On Sun, 28 Mar 2010 12:43:51 +0200, (Martin
Schöön) wrote:

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.

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 +/-.

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.

This is news"

Nearly all uses of new materials,or use of materials in a new way, has
resulted in failure. Not always catastrophic failure but some problem.
The first iron bridges were built with cast iron - they fell down. The
de Havilland Comet, the first commercial jet airliner, used to explode
in flight. Pressurization caused the fuselage to flex, the aluminum
cracked and the side blew out.

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.



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
Cheers,

Bruce

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.)