Re: [PSUBS-MAILIST] Lengthwise internal supports

["Sean T. Stevenson" <cast55@telus.net>]

Carsten - I think you misunderstood my message.  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 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.

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?

-Sean



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