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Microwaves to dry boat hulls
Bruce writes:
On Sat, 27 Mar 2010 10:07:40 +0100, (Martin Schöön) wrote: Bruce writes: For whatever reason the owner, or perhaps the surveyor, cut cores out of the hull and had them tested. The cores tested at 90-something percent of the calculated original strength of the hull material. I find it highly unlikely they could calculate the original strength with a 10% inaccuracy. The materials used were not characterized that well and the variation in the lamination process is much bigger. I have been told by a senior structural engineering consultant that the uncertainty in fatigue life for the materials we know best-- structural steels -- is roughly 6%. Composites, even aerospace qualities, are much, much harder to get good data on. I am not sure whether they had sufficient data to do accurate strength calculations although I had a book written back in he very early days of fiberglass boat building by someone who was described as an expert, that did list tensile strengths for various boat building materials and certainly there would have been tests made before publishing such a table. And the accuracy was stated as? All material data I come across at work is within +/- something. It is hugely important to make sure material data used for engineering calculations are for the stuff coming out of production and not from some lab. Material data should be for relevant ambient conditions, temperature, humidity or whatever applies for the intended application. The use of a new family of high strength steels in ship building in the early 1908s is a grueling case story. Ships and life were lost because fatigue life in the corrosive real world was so much worse than in the lab. Earlier steel qualities had not been affected by environment in the same way. Having said that, certainly there is a variance in strength of a fiberglass structure that varies with all kind of things - chemical makeup of the actual resin used, hardener/catalysis mix, amount of glass and resin in the structure and so on. I assume that why they said calculated strength. And I say that stating that the laminate still had 90 % of its calculated strength is nonsense since the errors in calculation and measurements stack up to far more than 10 %. There was no mention of the boat's history or how much time it had spent in the water, and in England many boats are hauled out for part of each year, so the testing was hardly a comprehensive study but, as the magazine wrote, it did show that fiberglass did not deteriorate greatly with age. This is pure nonsense. Fiberglass laminates have limited fatigue life as do all materials. I didn't say that it didn't deteriorate with age, I said it didn't deteriorate GREATLY with age. Which is what my statement below was all about. This boat has lived a pampered life relative to its scantlings. Leave the thing resting in a cool, dry place away from harmful UV radiation etc and it will last a long time. Hard everyday use will see it break down in a few years. That is at least what happen to the boats used for daily transports by the population of the Gothenburg archipelago. Three years is what they expect glassfibre boats to last. These are boats designed and built for recreational use. /Martin |
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