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