Richardw's Narrow Boat Project- PLATE THICKNESS

Discussion in 'Metal Boat Building' started by richardw66, Nov 27, 2013.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    material property
     
  2. SamSam
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    SamSam Senior Member

    Seriously? Material properties compared to each other on the basis of what?
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm..you clearly missed the link to the definition. For the hard of reading here it is again:

     
  4. Simonosteopath
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    Simonosteopath Junior Member

    Ad hoc and Petereng, is some of the confusion arising from strength per weight? And also, aside from material strength, it is important to consider what failure of one material means compared to another.

    If steel is highly stressed, such as during an impact, it will deform (bend, dent) well before it actually fails (fractures). then it can be beaten back to shape and welded and the repair will normally be as strong as the original condition, if done properly. If composites are compared, they often deform less before failure but when they fail cannot be repaired to a state of original strength.
    Please correct me if I'm wrong about this.

    Petereng, try to find a different way to describe toughness, besides the graph, which I find very interesting, I have to say!

    I think we all agree on the ballasting issue though!:)
     
  5. Simonosteopath
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    Simonosteopath Junior Member

    Sorry, I meant Ad Hoc, please find a way.
     
  6. TANSL
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    TANSL Senior Member

    I do not want to get into any controversy right now. Just point to what does exist are minesweepers in reinforced plastic. It is likely that the minesweepers are designed to withstand much higher impacts than icebreakers.
    That said, I leave the thread for you can continue with your semantic arguments.
    Cheers.
     
  7. SamSam
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    SamSam Senior Member

    Yes, you're right, I didn't read the link.

    Reading it though, I conclude that GFRP is about the same strength as, and a little bit tougher than mild steel.
    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-toughness/basic.html

    I assume those tension/compression figures the charts are based on are calculated using test coupons of similar size, say like a 1" cross section rod pulled and squeezed to failure.

    That's what I meant when I asked what the basis of the chart was. So I'm pretty sure the basis was size.

    If I go to the pound for pound comparison chart, it seems to show GFRP is roughly 4 times the strength of mild steel.
    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/spec-spec/basic.html

    Can a person extrapolate and conclude that pound for pound, GFRP would be 4 times tougher than mild steel?

    As for why there are no large vessels of GFRP, material cost is around 4 times higher than mild steel,
    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/strength-cost/basic.html
    and recycling cost are 10 times higher with no end use for recycled GFRP.
    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/recycling-cost/basic.html

    Minesweepers are fiberglass because of their electrical properties compared to mild steel.
    http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/resistivity-cost/basic.html

    Thanks for posting the link to the charts.
     
  8. TANSL
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    TANSL Senior Member

    A further problem associated with non-metallic materials, and probably what prevents any large passenger vessels built in GRP, is the difficulty or impossibility of getting enough protection against fire class A-60 or similar.

    No one says otherwise, but are designed to withstand instant loads stronger than an icebraeker.
     
  9. SamSam
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    SamSam Senior Member

    Yes, fire is probably the major reason for not using it. A whole large ship all aflame and no way to stop it. Docked, it would burn up the dock, nearby ships and part of the town.

    I wonder how the weight of minesweepers compare to other ships.
     
  10. SukiSolo
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    SukiSolo Senior Member

    As far as I am aware the UK Minesweepers are 50mm thick GRP laminate. they replaced the older wooden ones as both materials are less likely to set off magnetic mines. So the wooden ones would have been lighter! most likely.

    What I would like to see, is Richard coming back with some lines plans to share with us....

    It is an area of marine design that could do with a little careful thought regardless of hull material.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    No, it's not fully understanding or using the correct definitions.

    You’re getting confused and mixing up terminology and definitions, which is what Peter appears to be doing too.

    A simplified explanation.

    Any material which is placed into say a vice and fixed into position at one end and the other end is pulled will become stressed. By stressed this is the definition of “stress”, in this case it is simply, stress= force/area. The force is the pulling force, which is a known force as when the material is pulled, the amount of force pulling is very accurately measured. (This is all done in a laboratory by the way with highly accurate instruments). So we can establish the “force”.

    Before we start pulling the material, we measure the cross-sectional area (XSA). So if the sample was say a 50x5mm section, the area is simply 50x5 = 250mm^2.

    During the pull test, the force will increase, but the XSA will not. Therefore as the force increases, what occurs…..the “stress” increases.

    The term you’re using of “highly stressed”, all that means is that the material has a force being applied to a certain amount of area. And if the force is high, for the fixed XSA of structure it is applied to, the stress shall increase. Now this is true for any material, rubber, plastic, metal, it is all the same.

    So if you have a force of say (to keep the numbers simple) 1 tonne, or 1000kg or 10,000N (all the same value just different units) and if the XSA is say 100mm^2, what will the stress be?

    Stress = force/area = 10,000/100 = 100N/mm^2 or 100MPa.

    If the force is increased to 2 tonne or 20,000N, the stress is 200MPa. (Because the XSA remains the same).

    So, as you can see when the applied force is increased the stress in any material of a given XSA shall increase.

    Now, how does this relate to an actual known material?

    Well, in the above pull-test, what is measured is the amount it stretches, or correctly termed elongation. Since if we keep pulling but each time increase the amount of force pulling what shall occur, eventually the sample shall keep stretching and then… fail.

    What we end up with is a simple graph of stress v elongation. But now we call this elongation -> strain. Thus it becomes a stress v strain graph.

    The stress v strain graph is very important. The slope of the curve and where it suddenly changes. The slope up the point of failure, the linear region, if you dived the =stress/strain you get the Young’s Modulus of the material, E. All materials shall have a straight line of the stress v strain up until something happens to make it no longer straight. At this point, this is called the “yield” point. The behaviour of the material is linear (straight line) and is true for all materials. Thus when materials are quoted say like standard mild steel, what is the design allowable stress, it is 235MPa. In other words, yielding begins at 235MPa. For a standard GRP (CSM) this value is around 100MPa.

    Thus, in the above, if the force applied of 1 tonne, the stress is 100MPa. The steel yields, or begins to fail at 235MPa, so it is fine. It can “take the load”. If using standard GRP the ‘yield’ is circa 100MPa. Which means the structure is stressed up to its yield point, or it is just about to fail. If the force is 2 tonne, the stress is 200MPa. The steel is still working ok, and is now, in your words, becoming “highly stressed”, since the stress is approaching the yield point of steel. But the GRP has now failed, as the stress is greater than its yield point of 100MPa.

    BUT..and here is the other important point. Not all materials have the same stress v strain curve. The point at which yielding occurs is very different for steel as it is for GRP. And more importantly what occurs after this “yield point”.

    In the above, what occurs to the GRP after 100MPa and what occurs to the steel after 235MPa?

    In the graph below you can see:

    stress v strain graphs.jpg

    For steel, the yield occurs at around 3-5% strain, then after the yield point in steel it dips then continues to increase in both stress and strain. This is what is called the plastic region. So, even though the steel has gone beyond its yield limit the material, or structure, has not failed. You can continue to overload the steel it won’t fail, it just stretches. It can stretch until around 35% of its original shape, before complete failure.

    In the laboratory pull-test, what you will see is the XSA is now becoming less too. This is what is termed ‘necking’, as it looks like a neck. The XSA of the sample is slowly getting less and less, like pulling chewing gum until it fails. So the steel structure even though it has not failed, it has deformed, it has stretched!

    If we now look at standard GRP, its yield point is just a bit less than steel. It occurs at around 2-3% of strain. But here is now what gets to the heart of your comment, what occurs after this yield point has been exceeded? There is almost no more curve beyond its yield point as it fails at around 8-10% strain. In simple terms this is what we call brittle. The material does not stretch much before complete failure.

    So the slope, as previously mentioned is called the Young’s modulus or E. The Young’s modulus of steel is circa 210GPa and for typical GRP is circa 8GPa. This relates to the amount of deflection that is experienced in the structure. Since stress is just one part of the calculation.

    When designing a structure, you need to know what the applied loads are, and from that design the structure to ensure that the stress, from the applied load, remains below the yield point of that material you are using. And then calculate what deflection you have under that applied load. Without doing any calc’s, you can see that the Young’s modulus of GRP is significantly less than that of steel. Thus a steel structure will not bend, deflect, as much as a GRP structure of the same dimensions with the same applied load. To make the GRP deflect the same amount means increasing the amount of structure. And this relates to the EI, the flexural stiffness.

    The E = Young’s modulus and I = 2nd moment of inertia of the structure.

    So with I being the same for the steel and GRP as same structure, the only difference is the E.

    Does this answer your question?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    No, you need to fully understand the definition of "toughness".

    Here are some typical values taken from the source of those graphs.

    FToughness of materials.jpg
     
  13. Simonosteopath
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    Simonosteopath Junior Member

    Thank you, Ad Hok.
    I am not getting confused, but I may be using lay-terminology because not all people in this thread are engineers (I don't think).

    I used to work in the engineering arena and specifically with load measurement so I understand the definitions of stress and strain and I also know what Young's modulus is. This is partly why I joined the thread.
    I also think that we don't need to get quite so complex for a practical discussion on which are the best materials. I agree with your descriptions but boy, are they deep!
    My main point in my short comment was about practicality of material used, taking into account repairability, and the need for a lot of ballast if something like GFRP was used. You could certainly add cost to the list.

    If canals were deeper, or preferably, if all canal bridges and tunnels were higher I'd probably choose a composite hull for all the reasons you promote. I'd also have to be more wealthy!

    Simon
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.

    I designed an aluminum version some time ago for a client. It did allow more luxurious outfitting and carrying plenty of fresh water (far more than normal). But it still had 5 tonne of solid ballast.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    I was doing some research at the weekend on a totally unrelated subject and dug out one of my papers . And funnily enough came across this very subject. Anyway, the figures below clearly shows the superiority of blunt and sharp contact, as abrasion resistance, of metals. Where ceramics being the superior ones of course, followed closely behind by most metals, and the poor performance of composite.

    abrasion resistance.jpg
     
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