steel VS aluminum toughness at low temps?

I read someone commenting to Apex that aluminum would be better for ice because steel is more affected by "low temp".

I'd always thought that the entire range of liquid water(salt or fresh) would be "room temperature" to either steel or aluminum.

I've never heard of this in automotive design where structures are routinely subjected to far more extreme temps on both ends.

Then again with a car that crashes you WANT the metal to mangle and don't care if the car is at all viable afterwords (unless you are in boondocks during one of the extreme temps, but that is rare enough).

 

do some googling on Liberty ships that fractured from keel to deck in cold weather in WW2 - due to brittleness (opposite of ductility or toughness) in the steel. You will find some interesting pictures.

Other ship casualties since then have been due to the same cause

Steel in ships needs to be ductile rather than brittle. All ship structures will have some imperfections (cracks) which could (if the steel was not ductile) grow at alarming rates into life/hull threatening fractures.

Dangerous brittle fracture (very, very fast crack growth) in ships can happen when you have a defect in brittle steel, and one or more of low temperatures and high strain rate (e.g. impact loading).

This type of cracking was not a big problem in riveted ships (or aircraft) because the crack would stop at the end of the plate in question. With the advent of welding that was no longer a limiting factor. Hence the "unzipped" Liberty ships.

Steel toughness and ductility is definitely adversely affected in colder temperatures. Shipbuilding steels have to have Charpy values of toughness for different temperature ranges.

You have to consider the air temperature as well as the sea temperature - as the crack may start in the above water portion - and (for a large ship) be just as deadly if it started there as in the keel. I guess for a small boat, a crack in the deck is less important than one in the bottom. Also, in small boats the stress ranges and rates tend to be less than in big ships.

 

I was a shipwright in another life working the very shipyards that sent those Libertyships down the ways. A dirty little secret, part of the "tribal knowledge" of the shipyard subculture, workers were paid piecework during WW2 and it was common to fill large gaps with welding rod bundled together and weld over the top of it for a "perfect" fit.

A guy I worked with said that little secret came back to haunt his grandfather. Those boats were not that well engineered and had problems with notch sensitivity. Others will chime in but with modern technique, design, and materials I don't think you're going to have a Liberty moment.

 

Do´nt qoute me wrong please!

Though the statement was right in general (as JRMacGregor explained), I came to another conclusion! see:

http://www.boatdesign.net/forums/me...o-anywhere-trawler-build-side-side-28678.html

For a Icebreaker steel is still a good choice, provided you choose the right one!
You can build a icegoing boat from Aluminium, no doubt, and it is a good material, but there is a hughe difference between icebreaking and icegoing!

And seawater starts freezing at about -2°C, I would not call that room temperature.

Regards
Richard

 

Steel has come a long way since WW2 (or earlier). (and so has aluminium).

And steel used for cars is different than steel for boats or general construction. Actually, most cars nowadays are made from several types of steel, with different properties, to get the best out of crash protection, cost and weight.

Still, in the 70's there were still (older) workers who went crazy if someone dropped a heavy object on steel, in the cold. Due to the fear for cracking.

(and crazy enough in the 90's I learnt about all that 70's info at school. No information had been updated ever since.)

 

All larger Icebreakers of the world (as far as I know them, and that are many) are made of steel.

Though these vessels are quite different from average cargo ships, having much closer spaced framing, ice stringers, reinforced stem and thicker hull plating, they have been made from ordinary shipbuilding steel until WWII.

Even some of the oldest examples "Krasin" and "Suur Töll" survived for a century in the cold Russian and Estonian winters.

And of course it IS possible to make a icegoing, even icebraking yacht in steel. See my vessel as a example.

Cars are not a good example to compare. There is not only the skin, there are many additional structures we do´nt have and do´nt need in ships.

Regards
Richard

 

Squiddly!!!!
I am not an expert of aluminiums degree strength resistance.
But there is different steel grades for building Ice breakers.
"Det Norske Veritas" (DNV) does have gradual sheet lists for this.
For an example ordinary quality they do call grade "A".
For an example thay do also have grade "D" ,"E", "A-36", "D-36", "E-36".
This caused by different metal properties.

For the grade A there is an early strength decrease. At -20 Celsius the toughness is significantly decreased.

For the Grade A-36 the same toughness is about 36-40 degrees if I do remember this correctly.

I do assume you are not building a submarine.
kidding!!
But you do also have hull above the waterline. So lets say that the temp falling to -30 then maybe the grade "A" will brake like glass. When started to crack you also very easy to have a crack down under the waterline. If not then you do have to begging for low wind when sailing home.

So Iron actually is changing it´s properties a lot during temp diffs.

Hope someone is able to answer what happens with aluminium. Or maybe not happens!

Kind regards!
Janne

 

You are right nukisen.

"Dillinger Hütte" E 36 GL (EH 36 is LR spec) for plating, and D 36 for frames is the material of choice.

You find it here too:
http://www.boatdesign.net/forums/me...awler-build-side-side-28678-4.html#post294239

 

Tolly, Out of what...3000 built in the early 40's, I don't believe a Liberty Ship failed in this way before the end of its five year design life. They were built as quickly and cheaply as possible, to be fair. They were what they were.

 

Mark,
I'm not really sure what you're saying. I agree with you, they were what they were. Only a few fell apart and sunk.

I was passing along a little bit of the lore that was still part of the shipyard culture 25 years back.

 

Yes Apex1!
As even the letter A, D, E, Z, Also have a meaning of the properties of the steel.
The Z quality tells that the material is cross rolled and checked up so there isnt any laminated layer in the material, often used in dif kind of studs.

Also there is differents in the springy, also with the hardness of the steel.
with Springy means that you put up 10 meter long material and press it down in the middle. then how much you can force it to bend and when you release the pressure it will go back to its original shape.

So once again if my remember is fresch I suppose that the D quality is a little more springy then the a guality. And also the E quality is harder then the A quality.

Hahaha Tolly I do suppose with your signature you are a hunter also.

 

Hi Mark

The reference to Liberty ships was not a criticism. Just a fact. The problems with these ships KICK STARTED changes in the minimum specification of the chemical composition of steel for good behaviour in cold conditions - as summarised by Nukisen above.

But considerable numbers of these ships did break - some of them even before they were delivered. See this link.

http://school.mech.uwa.edu.au/~dwright/DANotes/fracture/maritime/maritime.html

But you are right - these ships were simple - built quick (not cheap) to do a jiob. The Liberty ships were an ancient British design of general cargo ship brought to the US for mass production.

 

Still waiting for someone can tell about the Aluminium specs!

Damn this made me curious!

 

That is a one liner (in general, we do´nt like one liners here).

Aluminium does not change its properties at low temperatures. (temperatures we still can navigate) So a 5086 is a 5086 even at -20°C

Regards
Richard

 

Then see what came out of it!
Then the question is solved once again if economic defensible, then use alu.
If not then use steel.
Else if you not want to use epoxy. hahaha

soon it feels like sniffed the accelarator. about my epoxy research.