Core thickness to compensate for re-enforcement change

[Idris]

I about to experiment with some industrial flax to replace fibreglass -

I'm not very good at Mechanical math - mattman or others can you help again.

(I've already gone from 650gsm to 750gsm glass witht the help of mattman)

Ski

Length 180cm

Core length 1550 (20cm tip 5cm tail)

Core thickness 2.5mm,11.5mm,2.5mm - smoothed pyramid in shape.

Tip width 130, 100 underfoot, 119 tail

Currently using 750gsm (22oz Glass)

Here are the calculations on both Glass and Flax for comparison

· Glass: 400 g/m2 fibers, 50% fiber volume fraction 50% matrix volume fraction, thus composite 596 g/m2, 0.31mm thick and modulus 39GPa, thus stiffness 12235 N/mm
· Flax: 400 g/m2 fibers, 50% fiber volume fraction 50% matrix volume fraction, thus composite 785 g/m2, 0.62mm thick and modulus 31.5GPa, thus stiffness 19385 N/mm

The material I will be using will be

400 g/m2 UD and 360 g/m2 biaxial +/- 45° flax

I am not looking for anything super accurate - although I know there are some rocket scientist lurking

Just trying to avoid shooting in the dark - not my preferred method of core calculation.

In future this may be a source of bio replacement for fibreglass for us all

Thanks in advance
Tom

( how the calc is done would be great to know so I can change all my other skis )

[chrismp]

i can't really give you any good tips on how to solve your problem...sry. i usually do this by trial and error.

what i'm really interested in is where you get your flax though! i've received some samples from lineo, but they don't have any +/-45 and only lightweight unidirectionals.
i've been wanting to try out flax for a long time, but i could never find a suitable cloth.

[Richuk]

Are you sure it's a full replacement? I was thinking in terms of a partial replacement. Replace E-Glass on the -/+ 45 degree axis; with a higher Young modulus, it will be a weight saving. Retain the E-Glass on the 0 degree axis and account the lower failure strain values for flax.

Anyway, I'm sure you know the best way round this - I'm sure the results are going to be really interesting.

Searching for 'rule of mixtures composites' will drum up some interesting Maths.

[SkiTheNortheast]

where are you getting the flax. I'd like to try it out

[mattman]

I would be happy to run those numbers for you...just need a little clerification first: Is the 31.5 GPa modulus the fiber modulus? I will have to write a new product ID into my program for 400g/360g triax. With this you would like to know how to change the core thickness to compensate (in stiffness, not strength) for the change from 22oz triax to the described flax triax - correct?

[mattman]

Quick side note: from looking at material properties for flax at lineo.eu (a supplier?) it looks like it is much weaker than E-glass but more brittle (similar to carbon - without the strength benefit). Whenever a supplier lists that a material is very lightweight, and has a similar specific strength to glass...they are basically saying "its strong for its weight and its VERY low weight" - thats a bad sign. Think about that, it is a very positive and crafty spin on a very bad material property. I would be concerned about using a weak brittle material in a product as dynamic as a ski....just my $0.02.

[Idris]

I would be happy to run those numbers for you...just need a little clerification first: Is the 31.5 GPa modulus the fiber modulus? I will have to write a new product ID into my program for 400g/360g triax. With this you would like to know how to change the core thickness to compensate (in stiffness, not strength) for the change from 22oz triax to the described flax triax - correct?

This is what I have on modulus

The modulus of Flax fibers is about 70 GPa, so about the same as glass (75 GPa), but the flax is two times lighter (density 1.3 while glass 2.6). Carbon fiber modulus varies between 200 and 600GPa depending of the type of fibers.
So flax is stiffer than glass, but less stiff as carbon. Also flax is a bit more difficult to compact than glass, so that you need a bit more pressure to have the same Fiber/matrix ratio.

Yes, to compensate for stiffness not strength, in the change from 22oz.

Thanks Mattman

[Idris]

I am going to do some mechanical tests before I commit to building a ski

When I started down the hemp route I built some ski core sections and did bending and twisting tests to compare the different weaves to glass.

In a ski I was under the impression that most of the time the glass, carbon etc is first used for dynamic stiffness rather than strength.

But now I might consider RichUK on the mix and match, we will see how the test go.

The strength for this flax 600-800mpa while E-glass is 2000

Both the ski failures I have had with my skis have been due to core failure - not composite - both were with very thin glass - so about the same strength as this flax should give me.

Both ski failures were good - if they hadn't failed the skier would have been injured / more seriously injured - the ski failed thus cushioning the impact

1 hitting a unintentional step up jump on a logging road - crashing into the step.

2 colliding with ice covered concrete wall

I don't want to build completely indestructible - it can be bad sometimes if the ski won't fail sometimes

[mattman]

-Assuming the core length is the contact length when carving
-Assuming 50% glass volume fraction
-Assuming poplar core
-Assuming Vectorply E-glass
-Assuming .053" thick triax (based on density)

Current Flexural Rigidity underfoot w/22oz: 9.47*10^4 in^2-lbs
Current Flexural Rigidity tip/tail w/22oz: 6.25*10^3 in^2-lbs
Equivalent thickness underfoot w/flax triax: 8.2mm
Equivalent thickness tip/tail w/flax triax: the thickness of the flax plys combined with the thin core profile makes this a bad fit for the assumptions sandwich plate equations make. Because of the thick facings, I would try to go as thin as you dare, ideally 1.4mm would create the same overall thickness as you have now.

I should point out that this assumes an ideal flax triax modulus of 53GPa based on the 65GPa fibers. For comparison E-glass fibers have an ideal modulus of 73GPa (slightly higher than flax), but the realistic modulus of the triax glass is about 37GPa. Anyway, the thickness of the facings should be pretty accurate and that makes the biggest difference. Hope this helps.

[Idris]

Many thanks for the calculations Mattman.

To mattman and others what size test pannels should I make up to test the theories and claims of the material?

I was personally thinking 60cm long (10 cm to clamp 50cm to bend) 5 cm wide 4mm or 5 mm thick core laminated on both sides.

No sidewalls or base or topsheet. Then bend twist and measure - making exactly the same in the two glass layups I have available so I can compare.

[mattman]

If you are trying to verify the material properties of the fabric only, then I wouldnt add a core. It doesnt need to simulate a ski. A thin plate of a few ply of flax will give you an accurately measurable volume fraction.

[Idris]

Taken from here.

After pressing at 40psi 25C for 24hrs

Thickness measured 10 times with digital callipers with a resolution of 0.01mm average thickness shown

Thickness Glass .62mm

Double glass 1.15mm

Flax 1.1mm

Double flax 2.07mm

[mattman]

If you measure some dry flax you should be able to get a pretty good volume fraction estimate (which has a dramatic effect on strength)...I think one of the sources you referenced mentioned the flax being more difficult to achieve high volume fractions. I'll be very interested to see what properties you find with the samples...the fabric looks great (not all course with pulls).

[Idris]

If you measure some dry flax you should be able to get a pretty good volume fraction estimate (which has a dramatic effect on strength)...I think one of the sources you referenced mentioned the flax being more difficult to achieve high volume fractions. I'll be very interested to see what properties you find with the samples...the fabric looks (not all course with pulls).

UD flax = 0.40 mm thick
Biax Flax = 0.66mm thick

Combined dry thickness 1.04

(measurement is an average of 10, using multiple layers, individual layers give really random results when trying to measure with callipers)

Fabric up close looks like aero grade glass (http://shop.r-g.de/en/Glass/Non-crimp-fabric-NCF/) just feels softer to the touch and colour of course is different.