Butt Joints For Hull Plating

Discussion in 'Metal Boat Building' started by bobk, Jul 16, 2010.

  1. tazmann
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    tazmann Senior Member

    Many ways to do it and with the newer machines with pulse, pulse on pulse with all the finite adjustments. There is no set rules, it's the end result that counts. In my case with the lighter sheets the bigest problem was shrinkage along the length of the weld. Tried several ways ended up just running short welds 2 to 2-1/2" long spaced about a foot apart, after they cooled did it again, when It got down to the last welds I gouged out the previous welds to get good fusion. After the one side was done I flopped it over and clamped it back in the fixture, back gouged and welded with the same procedures just a tad hotter. Slow and tedius but it was the only way I could get good results. Destructive test showed the weld to be stronger then the metal.
    Tom
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Tom, can you explain what you mean by this please? Since a weld is only as strong, in best case scenario, as the parent metal, more generally slightly less.

    And by 'strong', i assume you mean its mechanical properties?
     
  3. tazmann
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    tazmann Senior Member

    Hello Ad Hoc
    Welding with 5356 wire and done correctly on 5052 material the welds are stronger than the parent metal. You can bend the test peice any way you want continuesly til it brakes and the metal will brake and the weld will still look good.
    To give you an example Here is a picture from last year when I was learnin a new to me pulse welder, not the prettiest weld but heck I was learnin. anyway it's a simulated chine seam that I clamped in a vise and took a long wrench to it to get enough leverage to bend it as far as I could both directions, As you can see after several bends the metal broke. To me this makes for a strong chine seam ? A less than par weld lacking fusion-penatration will usualy brake down the middle of the weld first or second bend
    Tom
     

    Attached Files:

  4. Ad Hoc
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    Ad Hoc Naval Architect

    Tom

    Sorry you’re mistaken. What you show is a classic bend test, one of many required by Class. That is just a visual examination. The test must be independently mechanically tested for compliance with rules, to ensure that your procedure and sequence for welding does not promote a weak joint. The tests are mechanical properties of the weld, in terms of strength and hardness.

    What you have shown is what every welder/designer knows…the HAZ is the weaker part of the joint; hence the requirements for one welding sequence/testing to be tested and approved as ‘satisfactory’. Welds are never ‘stronger’ than the parent metal.

    Welds always fail in the HAZ, usually at the HAZ to parent metal region. The reason why it does not ‘generally’ fail in the centre of the weld as you suggested is because there is so much more surrounding metals or “volume/area”, per unit length. Stress = Force/area, thus more area, ie more weld, the amount of stress is less for the same applied force. Which means it is ‘stronger’ in the centre than at the ends of the HAZ. This is why the HAZ/parent metal zone fails, as noted in the attached.

    weld strength.jpg

    What should happen is shown in (c), with a good quality weld.

    weld strength-haz.jpg

    But what usually happens is (d)..failure of weld in the HAZ.
     
  5. tazmann
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    tazmann Senior Member

    Ad Hoc
    Yes I can see what your saying if I was building profesionaly and they needed to pass a particular code but for folks building there own I dont think it's nessessary , bend test, distructive bend test, and cutting into the welds from both ends and braking off the remaining to check for porosity would be the next best thing to find out if you have good welds or not ?
    I have not experianced the brake in the haz area all the time. That particular picture the brake was along the top of the vice jaws. I had it clamped in close trying to get the weld to brake.
    I did a lot of testing praticing on lap, one sided fillet, but, and temp backing bar welds and when I had the procedures figured out I could not brake any of the welds weather I bent them or beating on them with sledge the surounding metal would rip to pieces first. Tells me the welds are stronger then the materials I'm working with
    Tom
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Tom,
    I understand your position. You are not building to any professional code nor standards, so what you do is fine and for your own quality purposes only and do not need to know what your weld strength is from your weld sequence/set up.

    I’m sorry, but again, you’re 100% incorrect.
    It matters not what you do, welds are never stronger than the parent metal, whether fabricating in your back garden or in a professional shipyard. You have been fed some incorrect information, or, you interpretation of what you are “seeing” is incorrect.
     
  7. tazmann
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    tazmann Senior Member

    OK Ad Hoc
    I guess what I am seeing is incorrect LOL
    We will leave it at that
    Tom
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    I do not know anyone anywhere in the world with the ability to look at a weld and they can then tell you what the mechanical properties are of: the weld, the HAZ, and the parent metal simply by looking at it and thus provide a quantitative assessment of the weld quality/strength. That drifts in the “faith based” approached of science and engineering.

    Visual inspection of welds, only tells half the story. They can provide a guide to causes of failure as well as set/sequencing imperfections. They cannot provide weld strength values only mechanical testing can.

    What you subjectively see and what is independently proven by mechanical testing are generally very different. But that is your prerogative to think what you see is better than what is proven by tests and accepted by professional bodies and welding institutions the world over.

    welding ally mech props_Page_1.jpg welding ally mech props_Page_2.jpg

    As you say, we’ll leave it at that!
     
  9. alangluyas
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    alangluyas Junior Member

    Conventional wisdom on weld strength in 5000 series (so called work hardening) alloys is that they retain, at best, 85% of the strength of the parent metal, due, in part, to the loss of the work hardened properties in the HAZ - my understanding is that the metal in the HAZ reverts to its non-work hardened form.

    Mild steel does not suffer this loss of strength and is in many ways a more reliable material to fabricate. It is a shame about the weight and rust!

    Alan Gluyas
     
  10. tazmann
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    tazmann Senior Member

    I realy fail to see your point here Ad Hoc
    There is a difference between saying best or next best ?
    Yes according to the books the as welded specimen the welds are weaker than the metal but that is also calculated with the weld ground off to the same thickness as the metal. I do my as welded desructive test as the weld woild be finished in whatever I am building. In the case of the picture shown, chine seam- corner weld welded inside and out side with good fusion-penatration. the weld is at least double the thickness of the metal. Now if I can't brake that weld nomatter how I try. Is it weaker or stronger ? Faith based I don't think so
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    And there in lies the problem.

    Because you do not understand what the term ‘strong’ or ‘strength’ means in engineering you are basing all your discussions/points on a flawed assumption. To make matters worse, you are then, when pointed out that error, and being provided references to guide you so you may read up and learn why, yet you selectively choose to ignore this, I’m not sure why?

    When you arrive at a conclusion thus:

    This just demonstrates how little you actually understand regarding welding/engineering/structures, despite your protestations to the contrary, but for some reason you seem to imply that this ends the matter, as it is a closed ended statement!

    Stress = Force / Area

    If you apply a constant force, with a doubling of the area, the stress is halved. If you have 4 times the area, the stress is quartered, and so on. So, the more area, the lower the stress.

    A build up of weld, is providing more “area”. It does not alter the physical properties of the material, other than the localised changes that occur in the HAZ which are well known and documented. Steel is still steel as ally is still ally before during and after you weld it, it doesn't make steel ally or ally steel! An increase in the thickness of the plate results in the same effect, it increases the ‘area’, to lower the stress.

    All you are doing is moving the location of the failure by building up the weld bead. Building up the weld bead is like increasing the thickness locally of the plate, because the weld is still ally and the parent plate/metal is ally, thus it has the “appearance” of increasing the strength locally. Hence, the reason why it fails “elsewhere” is because for the same applied force, there is somewhere else with less area than at the build up of the weld bead, that is where there is “no weld” as you call it. But this is in region of the HAZ which can be as much as 25mm away from the toe, the physical mechanical properties of that region have been subtly altered owing to the welding, it reduces the strength. Therefore, this HAZ region has a lower strength, less ‘area’ and hence will fail at this location first.

    However, if you think that over welding is producing a better joint, you’re very much mistaken. All you are doing is promoting ‘internal flaws’ within the weld, which because sites for fatigue cracking and also the ‘external’ shape is now a localised stress concentration. The failure is then at the toe of the welds, owing to the change in cross sectional area and the sudden change in shape with promotes the stress concentration.

    Here is a classic example of over-welding by ignorance. The welders were lazy and ‘felt’ over-welding and gap filling was providing a better joint.

    over-welding.JPG excess weld bead.jpg

    As you can see, it is riddled with porosity, there is lack of fusion/penetration, the weld beads are over sized..and so on. You can also see the number of runs performed....way way too many. Very poor quality joint. This was a very costly joint to repair. No surprises then that it failed, it cracked at the weld toe, where there is less 'area'.

    Just some of the reasons why this joint failed within a few months of service are shown below:-

    porosity.jpg weld bead size.jpg weld overfill.jpg LOF-P.jpg

    The fatigue life of the joint is significantly reduced. And, as you can see placing too much weld down, in the false assumption that you're increasing the strength of the joint, the opposite occurs and caused this failure and is shown why in these graphs.

    It is faith based upon your personal feelings of what is happening and that you are right, rather than you trying to understand qualitatively what is really happening and has been well document for many years by institutions the world over. Yet, you seem hell bent on ignoring the all the evidence and actually learning more about aluminium, I’m not sure why.
     
  12. tazmann
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    tazmann Senior Member

    Ad Hoc
    Well I did try to concide and get out.
    First of all you did state that no one can do there own testing and that it wouldn't count. I disagree and so would a lot of folks that do it.
    It aint rocket sience and you do seem to overcomplicate things, most of us are not building mega yachts just small boats in the private sector.
    You read more into my statment on the weld strength than there was, to the point you want to argue, rather insulting on your last post there.
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Tom

    Well, I’m sorry you feel that way. For many reasons really.

    If you want to produce basic welds for basic structures for cheap and cheerful bits and pieces that is fine. If I did something that looked crap was crap and everyone else said it was crap….fine, my ego/pride may be dented but that is all. I did it and was good enough for me, but it doesn't alter the known laws of engineering, just my skill level!

    But when you want to take that “basic amateur procedure that is just fine for you” and use it carte blanch as a welding/structure statement that is applicable beyond your backyard play welding to the point that you are suggesting it is the same as any “industry standard”, that is where I must draw the line.

    You can do whatever you like…you test it how ever you like you can be pleased with whatever results you obtain by your won methods. I do my own too, but they are not independent results, thus must carry a caveat. So, you cannot take these methods and transfer them into the professional world where quality, regulations and standards are strictly adhered to and significantly higher than those you are happy to do yourself in your back yard. The two are chalk and cheese. Perhaps you do not realsie the distinction?

    If you stated “your destructive tests and results using your methods” alone give you XXX and in no way infer these to neither be the known industry standards nor infer that they are 100% correct, then that would be a fair and factual statement by you. It is not however, what is accepted beyond people’s garden/sheds. Thus, it was a correction, not an arguement. Your statement was factually incorrect.

    This is why people like Brent Swain appeals to so many amateurs….he “talks their language of “well…just roughly do this and do that etc.”, but none of you realise it is in a condescending manner, because his word is Law, not what is established and known the world over. Any and all of his procedures, for the amateurs that don’t know any better, do not equate to quality and professionalism, in the real world. But many of you do not 'work' in this field and thus unaware of what is done professionally. But, this is fine if you want to build in that way. Just do not suggest that they are applicable to both (back garden and real shipbuilding) which is why I responded with the simple question to you to seek clarification. I thought you would value some insight, I am wrong, my mistake.

    I didn’t read more or less into your statement, you are the one that has done that, because you have made the assumption that your statement is correct for all welding and structural design because that is what you see/feel, and I asked you to clarify it.

    Philosophers tell us that meaning is to some extent created by our view of what something is, for example, a table is only a table if you know what a table is. Thus a weld is a weld, whether that weld is on a mega yacht or a bit of tube to hold a flat bar for a bird table. The only difference is, your perception/view of that weld to mine owing to our differing views of what that weld represents. You have elected to view it from an amateur point of view, I have not.
     
  14. tazmann
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    tazmann Senior Member

    Gee Are you done yet?
    Niw youve brought Brent Swain into it
    Shurly you gotta have some more insults there ?
    This here does seem to be what your good at
     

  15. alangluyas
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    alangluyas Junior Member

    Ad Hoc

    From a technical point of view, I agree with most of what you say, but as another marine professional, I do not think the way you have presented your arguement does you any credit. It looks to me as though you are using your professional knowledge and intelligence like a club. Not a good look for the professionals.


    Cheers

    Alan
     
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