Stress Relieving Cutouts?

Discussion in 'Metal Boat Building' started by ACuttle, Dec 2, 2010.

  1. ACuttle
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    ACuttle Marine Design Engineer

    I was look at Alidesigner’s webpage (so he's maybe best placed to answer this question), and I read a section on stress relieving cut-outs. I haven't come across these cut-outs for stress relief in my career so I wondered whether anyone could give some more detail on their function and design?

    I'd asked around the office too, and none of us had seen these used either, I wondered what need there is of them and how best they are implemented. I normally design for stress minimisation through consistent structure, so I'm quite interested to learn more in this alternative direction.

    http://www.cncmarine.com.au/Design.htm
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    They are what they are called "stress relieving".

    I have used them in all my designs for over 20 years. If one does not pay attention to such details, especially in aluminium, then one is asking for trouble....Fatigue cracking!

    What in particular do you wish to know...unless Alidesigner wishes to chime in too :)
     
  3. daiquiri
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    daiquiri Engineering and Design

    They are also called stress-relieving notches. You can find an appetizer attached below, but don't hesitate to contact Ad Hoc for more info - also from the normative point of view.

    Cheers
     

    Attached Files:

  4. daiquiri
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    daiquiri Engineering and Design

    Ah... There he is, as quick as a flash!

    (and surely much faster writer than me...) :)
     
  5. ACuttle
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    ACuttle Marine Design Engineer

    Ad Hoc - thanks for the info, it's strange, it's something I've never come across, I've done a lot of Aluminium kits in the UK and with a lot of naval architects and never had it as a detail.

    Daiquiri - thanks for the pdf, one for the library.

    Looking at this paper, it seems that they are principally for use with hard butted-corners? This might well be why I've never encountered them as I don't have features like that, I'd go for a big-fillet instead. Would you say 90° corners like this are common then? I am suprised that the knotches as shown in Fig 7.b don't give you more problems as a site of stress-concentration.

    Either way, that's some professional development for me.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    No, they are used to reduce 1) stress concentration factor and 2) to eliminate biaxial and triaxial stresses in welds. In the image below, you can see the cut-outs in the corners. This does several jobs, but the principal reason is to ensure that the stress field in the longitudinal web, is not coincident with the web of the frame. Since where they “join” is a biaxial stress raiser, it is also very difficult for the welder to weld hard up in the tight corner. Thus leaving a poor welded joint. This leaves flaws, and flaws are sites of crack initiation, promotes fatigue and the actual poor quality lowers the fatigue life too.…and so on.

    angle bar cutouts.jpg

    As for the fig 7.b, has the semi-circular cutout above the butt. This lowers the stress at the weld toe. Weld toes are notorious locations for cracking, principally from fatigue, exacerbated by poor detailing/manufacturing. If you can relocate the higher stress region (as in the case of cut-outs and/or radii) or even reduce it as well, this is beneficial. A couple of examples of poor quality, even with cut-outs. The weld quality plays a major role in fatigue cracking, not just the detailing:

    poor butt joint.jpg frame butt joint.jpg

    Notch-toughness of the material also plays a part. Since the magnitude of the elastic stress field can be described by a single parameter K, the stress intensity factor. The applied stress, crack shape, size, orientation and the configuration of the structure subjected to this type of deformation affect the value of the stress intensity factor. Since the stress intensity factor, which governs the stress field at crack tips (this is all part of linear elastic fracture mechanics) is proportional to the stress at the crack and geometry it relates to the mode of failure, generally Mode I crack growth. (More explanations…more depth!)

    mode I-III cracks.jpg

    Whilst the reduction of stress, by stress relieving means, such as cut-outs etc are “simplistic” once the mechanism is understood, to explain the “hows” and “whys” requires far more depth and understanding, and more than a few lines of a reply.
     
  7. ACuttle
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    ACuttle Marine Design Engineer

    Thanks for the additional information.

    Yes, I use typical-details such as that on slots for stiffners - it was more as independent feature that I hadn't see a relief cut-out. It makes sense though I, and most designers I work with, don't use such hard weld toes for planer element. I would normally put such a join in away from corners - as the stresses in this regions are linear then persumably there is less of a problem.

    I use radius and scallops to take out stress concentrations and ease assembly (welds etc) without considing that it could be used between elements as shown in the paper.

    It's been a quite a few years since I did a lot with crack mechanics but it's coming back. I've always been given the rule of thumb to avoid any angled joints in aluminium and only ever have radiused corners which must have a much wider stress transfer. Alot of the structure I work on is capped, in which case any relief cut-out (not counting those for profile-slots) are going to loose their effectiveness (ability to be placed) unless they are a large enough size to take the capping bar.

    ---
    Mmm - the pictures didn't load first time. Those are some pretty untidy welds - I think I've been lucky for the most part that most of the yards I've worked with are pretty professional and the jobs well surveyed. In the second photo, have you got a hatch-opening just behind the join, so that's persumably a pretty high stress area.
     
    Last edited: Dec 2, 2010
  8. daiquiri
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    daiquiri Engineering and Design

  9. New Dawn Fades
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    New Dawn Fades Junior Member

    OK, then, if you want airtight compartments how do you make water tight bulkheads in an aluminum boat?

    Or do you not do that and use foam instead?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly can you separate your question. Are you asking about air tight or water tight?..the two are very different.

    Secondly....rephrase your question by adding more words, so we can understand what is you're trying to do.
     
  11. PAR
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    PAR Yacht Designer/Builder

    I've been calling the stress relieving bites "snipes" for a long time now.
     
  12. New Dawn Fades
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    New Dawn Fades Junior Member

    I call them limber holes

    Water tight and air tight, both,they work together. If the aluminum hull happens to get a hole punched in it, which seem unlikely, then it needs water tight bulkheads to keep the water from migrating to other compartments in the hull. The compartments also need to be airtight so that the pressure buildup of air in the compartment will restrict water entry.

    In order to have sealed compartments in an aluminum boat you can't have limber holes,but since you need limber holes in the bulkheads then the compartments will be neither airtight nor water tight.

    So how do you keep it from sinking in the middle of the ocean? Flotation foam?

    OK, show me wrong.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    So, your watertight bulkheads keep water from passing along the vessel in a fore and aft direction, once the water has entered. These WTBs will be fully welded and watertight, not air tight. They may well be air tight, if done by a very good welder, but you do not need the welds to be air tight. It is very difficult to get fully air tight. The molecules of air are considerably smaller than those of sea water, thus the passage of these molecules for air are harder to stop.

    If there is no vent in the void, water will find its passage into the void extremely difficult. Since if there is no outlet for the air, the air will just compress. It comes down to the difference in pressure inside the void compared to the hydrostatic pressure trying to enter. If you have a vent in the void, water shall flow into the void. What, then what prevents your boat from sinking are the WTBs.

    To keep from sinking...avoid hazards….failing that make sure you have WTBs. If these don’t satisfy you..then don’t go to sea, that is safest of all.

    However, I fail to see what this has to do with stress relieving cutouts!
     
  14. PAR
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    PAR Yacht Designer/Builder

    Limber holes and weep holes are drains, though they can naturally serve as a snipe or stress relief in certain locations. When you have plates meet at right angles, you cut the corners so the welds don't cross each other and set up a stress riser in the crossed welds. You always try to avoid this, for example a chain weld on a stringer to the hull plate is adjusted as it approaches, as to not intersect the plate seam weld. This doesn't require a snipe, but does prevent the stress riser.

    Naturally, in tanks and water tight bulkheads, etc., you can't avoid this, but this results in a very small percentage of the material intersections aboard.
     

  15. New Dawn Fades
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    New Dawn Fades Junior Member

    I'm sorry you had to go to all the trouble to write that because I already knew that, you just rephrased what I wrote and added the part I left out about air molecules being smaller.

    Thanks for the great advice!

    That is clear.
     
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