DNV-GL Design Pressure And ISO-12215 Online Calc Mismatch

Discussion in 'Boat Design' started by zstine, Mar 2, 2021.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Well, forgive me for say so, but when you make statements such as:

    Etc..does not help matters.
    Since this is suggesting you are "guessing" or unable to understand. It does not suggest a grasp of what you are doing... hence your Q&As.

    For example, you can have 2 steel plates separated by a foam core, and you can have 2 foam panels separated by a steel core.
    Are the responses to an applied load the same?
    If you answer - yes - then we have some way to go....and can take a significant amount of time, much more than a simple reply.
    If you answer no..great, this suggest you understand that structural design, is an application of structural theory.

    And as in the example above, it depends!...depends upon how thick the skins are relative to the core, and what material the core is.

    And that's the point in composites, you can "design" whatever laminate/stack that you decide to response to whatever applied loads you are expecting.. And if your objective is to make the cores be fully effective in taking the shear loads, both through thickness and in-plane, as well as taking all the tensile and compressive loads, than that is your prerogative. But it would beg the question... why???

    Thus, there is no "gotachya" statement - all you're beginning to do is understand the link between structural theory and its application in structural design.

    But you still have a long way to go....As you are still conflating structural theory with Class rules.

    With each step you are taking, it leads you to more questions. But you are seeking answers to questions, without fully appreciating the whole process, rather than, as you are, doing it piecemeal.

    And your citation is incorrect too. It is Pt. 3, Ch4 section 6...not Pt.3 Ch5. Sec6.

    And as I have stated from the outset, you cannot make a statement about this software until you have visibility of the rules upon which it is based.
    As you'll just be making incorrect assumptions. As you are conflating rules with theory.

    Rules are not theory.

    Rules, all rules, have fudge factors into them to provided a degree of confidence to the Class society that is issuing the rules, that the actual used in-service structure designed to satisfy said rules, will not fail when used as per the rules for calculation. Years of in-service experience owing to poor layups, workmanship or weather/seas conditions etc...all feed back into these fudge factors..... and that is where they depart from theory. And each set of prescriptive rules has its own "fudge" factors. And they may not even make sense or have any real link with theory. But that is not the point.

    Thus, if you are using a set of rules, to design a structure, you need to understand what the rules mean and apply both min and max examples, to satisfy yourself how the rules work and why.

    If you don't, you'll have endless Qs and they shall lead to more Qs. ....just as you have from the start of your thread. With each step you are taking you realised, for example, panel breadth and effective breadth are different. But didn't appreciate this at the start... ergo, you need to go through the whole process from start to finish before you can make statements with any degree of confidence.

    Once you have the rules that Vectorlam is based upon, you will be able to state with confidence:

    And you will know why too.

    And...it is generally nothing to do with structural theory...it is a "fudge factor" in the rules. Its own unique fudge factor.
    That's it... no magic.

    There are many on this forum that dislike one set of rules over another. So what?..if you don't like BV, or LR or ISO, so what, use a set of rules you do like. But each set of rules comes with its own fudge factors and are in general not applicable to any other set of rules, other than the one you are using.
    That's it. .. no magic.

    Well come to the discipline of actual real structural design - that must be rule complaint.
    Last edited: Mar 14, 2021
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  2. rxcomposite
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    rxcomposite Senior Member

    It depends on the formula used. Hexcel (I think) put forward a formula that estimate core thickness considering only face thickness. Core shear is not affected by skin thickness. It does determine the stiffness of the laminate though.

    LR on the other hand uses the modulus of the face as an input to determine core size. The formula is (Factor * b)^3 *3√(P Etps), where

    Core Factor= 0.114

    B= width


    Etps= Minimum modulus of face (use 14,000 minimum)

    It does influence the core thickness to a certain degree. Just another way of skinning the cat.

    To determine core shear, the formula to be used is basically Force/Area or (P*b/2Tc) *10^-3. Where:



    Tc= thickness of core

    Divide the result by the safety factor. This determines the allowable shear the core is going to handle. If the determined core size is unable to handle the load and you do not wish to use a higher shear strength core, you may opt to increase the core thickness, reducing shear stress.

    Sometimes, deflection is an issue (stiffness of a panel), so it is just a matter of bumping up the skin thickness a bit. If skin stress is an issue, say with your chosen materials and property, you are still exceeding the allowable stress on the skin, you have two choices. Use a higher modulus, higher strength on the facings or add more laminate. The latter is a heavier solution. The best solution would be to just increase core thickness as noted above.
  3. rxcomposite
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    rxcomposite Senior Member

    The material properties of LT 1800 seems to be a little bit off from a generic profile. I used it anyway and added what is missing. LT 1800 is a cross plied uni, only stitched together for ease of use. Its properties should be the same as Uni.

    DNV mentions that the ISO method of plate analysis is acceptable. Since ISO and LR uses the same tabulation with the exception of the Safety Factor, I used LR to demonstrate. This uses standard formula of Maximum stress = M/I/c

    Using a safety factor of 3, 0.0448 MPa of pressure, and the properties of LT 1800, I fed it into the LR software, a known standard. Here is the result by which you can compare it with Vectorlam.

    Note that in the first test 12 mm core the allowable shear stress is 0.345 MPa but the resulting shear stress is exceeding that. No problem with skin stress.

    Attached Files:

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  4. zstine
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    zstine Junior Member

    Thanks, This is very interesting. The Vectorlam provides FoS, and I ran the same laminates, same pressure. For the 12mm core, Vectorlam gives a FoS of 1.78. This is based on H80 full allowable shear of 1.1Mpa. So dividing that by the FoS (1.1/1.78=0.58 MPa) results in predicted shear very close to the 0.569 MPa of LR. The skin prediction is not so close. Vectorlam provides a FoS for 1st ply failure (3.17) and Ultimate skin failure (6.21). Even when using the higher FoS, to get the lower applied stress (368MPa/6.21 = 59.3Mpa) the result of 59MPa is significantly higher than the 17.9MPa shown in your 2nd table for 12mm core skin.

    For the 20mm (3/4in) core, Vectorlam predicts a FoS of 2.67 or (1.1/2.67) = 0.39 MPa, Compared to LR 0.34 Mpa, still shows a good correlation (3/4in is closer to 19mm which could explain the small difference). But again the FoS on 1st ply failure and Ult is predicated as 3.86 and 13.16 by Vectorlam. The FoS of 13.16 equates to a stress of 28.0 MPa, compared to a maximum of just 11 MPa shown in the last LR table.

    Of course, I assume the 0.345 MPa you reference has already taken the FoS of 3 into consideration since H80 spec allowable shear is 1.1Mpa. I noticed the allowable skin stress is 1/4 of Ult, so I guess LR has a FoS of 4 built in.

    This has been a great exercise and I have learned a ton! I have also done a bit of digging into what laminate schedules are actually being built in similar boats. There are many examples on the "oldmulti" thread. I found that 600 gsm E-glass skins on a 12mm(1/2in) ~H80 core is typical of this size boat. Generally there are no longitudinal stiffeners, just some tabbed in furnishings do the job. One ply of LT1800 is about 600gsm, so I ran 1 ply on 1/2in H80 through Vectorlam and got a core shear FoS of again 1.78, 0.58 MPa (same as 12mm above). The 1st ply failure FoS was 1.25 (294 MPa) and 2.81 (131 MPa) for Ult ply failure. While these results clearly don't meet any of the class specifications for Factor of Safety, these boats have been in service for many years/decades. I assume the specs are (over?) conservative since they don't want a spec-approved boat to break apart.

    I want to build a light boat that can foil and I am willing to risk structural issues by building lighter, though I don't want major failures (also saves money on glass and resin). I believe it is more desirable for me to mimic what is 'tried and tested' vice developing my laminate and structure based on class specifications. I'll finish with the below, which makes a great point that normal operational stresses on a boat of this size are generally not what would drive structure, but rather the stress when sitting on a trailer, bumped into a dock, or jumping on deck, etc. is going to produce the highest loads in the boat. Cost and availability is also a concern and I have found LT1800 overstock that is cheap and Gurit G-PET 90 - 13mm (19mm available also) thick core somewhat local (save on shipping). If those are still available in ~a month when I'm ready to pull the trigger, that will likely be the products I use to build... RXCOMPOSITE, I can't thank you enough for your help!
  5. rxcomposite
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    rxcomposite Senior Member

    If ultimate core shear stress is 1.1 MPa, then divide by 3, your factor of safety. That means you are only allowing it 1/3 of the ultimate force before it catastrophically fails.

    In reality, most metals. even composites yield at about 60% of its ultimate. That means it starts to yield or stretch to failure. We always design around the yield point.

    If you want a light go fast boat, use ISO rules. It asks only for a safety factor of 2 or allows 50% of the Ultimate Strength and lower skin thickness minimum. That means you still have an extra 10% before it yields or 40% before your boat becomes a total mess. Class Rules are conservative so they ask for FoS of 3.

    Aerospace would ask for 2, sometimes up to 1.87 near the limit of material yield. But that comes with a lot of material testing and actual part destructive testing. So unless you are willing to invest time and money, stick to 2 FoS.

    And use basket weave Woven Roving or twill weave WR. Stitched Uni is too strong.
    Last edited: Mar 16, 2021
  6. rxcomposite
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    rxcomposite Senior Member

    I checked Catsketcher ply schedule and it works. 1x WR600 outside, 15 mm core, 1x WR400 inside. Not biax. Skins not even strained, core nearing 50% of ultimate strength. Of course ISO rule not LR. I will add CSM450 on the outside and CSM300 on the inside to meet minimum skin thickness rule.
  7. zstine
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    zstine Junior Member

    Why do you say it is "too strong"? I don't understand why more strength would be a bad thing.

    I plan to build in epoxy resins, so I was avoiding CSM because the binder is typically not compatible. I could use something like an 1808, which is again a stitched 0/90 uni, but has csm stitched on one side and is compatible with epoxy ( https://vectorply.com/wp-content/uploads/2015/06/E-LTM-18081.pdf ). I understand that csm generally provides a better bond and would (i believe) typically lay against the core, though I think the csm would help with making the exterior fair. I plan to build on a male mold.

  8. rxcomposite
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    rxcomposite Senior Member

    Too strong means more than you need. A single layer of WR of same weight will do as above. Besides, I don't like Uni. Only 10% of strength remaining on off axis load.

    If you dont like CSM, put a light Biax near the core, not outside. Biax are like diagonals on a bridge. It just sits there doing nothing until load is applied. Good for twisting load but you need only about 30-35% to carry load. Don't use it as a primary layer.

    CSM inside is to promote adhesion during assembly or secondary bond. If you like sanding each time you have to bond, then WR will do.
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