Bismarck: Scuttled or Sunk?

[lwd]

Even if it breaks at a water tight bulkhead I would think the strain would likely compromise the water tightness to some extent.

[Herr Nilsson]

The watertightness was compromised by the torpedo hit anyway.

[José M. Rico]

I believe Bill Jurens has commented on the poor welding plan for Bismarck's bulkhead.

I will have to check this, but I believe the joints at frame 10.5 where the stern broke off were riveted and not welded.

[Herr Nilsson]

The bulkhead itself was riveted and it was riveted to the hull.
IMHO the reason of break off of the stern is that all plates of the stern are not staggered and are ending at 10.5. With 12 mm they are much thinner than the plates they are welded to at 10.5. The upper deck at the stern was just 6 mm surrounded by a 1200*10 mm stringer.

[Bill Jurens]

Herr Nilsson has it right. I found this particular failure quite confusing when I first examined the wreck in 2001; the stern was broken off very cleanly, but there was a narrow 'shelf' running around the entire perimeter of the break, i.e. the break actually took place a short distance abaft the after surface of the bulkhead. We imaged the break in extreme closeup and it was clearly due to a ripped weld. A few years later I got the plans that Herr Nilsson posted in the previous memo, and I was able to review additional videotape made by James Cameron. At that point the entire situation became crystal clear. The weld line showed signs of fatigue failure, which suggests that it was initially broken by the stern torpedo hit and subsequently 'unzipped' over a period of hours or days thereafter. It is likely that the stern shell plating separated close to the surface shortly after sinking. Had the stern remained intact all the way to the bottom, it probably would have been found already nearly intact, as was the stern of Hood. Some of the decking is still attached to the bulkhead at Station 10.5, bent downwards almost 90 degrees.

Bill Jurens

[Dave Saxton]

I concur that a discontinuity there created by the transverse armoured bulkhead represented the essential problem. However, the shell plating and the welds joining the shell plating would not be the primary structural strength members. Those would be the framing and the decks structures.

Here the problem turns out to be the fillet welds required by the prohibition of welding directly to armoured bulkheads prior to mid 1939, and the fact that an armoured bulkhead was there. It was not really known until the 1990s that fillet welds will fail if the adjoining plates become distorted by shockwaves. Otherwise fillet welds are the strongest possible practical joint design which could be used here. If the fillet welds used to join the internal decks are lost due the shockwave from the torpedo explosion, then all that's left holding things together becomes the butt welds joining the shell and the oberdeck. Obviously these butt welds alone would not be up to the task for long being worked by dynamic wave action, and of course they would not be asked to do this task by the joint designers, whom would not have expected the fillet welds to fail.

If the stresses and loads exerted are too great for the structure and the joint, the failure will always occur right at the weld, unless the weld is greatly over matching the parent metal in tensile strength. This is because the weld itself is always less ductile than the parent metal of similar metal and strength. Welds to ST52 to ST52 and welds of St52 to Wh would always be under matched or no more than matched. For example, 1942 tests of St52 weldments indicated that the parent metal had an ultimate tensile strength of 53.3kg/mm2, while the weld was 52.7kg/mm2. Although it is very close to matching, the weld is less ductile with an elongation of the weld metal was 27% and the ST52 elongation was 31%. Furthermore, it will show signs of fatigue failure because the yield strength of the center of the weld will be greater than the fusion boundaries.

For welds of St52 to Wh the weld metal will be an austenic stainless steel of much less tensile strength than the armour steel but SS has much greater ductility with elongation ratings of around 45%. Because the SS has much less tensile strength than armour grade steel the break will of course occur right in the weld if the load exceeds the tensile strength of the weld metal.

Weldments of mismatching section thickness require tapering of the thicker piece and this will mostly alleviate the problem of a discontinuity. This joint design calls for this so the particular failure was probably not caused by not staggering the joints. However, the amount of taper may not have been gradual enough.

[Dave Saxton]

Several years ago I ran across B&V's welding protocols for shipbuilding based on their experience during and since the war. For joint design:

* All butt seams must be (back) welded on their root side. Backing strips are not permitted

*No interrupted fillet welds are permitted on stiffeners

*The ends of doublings and stiffeners have to be carried out in accordance with the standards of the K2 class(this class quantifies permissable back and forth or fatigue stresses of the joint design and the materials)

*Where cross connections connot be practially avoided, K-web seams of special quality have to be required in the areas in which stresses at the level of K2 will occur (this is particularly germain to the Bismarck joint in question)

*Bending moments transmitting web connections have also to be carried in way of K2 stresses as K-web seams of special quality or K-seams.

* Structural misalignment of greater 3 and 10mm at butt seams has to be rough planed in an angle of 3:1

The allowable stresses for ST52 weldments are also given but I cannot reproduce the graph here. Obviously these rules for joint design are based on experience which could not have been gained when the Bismarck was designed. These protocols are about joint design and not about welding faults or the execution of the welds. The welds on Bismarck were done with welding technology, procedures, and materials not much different than used today.

[Herr Nilsson]

I'm not sure I understand everything correctly. Just two comments:
Did the fillet welds of the decks fail? Parts of the decks are still in place.
The half of the strakes were made of ST 42.

[Dave Saxton]

Yes, as I understand it the internal connections of the internal decks (stern side) were made with fillet welds and they probably failed. I have seen copies of the welding instructions specifying where to make the fillet welds and how long each bead is to be on the strakes although it has been awhile. I also have some photo's on CD ROM of the failed (internal) joints on the wreck which I can't seem to locate right now. The butt seams at the oberdeck and the outershell have been torn but this is to be expected once they were subjected to greater loads than they could handle.

Fillet welds normally are almost ideal for dealing with back and forth alternating stresses because the stresses either in compression or in tension are along the length of the weld which can be made long, rather than traversely through the weld. However if the adjoining overlapping plates are distorted (as can happen through shock waves we now know) the fillet welds can become torn or cracked, compromizing the primary internal connections.

The use of ST42 for joint parts would not be allowed according to the B&V K2 protocols, as that would not meet the allowable stress standards now in place, but this was probably not yet known in the late 1930's.

[paul.mercer]

Gentlemen,
I have posed this question before on another topic so I don't wish to reopen it to any extent. I have to admit I know absoloutly nothing about welding but as the subject of welds splitting due to stresses, I now wonder whether the theory of the Graff Spey running away due to her welded hull coming apart due to the stresses of firing her six 11" guns on opposite sides may have had some truth?

[Dave Saxton]

^^ Never heard of that before. I would expect if that was true the same problem would have cropped up with the sister ships as well, but I haven't heard of such problems with Scheer or Lutezow. Luetzow did suffer a collapsed stern from a torpedo hit but it was rivetted joints that failed there.

[RF]

Neither Scheer or Lutzow were in a position of having prolonged concentrated firing periods of the 11 inch guns together with rapid twists and turns as AGS had at the River Plate battle. Indeed the Scheer on its raiding cruise in 1940/41 only fired its main armament on a handful of occassions, even then the attack on HX84 didn't expend that much ammunition. Neither would the late WW2 firing on land targets in the Baltic reproduce the same stresses on these ships.

From what I recall from Rasenack's book there was no mention of the stresses on the hull during and after the battle. More crucial was the issue of the shallow depth of the Plate and the risk of AGS settling on or scraping the sea bed.