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Load vs deflection graphs are done that way mainly for historical reasons because most tensile test machines (well the old Instron machines that set the standard) move at a fixed rate to apply increasing strain or deflection. The old instron type machines can be considered as a big hydraulic cylinder that has oil puped into it at a constant rate by a fixed speed motor driving a fixed displacement pump. Load was measured by measuirng hydraulic pressure. (there is a whole can of worms for engineers to open up, but that the basics of it). Load was graphed by a pen connected to a pressure gauge, drawn on a piece of paper that moved as the ram moved (geared up so the paper moved a lot more than the ram). Its the most accurate way to find the yield point and the plastic behaviour. If you apply a gradually increasing load, once the material yields, the deflection (and strain) rapidly increases and the part will quickly fail without gaining any true insight into its plastic behaviour. Even though in real life, most loads increase over time (for example an increasing wind gust), rather than having a fixed deflection rate. When testing, particularly with hydraulic machines, its easier to apply a deflection rate (ie a flow rate of oil). When test loading structures with weights, the ting eventually just fails rapidly. When you do it with hydraulics, by applying a deflection you can see it gradually break and can see what happens, you can even stop it "half broken" to see what is going on, where as with an increasing load, like adding weights, you cant see the failure happen unless you have a high speed camera.
That also reminds me of a trick I discovered or rediscovered by accident when doing some wing main spar structural testing. I had tested some small structures previously with weights to failure and when they failed it was catastrophic, as in complete failure, often with a lot of secondary damage as parts fell in to other parts, or the weights crushed the parts. When testing a full scale wing on a full plane, the loads were big, like 8 tons big so I did it with hydraulics. The thing had to get to ultimate load or 225% of limit load. It got to about 220% load and failed. So the design was nice and light, but not quite strong enough to pass. This was with the wing tips bent up about 2 feet or half a meter on about a 6m wing span, so it was fairly bent. It was quite a complex carbon sandwich structure (20 years ago too, not that common then). But fortunately the hydraulics didn't cause any more deflection when it failed, so the damage was quite local. The main spar was repaired and slightly strengthened and then tested again and it passed. If we had done it "upside down", with 8 tons of weight on it (like bags and bags of lead shot or iron pigs), when it failed it would have gone to the dumpster. So using hydraulics accidentally saved perhaps $100k and a few months of waiting for new parts, and I wouldn't have so easily seen what part needed strengthening.
It's stress (y axis) vs strain (x axis) or more basically load vs deformation. The straight part of the curve is the elastic deformation (returns to original shape), the end of the straight bit is the yield point (point of no return for want of a better description) and the bent bit at the top is the plastic deformation (she aint coming back) and the end of the curved bit is the point of failure (she busts). Note the backwards bit of the curve towards fracture. As the material basically starts to give way with a lot less stress/load required for the same deformation before ultimately failing. The carbon fibre curve is what's called brittle behaviour, ie it's very strain resistant (resists deformation) right up until it fails without any warning or plastic behaviour. What sort of behaviour do you want in your boom, a high performance, deformation resistant member that can break without warning if it goes outside it's design use or a material that will deform slightly and maybe bend a bit before it fails? These are common engineering questions for all materials.
^ Explained beautifully.
I prefer the carbon fibre because it's ultimate strngth is so much higher than the alloy
John - nice graphs. How stable do they stay over time with continuous use within the working range? I recall a term called work hardening. Does this apply & result in a boom failure at low forces after a fixed number of hours use (as distinct from a high impact load causing failure)?
I should have also mentioned that I have had many "soft" failures not due to high impact (at the time anyway) including 3 from the one manufacturer all occurring at the exact same spot, B&J use only for these booms.
Exactly like John has described above. If designed properly, no component should deform to a point where it work hardens. Work hardening requires plastic deformation which you will definitely see in the case of aluminium alloys and therefore have some warning of a potential failure.
I'm still new to the sport and so go the opposite end of the spectrum when it comes to booms, I use a $220 on special aluminium alloy Sunshine boom from my local shop and it works fine for my intended use and I haven't bent it yet.
Ok, will try to find a fairly recent second hand carbon boom but will be passing on the enigma I saw recently ... going on the other thread here.
Otherwise I will go for aluminium monocoque - - the choice is limited to RRD M T9 V2, WH mikai, simmer white line, exocet XO , nautix Xfly or slightly more expensive , the chinook pro1 alloy, which I think might well be where I end up.
Yeah, excellent thread. I learnt something today.
On a related question, one thing I hated about aluminum booms was the salt react/welding and having to store booms disassembled. I noticed some boom brands were worse than others. My Simmer monocoque wave boom does not react at all to salt and can be stored assembled. I was wondering, do these different grades of aluminum have different salt susceptibilities ?
The Enigma is a good boom. I know plenty of sailors who have had them for a number of years without any problems
jn1 yes the different alloys do have different corrosion susceptibilities. Other factors that affect are other metals nearby - eg stainless steel. Tightness of fit - if it was on the loose end maybe water can drain away easier, so there is no electrolyte for the reaction? I'm interested that the Simmer boom didn't show any corrosion - - few of the manufacturers (including Simmer) tell what alloy they are using so its hard to know whether its due to the alloy or not. You like your Simmer boom? - its one option i have been looking at.
Different alloys of any metal (including aluminium alloys) will have different energy levels required for corrosion to commence. Corrosion can occur in a few different ways but a couple of more basic ones are the galvanic cell (think of a battery) formed by dissimilar metals (avoid stainless steel and alloy in an electrolyte for example) and also avoid moisture sitting in a gap or in dirt/grime that will allow corrosion to occur. A great gap is the little gap between the 2 sliding parts of your boom, that's why I pull mine apart to store it. The thing I don't like about my alloy boom is that the back bit is filled with a foam, presumably to keep dirt out but it'll hold water and assist corrosion too. The only real way to avoid the corrosion is by using a coating to stop contact with oxygen so maybe the Simmer boom has a very good anodised or painted coating on it?
So I have come down to deciding between the North silver hd 150-200 or the chinook pro 1 alloy 135-197. North is 7075 alloy bonded technology, Chinook is unknown alloy and T8, and monocoque. Probably quite a difference in alloy strength, but that is not the whole story as the overall design is important. Anyone tried both? for a 5.5m183boom freeride sail, bump and jump.
If I had to choose between a monocoque and 2 peice I'd go monocoque (and I did) only because there's no joint/connection with the mast clamp bit which I personally saw as a weak point, but I've got no evidence to back that up.
Back in the '80's, I got into making Aluminium booms for myself and friends. I just bought the commonly available 32mmx1.6mm alloy tube and bent it over a jig. They were flexy, but no different from the bought ones, but they did the job with the technology we had then.
I was given a length of tube by Amac from Wild Winds. His comment was that this tube was super stiff and would make a great boom. It was/is incredibly stiff! I broke my jig swinging on it and could not get it to bend at all!!! It is still in the rafters of my shed 20 years later and I still can't bend it! I suspect that if I found a way to bend it it would just snap anyhow. I wonder what sort of alloy it is?
But, I am also confused by the engineering discussion. There is no doubt that modern Alu booms are much stiffer than the early ones. I always put this down to them being tempered after bending, like the way Hydrodynamics used to do it. ( and why did that work then?) The tube diameter of modern Alu booms is smaller (around 29mm compared with 32mm) but they are far stiffer and no heavier. Is this all down to increased wall thickness? What else is going on?
Andrew,
You got most of it:
- greater wall thickness
- stronger material
Also
- monocoque construction
- more overlap of rear and front section
- better fit between rear and front section diameters
- better quality heads
- shorter booms for the same size of sail
I've had 4 booms over the years - none of them actually broke , they just got loose at the head and at the tail. What I'm surprised to hear is how many people are snapping their aluminium boom. It seems modern aluminium booms have got stiffer but the price is they fail catastrophically. I suspect the old ones survived because they could flex a bit.
So maybe aluminium is no longer suitable...
Maybe somehow design some flex into the boom so that under high load as in a catapault there is some give... Or go to a stronger material like carbon fibre...
Most of my broken Alu booms have been massive high speed crashes. That was fine with me because I would rather have the boom break than me.
A few of my Alu booms have broken in the head where salt water has got into the join and corroded the Alu. It seems that the ones that have a good seal, last. Those that don't, eventually corrode and break.
