Surely repeated dives on an aluminium structure would deform it, retruning to the surface allow it to go back to the original shape and therefore work harden? The same as aircraft fuselages eg Comet, famous for it's metal fatigue
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Ok, it is a long time since i studied any of this sort of thing, but so far as i can remember for work hardening to occur, there has to be "plastic" deformation, as opposed to elastic deformation. So, you would have to stress the material beyond its yield point. Obviously, a pressure vessel would be designed such that the max stress did not come anywhere near this level.
I do not think aluminium exibits a clearly defined yield point though and i cannot remember how they decide upon the max allowable stress for the stuff.
Anyway, as regards "metal fatigue", again, so far as i can remember, this, once again, involves stresses in the range that induce plastic deformation and without plastic deformation, you cannot have metal fatigue..
This is complicated by two things, once again, so far as i can remember, aluminium does not have a "fatigue limit", that is, a stress below which fatigue life is indefinite, the stuff will break at some stage, but probably a much more important factor, would be bad design, or damage, giving rise to "stress raisers" such as cracks and sharp corners and the like.
The presence of these would mean that although the stress in the bulk of the structure was in the elastic range, at the sharp tip of a crack, the stresses would be concentrated such that plastic deformation WOULD occur, complete with the attendant work hardening, which would render the material at the tip of the crack less ductile, or more brittle if you like, and therefore, this cycle of hardening and cracking would continue and the crack would spread until failure occured.
There are mechanisms by which "brittle fracture" occurs although this is a bit beyond me, although it is something along the lines of, the spreading of a crack involves the production of more surface area [the sides of the crack] This involves energy, the production of the additional area i mean, although i cannot remember why and the spread of the crack is arrested.
Basically though, if or when a stage is reached, where more energy is released by the spreading of the crack than is required for its production, then the crack will proceed through the material driven on by the elastic forces in the material at about the speed of sound and obviously the structure fails.
This is why when they first started welding ships together that had failures where the entire ship would fly into two pieces instantly, where as, in a riveted ship or structure, the crack would proceed to the end of that particular section of plate and then stop..
john..
You may well be right, I did a bit of metal inspection work (3.4u qualified) when I was piloting ROV's, inspecting sub sea structures around oil rigs etc. Quite a bit of the work was finding cracks that were then later repaired by welders. Nightmare work, you had to be absolutely meticulous staring at the screen for hours on end. This course was steel rather than aluminium which again will react differently. Some of the defects did occur through work hardening according to the boffins, sometimes whole sections were replaced where they were subject to constant pounding by the North Sea.
novelsmike
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At last another metallurgist gives us a complete answer on the materials issue. Mind you, my degree was in 1970 so not exactly up with all things metallurgical!
The problem being of course, Titan wasnt made of metal
Apprentice87 certainly gave us all a good insight to metallurgical issues, every day a school day!.
A good example of the ships that fell apart was the ' Liberty ships' built in the USA during ww2. These were the cargo vessels used to transport supplies to the UK. They had been welded for speed of construction, with square hatches in the deck to access the hold, the corners of the hatches is where the cracks started. Cured by simply rounding off the corners. Airplane windows have rounded corners for the same reasons
Exactly. Certain steels are not so good at low temperatures. They have a "ductile/brittle transition temperature" [This is fairly self explanatory] Once below this temperature, bad things happen!!
The problem with a ship i suppose, is that there will be a lot of residual stress from welding, as you are hardly going to be able to "stress relieve" an entire ship. I read once, that during construction a large ship will end up about a yard shorter than it was, thanks to shrinkage caused by welding.
Anyway, the entire ship will be like a "coiled spring" if you like. Thanks to in the first instance, plastic deformation [as opposed to elastic deformation] because, although the overall stress within the structure is within limits, thanks to the stress raiser [like the square windows you mention] the stress in these areas has gone above the yield point, before long cracks will form..
Think about this; If you got yourself a flat steel bar and did one end up in a vice, you could bounce the free end up and down as long as you liked. Nothing would happen as the deformation would be within the elastic range.
If however, you forced the free end up and down, PAST the point, that when you let it go it returned to the original position, then the stresses would be such that you were now causing deformation in the PLASTIC range, [which is why you have now permanently bent the bar] and you all know from experience what is going to happen with repeated stress cycles..
In the case of the ship, when you get to the point that the energy released from the system by the crack spreading is greater then the energy required to form the crack itself, the crack as i explained before, will then propagate at about the speed of sound as the "coiled spring" ship "twangs" back to an unstressed state..
When divers go to have a look they can clearly see this, as a normal tensile failure will have "necked off" with a reduction of area, bit like if you pulled a stick of toffee apart. The brittle fracture failures, will not be like this, they will look more like if you snapped a thick pane of glass into two pieces with no reduction of area..
My poor old brain has had its biggest workout for about 30 years remembering all this sort of stuff!!
john..
How did they do the crack detection at the bottom of the sea?? How did the welds last?? We were taught that if something fails as a result of abuse, then you can probably repair it. If however, it fails as a result of normal use, then it is a design fault and if you merly "repair" it, it will go on to fail once again..
Was the welding the "dry" sort where the welder is in a chamber or the sort where a bloke in a diving suit goes down with a handful of welding rods with varnish on them!
john..
Your poor old brain is doing better than mine
There's something about how sq corners propagate fractures lurking in the back of my mind, but I will have to go look that. Such concerns have been of little interest to me since becoming a sparky
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MPT wrap a coil of cable around the item, switch on a highish current, MPT shows the crack
It varied
Brae Bravo was falling apart literally but it was almost end of life
I can see that.
No, it was relatively shallow, less than 30 to 40 metres, after that you had to get sat divers on location with a support vessel. The welding was just done in open water, impressive to watch.
With the magnetic particle inspection then, they must have turned the current on and sort of rubbed the iron filings stuff on with their hands to see where it stuck??
Must have been hard to see as on dry land they spray the area with white paint first. I suppose that in the middle of the north sea you just have to do the best you can!
john..
Your poor old brain is doing better than mine
There's something about how sq corners propagate fractures lurking in the back of my mind, but I will have to go look that. Such concerns have been of little interest to me since becoming a sparky
Yes they will; Anything that gives rise to "notch effect" and concetrates stress will do it. Notches, cracks, surface marks and scratches, anything that acts as a "discontinuity" This is why years ago engine tuners used to polish con rods and the like to remove such things.
One thing that winds me up, is "would be expert welders" that all try to imitate what they call the "stack of dimes look"
All they are doing, is creating hundreds of stress raisers. Excessive ripple is a fault in welding, the smoother a weld is, the better. Go look at the welds on pressure vessels. They are very smooth and blend nicely into the parent plate..
A serious welding fault is "stray arcing" This is where someone touches the rod down outside of the weld itself. This, in most steels, gives rise to a small very hard spot that acts as a very serious stress raiser with predictable results..
john..
We use various methods, ultrasonics for metal thickness and flooded member detection, x-rays (hated that, really dodgy radiation source IMHO)
Me at work!! (rare occasion LOL)
Sharpend
"It Just Is"
That’s not work that’s watching telly!!
LOL, believe me, $1,000,000 ROV, a multi million dollar blow out preventer and you're in the hot seat placing gaskets on wellheads the pressure is unbelievable, especially when you find out everybody is watching the operation - on the vessel, on the beach at HQ etc etc, it was horrific. When it went well it was brilliant, a real feeling of elation. I'm glad I had the experience at the end of working life, looking back it was amazing and I'm privileged to have been part of it.
Clearly that was a well designed primary use, it's secondary use as a porthole / viewing window however needs further work.
also noted that nothing recovered appears to be of the carbon fibre tube. if the porthole window did fail first then id expect that the tube may have survived in a reasonable shape, maybe broken into a few pieces, but at least something there to bring back. but there wasn't anything resembling the tube in the video which would probably imply that there isn't anything left of it to recover, so most likely it was the tube that failed first
From some of the later reports it sounds like they have recovered some large chunks of the sub with presumed human remains.
It certainly looks like they are going to recover as much as the can and investigate the failings of this experimental submersible
It certainly looks like they are going to recover as much as the can and investigate the failings of this experimental submersible
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