Carbon Fiber Chainplates

Discussion in 'Fiberglass and Composite Boat Building' started by david@boatsmith, Feb 24, 2014.

  1. redreuben
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    redreuben redreuben

    LOL, pmsl, so true !

    Mouse meet Camel, Camel, meet Mouse !
     
  2. rxcomposite
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    rxcomposite Senior Member

    Must be the terminology used.

    "Working load limit
    From Wikipedia, the free encyclopedia
    Safe Working Load (SWL) sometimes stated as the Normal Working Load (NWL) is the load[disambiguation needed] that a piece of Lifting Equipment, lifting device or accessory can safely lift, suspend, or lower without fear of breaking. Usually marked on the equipment by the manufacturer and is often 1/5 of the Minimum Breaking Strength (MBS) although other fractions may be used such as 1/4, 1/6 and 1/10[1][2][3]
    Other synonyms include Working Load Limit (WLL), which is the maximum working load designed by the manufacturer. The load represents a mass or force that is much less than that required to make the lifting equipment fail or yield, also known as the Minimum Breaking Load (MBL). The SWL is calculated using a safety factor For example, 5:1 (5 to 1, or 1/5). An example of this would be a chain that has a MBL of 2000 lbs would have a SWL or WLL of 400 lbs if a safety factor of 5 (5 to 1) is used."

    And from Engineering toolbox

    "Equipment Factor of Safety - FOS -
    Aircraft components 1.5 - 2.5
    Boilers 3.5 - 6
    Bolts 8.5
    Cast-iron wheels 20
    Engine components 6 - 8
    Heavy duty shafting 10 - 12
    Lifting equipment - hooks .. 8 - 9
    Pressure vessels 3.5 - 6
    Turbine components - static 6 - 8
    Turbine components - rotating 2 - 3
    Spring, large heavy-duty 4.5
    Structural steelwork in buildings 4 - 6
    Structural steelwork in bridges 5 - 7
    Wire ropes 8 – 9

    Source: http://www.engineeringtoolbox.com/factors-safety-fos-d_1624.html"

    In composites especially class Rules, these terms are not used. I find maximum stress and maximum strain rule applies but not FoS.
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    Re the slots--by having the slots well down the bulkhead, this becomes a "belt and braces" sort of construction--two additive components that add to the strength. The 24" alone would probably be more than strong enough.

    So you have thought out the design fairly well and are comfortable with your analysis, and you are capable of building them--go for it.

    Yes, you want to check that tension area either side of the hole. But if you have a combined chainplate for the three shrouds, and the pin holes are close together--say all the same width, then you have to be careful not to over-stress the space between any two adjacent holes. So space them out a bit and make the spaces between the holes, say, twice as wide as the spaces at the either end of the line of holes.

    Eric
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    In response to Ad Hoc and rxcomposite--anyone can set any sort of FoS that they wish. I could comment about the British, for example, on the lifting gear of Tower Bridge in London. One is amazed at the massiveness of this Victorian era bridge and its lifting gear--I think that bridge will last forever! The safety factors must be very high--and admittedly, I think our engineering skills in general are a lot more sophisticated than what19th century engineers had back in their day.

    I'll make another general statement that I used in an IBEX lecture some years ago when talking about Factors of Safety: The factor of safety that you use is a direct measure of your uncertainty about the engineering problem, or of the unknown loads that you may expect the article at hand to experience. If you have very highly defined loads and a very solid idea of the performance of the structure, then your Factors of Safety will be very low. This is why the FoS of rigging loads in an America's Cup boat are usually little more than one. But if you are more uncertain of the magnitude of the ultimate load, or the fatigue loads over time, your FoS will be much greater, such as 9 by the British MoD for lifting loads.

    Choose your FoS that you are comfortable with, and recognize that in some situations, the cost of your choice may be prohibitively high. The higher the FoS, the more expensive the structure will be simply by the amount of material and labor that it takes to create it. Sometimes this is important, sometimes not. You make your choice and live with it.

    Eric
     
  5. Steve W
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    Steve W Senior Member

    Thanks RX, that's very interesting info, it pretty much confirms both Erics and Ad Hocs suggestions, I read on another forum thread somewhere that a fos of 10 is used when rigging for lifting humans in theatre. I have refigured my boat and it looks like they have a small fos of less than 1.5 for the chainplates based on the rigging sizes, in fact the reduced the strength of the shaefer chainplates by drilling out the pin holes to accept a 1/2" pin. Shaefer give a maximum wire size of 7/32" for this chainplate with a pin size of 7/16", my caps are 9/16" with a 1/2" pin. A few of these boats have broken chainplates. It looks like I can achieve a fos of 5 quite easily using e glass uni so that's what I will use, the clincher is i scored a roll of heavy, 4" wide e glass uni yesterday for cheap, i don't know the weight so i will have to weigh it but being a lot heavier than anything else i can find it will take less wraps which will be nice. Im surprised how hard it is to find uni of a decent thickness in any of the fibers.

    Steve
     
  6. Steve W
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    Steve W Senior Member

    Hi Eric, very interesting re the spacing of the holes, i had not thought of that, the chainplate will be 4" wide with 3 slots for the ss tangs that the turnbuckles will attach to, i will be using a piece of 316 ss tube to wrap the uni over which will stay in to bush the holes, the horizontal pin will be 5/8 or 3/4", this will be the pivot that lines up with the mast pivot pin and the 3 tangs will rotate down with the shrouds when lowering the mast forward. Now i have to wait for the weather to warm up and the snow to melt. Thanks for all the help, it is very much appreciated by all on the forum to have truly knowledgable folk such as yourself to bounce ideas off.

    Steve.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Hi Steve. The FoS is generally increased as the dynamic effect of suddenly applied loads or unpredictable uniformities in the material. Like Eric basically said, if you are certain about the loads you will encounter and are sure of the quality of the materials you have, then the FoS can be reduced if the rules allow.

    Generally, most metals have published yield point, working stress, and or allowable stress and very easy to predict. With composites, this yield point is very elusive and is affected by the choice of matrix and fiber combinations.

    I will prepare some illustrations tommorow to expound on this.

    Eric was correct in using metal to line the contact side of the composite chainplate. Metal is dense and resistant to abrasion. It prevents the other material from chafing the otherwise easily abraded carbon composite. Metal is also better in shear than composites so it acts as a shear plate.
     
  8. Steve W
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    Steve W Senior Member

    Thanks RX, it was interesting to note on the info you supplied in your last post that the fos for aircraft components is shown as just 1.5 to 2.5. There simply has to be a limit to how much you overbuild when weight is critical. When I was a young boatbuilder I was told by a mentor to "take care of the ounces and the pounds will take care of themselves" meaning you always look at every individual part and try to strike a reasonable balance of strength vs weight. I don't know how many times ive seen boats advertised with oversized rigging or overbuilt this or that as if its a good thing. I would rather plan on a fos of 5 than 10, its reasonable but not overbuilt I feel.

    Steve.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Yes Steve. When I worked in the aircraft company before, the FoS is 2. Not that we design airplane but we have to produce the calcs in order to register as a regular airplane.

    Keeping that FoS was expensive. Lots of coupon test and sometimes we have to scrap parts costing $2,000 apiece because of cosmetic defects.

    Composite boats start at FoS of 3.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Hi Steve,
    Here is the Illustration that of the different resins when combined with a specific fiber. I am using the Volume Fraction method as it is more versatile than the LR, ISO, BV formulae for determining laminate strength. Though the approach is different, the results are very close to the Rules formula.

    Here are the basics;
    VF method works by combining the proportions of the matrix and fiber. It follows the straight line method and only linear elastic line, the most straight line, the proportional limit, is used. The upper end is the working stress, the allowable stress, or the maximum work limit. It has to work in the straight line so as not to violate the stress-strain relation of the Hooke’s law. The yield point is just slightly above this limit and the ultimate strength is way up the scale. Terminology is important as this got me bogged down for several months. LR is calling its allowable stress the ultimate strength and uses it interchangeably.

    Here are the basic assumptions:
    Failure means the component has fractured or yielded. Elongation at break or failure at breaking stress is the final breaking point occurring after the ultimate stress.

    For our intention, when the resin has fractured or yielded, the laminate has failed. The fiber has no yield point. It breaks catastrophically. The resin exhibits a plastic curve, having an elastic, yield and plastic zone. When the fiber breaks first, it is catastrophic but when the resin fractures first, the slack is taken up by the fiber but the laminate is useless.

    The stress-strain curve of the resin follows a certain pattern and can be defined by the boundaries set by the tensile modulus, the ultimate tensile strength, and the elongation at break. These data are easily available. Neat resin curves are rarely published, only generalized. Fiber curve is simple. It just goes straight up, curves a little at the top then break.

    Illustration 1 is Eglass Uni (4.6% elongation) laminated with polyester resin (2.5% elongation). Polyester has a standard curve, a little sharp at the top. The elastic limit is when the curve separates from the straight tensile modulus. Notice that the resin breaks first and its elastic limit is 2/3 of its height indicating a high yield point. When the resin and the fiber are combined, the resulting curve is the laminate curve. The fiber curve is way off the chart and is not shown for clarity of presentation. Note the notch in the laminate curve where the resin has failed completely but the slack is taken up by the fiber.

    The elastic limit curve is projected vertically upward and intersects with the laminate curve and the fiber curve (not shown). The intersection at the laminate curve is the allowable stress. It is very close to LR predicted strength.

    Illustration 2 is Eglass Uni (4.6% elongation) laminated with Vynil Ester resin (5.0 % elongation). Notice that this is a double curvature curve to fit the constraint, rounded at the top and has higher elastic limit indicating a better resin. It also fails after the fiber. Again, projecting this elastic limit upward shows that we gained a higher usable tensile strength of the laminate.

    Illustration 3 is Eglass Uni (4.6% elongation) laminated with Regular Epoxy resin (4.4 % elongation). Regular epoxy has almost the same curve as VE. I just happen to choose one with a lower Tensile strength so its performance is not up to par with VE. There are other formulations available.

    Illustration 4 is Eglass Uni (4.6% elongation) laminated with Toughened Epoxy resin (8.0 % elongation). These are high modulus, high yield point resins which resist final fracture long after the fiber has failed. Quite an excess really but the high yield point pushed the limits of the performance of the laminate. I used Silver Tip Epoxy data sheet (not sure about the curve) for illustration but there are other epoxy formulations with higher tensile strength. Most epoxies are characterized by this curve at varying degrees of yield.

    To get the best performance from your Uni, choose a high modulus, high strength epoxy whose elongation to break is longer than 4.6%. Lastly, have a coupon test made. These predictions are at best theoretical but guide you into an intelligent choice.
     

    Attached Files:

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  11. Steve W
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    Steve W Senior Member

    RX, your post is about as informative as any ive seen on these forums, thank you.I have acquired a roll of 4 " wide E glass uni, I don't know the weight, its pretty heavy stuff that a local windmill manufacturer was using for blades, the weight is irreleveant really but ill cut off a foot and weigh it and extrapolate, just so I can figure how much is on the roll. Since the thickness of the plies is the important number for this application im wondering how others approach this. I have found that when i measure the thickness myself it is nearly always thinner than the manufacturers published thickness, with my number being more accurate when measuring the finished part. Most of the time it dosnt matter as i am just shooting for a finished thickness and can always add material but for these chainplates I need to get the right number of plies to get the cross sectional area im after. Im not sure what epoxy I will use yet but it will be a higher elongation one, it will probably be difficult to vacuum bag these parts in situ but I can probably post cure.

    Steve.
     
  12. rxcomposite
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    rxcomposite Senior Member

    Thank you. I thought it would be easy to explain in a few words but ended up writing a novel.

    You can make a test piece if you have a postage stamp meter, a caliper, and a set of weights.

    For the weight proportion test, you can make it one layer thick, 50mm wide x 600 mm. long. Lay it flat on a thin plastic, wet the uni, squegee out the excess resin and let it cure. Weigh the piece before wetting and after cure. That is your weight ratio. Epoxy is heavier than poly so the usual weight ratio formula may not be so accurate.

    For the strength test, cut the test laminate, say 25mm wide, 300 mm long. Dog bone the flat shape so that the middle portion is 10mm. wide. Measure the thickness and the narrowest width. This is your cross sectional area. Glue and clamp one end and the other end, secure a hook where you can suspend barbell weights. These epoxy laminate are known to have ultimate tensile strength of 1,000 MPa with the right epoxy so make provisions to add lots of weights.

    Keep adding weights until the laminate cracks. That is you yield point. If you can't break it, use a smaller cross sectional area, say 5 mm wide.

    Pressure=Total weights(Force)/cross sectional area. That is the strength of your laminate at yield.

    Crude but it works. Saves you $50 for a coupon test.
     

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    Last edited: Mar 23, 2014
  13. Steve W
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    Steve W Senior Member

    RX, again, great info, its going to be a month or so before it warms up enough to do this so I do have time to do some coupon tests. If I were doing the chainplates over a composite bulkhead i would make them in the shop as one part and infuse it and then cut it in half to make the 2 chainplates and then epoxy into place, it would be a lot easier and easier for QC, but im concerned that it all comes down to the strength of the bond,not of the chainplate to the plywood bulkhead but the wood to itself. I may still go this route and bond and bolt, it would still allow for a no corrosion, leakproof part with the advantage that i don't have to wait for the weather to break. As im writing this im talking myself into leaning that way,.

    Steve.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Yes you are correct. The weakest link is the bond as this is a secondary bond. It helps to splay out the fibers at the ends to increase bond area but there is a limit. Once the fibers deviates from the vertical at 7 to 12 degrees, you start losing strength (fiber directionality factor). E glass is more forgiving because of higher elongation strength but carbon is not. Degradation starts much sooner, before 7 degrees.
     

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  15. Steve W
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    Steve W Senior Member

    I don't have the space to splay out the fibers anyway due to the proximity to the door openings but i will have about 192 in2 of bonding area but like I said, I can bolt as well.

    Steve.
     
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