Reversing the fittings on the top mast section

Thread starter #1
My stop mast has become soft so I want to reverse the fittings. Should I keep the fore/aft orientation of the tube or reverse that as well?
 
#4
I expect it is because Steve will have done many of these in his career :) It would also seem to be correct as it will be less likely to break if the stress goes the same way - Reversing the stress could make it brittle :)
 

AlanD

Former ISAF Laser Measurer
#5
It would also seem to be correct as it will be less likely to break if the stress goes the same way - Reversing the stress could make it brittle :)
As a metallurgical engineer, I'd love to see any scientific or engineering explanation of this.
 
Thread starter #6
Good to see two metallurgical engineers sitting on the fence Alan! I asked a ME friend last night what his thoughts were and he shrugged his shoulders;). I think you could box this one with the so-called work hardening of our masts. What are your thoughts on that Alan, truth or folk-lore?

In the interests of research, I think I'll reverse the orientation and put the old compression side facing forward. If it breaks, you'll all be the first to know :)
 
#7
As a child, I was making an assumption. Also the word 'seem' suggests that I do not know the correct answer, but am making an assumption to try to help a fellow sailor :)
 

AlanD

Former ISAF Laser Measurer
#8
Good to see two metallurgical engineers sitting on the fence Alan! I asked a ME friend last night what his thoughts were and he shrugged his shoulders;). I think you could box this one with the so-called work hardening of our masts. What are your thoughts on that Alan, truth or folk-lore?

In the interests of research, I think I'll reverse the orientation and put the old compression side facing forward. If it breaks, you'll all be the first to know :)
In part covered this a few weeks ago (see below). But essentially I do not believe that from ordinary use, Laser masts work harden, there for lining up the rivet holes etc will have no impact on on the mechanical properties of the spars. You could put forward a very weak argument that a rivet head 305mm from the top of the mast might increase friction of the sail preventing the luff adjusting easy but considering the amount impact the cunningham has on a sail that high up, the friction impact would be neglible. So the only real impact of a rivet filling the old hole is cosmetic.

The crux is:

The builders / dealers theory requires the mast to be bent permanently, gets straightened, get bent permanently, straightened again .... get bent permanently, straightened again before it begins to gain strength where it resists getting permanently bent. A mast flexes in the elastic region and blockages required that inhibit dislocation just don't form, so the mast won't require more force to bend it permanently, than when it left the factory.

Theory

A metal grain consist of atoms which are in a crystal structure, each atom positioned in a precise location relative to other atoms in the structure, think of each corner of a cube (slightly more complex in almost all metals). However, sometimes there are defects with the crystal structure called dislocations. Dislocations are important because it allows rows of atoms to move relative to adjacent rows. Whilst not the same, but just from a mental perspective, think of two platonic plates sliding next to each other on an earth quake fault. Anyway, if the dislocations can move freely, the rows of atoms within the crystal structure can slide smoothly past each other allowing this permits elastic (non permanent deformation to occur) i.e. your mast flexing.

When the metal is in the elastic region, the concentration of dislocations is very low. As the loads increase, more dislocations are generated and this permits permanent deformation to occur the number of dislocations can increase, they multiply. When you reach the point where the number of dislocations begins to increase, permanent deformation or yielding will occur. For your mast, this is where it will become permanently bent or at least until you straighten it.

Dislocations can move along different planes, exactly which ones will depend upon the direction the metal is being loaded. When two dislocations meet they can cancel each other out, but more importantly they can also block each others movement and that will block following dislocations from getting past. For the following dislocations to get past, more force is required and this is really what work hardening comes down to. If you generate enough blockages, dislocations can't move easily unless a lot of force is applied and then the material will not easily deform easily.

For work hardening to occur, you need a high density of dislocation to be present and this just doesn't happen in the elastic region, you need to be in the region where permanent deformation.

I suspect you didn't really understand to much of that, but without lots of drawings and siting down with you for an hour or two it's a difficult thing to grasp.
 

Merrily

Administrator
#9
Alan,
Any idea why the instructions that came with my Vanguard Laser said to always line up the top mast rivet with the gooseneck? They really meant this as there were also red arrows on the masts pointing to where they should line up. I always thought it had something to do with work hardening, but you say it ain't say so, n'est pas?
 

AlanD

Former ISAF Laser Measurer
#10
Yes, it's because the rivet / rivet hole are stress concentrators, which basically means that it a localised point that the forces are magnified at to the point where if the rivet facing the opposite way, the forces can potentially exceed the stength of the material which will result in the top section breaking. This is completely unrelated to work hardening and whats being discussed in the rest of this topic. :)
 

AlanD

Former ISAF Laser Measurer
#13
As much as I want to believe in whats being said, I cant help thinking about Uri Geller & his spoon bending.
There are a group of metal alloys called "shape memory". If you bend them (or straighten them in this case) below a certain temperature, when the temperature is raised above a certain point, they will return to their original shape. If you chose the right alloy, just holding them in you hand is sufficient to cause the metal to warm sufficiently to change shape. These alloys are used quite a bit in electronics, but Uri just uses slight of hand.
 
Thread starter #14
That is the best explanation I have come across Alan, and at least we all now have a basic picture of what is occurring at the molecular level when our masts bend. I missed that earlier post, so thanks for repeating it here. Great stuff!
 
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