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Normally I wouldn't cross-post anything from rec.scuba, Hell I barely admit
reading it, but this is such a fine piece that I think is more relevant to
us than Yankee pizza and the defamation of grits, the southern version of
manna from God.

The author obviously has his shit together, and I thank him for his time
writing this piece.

Drew


-----Original Message-----
From: Matthew MacLean <Matthew.MacLean@bi*.co*>
Newsgroups: rec.scuba
Date: Saturday, April 01, 2000 9:28 AM
Subject: The truth about cracks and scuba tanks (was Re: It was a Aluminium
tank that blew)


>The truth about cracks and scuba tanks (I think)
>
>
>The following is very long and attempts to answer as many question a
>possible about Al scuba tanks, cracks and testing mechanisms. It is a
follow
>on from the " It was a Aluminium tank that blew" thread by Skip of E Force
>fame. It is written as simply as I can make it but it is still a fairly
>technical subject. To that extent I apologise to people who have a good
>understanding of this field for simplifications inherent.
>
>
>What is 6XXX aluminium?
>
>6XXX grade Al is a wrought alloy aluminium. Wrought alloys differ from cast
>alloys in that they can be shaped by deformation.
>
>It is an age hardenable ternary alloy.  All ternary means is that it has 3
>primary constituents, these being Al, Si ,and Mg. They may also include
>trace elements of both copper and Cu and Cr [2].
>
>Age hardenable means that the material can be treated to produce a fine
hard
>coherent precipitate in the softer more ductile matrix. More on this later
>
>A typical 6XXX grade alloy is 6061-T6. This is the current industry
standard
>grade for aluminium tanks
>
>
>Composition of 6061- T6
>
>1% MG, 0.6% Si, the rest Al with trace elements of Cu and Cr. Typically
>these trace elements are less than 0.5% combined. [2]
>
>
>So what does the T6 stand for?
>
>T6 is the designation for the type of material treatment process that the
Al
>has undergone. T6 stands for solution treated artificially aged. [1]
>
>
>Solution Treated Artificially Aged:
>
>To achieve this the material is raised to a temperature above room temp to
>allow different phases within the material to form. At some point when the
>desired precipitates form (ie come out of solution), the alloy is quenched
>to lock the new microstructure in place.
>
>Over Ageing:
>
>occurs when a material is held at a process temperature past it's optimum
>material properties. At this point after quenching the yield and tensile
>strength are lower than expected. This may also occur at room temp due to
>diffusion but the time periods are huge and depend on the precipitates
>formed. It will also occur if a material temp is elevated at any point in
>the materials life.
>
>6XXX series alloys are usually raised to 190 degrees C and held at that
temp
>until the desired material properties are achieved and then quenched in hot
>water (80 degrees C.) [1]
>
>Typical properties of 6061-T6 Al is tensile strength 45000psi and a yield
>strength of 40000psi, this is in vast contrast to pure Al (annealed), with
a
>tensile strength of 6500 psi and a yield strength of 2500 psi [1].
>
>The material is basically a matrix of Al with Mg2Si precipitates spread
>evenly through it. It gains it strength from the interaction of the "hard"
>Mg2Si precipitates with the soft Al matrix as dislocation attempts to take
>place when the material is under stress.
>
>
>
>Why is Al used in scuba tanks?
>
>The primary design characteristics for a pressure vessel material are a
high
>yield strength and high fracture toughness (K).  More accurately we wish to
>maximise the material ratio property M 1 = {K(squared) / maximum yield
>strength} and the second material constant M2 = high yield strength. [4]
>
>Fracture toughness (K) is a measure of the materials resistance to cracking
>and is a property of the material the same as yield strength or hardness.
>Typical valves of K for Al/Mg/Si alloys (T6 treated) are 25 Mpa/ square
>meter [3]
>
>Secondary material considerations which may or may not have validity when
>designing scuba tanks are final weight, corrosion resistance and cost of
>fabrication.
>
>Materials that exhibit the primary material selection behaviour are some Al
>alloys, steels, Cu alloys, Ni alloys, some Ti alloys and other specials,
and
>GFRP (carbon fibre and like). [3]
>
>Taking in manufacturing costs and corrosion issues into the design
>consideration we get left with the choice of steel and Al for our tanks.
>
>In practice weight consideration for Al and steel tanks are not as great as
>you would think, but the buoyancy characteristics alter significantly for
>the 2 classes of tanks, and this too comes into consideration for many
>users.
>
>
>So where is the Problem?
>
>Often metals when subjected to cyclic loading at stresses far less than the
>material yield strength fail in a catastrophic manner. This is called
>fatigue [6]. The point of using fracture toughness of the material in its
>selection from a design point of view is to limit this event. The
difference
>between Al and steels is that Steels have an "endurance limit"[3]. Stresses
>applied to the steel below this limit will never create fatigue. Al on the
>other hand has no endurance limit, so even small repetitive stresses will
>eventual cause failure in the aluminium.
>
>
>Does this mean that Al tanks crack and Steel Tanks don't ?
>
>No, both tanks materials due to the level of stress induced in service will
>develop cracks eventually, just the time periods are different. We don't
see
>cracking as a major issue in steel tanks because other failure modes occur
>faster (ie rusting) and tend to remove the tank from service first. Also
>Hydro's are more effective at picking up cracks in steel due to the slower
>crack growth rates (more about this later), and the better chance for
>plastic deformation of the remaining uncracked material during a hydro
test.
>
>Also scuba tanks are designed to one of two design philosophies, these
being
>"leak before burst" and "deform before burst". These philosophies have to
do
>with the cracking nature of the material used and the stresses under which
>it operates. Basically they say that any crack in the tank should either
>result in the tank deforming before it fails catastrophically (ie explodes)
>or it should leak before a crack will get to the size that will result in
>catastrophic failure (ie the crack length required for catastrophic failure
>is longer than the wall thickness of the tank). There will be a lot more on
>this later.
>
>
>Why is this of interest to scuba divers?
>
>Because certain older Al alloy tanks have not done any of the things that
>the designers have intended (ie leak or deform) and have blown up severely
>injuring people.
>
>This is called catastrophic failure.
>
>To understand this we need to understand Fracture mechanics and crack
growth
>theory.
>
>
>
>Crack Growth and Fracture Mechanics (as it applies to scuba tanks)
>
>Cracks.
>
>Cracks form when localised stresses in the material exceed the plastic
>deformation limit of the material. Cracks can initiate from many things,
>including, foreign inclusions, surface defects, micro voids, rust,
>manufacturing induced stress concentrators (sudden changes in wall
thickness
>etc). Whether one of the above mentioned factors results in a crack depends
>on local stresses, orientation and a bunch of other things. The same
>mechanism in one spot will start a crack and in other act as a crack
>inhibitor [5]. All we really need to know here is that the conditions for
>crack formation will be meet in service at some point and the crack exists
>[5]. At this point cracks can either be non-propagating or propagating. In
>the course of this discussion I will concentrate on propagating cracks.
>
>Basically once in existence the crack will grow relative to the energy
>applied each cycle, the geometric properties of the crack itself, and
>certain material characteristics like the size, number and distance between
>mirco voids in the material. The more voids the faster the crack will grow
>for a given energy/crack geometry configuration [5]. Cracks in this state
>exist in everything from bridges and ships to Nuclear reactors [6], and
>scuba tanks.
>
>
>
>So What Happens Next?
>
>Cracks in this state continue to grow until one of three things happen.
>
>1: the remaining uncracked material yields due to the increase in stress
>resulting from effectively less material carrying loads. (Your tank fails
>hydro due to plastic deformation).
>This is a result of the "deform before burst" design philosophy coupled
with
>a proof test
>
>2: Cracks continue to grow until they exit the surface of the tank and
>hopefully you notice a leak (pin hole leaks in tanks noticed during fills
in
>baths). Or visually/remotely detected during hydro and visual inspection.
>This is the "leak before burst" design philosophy
>
>3: the crack continues to grow until the critical strain energy release
rate
>condition is met and the tank blows up. This is a fuck up!
>
>Relative to this discussion it's event 3 which interest us.
>
>Quick note on the 2 design philosophies, you really only use one of them in
>design, and apply it to cracks running in one direction (usually straight
>through the wall), but occasionally due to geometry a crack will run around
>the inside of the tank, and then the other can come into play through luck
>more than good management, since critical length of surface cracks can be
>quite long.
>
>
>
>Fracture Mechanics
>
>In order for the tank to blow up it need to meet the following requirement.
>
>Kic =<  Stress( induced) x Square root (pie x a)  [5],
>
>Kic: is the material fracture toughness described earlier (critical point)
>
>Pie is the mathematical constant 3.1415.
>
>Stress (induced) is the stress present in the material due to it's current
>loading.
>
>"a" is the dimension of the crack in the principle direction (ie direction
>it's growing)
>
>Once this equation is met our crack suddenly grows through the material at
>speeds approaching the speed of sound, with the resultant release of
energy.
>
>From this equation you can see that the guiding requirements are Stress and
>crack size. More stress and means a smaller crack size to meet this
>condition and vise vera. It also says that as cracks get larger they grow
>faster (the size 'a' increases which affects the mechanism for propagation)
>
>The equation is the same for both Al and Steel tanks. Just the material
>properties are different
>
>This is not quite the whole story, as there is a shape factor that goes
into
>the equation, it is dependant on crack starting point/orientation/type, but
>becomes a constant for the crack in question. Typical shape factors can be
>found in [5]
>
>Also engineering design factors for stress concentrator factors can be
>introduced here for design purposes (a typical one is a nipple in a
pressure
>vessel raises the local stress(induced) given by a hoop stress equation by
a
>factor of 3 in the area of the nipple fitting). [6]
>
>Like I said before tanks are designed so that this condition can't be met,
>either the wall thickness is such that the crack goes through it and leaks
>or the tank deforms first (depending on design philosophy). Currently due
to
>hydro's, visual inspection and other means tanks that are approaching these
>conditions are detected and removed from service. This is what is supposed
>to happen, tanks do not have an infinite life.
>
>
>So Why Did Those Tanks Explode?
>
>
>6351- T6 alloy, sustained load cracking (SLC) and corrosion cracking (CC)
>
>In the case of 6351-T6 alloy, it displays a nasty property called "Slow
>crack growth".
>Both SLC and CC are subsets of slow crack growth theory[5]. What happens
>here with SLC is that once the K valve has been exceeded locally and our
>normal crack propagation starts, the value for K in the local area drops to
>less than the design engineer thinks and material testing indicates. The
>best analogy I can come up with is static friction and dymanic friction. In
>your car you stop quicker when you hit the brakes if the wheels keep
turning
>cause you are braking using the static value of friction between the tires
>and the road. Once you lock up the wheels you are using the dynamic valve
of
>friction and the car takes longer to stop because it is lower than the
>static valve. Something like this happens with SLC, it results in effective
>crack length a lot longer/quicker than we would otherwise expect. But it
>resets itself every time you empty the tank. So the next time you fill the
>tank you have the higher valve for K present, until the crack starts. The
>tank still has to meet the higher valve of K for it to explode (I think,
I'm
>not 100% sure on this bit, it has to do with the energy model for crack
>growth/plastic deformation in the local region around the crack tip and
>total crack energy).
>
>
>So why did the tank explode in the E-force shop?
>http://www.evcom.net/~n4mwd/chris.htm
>
>My guess is that the tank did have a fairly standard sustained load crack
>that was not detect for what ever reason, the reason for the initial
>confusion put out by the DOT inspector about Fast fracture of a new type is
>that it did not seem that the energy requirements had been met for fast
>fracture as described earlier. I propose that the increase in stress in the
>tank came from thermal conditions imposed when the tank was placed in the
>filling chamber. From what I understand E-force used a cooled water bath to
>fill tanks in and this created enough thermal stress in the tank to tip it
>over the balance of the strain equation. This stress can be quite high,
>especially if the tank had been in the sun for a period of time and
suddenly
>introduced to water at 30 to 35F. (13 to 16 degress C). I am not implying
>that thermal shock resulted in the explosion but that the energy
>requirements for Fast Fracture was meet due to a combination of thermal
>stress and internal stress due to the tank being almost full at the time of
>being placed in the bath.
>
>Also I am not in anyway implying that procedures at the shop caused this
>accident, I am just trying to explain the mechanism, this was really just
>the straw that broke the camels back.
>
>What I haven't mentioned so far is why neck cracks are so bad, In the side
>walls a design before leak philosophy results in leaks, but in the neck due
>to the way tanks are made the material thickness can exceed the thickness
>that a design before leak concept would indicate. Coupled with possibility
>of incredibly high stress raisers and events that occur during filling,
tank
>abuse etc the potential for the fast fracture condition to be met, exists
as
>a statistical probability.
>
>Corrosion cracking  [5, 3]. Similar to SLC but uses the higher valve of K
>(the one we think the material has), and comes about because environmental
>elements and residual stresses combine to continue crack growth after
normal
>propagation methods have stopped. In the case of Al tanks one agent is Cl-
>ions found in seawater. Really it just means wash your tanks, and inspect
>them for pitting. (got this bit off some dodgy web page J )
>
>
>Dag's 5283  grade Al tank
>http://www.evcom.net/~n4mwd/scubadag.htm
>
>In one way this one is even worse, the material properties of 5283 Al are
>believed to alter in relatively short time periods, Since K is a material
>property and gets lower as the material ages then the length of a critical
>crack for a Fast fracture becomes less and eventually will be less than the
>wall thickness, On the day in question it was estimated the temp in the
shed
>was 40 degrees C so the pressure in the cylinder would have been greater
>than when filled, couple with the lower K value a crack that would have
been
>stable when the tank was made became unstable and blew up. Also this tank
>material looks like shit even from photo's on the web page
>
>
>Testing For Cracks
>
>Eddy current test:
>
>If you want to know how they work go to the following site
>http://members.aol.com/flare439/myhomepage/visualeddy/Visual_Eddy_Brochure.
P
>DF
>
>What does it mean. Eddy current tests determine changes in resistance
>inherent in the metal. It is considered a technique requiring high operator
>skill for correct interpretation [7]
>Due to these consideration it is usually used for go-no-go checks on like
>components [1] (if one reads different than all the rest it is rejected).
It
>is good for picking up surface and subsurface defects. Due to the fact that
>it detects changes in electrical resistance in the material it will detect
>cracks, inclusions, changes in material density and get different results
>for paint and unpainted parts. Because of this in industry when used as a
>board based search techniques it is always backed up by a second technique,
>usually ultrasonic testing to correctly identify the flaw type [1].
>
>Having said all this the manner it is used for testing cracks in the thread
>area is almost an ideal usage model for the technique. It is repetitive,
>very localised and tailored for the operation at hand. But it still can't
>tell you if what you are looking at is a crack or another kind of fault (it
>can tell you where and depth). If a tank failed testing due to this after
>several years of testing by the same technique I would have no problem in
>scraping the tank. On a new tank I would require the tank to be tested by
>other means to determine whether it was a manufacturing induced crack or
>other defect from manufacture or just a statistical anomaly in material
>thickness/some otehr feature in the tank.
>
>Note, this technique would not have detected the crack in Dag's tank, but
>may of detected the one in Force-E explosion. (if it was what was mentioned
>by the inspector on the web site, it wouldn't detected the crack, since he
>claimed it didn't exist prior to the onset of fast fracture mechanism)
>
>
>Where does this leave us?
>
>If I owned a 6531- T6 grade tank I would be getting it Hydro and then crack
>tested yearly. Why, the hydro acts as a proof test and tells me I don't
have
>cracks of a certain size present as can be calculated from the equation for
>fast fracture (reverse the equation and solve for "a", put in the hoop
>stress for the hydro pressure and K from a material hand book and you have
>critical crack length that is required for the tank to fail. If the tank
>doesn't fail than that length crack doesn't exist. I would then follow up
>with a eddy test for crack growth in the neck since this stress is greater
>than normal operation stress the tank should be OK for service for the it's
>normal cycle life in one year. I would also be storing it at 40bar. If this
>is uneconomical in the States I would be trading it in and buying a new
>tank.
>
>On a side note: it appears that walter Kidde tanks are more susceptible to
>exploding, probably due to the fact it is a shitty design more prone to
>stress raisers in the neck area. This does not negate the problem for other
>6351- T6 grade tanks
>
>
>If I owned a 6061-T6 tank. I would be getting tanks over 10 years old eddy
>tested for neck cracks yearly, other than that I would be ensuring hydro's
>are in order, and making sure the local visual inspections were being done
>properly. This should give a high level of confidence to users as no
reports
>of 6061-T6 tanks exist of them exploding. It does not suffer from SLC, but
>being aluminuim it will crack at some point
>
>Note: In Australia tanks are hydro'd yearly so the proof test concept is
how
>this cracks are discovered.
>
>If I owned steel tanks, I'd fear rust, tanks used with high 02 can rust at
>accelerated rates and should be inspect internally regularly. Especially if
>saltwater contamination is a possibility.
>
>Also ice divers, if I were you I would look at the ductile to brittle
>transition of BCC (body centred cubic) materials and it's effect on K
values
>for steel. (saw this lovely photo once of scuba tank stored on the ice at
>about -30 degrees C, scared the shit out of me). This doesn't affect Al
>tanks.
>
>As for cracks, because steel has a higher K valve than Al cracks in steel
>for the same stress characteristics and shape factors as in a AL tank grow
>at a slower rate. Hence you have more time to detect them. But remember
they
>will crack eventually if nothing else gets them first. (Look at the history
>of LP versus HP steel tanks for conformation of this)
>
>
>
>Why am I happy to say I would condemn a tank on a on the basis of an eddy
>current test? (if you had followed the originals line I was critical of
>this)
>
>Because in the world of equations and testing I can determine the length of
>the crack I have, the length of the crack needed to go critical, and the
>number of cycles required to grow the crack to that length and the amount
of
>useful life left in the tank.
>
>Unfortunately in the neck region some very complex actions take place and
>the one thing I can not be assured about is the actual stress that being
>applied to cracks in that area. So hence in this situation once I have
>detected the crack (and made sure it is a crack) I will happily scrap the
>tank, Because the real stresses in this region can be far higher than
>predicted. (Ross B. put up a nice model showing some of the complexities
>involved)
>
>
>Remember the next time someone scraps one of your tanks, be happy, he just
>made your day!
>
>
>Just for clarity I have no intention of giving up using AL tanks, but I
>might look at them a little more closely in future.
>
>
>If you have actually read this far I congratulate you.
>
>Matthew MacLean
>Mackay Qld
>
>
>
>A few loose points from the original thread,
>
>1: All metals work harden to some degree, just the amount varies
>
>2: Comments that steel barrels before bursting is a result of design issues
>only and not a product of material property as implied by the original
>poster. Once the energy requirements are met the material will fast
fracture
>no matter what it is (there is a case study on human skin and this property
>if you really want to read it!)
>
>About the author
>
>I am a practising Mechanical Engineer, with a degree (B.E Mech.) from the
>University of Newcastle NSW, Australia. I have my own consulting business
>(MacLean Engineering & Management Services) and I primarily work in the
area
>of Maintenance Management and the development of maintenance systems for
>capital intensive industries. I studied materials, scheduling systems and
>maintenance management in my final years at university (basically cause I
>had no idea what I wanted to do) and ended up with a job where I
>occasionally use all three. I am also bored cause I just had my 4th diving
>weekend in a row cancelled due to the weather in QLD.
>
>Oh Yer, to the guy who referred to me as totally clueless in relation to
>engineering issues in a previous post, well other people pointed out your
>errors, but have a nice day any way!
>
>
>[1]  The Science and Engineering of materials, Donald R Askeland, 1984 PWS
>Publishers
>
>[2] Engineering Material and Their Properties 2nd edition, Flinn/Trojan
>Houghton Mufflin Company / Boston 1975
>
>[3] Mechanical Metallurgy, Geoage E Dieter, McGraw Hill Book Co, 1988
>
>[4] Materials Selection in Mechanical Design, M.F Ashby, Pergamon Press
1992
>
>[5] Elastic & Plastic Fracture, Metals, polymers, ceramics, composites,
>biological materials. A.G Atkins & Y-W. Mai, Ellis Harwood Publishers 1985.
>This is very much a definitive text for general use.
>
>[6] Engineering Materials 1, An Introduction to there Properties and
>Applications, Michael F. Ashby and David R.H Jones Pergamon Press 1980
>
>[7] Maintenance Management, APESMA Short Course Notes, Prof. John Chambers
>and D. Wilkinson, University Of Newcastle Press, Adapted from the course
>notes for the Maintenance Engineering Post Graduate Course . 1990
>
>

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