All Discussions Tagged 'sandwich' - Wharram Builders and Friends2024-03-29T01:52:19Zhttp://wharrambuilders.ning.com/forum/topic/listForTag?tag=sandwich&feed=yes&xn_auth=noEXPERIMENTS WITH FOAM SANDWICH CONTRUCTION MATERIALStag:wharrambuilders.ning.com,2017-04-30:2195841:Topic:1509182017-04-30T15:44:48.954ZRobert Hugheshttp://wharrambuilders.ning.com/profile/RobertHughes
<p class="western" style="margin-bottom: 0cm;">A while ago I uploaded a discussion of the engineered carbon fibre crossbeams I've fitted to my Imagine Multihulls glassfibre Tiki 26, Zest.…</p>
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<p class="western" style="margin-bottom: 0cm;">A while ago I uploaded a discussion of the engineered carbon fibre crossbeams I've fitted to my Imagine Multihulls glassfibre Tiki 26, Zest.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://wharrambuilders.ning.com/forum/topics/engineered-carbon-fibre-beams-for-a-tiki-26?commentId=2195841%3AComment%3A142156">http://wharrambuilders.ning.com/forum/topics/engineered-carbon-fibre-beams-for-a-tiki-26?commentId=2195841%3AComment%3A142156</a></p>
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<p class="western" style="margin-bottom: 0cm;">At the time, I mentioned I was experimenting with materials for a new cockpit. I wanted to build a replacement that was significantly lighter than the existing one, while incorporating lockers under the seats, eliminating places where water could splash up in rough weather, and modifying the engine well. I had decided to use a foam sandwich construction, with structural PVC foam cores, but was undecided whether to use glass or carbon fibre, and polyester or epoxy resin. I did consider kevlar, but experiments showed that cutting, finishing etc would be more difficult, and it's not as stiff as carbon. I had received conflicting advice from various sources as to which materials would be best, and concluded that the choice really depended on how I would actually use them in practice. As an example, I would be using a wet layup method without vacuum bagging, and no oven. I thought the best way to decide would be to prepare some samples and test them.</p>
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<p class="western" style="margin-bottom: 0cm;">The first picture shows some of the samples being prepared. The fabric is laid up on the foam cores by hand, just as you would normally lay up fibreglass. The fine white fabric is peel ply, which I used to help consolidate the matrix and exclude air. When it is peeled off it leaves an ideal surface for further bonding processes.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799499779?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799499779?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">After the samples were laminated they were all trimmed to a uniform size, using a small bandsaw with a fine toothed metal cutting blade.</p>
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<p class="western" style="margin-bottom: 0cm;">After trimming I examined the cut edges. This picture shows two samples of carbon fibre, the only difference being that the upper one is epoxy resin, whereas the lower one is polyester. It looks like the epoxy holds the carbon fibres more strongly so they cut cleanly, but the polyester gives a more ragged edge as short bits of fibre pull out.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799499911?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799499911?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">This shows the test rig I used to measure the stiffness of the samples. Ignore the mug of tea, it's nothing to do with the tests, it's just there because I'm British and can't work without tea.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799502263?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799502263?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">Each sample was supported on a pair of wooden blocks on a work bench, then progressively loaded with lead weights in the middle. The deflection was measured from below, through a hole in the work bench.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799502397?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799502397?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">Each lead block weighed around a kilogram, 2.2 pounds. It was surprising how much weight some of the samples took, and it got a bit scary crouching down below to take the measurements!</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799502567?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799502567?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">The first graph is really just a test of the method, and illustrates the theory of sandwich structures or I-beams. Glass was laminated with polyester resin over three different thicknesses of core. You can see the dramatic increase in stiffness as the core thickness goes up, due to the load bearing faces being further apart. Thicker sandwiches are much stiffer for their weight, particularly as the core material is light. The fact that the data points fall on fairly smooth lines shows the measuring method works well enough for what I'm trying to do.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799502595?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799502595?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">The next graph shows the comparison of polyester vs epoxy resin, for glass and for carbon fibre. In both cases the epoxy structure is slightly stiffer. You will also notice the lines for polyester stop just above 10kg applied load. The reason is that I stopped adding weights at that point, because very sharp high pitched snapping noises (“tink!!”) started to occur and I felt it was unsafe to continue. I believe these were individual fibres snapping as microscopic distortions took place in the matrix and the load was no longer shared properly across all fibres. The fibres that became more heavily loaded then failed. This never occurred in the tests of the epoxy samples, even with a further three weights added. This suggested that epoxy would certainly give superior results, in the application I envisaged.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799504861?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799504861?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">The next graph compares the results of the carbon and glass fibre samples, both using epoxy resin. You can see the carbon is very much stiffer, the glass bending about 50% more under the same load. It should be noted that the glass cloth is much heavier.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799505002?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799505002?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">To allow for the differences in cloth the results for carbon vs glass were recalculated by adjusting for weight, the results being shown below. The glass now bends about 2.5 times as far as the carbon for a laminate of the same weight. Please note that this is a fairly crude comparison, because there are many other potential variables such as different grades of glass, weaves of cloth etc available. It was sufficient for my purpose though, as it demonstrated a carbon structure would be much stiffer for the same weight, not just a marginal improvement.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799505063?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799505063?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">I wanted to test a sample to destruction, but in view of how much weight was required in the bending tests I decided to use a much smaller sample, for safety reasons. This rig was set up on my main workbench, with a simple piece of wood laid across the test piece so that the weights did not fall off sideways. To give an idea of the stiffness of these materials note that the sample uses 10mm foam and is only about half an inch wide, weighing very little. For the same weight of material, mahogany could be 2.4mm thick, or aluminium 0.7mm. The wood would have snapped by now, and the aluminium would have bent down to the bench. To achieve good stiffness with those materials other approaches are used, such as ribs and bulkheads in wood, or clever extruded sections or tubes in aluminium. I can now understand why the America's Cup boats are built in carbon...</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799505138?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799505138?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">This picture shows a carbon/epoxy sample after loading to failure. Note the break is in the face under compression, not the one under tension. This suggested I should make the upper surface of the cockpit floor a bit stronger than the lower surface. The break is very clean and sharp; I also tested a carbon/polyester sample and the break in that was more ragged.</p>
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<p class="western" style="margin-bottom: 0cm;"><a href="http://storage.ning.com/topology/rest/1.0/file/get/1799507214?profile=original" target="_self"><img width="500" src="http://storage.ning.com/topology/rest/1.0/file/get/1799507214?profile=RESIZE_1024x1024" class="align-center" width="500"/></a></p>
<p class="western" style="margin-bottom: 0cm;">In conclusion, these tests gave me the information I needed, to decide to use mainly carbon fibre and epoxy resin for my new cockpit. While working with the materials I also saw that the carbon can be quite brittle in some circumstances, so I used a hybrid laminate for the floor, to avoid damage from anything like a dropped anchor or outboard motor. The tests were not comprehensive, nor were they rigorous enough to publish in a scientific or engineering journal, but they were sufficient for my purposes and I hope you find them interesting.</p>
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<p class="western" style="margin-bottom: 0cm;">Construction of the cockpit is now well under way, and I'll write that up in due course. I will say two things at this point though, it's feeling impressively light, but it's taking a lot of work...</p> Putting fittings through balsa and foam sandwichtag:wharrambuilders.ning.com,2010-01-30:2195841:Topic:126152010-01-30T12:41:19.000ZRobert Sheridanhttp://wharrambuilders.ning.com/profile/RobertSheridan
We are about to put some new deck fittings on the boat and need to reinforce the existing decks where they will go. <br/><br/>The hull decks are foam sandwich and the pod decks are balsa sandwich. I believe that the way to deal with foam is to use an Allen key to pulverise it and then pour in an epoxy mix, but I have not found any guidance on what to do with balsa; any advice?<br/><br/>Thanks<br/>Robert<br/>
We are about to put some new deck fittings on the boat and need to reinforce the existing decks where they will go. <br/><br/>The hull decks are foam sandwich and the pod decks are balsa sandwich. I believe that the way to deal with foam is to use an Allen key to pulverise it and then pour in an epoxy mix, but I have not found any guidance on what to do with balsa; any advice?<br/><br/>Thanks<br/>Robert<br/>