Load Paths in a Catamaran

Discussion in 'Multihulls' started by AndrewK, Feb 24, 2009.

  1. sailor2
    Joined: Jan 2009
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    sailor2 Senior Member

    Please take a look at the original post in this thread. Do you think that statement is correct regarding that boat ?

    If I assume 30% of the full load displacement is reserved for payload, then 70% is available for interior & all structures. It seems to me that if only 30% is used for structures and 40% for interior either high tech construction is used or total weight is large for it's length, making it perform no beter than a monohull of the same length. Making the cat narrow would help on keeping percentage of structural weight low.

    I think it is much easier to keep percentage of weight used for structures low for monohulls than for multihulls.
    I don't think so, designing multihulls and doing structural engineering work for those is completely different task and should preferably be done by different people with different knowlidge in those very different fields.
    In most cases it might be beter if those who design multihulls, and know how to do that well, would not try to also do composite engineering by themself.

    Doing structural ingineering for metals is certainly much simpler than composite engineering if details can be left out for those supervising the construction. Perhaps so much simpler that multidesigners can do it succesfully.

    Ps. In the original post in this thread poster asked about load paths, doesn't that imply not using isotropic laminates, otherwise what difference does load paths make if isotropic material is used, why even ask that ?
    The only reason making sense to me would be the intention to place fibres along the loads for the resulting weight saving, do you see some other reason for the question about load paths rather than the magnitude of loads ?
     
  2. sigurd
    Joined: Jun 2004
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    sigurd Pompuous Pangolin

    S2,

    You are right, I wasn't clear on the meaning of the word strain. "Strain is a dimensionless quantity, which can be expressed as a decimal fraction, a percentage or in parts-per notation." (Wikipedia) So micron doesn't come into it, ever, in my interpretation. Thanks for clarifying.

    Are you sure? If something is dimensioned for less strain, that means it is stiffer, means more laminate, means heavier?

    Could not wrap my head completely around the first part either.
    case A)
    load 0' = 1
    load 90' = 1
    allowed strain 0' = 90'
    fibers 0' = 1
    fibers 90' = 1
    both directions elongate the same; same load, same amount of fiber.

    Case B)
    load 0' = 2
    load 90' = 1
    allowed strain 0' > 90'
    fibers 0' = 1.5
    fibers 90' = 1
    0' elongates more than 90' since the loads are twice, and the fibers are only half again.

    Yet you say the 90's will be fatigued before the 0'. Is it the 0' elongation or the 90' elongation that fatigues the 90' fiber/matrix?
    I guess the first. Then why did you allow it to elongate more in that direction? In short, if my trying to express what I don't understand falls short; could you repeat that, in other words?

    Ad Hoc;
    Maybe this is the same thing I don't understand?
    Case C) A given +-45' laminate is elongated a given amount in the 0' direction. Fatigue is "initiated".
    Case D) A layer of uni is added to an equal laminate, and it is elongated the same amount. It takes more force to do so, but how can the fatigue NOT be initiated?

    Is this due to stress raiser fiber-matrix-fiber discontinuous strain, or something?
     
  3. sailor2
    Joined: Jan 2009
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    sailor2 Senior Member

    Heavier in that direction where loads are low, while being lighter in the direction loads are high resulting lighter overall.
    Correct guess. First cause for fatigue is the local strain of matrix between 90 deg fibres when elongated in 0 deg direction.
    That local strain is higher than the average strain in that ply having 90 deg fibres because E-modulus of those fibres measured in 0deg direction is much higher than E-modulus of the matrix withing the same ply. If linear model would be perfect (it isn't) one could say that E-modulus times strain is the stress and with same strain the matrix would have much less stress than the fibre. Multiply that different assumed stress by the cross section (same for both) and you end up different force. Do you see that has to be erroneus idea ? The force can't change suddenly at the matrix fibre interface. That's like assuming that when you pull a rope at end with force F the load at the middle would be different even the rope is only in contact with anything at the ends. Clearly a wrong idea.

    It's the max local elongation that matters for initiation of fatigue process.
    But how critical that process is to possible failure is not same everywhere.
    It makes a huge difference if matrix suffers locally or if fibres break.
    In the former case the result can just be shifting of shear load to elsewhere in the laminate in some cases while fibres still can take full load in the same area as they are intact. That is allowable if some matrix is left undamaged in every place of the laminate, meaning the damage is in one ply only while laminate has several plys, and fibres in that damaged ply can transmit the shear loads to other plys just fine by the matrix still intact at some interfaces.
    The result can still be as stiff in all directions as it initially was.


    Rewrite case A) as follows :

    Case A)
    load 0' = 2
    load 90' = 1
    allowed strain 0' = 90'
    fibers 0' = 1.8
    fibers 90' = 0.9
    total fibres 2.7

    Compare the result with case B now when both cases have same loading !!!

    Case B)
    load 0' = 2
    load 90' = 1
    allowed strain 0' > 90'
    fibers 0' = 1.5
    fibers 90' = 1
    total fibres 2.5

    Which one is lighter ?
    isn't 2.5 < 2.7 ?
    The answer to that should answer the following question of yours:
    I may have made a mistake there, don't recall what I wrote and why.
    But:
    If you put same load to both cases the elongation is not the same.

    Or if you use 0/90 laminate of same weight in the latter case with same load at 0degs in both cases. In the +-45 laminate there is elongation also in 90deg direction caused by the fibres (not any external force in 90degs), but almost neglible in that direction in the latter laminate making a significant difference in fatigue properties.
     

  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Sailor2

    “…Do you think that statement is correct regarding that boat ?..”
    The statement wasn’t made about ‘that’ boat, the statement was regarding the boats I design to give context, in the sense the whole design philosophy you’re referring to is totally different from mine.
    “..It seems to me that if only 30% is used for structures….either high tech construction is used..”
    This just highlights my point. Take the latest Incat RoRos (commercial field that I am in), their latest generation craft have a payload carrying capacity almost 100% of the ship’s own weight. This is using bog standard aluminium, nothing high-tech at all.
    “..I think it is much easier to keep percentage of weight used for structures low for monohulls than for multihulls…”
    Not in my filed of design. Monohulls generally have a higher structural weight owing to their wider beam given the same SOR and to carry the same payload. The span of the ‘frames’ say on the bottom is effectively doubled so the modulus required is 4 times that required. But as always in design, this is not a fixed statement, there are always exceptions to the rule.
    “..In most cases it might be beter if those who design multihulls, and know how to do that well, would not try to also do composite engineering by themself..”
    To an extent that is true. However, most composite engineers sit in air conditioned offices looking at their FE results and fail to appreciate the nature of the whole structure the total requirements of the boat and the environment. They also are not naval architects and not bound by endless restraints which tend to over show extremely detailed analysis. As such their engineering analysis is almost an academic exercise or at least a nice research project for a masters or PhD thesis. I’ve come across many of these “engineers”, some good many not so. I have learnt a lot of theory from them, but not much actual practical application since other design constraints generally render their analysis and recommendations cost or time prohibitive. There is no ‘real world’ context.
    “…Ps. In the original post in this thread poster asked about load paths, doesn't that imply not using isotropic laminates,…”
    Yes it can be. But it can also be because the person lacks the knowledge and understanding of knowing how to analyse a multihull structure. It is a 3D structure with 6 degrees of freedom. Just wanting to know which direction the lay fibres does not address if there is another load path that needs to be considered. As such requires some assistance and understanding where the load paths come from and making sure nothing is over looked out of ignorance. This is common when someone has not designed a multihull before or is not a naval architect. I give him plenty of credit for asking such a basic question, but is far from basic in its understanding.

    Sigurd

    “..It takes more force to do so, but how can the fatigue NOT be initiated?..”
    As I have noted above, fatigue is very complex with composites, far more so than metals. As such, the statement you quoted high lights that, since the qualifier is once all the facts are know from the analysis.
    One cannot take part of the structure and say oh, I’ll assume fatigue is going to occur here and in this manner. It requires significantly more thought and understanding than that, there are so many variables to consider. But one needs to consider them all and place in order of merit those which affect your structure the most and then what can be done to mitigate them.
    “..Is this due to stress raiser fiber-matrix-fiber discontinuous strain, or something?..”
    It can be owing to many sources. That is the point. You need to address each one, as noted above and then decide yourself, for your structure which is most appropriate. Based upon many factors, one of the most important being the quality of the yard/laminator being used. All your nicely thought out fatigue/structure calculations can go out the window if the laminator has no proper quality control and is generally inexperienced. Everything must be taken into consideration when designing with fatigue.
     
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