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#1
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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 |
#3
posted to rec.boats.building
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Microwaves to dry boat hulls
Bruce writes:
On Sun, 28 Mar 2010 12:43:51 +0200, (Martin Schöön) wrote: snip 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. It is? Here is a section of a specification sheet: "The standard requirements for ASTM A516 physical and chemical characteristics are given in the tables below." Mechanical Properties: A516 Grade 60 A 516 Grade 65 A16 Grade 70 Tensile Strength (ksi) 60-80 65-85 70-90 Tensile Strength (MPa) 415-550 450-585 485-620 Yield Strength (ksi) 32 35 38 Yield Strength (MPa) 220 240 260 Elongation in 200mm (%) 21 19 17 Elongation in 50mm (%) 25 23 21 Max Thickness (mm) 205 205 205 The supplier is Oakley Steel, " a specialist steel supplier focusing in boiler and chrome moly steel plates. We stock pressure vessel quality plate in ASME and ASTM grades principally for use in the oil and gas industry." Hard to find a mention any tolerance, any +/-. No, in your example I find that right away for tensile strength. If you find it hard to get the information it is either because you use the wrong suppliers or because you are not important to them. Having written that I think I have to point out that published data sheets seldom are really helpful because the tolerances found there are cooked up by the sales department... you have to go to the next level in most cases. snip I have sent quite a number of coupons for testing and processed quite a few materials certificates, furnished by steel makers, and I have never seen a tolerance, never; "Tensile strength 50,000 psi +/- 10%". Every materials certificate or test coupon has listed only the results of the test as "tensile strength 50,000 psi" without a tolerance. If I got such an answer I would start looking for another tester. Someone who knows and acknowledges that test equipment and test procedures have limited accuracy. Several samples should be tested since all manufacturing processes have variations. Trying to insert some boating content: The other day I revisited a report on the structural modelling of a racing yacht. One chapter is dedicated to material testing. They manufactured laminate samples and had them tested at a test institute to get data for the modelling. The data they got came with a mean value and a standard deviation number. The standard deviation for the material parameters of those carbon pre-preg laminates were roughly 3%. (You see were my skepticism regarding that 90% figure comes from.) Back to my rant: It is *very* important to realize that real world materials and production are subjected to random variations and modern engineering must acknowledge that and take it into account. "If you haven't done a proper yield analysis you aren't done." Yield in this case is not yield as in yield strength, but you realized that for sure. http://en.wikipedia.org/wiki/Six_sigma http://en.wikipedia.org/wiki/Cpk_Index http://en.wikipedia.org/wiki/Statist...rocess_control http://en.wikipedia.org/wiki/Design_of_experiments http://en.wikipedia.org/wiki/W._Edwards_Deming /Martin (sorry for the late reply, it has been busy times. I have been studying the impact of mechanical tolerances on a proposed design, among other things.) |
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