains the capacity difference, but the tank is about 3lbs lighter, and if you made it round bottom, or made the lp85 flat bottom, it would be about 6lbs delta. So of comparable physical dimensions, the aluminum tanks are considerably lighter, but because there has to be more material to resist the forces, the capacity is reduced. With this specific example, the wall thickness causes the aluminum tank to have a capacity about 75% of the steel tank.

If we look at aerospace, bike frames, etc. there is a minimum thickness required to resist bending. This is why composites are so brilliant because you can make incredibly thin structures that are incredibly rigid. There is no need for the actual strength of the materials in terms of tensile strength, or in this case resistance to pressure, but you need the thickness for the rigidity. Aluminum being less dense than steel is lighter for comparable rigidity, fiberglass even better, and carbon fiber one of the best materials out there. We don't really care about rigidity in scuba tanks, but it is inherent to the material thickness required to maintain that pressure.

We can make tanks out of carbon fiber, and some do, that are idiotically light, but idiotically light also means idiotically buoyant so the weight advantage on land is instantly negated by the need to put ballast on your body so you can become neutrally buoyant.

That all make sense?

 

[Superlyte27]

Also gotta keep in mind that these are stretched and released over and over and over again. Steel likes that. Doesn’t care at all. But stretch and contract aluminum and it gets brittle and weak. It’s why we can put 4000psi in a 2640 tank over and over and over again, but we don’t dare try 4000psi in an Aluminum tank.

I used to run a hydro facility. I used to find it interesting seeing just how much a tank stretches. In order for the tank to pass hydro, it would need to come back to its original size +/- 10%.

 

[BRT]

If you made steel parts for airplanes that were as strong as the aluminum parts the steel parts would be so thin they would be likely to corrode away.

 

[Johnoly]

...That all make sense?

Just tossing this in the thread salad but...........
Cousteau used 40's Titanium tanks for a while.

 

[Steve_C]

There is a framework with an aluminum skin over it. It only has to hold in a pressure of a bit less than 1 atmosphere for the central tube. Goal is to keep weight down. Plane undergoes major temp changes each flight. Using dissimilar metals could create issues. So best to avoid doing it except in critical areas.

 

[Charles2]

This may be a bit off-topic, but I was always under the impression that aluminum was stronger than steel, for a given weight. Thus why aluminum bike frames are preferred to cro-moly steel, airplanes are 80% aluminum, even car engine blocks are aluminum instead of steel, which was practically unheard of until the 1990s.

And yet a steel tank of equivalent capacity weighs less than an aluminum one and is smaller. What's going on with the material properties here? Is this tensile strength vs compressive or shear? If anyone can enlighten me, I'd appreciate it. Thanks!

Speaking in generalities, aluminum is about one-third the density of steel and has about one-half the strength of steel. Therefore, your impression that aluminum is stronger than steel, for a given weight, is correct.

The specifications for 3AA (steel tanks) and 3AL (aluminum tanks) are very similar and, in fact, use the same calculation for wall thickness. In case you are interested, that calculation is: S = [P(1.3D²+0.4d²)]/(D²-d²) Where: S = 67% of the ultimate tensile strength of the material in psi ; P = test pressure in psi; D = outside diameter in inches; d = inside diameter in inches. If you will do the math, you will discover that the wall thickness of an aluminum tank will be 2.5 to 3.5 times that of a comparable steel tank. There are other factors that determine the final weight such as bottom shape, but you get the idea.

 

[Rred]

The OP's mistake is using the word "stronger". There are many different "strengths".
In engine blocks, aluminum corrodes more easily and the engine coolant often must have extra corrosion inhibitors to compensate. On top of that, the cylinder walls need to resist "scuffing" abrasion from the pistons. Doesn't matter how "strong" the block is, if the piston rings scuff it away. And that surface isn't just drilled, it may be hardened or laser etched in many different ways.

Stronger? When they were building the Alyeska Pipeline, mechanics learned that the finest stainless steel tools became brittle and shattered in the deep winter. "Weaker" metals performed better.

Strong? That's like asking how long a piece of string is. You'd have to go into engineering properties to evaluate and compare two specific alloys.

 

[rhwestfall]

Did you know that the aluminum block in the 70's Chevrolet Vegas had wear issues requiring sleaving the cylinders with steel?

Modern Nitro burning top fuel dragster and funny cars are aluminum blocks with steel sleeves...