SS Cables Strengthen Structure

Hi Guys,

Is there any reason why one cannot use stainless steel cable embeded in a fiberglass structure / construction to enhance stiffness and prevent possible overload breakage ?

For instance on a weight (like mast) carrying beam, if some stainless steel cable is embeded in the fiberglass on the bottom and cable-ends are adhered to the beam end's, it seems logic that the force carrying capabilities of such a beam should increase by quite a lot. It should also have the effect of stiffening the beam since the cable(s) should prevent stretching of the fiberglass on the side of the surface the cable(s) is embeded in.

Am I missing something ?

 

You're not missing anything as far as a simple structure would go, Fanie. The issue of tha cable's uniqueness gets involved when you consider that the round cable will try as hard as possible to roll off the bent surface of the beam to which it is attached, if not embedded.

This means that you will have to build and confine the cable's tension path along the same path as the beam is being subjected, or the cable will roll off axis and its value as a structural member will be lost.

Picture the diamond wires on a mast, running trough the spreaders as a structural stiffener and you get the general picture. A taught wire running along the surface of the mast would also provide a degree of stiffness, but it would tend to roll off the surface of the mast and take the path of least resistance.

If it is embedded, as you suggest, then the materials need to be matched to one another, or the load will be completely supported by the stiffest one and only engage the other once the first material fails. This is one of the reasons why carbon additives to fiberglass structures need to be properly engineered for best benefit.

My take on this concept is that you will be inviting corrosion along the axis of the stainless wire within the composite structure. This corrosion will be essentially invisible to the eye and only make itself known when the loads exceed the carrying capability of the base composite structure, revealing the worthless stainless wire within.

 

Hi Chris,

The idea is to indeed ebmed the SS Cable in the fiberglass, to prevent it from moving firstly and secondly to isolate it (for what it's worth with fiberglass) from water. Finishing and painting should further isolate possible damp to get into the cable.

I also have the idea to use more than one cable and to adjust the tension in the cables so that they all carry an equal load.

The idea would be to use say three 8mm SS cables on a surface. If a good quality cable is used it may take a while for it to corrode within the fiberglass. If you note some SS items last quite a while on a boat, so there's no reason the cables shouldn't last so long either.

Thanks for the reply, I kinda knew the answer, just didn't know if there was a reason why it should not be done.

 

Good idea but it in composite

Hello Fanie

Your idea is what happens in mast beams in cats already. The difference is that modern cats use unidirectional glass instead of stainless. Stainless has lots of problems combining structurally with a glass beam but if you laminate up the unis as you make the beam the whole assembly is one piece.

Stainless chainplates are less common now as people use unis to do lots of the things stainless used to do - fewer leaks and lower stress concentrations.
By spreading the unis over the whole bottom surface of the box beam the shear load of the core or glass laminate is reduced. If you apply a load over a small area such as with a stainless wire then the shear and compression loads carried by that small "touching" area can be too high and cause a core shear failure.

It is also easier to engineer a homogenous beam - go composite - far fewer headaches.

cheers

Phil

 

Fanie,

Stainless steel does not like to "be embedded" in anything. It seriously relies on oxygen from the air to prevent it suffering from crevice corrosion. Chainplates in glass are often areas of failure as crevice corrosion start working, as are keel bolts sealed in hulls.

Stainless steel's corrosion resistance depends on the formation of a "passive" chromium oxide film on the metal surface which is highly resistant to corrosion.

The MEKP in poly resin is another source of corrosion, but of the chemical nature.

 

Ok, I hear what you say. Makes sense.

I should then use unidirectional glass instead for more strength.

I can do that.

Thanks everyone !

 

Uni Carbon would have to be the easiest option I would have thought.

 

Fanie
I would say yes - you should look at the numbers!

The main reason would be added weight. Glass fibre has an order of magnitude greater tensile strength than steel of the same weight.

Why would you choose to reinforce with a much lower strength material in what is likely to be a high stress area???

Rick W.

 

Good to know...

 

Not so sure

Rick may have more accurate numbers than me but my reading has Stainless being stronger in most situations in a real life sense. Witness the large composite chainplates in modern boats compared to their stainless predecessors.

The SP systems book - download from SP - has E glass composites having a Tensile strength of 100-900MPa. Stainless steels have a tensile strength of about 600MPa but and there is a huge but, the stainless can be pushed much harder than the composite can.

Composites exhibit a behaviour called micro-cracking where repeated stresses soften them over time. Stainless also has a yield point below its ultimate breaking strain but this is much higher than the E glass composite. Meanz advises for carbon which has a tensile strength of 700-2200MPa. You have to be careful that the carbon has enough bonding area to feed in the loads it has along the beam.

Allowing for fatigue - which produces micro-cracking - most composites need to be kept below the tensile figure they attain after a million load cycles. For an E glass laminate this can be about 350MPa for a lab controlled laminate - less in our boatbuilding sheds. I would assume less than half for any beam I was designing unless you can test a heap and find they all exceed these values.

Fanie - get yourself a couple of my fave books - anything by JE Gordon and Stress without tears by Bernard Rhodes. Then read Marine composites at http://www.marinecomposites.com/

Have fun

Phil Thompson

 

A bit extra

A quick re-read had me back pedalling a bit - Stainless is very dense about 7 times heavier than water and E glass laminates about 2 times so that has to be taken into account in design.

For those who want to know about composites and fatigue Mike Johns started a good thread (from which I learnt a lot) on composites here

http://www.boatdesign.net/forums/showthread.php?t=13174&highlight=fatigue composites

cheers

Phil

 

A table at the bottom of the page on this link gives some good comparisons;
http://www.islandone.org/LEOBiblio/SPBI1MA.HTM

Some things to note from the table
1. The strength properties are not specific properties they are based on actual areas. So when you bring density into the equation you see something like S-glass and spectra are far superior to steel.

2. I do not know of any steel with tensile strength near 5GPa. The best stuff I have used runs to around 2GPa.

3. Carbon fibre is not so impressive in its tensile strength but look at its stiffness. Up to 4 times better than Steel at 1/3rd the weight - 12 times better for given weight. So for low flex go for CF. That is why it goes bang when it breaks. If does not have a lot of give for elastic energy absorption.

Some useful rules of thumb for steel:
1. Well terminated steel can take cycle loading to 40% of yield for ever. The range means the sum of compression and tension if the stress reverses.

2. If you weld steel allow a 10 times safety factor on UTS for any cyclic loading range. So 500Mpa welded steel should not have greater stress than plus and minus 25Mpa if you want it to survive.

If stainless had better strength to weight ratio than composites we would see the ultimate racing machines like F1s, maxi yachts and globe-circling tris using more of it. In fact the opposite is true. We see composites like CF being used in all sorts of high tech racing machines. (I know the price of stainless is through the roof but CF isn't cheap either - and money would not stop the top racing teams from using the best material irrespective of price.)

Rick W.

 

SS and glass

Rick, I am quickly coming to the limits of my knowledge here and thanks for the guff on stainless but the data in the table is not for a laminate. The values for a glass laminate should lower than that in the table. The fibres are the higher load bearing part of the compsote matrix.

To be truthful that is one thing I have been meaning to do for a long time. I need to get into my shed and make some test pieces and find out what I, and some friends, can achieve with a tensile test in our sheds. I have always been suspicious of the test data form shinky labs and guys in white coats - they get it so easy.

Cheers

Phil

 

You are correct about the data being for the fibres but my understanding of the original question was that Fanie intended to use stainless steel fibres in the composite structure. Hence my question to him, why would he want to substitute a material with much lower specific strength than the normal fibres used.

Rick W.

 

Here's what I have been looking at -

A 9.5mm (3/8") 316 woven 7 x 7 SS cable can handle 5136kg (11940 lbs) pulling strength max, which is quite a lot. Weight is about 243 lbs for 1000 feet or about 400 gram per meter if I calculated right. (110kg for 303m)

Now if I have a beam say 400mm x 300mm and 8m long, and the glass strands run over the length of the beam, you should get a fairly strong beam if a few layups have been made. I have actually taken a strand from mat and it is amazingly strong, even when not polyesterized or added bondage between the fibers.

When you have a fiberglass beam like that, although very stiff and hugely strong, when you load the beam in it's centre of length, like a mast under sail sitting on it and pushing down with huge force, you get a couple of things happening.

The bottom of the beam tries to stretch around the horizontal centre line of the beam,

The top of the beam tries to compress to become shorter as it is folded in, also inside the beams horizontal centre line, hence it bends some.

Now if you look at all breaks in fiberglass, it occurs always on the side that stretches. I have never seen the top imploding while the bottom stays intact.

Now I think one can argue that you have to or could add more material to the bottom to make it even stronger, but the weight you add to double the strength on such a beam could be quite a lot.

Now assume you have three evenly spaced SS cables running under the beam, each saddled into place so they cannot slip out of place under the beam and the cable ends are adhered to some holding plates on the side of the beam.

These three cables will add 15 tons of pulling strength that can be loaded on top of the beam.

So what should happen is that when under normal load conditions the fiberglass beam shoud carry most of the weight due to it's stiffness. The SS cable should also be stiff but tension in it would be lowish.

In the event of a sudden hard gust which could for argument sake double or tripple the mast's down force, the beam would attemp to bend further down, but now the SS cable would tention up and retain the amount the bottom of the fiberglass would stretch around it's horizontal centre line.

Now it is also my argument that since the SS cable now carry the pulling force and preventing furter stretch along the beam bottom, the whole beam would be compressed towards the middle, and not only the top part of the beam. Remember the SS cable doesn't allow further or very little bottom stretch.

This should imply that the beam should be able to carry the 15 000 kg pulling due to down force plus the strenth the beam stiffness adds.

The cable should add 400g x 8m x 3 cables which is around 10kg plus end plates and bolts.

How much weight would more layups add to withstand such a force ?