Re: [PSUBS-MAILIST] Lengthwise internal supports

[Carsten Standfuss <Merlinsub@t-online.de>]

Sean T. Stevenson schrieb:
> 
> Carsten - I think you misunderstood my message.  

Yes I did.. 

>I understand about stress
> analysis and section properties, and also how the stiffener can
> (theoretically) be used with no weld at all.  Your design is excellent from a
> strength point of view.  What I am wondering is if artificial cracks, such as
> the contact gap between two partial penetration welds on either side of a T
> joint, or in your case between the welds where you have welded the outer
> flange to the pressure hull, could potentially be an initiation site for a
> fatigue crack which could compromise the main pressure hull?

In theory - yes. 
 
> In the case where stiffeners are put in place with no weld at all, any crack
> which initiates within the stiffener has no path to propagate into the
> pressure hull; thus, any such crack is limited to the stiffener and may not
> be a serious problem as it is loaded in compression.  Once you weld the
> stiffener to the pressure hull, now there is a path for such a crack to take.
> 
> My question is whether or not this risk is significant or not?  Strength and
> fatigue life are two distinctly different analyses.  As an example, two
> approaches could be used to locally stiffen and strengthen an area in a steel
> plate - you could weld another one on top of it to double the thickness
> (fillet weld around the perimeter), or you could cut a hole and weld in a new
> plate which is twice as thick with a full penetration perimeter weld.  Both
> approaches achieve the strength objective, but the latter approach is far
> superior from a fatigue life analysis since no artificial cracks are created.

Also yes. 
 
> This may not be a significant risk with submarines, since they are not subject
> to cyclic loading at anywhere near the frequency of industrial machinery or
> other applications where this is an important consideration, but I don't
> actually know.  Is this issue discussed in any of the sub related texts?

The life time of a submarine is normaly below 1000 dives. 
And most dives goes not to the max. deep. 
So there should be no liftetime risk. 

I see no risk to this small 8 mm welds to the 22 mm shell. 
The shell is high tensile steel - and the I-Beams are mild steel. 
Because of the two chord the I-beams are 50% stronger than they need
according to the presure hull calculation. But 30% of this "more
stronger" is eaten by the rolling process with cause some stress in the
frames. But our calculation shows that the connection for the stress
flow from the shell under pressure to to the frame is much better with a
I-Frame and small welds than with a full welded T-Frame ! So dive deep
is deeper. Yes, a I-frame is heavier than a T-Frame. On the other side a
I-Frame can reduce the high of the frame a little against a T-Frame - so
miore internal space is possible. 

I think on the other side that if you make the 20 mm strong frames chord
with "full" 20 mm welds - means this generate a lot of heat and weld
stress to the shell. You can delete this stress during the production
process by normalize the hull pressure hull. But I have not the
possiblity to normalize a 52 x 8,2 feet hull. 

Most reason to use I-frames was to reduce cost. Less welds. And the
I-frame is one single pice of metall - no welds inside the Profile ! A
T-frame of this tickness is made from a plate of meatll and a rolled
chord and full welds. A lot of heat, welds a place for failtures in the
welds.. And expensive.. 

Best regards Carsten

> -Sean
> 
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