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.

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.

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.

Sail_Latte said..
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)?

Work hardening requires plastic deformation. Carbon fibre does not plastically deform, hence it does not work harden.

It is possible that cyclical elastic loading like you have described may possibly cause fatigue, but only if the frequency of the cyclical loading is near the harmonic frequency of the boom. It is highly unlikely for this to be the case.

The anecdotal evidence is that carbon booms, if made well, last for many years.

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.

The Enigma is a good boom. I know plenty of sailors who have had them for a number of years without any problems

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?

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.

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...