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Aluminum Work Hardening
5

Aluminum Work Hardening

Aluminum Work Hardening

(OP)
I've been involved in a long-term debate with people who in my opinion do not have the technical background, over whether Aluminum mast (for sailing) will work harden without the masts becoming permanently bent. The masts are a tubular section of either 6061 or 6063 grade and heat-treated to T6

From my knowledge of dislocation movement work hardening will only occur when the material yields. But below the yield point, the dislocation is unimpeded.

The masts do flex, but it seems wrong to me to think that they work harden while they flex. To make things more difficult, the mast suppliers are now labelling the masts to indicate that new masts should not be used in strong breezes, until the masts have been "broken in".

Does anyone have any comments? I'm particularly interested in any papers written on the subject.

RE: Aluminum Work Hardening

Perhaps the more likely term should be aged harden. Aluminium with a Mg content approx above 0.5% will age and become stronger through time, typically components made of this material are laid down for a period and allowed to age before eing put into service. One way is simply to conduct hardness tests every few days untill a stable result is obtained.

RE: Aluminum Work Hardening

(OP)
I don't think that is likely. The alloys mentioned are solution treated and artificially aged (T6 temper). The artificial aging temperature from my understanding is between 160-175°C (approximately 310°F. If the alloys are correctly aged, the aluminium should be in the fully aged condition.

At room temperature these alloys shouldn't be naturally aging to any significant extent and if they did they should become overaged and hence softer.

RE: Aluminum Work Hardening

2
It sounds as if the people with which you are discussing this issue do not understand Materials Science and Engineering.  Work hardening, more properly called strain hardening, occurs when a material is plastically deformed, i.e., when stresses beyond the yield strength cause dislocation motion.  In the case of a sail mast, the forces should be causing only ELASTIC deflection/deformation, and not PLASTIC deformation.  Strain hardening most certainly does not occur under elastic loading conditions.

The following websites have some additional information on the subject:

http://www.tpub.com/doematerialsci/
especially this chapter:
http://www.tpub.com/doematerialsci/materialscience59.htm


http://www.virginia.edu/bohr/mse209/class.htm
especially this chapter:
http://www.virginia.edu/bohr/mse209/chapter7.htm

For an expert-level discussion, you should refer to R.W. Hertzberg's excellent book Deformation and Fracture Mechanics of Engineering Materials, available from Wiley using the following link:

http://www.wiley.com/cda/product/0,,0471012149,00.html

RE: Aluminum Work Hardening

I don't think I agree with the people to whom AlanD refers, but I will play devil's advocate here.

Since masts are long tubes, the stresses are concentrated at the base.  This also concentrates any deformation.  Is it possible that initial loadings cause small deformations concentrated in this area.  This may not cause gross mast shape change, but may cause sufficient strain hardening to prevent more deformation from future loadings.  One thing that seems to disprove this idea is that 6061-T6 has little strain hardening capacity: yield strength = 275 MPa & ultimate tensile strength = 310 MPa.  This small strength change wouldn't allow significant improvement.

RE: Aluminum Work Hardening

Instead of speculating why sailors think masts need breaking-in, I visited boaters' sites including a forum at http://www.briontoss.com/wkstone-0.7.5/webkeystone.py?UserID=biz_briontoss&Profile=message_board/showMessages.prof

I didn't find the exact Q & A, but I drew the following conclusions:
1) Sailors are definitely not trying to work-harden their masts by plastic deformation.  It is much trouble to straighten a bent mast and then fit a concentric sleeve over (or inside) that section for reinforcement.
2) They know a lot about the metallurgy of Al 6063-T6, how welding weakens it, galvanic corrosion, anodizing vs. polyurethane coating, etc. They know some engineering failure modes, and space and size holes to avoid stress concentration.  Also, masts are pretty efficient beams, with a pinned bottom connection, tables of Ixx & Iyy for various extrusion sections.
3) While sailboat masts all have a strong, hinged bottom connection (a 'tabernacle') to the hull, there is a lot of variability in the strength & flexibility of decks.
4) Between the mast & deck, boaters may use shims, blocks, wedges, etc., of wood, hard rubber, cast acrylic, etc. To get one of these seated, it is pounded in while loading the mast in the opposite direction.
5) Some boats use a threaded 'tierod' to connect the mast to the deck.
6) The maximum mast stress is not at the bottom, but probably at the sail spar(s) or deck.
7) There was mention of 'tuning the mast' which apparently means getting it seated with the deck and having all the cable stays properly tensioned. This gives the mast some rigidity to prevent it from flopping around when the direction of the sails change.
8) Finally, my opinion is that masts do need a 'break-in' or 'tuning,' the reasons being that a) the mast/deck connections need to be adjusted and seated uniformly after some flexing, and b) the cable stays need to be properly tensioned. With too much 'play,' a sudden shift in wind direction could create a whiplash effect which would bend the mast.  Both reasons allow for more uniform stresses on the mast.

Work hardening and age hardening are not involved.

RE: Aluminum Work Hardening

(OP)
Unfortunately Kenvlach, those comments are not relevant in this case. The boats in question are the International Laser Class (one of the Olympic classes). Although people in the past have thought that work hardening is occurring, the people currently pushing the issue are the 5 manufacturers around the world who are trying to avoid replacing masts that are deforming far too easily. The real issue is that the specifications are not up to scratch.

My background is as a professional metallurgist not currently working in the industry, however I'm also the Australian Measurer for the class. Most people in the class will accept what the manufacturers say with regards to "breaking in" the mast, but as a metallurgist I cannot. The manufacturers are informing us that the masts should not be used in winds over 12 knots, until the masts are "broken in". A good quality mast can easily survive 30-40+ knots, which is beyond the skills of most people in the class.

The mast are a two piece arrangement. Both sections are circular anodized aluminum tube, with the bottom section being of a larger diameter than the top section. A plastic collar locates the bottom end of the top section within the top end of the bottom section (305mm/12" overlap). The fittings on the mast sections are all under compression (unless you do something stupid on the water).

In my opinion I believe that the real issue is that tolerance specified by the boat manufacturers are inadequate. The problem lies with the wall thickness, alloy or properties after aging.

I hope this background information assists.

My original problem still remains, does anyone believe that work hardening could be occurring below the yield strength. In my opinion it cannot occur.

RE: Aluminum Work Hardening

AlanD,
     I don’t fully understand the boats, and there may be signicant differences between Olympic class and ordinary sailboats that I drew conclusions from.  However, I feel that you haven’t disproved my basic premise [the mast needs to be uniformly seated and the rigging ‘tuned’ in order that the mast transfer stress to the deck and hull without any excess play that could create localoverloads].
    However, you have brought up some points for consideration.  Also, I have read that some sailors use stainless steel cables while others use coated, high strength steel, and I presume that some people use polypropylene rope, and maybe racers use some Kevlar type.
     So please enlighten us a bit more.
1) Are the masts similar to those shown by Dwyer http://www.dwyermast.com/ or is there a geometric difference between ordinary & racing masts?
For example, do the latter omit the internal conduit for electrical?
2) At what location on the masts does failure typically occur (relative to hull, deck, splice, attachments and loading points)?
3) Do the masts fail in strong gusty or change-of-direction winds (rather than strong steady wind)?
4) What kind of rigging cables are used?  At what tension? Maybe some work hardening?
5) What kind of mast-to-hull connection? What material? Does it need adjustment?
6) Do the mfrs. recommend break-in for shorter, one-piece masts?,
7) By “The fittings on the mast sections are all under compression” do you mean that through-bolts are used, with a piece of rigid tubing inside the mast to avoid collapsing it?
8) You say “the real issue is that tolerance specified by the boat manufacturers are inadequate. The problem lies with the wall thickness, alloy or properties after aging.”

Since you believe that no work hardening occurs via break-in nor that age hardening occurs after receipt of the mast, (and also disbelieve my idea that ‘break-in’ is getting rid of slack in the mast connections), you believe that ‘break-in’ is nonsense.  Hence,  some masts will fail at 16 knots due to mfr. defects regardless of break-in, and some better made ones will withstand 40 knots regardless of break-in, contrary to the mfr. spiel.    Is this a correct statement of your opinion?

RE: Aluminum Work Hardening

(OP)
Photos in the link below
http://www.mhasc.com/content/gallery/group1/index.html

Measurement diagrams here (note these do not specify diameter, wall thickness etc),
http://www.laserinternational.org/rules/measdiag.htm

1. Lasers are a one-design class; we can only purchase equipment from authorised dealers, who obtain that equipment from licensed builders. They are not tapered sections, they are just like the tube you could purchase from your local aluminum supplier, however the diameters/wall thickness etc are obtained by using class owned dies.

2. The usual point of bending and failure is near the collar 305mm position on the top section). The bottom section will usually bend near the 945mm fitting where the boom attaches, but fail near the 445mm fitting just (25-50mm) above the deck level. Obviously these are stress concentration points, but usually any failures are the result of corrosion.

3. The general use failures tend to be in stronger winds and often associated with the masts hitting the water i.e. capsizing. The bending of spars is a well known phoneme and would normally occur in stronger winds, but we are having problems with masts bending in light breezes when they are first used (IMO a quality/specification issue).

4. In the case of lasers, the masts are what are known as free standing, in other words there are no cables (stays) holding them up.

5. They are seating in a loose fitting fibreglass tube, which joins the deck to the hull, approximately 400mm long. These tubes are fixed and do not require adjustment.

6. They make the same recommendation for all mast/boom section. A small note, the sections used for the radial bottom section shown in the second link, is of a thinner wall construction which has a smaller diameter sleeve inserted, these sections are currently causing the most concern at the moment, but the problem still exists for all other sections.

7. The fittings are riveted onto the tube, but all rivets are located where the tube when bent (flex load, not yielding) would be under compression. By that I mean that the mast flexes backwards and the rivet holes are on the backside of the mast, there is also some sideways bend. In the 18 years I have been sailing this type of boat, I have bever seen a crumple failure, where the mast wall has collapsed.

8. Hence,  some masts will fail at 16 knots due to mfr. defects regardless of break-in, and some better made ones will withstand 40 knots regardless of break-in, contrary to the mfr. spiel.    Is this a correct statement of your opinion?

Yes (and the wind strength can be as low as 8 knots).

RE: Aluminum Work Hardening

I may be completely wrong with this but I'll give it a shot anyway.

The masts are constantly being loaded and unloaded.  The manufacturers anticipate loads which may cause concentrated stresses beyond yield and subsequent workhardening.  With the load removed the mast may obtain a slight permanent set which after first usage may be localised at part of the stress concentration.  

As time goes on and the load is applied in different directions the permanent set would become more uniform around the stress concentrated area.  Subsequent reloading would then result in a more linear relation between the strain and the force applied and workhardening would not continue until a load higher than the yield stress were reached.  The unloading of the mast would also result in a return to a state that appears undeformed.  

RE: Aluminum Work Hardening

My opinion is that strain hardening cannot improve significantly the performance of these masts.  The strength change is only ~ 10%, and that would be after significant plastic deformation.  I don't see where the manufacturer can claim any improvement vs. time.  Has there been evidence that "broken-in" masts still fracture at relatively low stress?

RE: Aluminum Work Hardening

If alum. masts had to be "broken-in" metallurgically prior to "full loads", I would imagine that the same idea would apply to aircraft wings-but it doesn't.

RE: Aluminum Work Hardening

AlanD,
Thanks for the details and photos. You've given us a much better idea of the situation.

Any close-up photos of bent masts in-situ (not removed from boat, with all fittings still attached)?

Re degree of plastic deformation: with the 6061-T6 numbers given by Corypad and an elastic modulus of 69 GPa, the yield strength is reached at a deformation of 0.40 % and the UTS at 0.449 %. Not much leeway. The work hardening would occur during while the tension side of the mast is stretching on its way to failure.

While corrosion is certainly a factor in long term failures, we should restrict this thread to the short term failure issue: Is the 'breaking-in' of masts scientifically sound or the mast mfr.'s hocus-pocus to cover-up material problems?

RE: Aluminum Work Hardening

(OP)
BespinSunset: I know where your coming from and in my opinion it's basically the only way you could achieve work hardening, except that the work hardening is occurring during the natural flexing of the mast section. So when we come off the water and remove the masts, no permanent set may be observed under normal circumstances. Class rules prevent us from sailing with no straight mast sections, so we have become adept at straightening them.

Corypad: No evidence of "broken in" masts failing at low stresses, except through corrosion problems later in their life.

Kenvlach: Sorry no photos are available of bent mast sections in situ. I'll see if anyone bends one this weekend where I normally sail.

"Is the 'breaking-in' of masts scientifically sound or the mast mfr.'s hocus-pocus to cover-up material problems?"

Hence why I asked this question here.


  

RE: Aluminum Work Hardening

"Breaking-in" of the mast assembly may be happening, but strain hardening of the mast due to elastic flexing is not.  I doubt that strain hardening of the mast due to plastic deformation is happening either.  The scientific principles of materials science explain why strain hardening does not occur under elastic conditions.  It sounds like mast manufacturers need to investigate:

a) stresses on the mast during operation, including a detailed analysis based on the exact degree of fixity and constraint imposed by the connections

b) statistical distribution of incoming mechanical properties (mostly yield strength)

c) potential variation in heat treating of alloy 6063, which by the way, is a great alloy for extrusion, but not necessarily a great alloy for end users.  Response to heat treating is questionable in my opinion.

RE: Aluminum Work Hardening

AlanD

It's a good query!

It is the only way I can imagine work hardening specifically to be of benefit in this case.


RE: Aluminum Work Hardening

Just a few thoughts that nobody has mentioned above:
As I understand this, these boats are designed at the edge (without much of a factor of safety).  I make this presumption because speed (hence mass) is enough of a factor that one would not any more mass than is necessary. Please correct me if I am wrong on this.

Given this assumption, one would presume that this is designed to operate at or near "yield point".  Keep in mind that classic "yield stress" is at 0.2% plastic strain.  All materials diverge from Young's modulus (i.e. "go plastic") at a point lower than the book "yield stress". I would presume that these masts are designed to operate near yield strength, hence there may be some amount of work hardening. I make this statement not as a sailor nor nautical engineer; I am building up from the problem as I understand along with an understanding of mechanics.

I would argue that work hardening CAN happen below "yield". I cannot state how significant this could be.

Note--MetalGuy conjectured that this would be expected to also hold for aircraft wings (therefore proof that this is not the case).  I contend that this is an apples-to-oranges comparison, as the factor of safety on aircraft wings is certainly much greater than the factor of safety for this application (hence the operating stresses are MUCH lower for the aircraft).

Brad

RE: Aluminum Work Hardening

(OP)
Interesting point Bradh, which I really can't comment on, I lack the knowledge of the original designers. The boats were initially designed as a "fun" boat, rather than a "racing" boat. The Olympic status was gained 25 years after the boats were first designed, but with the exception of tightening up some of the tolerances of the fittings (+/-12mm reduced to +/-5mm). My assumption would be that the original designers selected the alloy, mast diameter and wall thickness on what wouldn't bend in ordinary usage as a fun boat.

RE: Aluminum Work Hardening

There have been many good comments, but none directed at the actual amounts of bowing of the masts in the photos and dimensions in the 2 links provided by Alan.  Maybe we need more understanding of the particulars; at least, we non-sailors do.

Alan,
I was rather amazed by the dimension schematics; straight mast sections, but the lower figures with sails show considerable bending (unfortunately, not to scale).  Observations and questions:
a) Is there a Class numerical value for max. bending allowed in a strong wind? Or
b) Is there a max. allowed ‘preload’ on the mast that bends it  backwards (from tightening the really strong sail (maybe with tension cables sewn into the sail) connecting the boom & mast)?
c) The sailor for a given  wind) controls the amount of bowing with the boom; with loosed boom and almost slack sail (photo marapov, lower left with green buoy) the mast is straight, while with a taut, bowed boom with a strong wind  (photo 11agstrt, just left of center) the mast shows incredible bowing.  So, I presume the boom has a swivel attachment to the mast there is a lot of furious cranking on some winch during racing.  Correct?
d) Last but not least, with all tension is removed, does the mast return to its initial straightness?

Suggestions.
As a beam model for loading of the mast, I suggest that the mast, boom and sail act as a cable-stayed jib crane. The tensioned stays (integral with sail) from the end of the boom radiating to different sections of the mast account for the bowing because the vertical component increases with the sine of the boom-stay angle.  This keeps the rear of the mast in compression despite a rearward, local tension upon the mast due to wind in the sails (opposite of a gravity load upon a jib crane).  

Maybe Alan can confirm this, give us the mast ODs and someone with a suitable computer program can input some strains and ‘back out’ a reasonable wall thickness for the mast and a presumed stress at the elastic limit.

RE: Aluminum Work Hardening

Unless these masts are guyed in some way such that they are under constant bending loads, in all probability they are designed primarily to prevent fatigue cracking.  If this is true, they would not be loaded anywhere near their proportional limit or yield strength.  The designer must decide the approx. number of cycles and stressing conditions the mast should withstand.

Given the orders of mag. more loading cycles on aircraft wings during the expected life of an airplane, the comparison would then be valid.

RE: Aluminum Work Hardening

Please drop the notion of fatigue design stress or failures.
We are dealing with failures of ‘fresh, unbroken-in’ masts, and (to me at least) the amount of mast bowing in some of the photos demonstrates enormous strains impossible for 6061-T6.  I agree with bradh’s presumptions although not about work hardening below YS; rather think that there’s variation in the material or an unnoticed amount of plastic yielding.

TVP & CoryPad, I think the mast mfrs. have investigated and aren’t talking.  Suppose there are some lower strength areas in the mast due to unevenness of heat treat or wall thickness.  Wouldn’t these areas plastically work-harden to bring them up to strength of the elastic regions? Although I would expect some permanent set because I feel the loading is fairly unidirectional (unlike BespinSunset), it may be ~0.5% over a limited span and unnoticed.  

Also, with stress concentrations at mast fittings (some of the masts look like rounded vees rather than smooth curves [due to splice?]), so there may be a little bit of plastic deformation of the order 0.05% for stress redistribution.  And,  a 12+% increase in strength is significant with mast stresses at the point of breaking.

I have persuaded myself that there are some valid reasons for a ‘break-in.’  
Alan, any more info on straightness & thickness would be helpful.
I hope someone with more structural expertise than myself can demonstrate a convincing answer.

RE: Aluminum Work Hardening

One more time, IF the assumption is correct that these masts see varying stresses during operation, can you understand why avoidance of fatigue cracking would be important?  Can you also see why the mast would/should be designed such that the bending-imposed stresses would not be anywhere near the materials yield strength?

Therefore a sudden "overload" fracture without a higher-than-normal stress just screams "defective mast".

RE: Aluminum Work Hardening

Metalguy,
You are pursuing a non-issue.  Obviously, the masts see varying loads (see photos).  Yes, the masts should be designed to avoid fatigue cracks.
Fatigue design implies S < YS which the bending masts in photos show is untrue (since elongation is only 0.4% at YS).  IF the masts were originally designed for a high fatigue life, then either the design was faulty or else inferior material.

 “can you understand why avoidance of fatigue cracking would be important? “
 -- If masts fail catastrophically when new, fatigue cracking is irrelevant.  What does a cure for arthritis matter if the baby is murdered?

“Can you also see why the mast would/should be designed” –- you are proposing a redesign to lower the stress, this thread concerns the actual existing design.

What possible relevance is fatigue design to the manufacturers' recommended ‘break-in’ for new masts?
--maybe S-N curve is upside down????

Alan,  wouldn’t the development of stronger sail material over the past 25 years allow letting out more curvature of the sail to catch more air (sorry for non-nautical phrasing)?  And, also allow use in stronger winds?

RE: Aluminum Work Hardening

I'm not proposing any "redesign to lower the stress".  I simply pointed out that the ORIGINAL design must/should have taken fatigue limits into account, and therefore the levels of stress should be far below the YS.  So why do some new masts bend permanently in winds much slower than what other masts (apparently same design/size)?  Because the stresses exceeded the YS in the area of bending, or there is a buckling collapse on the compression side.  So either something other than the moderate winds caused these high stresses, or the mast had lower than normal YS in that area, or there was a buckling problem with the bent masts.

In re-reading most of the earlier messages, it seems that there haven't been any fractures, only bending.  So I will change the words "sudden overload fracture" in my earlier post with "bent mast"--the meaning remains the same.

RE: Aluminum Work Hardening

Metalguy,
Glad to have you aboard the main thread & in agreement that the design is inadequate, etc.
Ken

RE: Aluminum Work Hardening

Thanks, Ken

Glad I found this website-some excellent answers in every area I've visited.

Metalguy

RE: Aluminum Work Hardening

Alan,
I read through the Laser rules (earlier, had stopped due to nautical terms I don't understand) and have some questions for you, plus some points for everyone to consider.

"FUNDAMENTAL RULE
The Laser shall be raced in accordance with these rules, with only the hull, equipment, fittings, spars, sail and battens manufactured by a licensed builder in accordance with the Laser design specification (known as the Construction Manual) which is registered with ISAF."
   
Any chance to obtain one of these Construction Manuals?  We especially need the wall thickness for the mast and verification that the material is Al 6061-T6.

"5. MAST
No mast which has a permanent bend shall be used at any time."
    
Below, I found a rule referring to a mfr.-provided 5 degree aft pre-bend.  Are there 2 classes of masts (if so, do the mfrs. recommend 'break-in' for both) or do they all have 5 degree prebend?

"PART FOUR
LASER RADIAL RIG AND LASER 4.7 RIG OPTIONS
Part 4 of the Laser Class Rules shall be read in conjunction with the remainder of the Laser Class Rules.
28. LASER 4.7
(f) MAST
  Rule 5 shall be amended to read as follows:
  5 The Laser 4.7 bottom mast is supplied with a pre-bend aft of approximately 5 degrees. The pre-bend shall not be increased or decreased. No top mast that has permanent bend in it shall be used at any time."

Alan, for both rules any permanent bend is prohibited (other than by mfr.).
a) Do people try to gain some advantage by cheating a little on this rule?
    Or maybe bend & then straighten???
b) There must be some enforcement of this rule, and there must be some tolerance due to original variation as manufactured?  ½ degree?  What happens to out-of-tolerance masts?

To all:  This approximately 5 degree prebend was probably conducted using some type of giant tubing bender to prevent any buckling of the aft surface. This amount of bend is about 1/2 way to the breaking point for Al 6061-T6 and indicates that work hardening to the UTS did occur, and furthermore, there must be some material thinning of the forward mast surface.  My interpretation of "approximately 5 degrees" is greater than 4.5 and less that 5.5 degrees, and this variability is a significant matter.  Comments?

To all: I measured a total mast curvature of 9.0 degrees from photo 11agstrt.  Has anyone else tried to measure mast bend from the photos?

Alan, can you tell us the radius of curvature of this bend or more simply, the length of the non-straight portion of the mast?

Alan, I noticed earlier that non-racing masts were either anodized or painted.  Do the Laser masts have anodized finish?  (If so, I sure hope it was done after mfr. did bending).  Any idea how thick?  If no data, I’d guess 0.0005 inch.  This would reduce the Al 6061 wall thickness by about 0.00025 inch (presuming only the outer surface was anodized).  

To all:  (If anodized masts) The anodize could crack and offer lots of crack initiation points if the mast flexed significantly during racing, but how relevant that is for overloading-type failure?  My guess is less important than wall thinning.   
[it would certainly decrease fatigue properties if those were being considered].

Please, will some mechanical or structural engineer do some computer modelling (maybe backcalculate the wall thickness from the degree of mast bending and Al 6061-T6 props.); I don’t have the means.

Ken V.

RE: Aluminum Work Hardening

Alan,

99% of all aircraft structure is 2024 T3.  There is some 2011 or 2017.  The loads that a plane fuselage sees are obviously a function of the load induced, by either flight profiles, landing loads or manufacturing/assembly loads.  6061 is not cosidered to be "structural" or aircraft grade aluminum at all, though some people refer to it as such.  The idea of breaking in a mast is proposterous, as someone mentioned earlier there is no break in for a plane.  I can  however reccomend 2024 T3 or any 2000 series aluminum for a mast.  2000 series aluminum's major alloy is copper, which does work or strain harden over time, but only if flexed or worked past its yeild point.  If that point is not ever reached, it could last forever.  I can recommend a good friend for this, world famous One design boat racer/builder Mike Mahar of Mahar Spar here in Cleveland OH.  His number is 216-249-7143.  Tell him Mike Moore said to call him.  He loves cheap wine and good cheese.

RE: Aluminum Work Hardening

mikemoore,
Al 6061- or 6063-T6 is commonly used for masts due to superior corrosion resistance (2024 is perhaps the absolute worst Al alloy w.r.t. saltwater corrosion).  Maybe 2xxx or 7xxx aerospace alloys or Kevlar, etc. are used for serious racers like for America's Cup where masts are frequently changed & owners don't worry about lifetime & cost.
Your engineering comments are appreciated, but the mast design & material are 'locked-in' by rules, and it appears that the masts are being used beyond the yield stress (there are even rules regulating bent masts).

To all: Re: "This approximately 5 degree prebend....is about 1/2 way to the breaking point for Al 6061-T6 and indicates that work hardening to the UTS did occur."
I meant to say about 1/2 way from YS to UTS and then I wondered about necking and further, whether I was being presumptious; could the masts have been bent in the solutionized state and then aged? Comments?

RE: Aluminum Work Hardening

From Alan-

"The manufacturers are informing us that the masts should not be used in winds over 12 knots, until the masts are "broken in". A good quality mast can easily survive 30-40+ knots, which is beyond the skills of most people in the class."

Too bad the boats move-makes it much harder to calculate the mast bending stress!

But I daresay that there would be a LOT more stress at 30+ knots than 13.  No way would a slight/moderate increase in strength cope with it.

Y'know, all "we" need is to perform a decent failure analysis on a bent mast, and compare it with a chunk of a good one.

"Mast breakin" indeed.  That's better than some of the BS I get over here in Italy, home to "cheap wine and good cheese"!!!

RE: Aluminum Work Hardening

Metalguy,
Is that the EtOH speaking?

I did some thinking outside the box and figured it all out.
The sailors are breaking the masts.  See what I wrote Jan 17 (1st) about preloading the mast by tightening the sail.
Now, if the sails are strong enough and the masts pretty thin, that's a significant stress. And some sailors are stronger than others, so differing preloads. Ergo, the stronger sailors better go sailing for a while in some very light breezes to loosen up the sails and maybe distribute stresses at the fittings before a little bit of additional wind load becomes 'the straw that broke the camel's back.'
Hence, 'breaking-in' is due to the human factor.
And, if they take the sails down, maybe have to repeat the process.

Alan, does this make nautical sense?
An inverse relationship between sailor strength and wind speed for the broken masts?

RE: Aluminum Work Hardening

(OP)
To answer some of the questions

Wall thickness and outside diameters:
Top section 1.5-2.0mm wall thickness, diameter 51.0-51.5mm
Bottom section (standard rig) 2.5-3.0mm wall thickness, diameter 63.5-64.0mm
Bottom section (radial rig) 1.8-2.0mm wall thickness, diameter 63.5-64.0mm
No measurement obtained for the sleeve wall thickness or diameter, but I would estimate it to be 1.5mm, 55mm.

It is my belief that the wall thicknesses are not sufficient when the extrusion dies are new.

There are no wires etc in the sails and no class limitations on the amount of bend you can generate. However the real limitation is how for you can pull the end of the boom down to the deck, which is part of the way the mast is bent.

15 years ago there was a change in the cloth weight of the sails, which increase the stiffness of the sail. However the materials haven't changed since then. However I believe the problem existed before this change and the wind strength we sail in also has not changed.

Even as the Australian measurer, I do not have access to a construction manual. I definitely know that the alloy is either 6061 or 6063 with a T6 ageing process, but no specific knowledge of the YS, UTS or hardness specifications.

There are 3 types of mast, depending on the sail area. The standard rig and radial rig bottom sections are mentioned above. The radial section is shorter and has a sleeve in it to permit a better sail, if a cut down standard section is used, the bottom section is too stiff to permit a fast sail shape to be achieved. The cut of the sail can achieve in achieving a fast sail shape; the reduced length of the mast did not permit enough bend for the sail to achieve this.

The 4.7 mast is smaller again and is designed for the use by young teenagers or very light adults. In order to achieve a fast sail shape, the designers found it necessary to pre-bend the mast, as even a thinner wall thickness was inadequate. Only the 4.7 mast has a pre-bend, the other two are both straight.

Part of the characteristic of a fast sail shape, comes back to the balance and having the tip of the mast back far enough (a bit hard to explain).

"a) Do people try to gain some advantage by cheating a little on this rule?
    Or maybe bend & then straighten???
b) There must be some enforcement of this rule, and there must be some tolerance due to original variation as manufactured?  ½ degree?  What happens to out-of-tolerance masts?"

There is no deliberate cheating to my knowledge on this rule. When people bend their masts, it becomes fairly obvious and other competitors will comment. Most competitors will attempt to straight the masts, particularly top sections that straighten easily. We don’t have a tolerance measurement unfortunately, my general rule is if I can observe that the mast is not straight, I will ask the competitor to straighten it.

The 4.7 bottom section is bent after ageing, using pipe bending equipment, however as the sail is so small; they do not appear to bend accidentally like the other mast sections.

"Alan, can you tell us the radius of curvature of this bend or more simply, the length of the non-straight portion of the mast?"

The top section will bend more than the bottom section and they will be a bit more distortion where the two sections join.

All mast/boom sections are anodized where they are produced, the bend in the 4.7 rig done at the laser builder, so post anodizing. I haven't ever checked the thickness of the layer, but it would be about the standard for commercial anodizing (not hard anodizing).

"To all:  (If anodized masts) The anodize could crack and offer lots of crack initiation points if the mast flexed significantly during racing, but how relevant that is for overloading-type failure?  My guess is less important than wall thinning."

I've never observed any cracking of the anodized layer caused by ordinary flexing or when the masts have been slightly bent. However when the masts have been badly bent or dented, yes the anodized layer has crazed. I don't think the anodized layer will significantly effect the properties of the mast.    

I did my thesis on the corrosion aspects of anodized aluminum copper alloys, the corrosion issues are extremely significant. The way aircraft avoid the problem is by cladding the parts in a layer of pure aluminum.

I apologise if this post doesn't make to much sense, my mind is else where; we are having significant bushfire (wildfire) problems in Australia. Where I am is ok, but I have friend in trouble.

RE: Aluminum Work Hardening

(OP)
"Alan, does this make nautical sense?
An inverse relationship between sailor strength and wind speed for the broken masts? "

I see what your aiming at, but most of the sailors will not treat there masts diffenrently irrespective of their strength. I'm one of the stronger people and as I do not believe in this "break in" concept, I will just use a new mast or sail in the conditions presented on the day. I will however aim to purchase a heavier mast section. A heavier mast section should have a thicker wall section.  

RE: Aluminum Work Hardening

Alan,
Thanks, and congratulations on being correct:  
“It is my belief that the wall thicknesses are not sufficient when the extrusion dies are new.”  
Also, there is no reason to speculate about work hardening below the yield stress; the YS is clearly being exceeded.

From your information, I think we can accurately say that the masts are failing due to gross overlading.  The combination of inadequate mast wall thickness, preloading by tightening of sail for racing advantage, and wind loads can cause stresses beyond the YS and in some cases, beyond the UTS.  The description of masts that are bent and straightened shows clearly that the YS is exceeded.  Work hardening occurs via plastic deformation during both the bending and the straightening processes.  
The variations in mast thickness, tightening of sails, unnoticed small but significant bending (remember, the YS is reached at only 0.4% deformation) and probably similar variations in thickness and tightening of fitting materials no doubt made it more difficult to figure out.

I’d like to point out that this bending and straightening of masts, with work hardening occurring both ways, can explain the rationale for ‘break-in,’ although probably not one that the mast mfrs. would admit to.  Due to sail preload and then wind loading, the thinner masts bend when the YS is exceeded in a light breeze.  If this had happened in a strong wind, the UTS would have been exceeded at the highest stress concentration in the mast, and the degree of plastic deformation (which I’ll call ‘necking’ for simplicity, although there may also be buckling) would have decreased the mast cross section enough that rapid necking and gross failure would have occurred.  During the straightening, some recovery of the cross section occurs (at least, any buckling is removed), and the work hardening results in this section being stronger than other.  During the next sailing where bending occurs, the next most stressed section ends up getting work hardened.  After enough such bending and straightenings, the YS approaches the UTS for all of the most stressed sections of mast, and the mast is now ‘broken-in.’

I’ll repeat my request, “Will some mechanical or structural engineer please do computer modelling for the mast; Alan has provided the necessary info.”

Ken V.

P.S.  When you mention purchasing a heavier mast section, do you mean taking along a micrometer and tape measure to select a standard bottom mast of 3.0 mm wall and 64.0 mm diameter, i.e., at the upper tolerance limits?

RE: Aluminum Work Hardening

(OP)
Obviously once the masts have been bent and straightened again, they do work harden. However the manufacturers are claiming that if we just sail with the mast in light winds, that the masts will work harden and this will prevent them from bending in the first place, it this point I don't understand metallurgically.

I usually take a set of kitchen scales actually. Weighing them is a fairly accurate way of measuring, for our purposes. I just take the heaviest sections in stock.

Thanks for your help.

RE: Aluminum Work Hardening

Alan,
It's been interesting, kind of using the Socratic Method to bring you to an answer that you really knew.
But there may be further developments.

I've got a subtly to refine the 'break-in' rationale in my last posting.  Recall that the YS for longitudinal tension is slightly higher than for longitudinal compression [or better, see the Al 6061-T6 curves in MIL-HDBK-5H]. When the mast flexes near the nominal YS value, it is possible for the inner, compression side to begin plastic deformation while the tension side is still elastic. To further make this case, I point out that the radius of curvature on the compression surface (aft) is smaller than for the tension side, so the stress and strain are both higher. For the case of just slight plastic yielding in compression and none in tension, it is possible for the compression side to work harden a bit and then, when the stress is reduced or removed, the tension side pulls the mast nearly straight. Then, as I explained above, this section is slightly stronger, so the next bending and straightening cycle work hardens the next highest stressed area. Ad infinitum.

So, my 'break-in' rationale can explain the mfrs. claim. Scary thought, isn't it? Or, as Alice said, "It gets curiouser and curiouser."

RE: Aluminum Work Hardening

Just a few comments about aluminum (and aluminium!).

1. All aluminum alloys can be work hardened - heat treatable or not.

2. Aluminum does not have a yield strength. Under stress, it starts to deviate from the Hook's law line right from the get go.  The "yield strenth" is the 0.2% proof stress - that is, it has already accumulated 0.2% of non-reversable strain at that point.

Regarding the discussion about aircraft.  I served on the US Air Forces MIL-HBK-5 committee for many years.  All aluminum structure on an aircraft has a life expectancy and must be replaced at some point.  Low cycle and high cycle stresses inflict damage at stresses well below the "yield strength" which is why old aircraft are retired.  In recent years, advances in aluminum metallurgy have produce many damage tolerant aluminum alloys.  Unfortunately, none of them like sea water.

RE: Aluminum Work Hardening

I thought the aircraft industry used a plastic strain of 0.02% for their def. of YS (10 times less than others).  Perhaps they have abandoned this?

Your comments on cyclic stresses are why I brought up the subject of fatigue cracking here even tho the masts don't fail via fatigue.  

But maybe Kenvlach is onto something here.  Will be interesting to see just how much increase in YS (0.2%) 6061-T6 will give during cold work.  I don't have any decent alum. books with me.

RE: Aluminum Work Hardening

Alan & Metalguy,
I'm not a mechanical engineer, but I think that what aluminumphil said about YS broadens the stress range to which my proposed mechanism applies. So, even more likely that low-cycle work hardening occurs.
Agree?

RE: Aluminum Work Hardening

Ken,

I suspect there is a only a slight increase in YS given the very small strain involved.  The differences in wind speeds would seem to indicate a large increase was required, even tho the boat is not anchored.  

I'll still go with the idea of defective masts, either because of thin/marginal thickness or a problem with the YS/HT.

A good met. lab. would be able to tell what is going on.

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