Left: this is a microscopy image of the bond between a carbon skin and Nomex honeycomb core. The vertical stripes are the cell walls of the Nomex honeycomb, and the distance between each cell wall is 3.2mm. We want to see the quality of the resin fillets that form between the Nomex cell walls and the laminate skin. Ideally we like to see fillets that are completely clear of any air bubbles, that are full fillets as opposed to shallow indentations in the adhesive film, and that extend up (or down) the cell walls as required. We can vary the fillet size by changing the amount of adhesive used, or by fine-tuning the cure schedules, and also by changing the way in which the adhesive itself is applied
Skin-to-honeycomb core bond testing. To check the strength of the bond between a carbon skin and honeycomb core we pull a small disc of laminate directly off the core with a hydraulic pull-out tester (left). First we cut through the skin with a hole saw, then attach the test rig to the laminate skin. The test rig measures the force required to separate the skin from the honeycomb. The first thing we look for is that the bond is stronger than the core; then we look at the size and quality of the fillets formed between the two
We often use the same specialist engineers whom the naval architect consulted at concept stage – but the engineers work for us as builder and take account of the materials and techniques that we have developed. Where that is not an option we employ a third party engineer to double- check the drawings and verify the calculations behind them for key areas of every boat we build. There have been at least three occasions when we did not do this, and we lived to regret each one. Second, we apply commonsense and
experience and reject structures or laminates that we don’t like; there are about a dozen common details that we will always reject or modify. For example, we never accept 300g cloth as the first ply into a hull mould because the overlaps are too thick and the unidirectional fibres that run over the top form a void at the edge of the cloth overlap. Third, we can agree a non-destructive
test criterion that sets out the level of porosity or size of void that is acceptable at the start of a project so that we can use non-destructive test methods to assess the laminates we build against the agreed standards. Fourth, while building a boat we can
address all the technical issues that we know cause laminate defects. Which brings us to the next section of this piece: in ways that are often far from obvious, apparently
small evolutions in boatbuilding technique have improved the quality of the laminates and joints that make up the structure of a racing yacht in 2021.
A racing boat structure is just a stack of carbon, resin and foam or honeycomb core… But if you put these three ingredients together in the wrong way or in the wrong combination there are far more than three things that can go wrong. When we are evaluating a laminate or
structure the main things we are looking at are the correct ratio of fibre to resin, air voids in the laminate, laminate porosity, fibre wrinkling, contamination, state of resin cure, honeycomb fillet size, skin debonding or blistering, core bridging over laminate overlaps. And we look for these at three levels. First level inspection is visual. Some of
our most experienced team leaders have been building boats for decades; it’s surprising what an experienced composite boatbuilder or laminator can pick up. A visual inspection is a much underestimated first line of defence. Second level inspection uses non-
destructive techniques. There are many pieces of NDE technology available: ultrasonic transducer inspection, core bond testers, laser shearography, thermal imaging etc, and in the right hands each of
these has its place. The best surveyors have years of experience with boat structures, they know what the engineer requires and they know how the laminate was built. As with visual inspection, the best practi - tioners can tell you more about the laminate than you’d think possible. Ultrasound is mainly used to find voids
and to characterise the porosity level of a laminate, though it is generally only practical to test large areas by grid sampling points 100mm or 200mm apart. By using a dual-element ultrasonic probe, the bond between skin and core can be inspected, though many would argue that laser shearography is a better method for this. An experienced ‘shearographer’ can detect voids and dis-bonds deep in a laminate, even between layers of core material. A thermal camera can inspect a large
structure in a short space of time, but only in a superficial way so it is often used in conjunction with ultrasound – a coarse and fine approach. The thermal camera has some unique strengths, however: for example, it can pick out air voids within a tight resin fillet and, because it is not just point sampling, it can identify areas of dry laminate where the vacuum has removed too much resin during cure. Third level inspection is destructive
testing. We build test panels alongside all main components so that we can do mechanical testing on a test piece that was
SEAHORSE 53 w
PIERREPONT ANALYSIS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120
