hi

does anyone have answer to how to define nr. of layers to obtain desired strength of object.

i would like to do seat post tube for my MTB.
so how to define what would be the minimum amount of layers that tube will still support my weight and not to have too much of not needed weight.

i know there is no 100% accurate way, but at least a point to start

[I'm new to this too, so don't rely on anything I say, but I think I've read a little bit more than you have.]

Layers of what exactly? You've got a wide choice of types of carbon fibre cloths to choose from with different weights, so the number of layers will depend a lot on the type(s) you use. It's also hard to predict in advance how strong a part will be, so it's always best to make prototypes and test them to destruction. Wherever you cut the cloth, you substantially weaken the effective strength of the cut fibres when in tension for many inches away from the cut ends, so you may not get anything like the strengths stated on datasheets. Given that you're going to be sitting over this thing and it may break under you, potentially leaving lethal shards pointing upwards, you want to over-engineer it heavily, bringing its weight close to the weight of the metal you're replacing, and that means you should be aiming for a modest weight saving at best. Weigh the metal part and then try to use perhaps 90% of that weight in cloth+resin (assume equal weights for each of these). Whether a core material like Soric would help in a case like this, that I don't know, but someone who does know ought to comment on that - a fatter tube wall would be stronger and Soric would help get the resin through all the cloth, but it may not be possible to work it into such a tight, curved space and it may lead to broken outer layers when the post is clamped in the bicycle frame if the give is different under compression.

On a seat post, the back part will often be under compression while the front is in tension. I would think about using 45 degree biaxial 300gsm cloth aligned along the post, alternating it in layers so that you get half the fibres aligned up and down the post while a quarter are aligned 45 degrees to one side and the final quarter at 45 degrees to the other side. Put three or four layers of kevlar/cf cloth in between the outer layers of biaxial cloth (using the plain weave 200gsm) and the outermost layer should be twaron cloth to give the part a more attractive appearance (so from the outside inwards that's twaron, biaxial, kevlar, biaxial, kevlar, biaxial, kevlar, biaxial, biaxial, biaxial, biaxial...). The lower part of the post should perhaps be thicker as it will be clamped into a frame, but how much thicker is hard to know without testing to destruction.

Finding a viable construction method may be your biggest problem though. You'll probably need to pack all the cloth you use into a cylindrical mould with a tubular bag running through the middle which you can either inflate to a high pressure or use it to seal the cloth in against the mould so that you can infuse resin through it (the cloth under a vacuum). I can imagine a lot of failed attempts at this with the resin failing to infuse all the way through, so it may be an expensive business. I have searched for tips on this and found very few, and I still don't know how you can get all the cloth into the right place for the vacuum to be able to compress it outwards against the mould properly without it pulling tight everywhere and leaving voids where it can't reach the outside. Perhaps there is someone here who has done it and can explain how, but that's something you will certainly need to get answers on.

I would disagree with building it to the same weight as the metal part it is replacing, though to some extent that does depend on the mechanical strength required. The general rule of thumb is to build it to the same THICKNESS as the metal part it is replacing. So if the original part is a 2mm thick steel tube, then a 2mm thick carbon tube should be ideal. 

i agree with Hanaldo.
its no point of putting all that work to make same weight part as metal. but thanks David for answer.

and as i read is kevlar only crack arrestor. and if not cured at exact right temperature it doesn't bond. so i think i will skip it.

i was thinking kinda a same direction. same thickness. but if i would like to go more extreme.
carbon fiber have twice the strength as aluminum.
so in theory it can be halve the thickness of alu ? of course its not that simple...
and of course depends on layup and type of fabric.

My main worry is that it's potentially a dangerous thing to make, so you don't want to take any chances with it. Maybe it would be safe though if you deliberately make it weakest just over the point where it leaves the frame so that if it does break it will go there without leaving a shard sticking up. If you can do that, it may be okay to risk going for a big weight saving.

agree. it is potentially a danger. i don't wont to go to extreme. but i would still like, if i do something from carbon, that will look better than alu and at same time save weight.
but not to go in extreme. that's why i wont made handlebar
i think if i do same layup trough whole length, and if breaks, it will break just over the frame. there is the most stress.

i was thinking using UD carbon 100g/m. 5-7 layers. and every layer little bit different orientation. that all of them cover bit less than 45 degree.

Typically 1000g of carbon used with vacuum or shrink tape is 1mm so you would need around 2000g for 2mm.

7 x 100g is just 700g and less than 1mm thick, not very strong especially if this is not the usual pre preg that most tubes are.

agree, but my momentary alu seatpost has only 1,3mm thick walls. so if i go with 7-8 layers plus 1 spread tow finishing. it will bo close to 1mm. what should be enough.

i was thinking of wet layup on bladder. and than whole mould in vacuum bag. hopping to get less bubbles in part.

Like Hanaldo said you should at least match the existing 1.3mm alloy tube and then I think that considering you are doing this wet laid I would still go for 2mm.

I work a lot with carbon tubes and boats. Most tubes 20mm dia to 70mm dia are usually around 1.6mm wall with about 70 % fibre longitudinal and 30% in the hoop orientation. Smaller tubes 10mm-20mm dia are usually 1-1.5mm wall.

thank you for advice. 
also percentage will help.