hybrid plies 4

[tstanley]

I've run across a few manufacturers that make a fabric that places unidirectional fibers in alternating materials in a single ply. For example alternating fibers of carbon and kevlar, or kevlar and glass. Then they take this ply at 45 degrees and stitch it to another ply at minus 45 degrees. An example manufacturer is Vectorply.

What are the advantages of alternating the material of the fibers in a single ply? I would expect that the fiber that is the stiffest fiber (higher elastic modulus) would end up carrying most of the load.

Tom

 

[Compositepro]

You are correct. But hybridization might be useful to make failure safer. An example is bicycle handle bars. Personally, I'm not aware of an example where hybridization has a clear technical benefit.

 

[berkshire]

One example of this type of stitched fabric is to save lay up times in wing skins on composite aircraft.

In the past, Unidirectional fabric was laid at 45 degrees to the direction of flight in the mold, then a separate layer was laid on the opposite axis. Now with stitched fiber it is possible to do this same operation by simply un rolling the pre laid stitched fabric down the length of the wing. However I am unsure that hybridizing the materials help, except in perhaps preventing an abrupt failure.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[tstanley]

Hi:

I understand about the labour saving aspects of the stitched double ply part of it. I've heard that when carbon fiber fails it breaks into a lot of sharp small bits and the kevlar helps to keep them contained. That wouldn't explain the kevlar/glass hybrids though.

I wondered if the alternative fibers helped to wet out the kevlar?

However, I was mainly wondering if there was some structural advantage for people to use it.

Tom

 

[RPstress]

"Most" hybrids increase toughess over a single reinforcement type (there may be ones that I'm unaware of with different purposes plus what I've noted below). The toughness may increase strength after a low speed impact has made a dent, or it may be more for strength after a high speed impact has made a hole. The roughly 50:50 carbon and E-glass hybrid used for composite helicopter rotor blades by (at least) one manufacturer was/is mainly the latter, with battle damage from a 20 mm cannon shell being resisted much better than pure carbon. Although there is not much information available, carbon/aramid hybrids are rumoured to have a similar effect. The carbon and E-glass materials were prepregs and wet out was not a problem, that I ever heard. Not entirely sure why someone would hybridise nice cheap glass with expensive aramid. You could get higher tensile strength and stiffness with lower compression strength, all with lower density. Maybe you'd also improve toughness and compression strength might be adequate. It might also be a bullet resistance issue, where resistance to penetration rather than performance after penetration matters.

There are other hybrids experimented on such as a layer of threads of SMA (shape memory alloy) in carbon (SMA can have a very high elastic strain and plastic strain to failure, as can Toyota's Gum Metal). As well as increasing laminate toughness, 10% volume fraction of these metallic threads can also conduct electricy, greatly increasing resistance to damage by lightning, especially if they are in the outermost ply. Fibre metal laminates (FMLs) also hybridise metal and continuous fibre polymer composite (the classic example being Glare). As well as almost removing lightning strike damage as a source of concern these also modify the low speed impact behaviour, making it very easy to spot a dent before it is dangerous damage.

For something like carbon/glass or carbon/aramid putting both fibres in the same ply is a natural thing to do if the technique is affordable (I'd have thought it was petty easy for a well set up weaver/converter). Intuitively one can visualise it making a more finely mixed material maybe with more toughness (if it worked). I can see it being awkward to set up such that both material threads were the same thickness, but surely not a severe problem for an able converter.

NB: toughness is a bit of an emergent property and is very hard to predict from knowledge of the toughness of laminates made of all one fibre type. E.g., see

http://www31.ocn.ne.jp/~ngf/english/product/a3.htm

and click the Impact Demo button for a video. This is a hybrid of ordinary high strength (HS, aka standard modulus) carbon and ultra low modulus (ULM) carbon fibre. A lot of hybrids seem to rely on getting more of one materials' beneficial properties than the other one's disadvantageous ones. You can never tell (well, I can't) which may be most advantageous.

 

[tstanley]

Thanks to all:

RPstress, the applications you describe are exactly where I've seen this used. The applications are drag boats and canopies for offshore boats, so the application is for impact although not at the speed of bullets. I've been doing fea on offshore stuff where the kevlar glass hybrid was used and it didn't stand up in real life. As part of my analysis I did an fea of a unit cell and that is what got me thinking. The drag boats are using carbon kevlar and that seems to be working OK, although there is a lot more to it than just these plies.

Thanks again.

Tom

 

[berkshire]

tstanley,
I think the thing you are not looking at is the failure mode of Kevlar.
Glass and carbon take a load until they fail abruptly somewhat like a steel spring.
Kevlar 29, 49 and to a lesser degree 149 tend to stretch and yield a little before breaking.
They also take a set in compression which Glass and Carbon do not do.
This makes Kevlar ideal for things like roll cages and cockpit protective cages, while being too floppy for other uses.
Now bear in mind that this is somewhat of a generalisation and dangerous to post here, I will most likely get beat up for this.
Hybridizing two different materials is supposed to offset the dis advantages of each, and form a symbiotic advantage ,it does not always work in practice, but sometimes it does.
B.E.


The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[RPstress]

tstanley: Interesting that you say the glass/Kevlar hybrid didn't work out. Analylitical modelling of actual failure is very hard to do with metals, and harder still with composites and even harder still with hybrid composites. It's devilish difficult with static (so-called monotonic) loading and impacts just make it all that much harder. You also say that the carbon/Kevlar hybrid worked 'ok.' Hang on to any data you've got on those failures, including the amount of carbon vs. Kevlar by volume fraction and the resin used. It's hard and expensive to get failure data of any sort for that sort of thing.

PS: any tips you can give us on how these laminates behaved without violating IP would be fascinating. For instance, in what ways did the Kevlar/glass fall short of expectations? The tensile failure should be increased compared with just glass by having Kevlar which is strong in tension and usually stiffer too. Kevlar's crap compression strength should drag the laminate's compression test strength well down compared with just glass, but who can say about the toughness? Even if static test strength is reduced, the energy absorbed during failure might go up.

 

[mrsdesign]

For some good information on the hybrid synergystic strengthening effect:

http://www.tech.plym.ac.uk/sme/mats324/mats324A6 hybrid.htm

 

[RPstress]

Thanks mrsdesign. Interesting. "...This synergistic strengthening is known as the hybrid effect (but is as elusive as the Philosopher's Stone)." Yeah. The refs on that Plymouth U page seem to be from the early '80s at the latest. Is it an old page or are the refs out of date? Does anyone know of more recent stuff on hybrids? (Not trying to hijack your thread tstanley; any answers might presumably be of interest to you too.)

 

[tstanley]

Thanks again. It is a very educational and interesting discussion. It is another case where fea doesn't tell the whole story and good engineering judgement is required. Also, per the world wide failure exercise, failure is yet to be easy to predict from fea results.

I may have been a bit harsh to say the kevlar glass hybrid didn't work out. While it did fail and the results were catastophic, I don't believe the failure initiated with the composite parts. The complete failure occured between frames of the onboard camera (.04 sec) so it isn't easy to be certain. The hybrid plies were only part of a sandwich construction with balsa core and triaxial glass. What was notable was thet while the glass plies looked resin rich, the hybrid plies seemed quite dry. I don't have the equipment to check the resin content unfortunately. It would be good to know since my calculations depend on it. My judgement for the RC was based on experience modelling a standard test with a known laminate schedule and known results. There was lots of delamination going on where the kevlar glass ply met other plies. There were other areas where the core was eleminated like edge flanges, and the two skins delaminated where they came together, and that was between the glass plies. The kevlar glass plies were used for tabbing and these showed a lot of delamination. Other than that there were mostly tension breaks. It seems to go back to being a dry layup for these plies. That is why I was wondering about helping the kevlar wet out.

The carbon kevlar was not related to this accident. It has undergone some testing using our standard four bar test but I'd rather not comment on the results. I haven't personally seen the failed parts.

Berkshire, in this case there was a minimal aluminum roll cage, but it was integrated to the shell using the hybrid plies to tab it in place. There was delamination and tearing of these tabs. There was also one compression failure where the roll cage tube likely kinked underneath it. Your comments make me think that it would be important to consider where in the laminate schedule the hybrid ply would be placed in order to take advantage of the properties.

mrsdesign, thanks for the link, I hadn't seen the equations before so I gave you a star too.

RPStress, don't worry about hijacking the thread, I am interested in the potential answers.

Tom

 

[RPstress]

Can you tell us a bit about your materials and manufacturing method? What resin is used, and is it infused? Are all plies unidirectional or are some woven?

 

[tstanley]

My look at the kevlar glass layup was in the nature of forensic work so I don't know the manufacturing methods. Because of the age of the part it is highly unlikely to be infused, some areas may have been vacuum bagged though. Similarly I couldn't say what resin was used. The kevlar glass was not woven but was a stitched double bias unidirectional fabric. The plain glass was stitched triaxial unidirectional fabric. There weren't any woven plies in the main parts, only on some gussets that were added later that didn't contribute.

The carbon kevlar parts are for a new product which will be vacuum bagged. The epoxy resin will come from Applied Poleramic. There will be some woven carbon as well in this layup. I'm not involved in the manufacture of this product either though.

Tom

 

[tstanley]

Hi again:

Today I ran across a paper that studied the strength of a carbon/kevlar hybrid braided material. It looked at the strength of the ply with varying percentages of carbon/kevlar from 100% carbon to 100% kevlar. It showed that at certain percentages of kevlar the flexural strenght was actually stronger than 100% carbon. The link to the paper is:

http://hosocon.com/image/standard/2.pdf

It is an interesting paper.

Tom

 

[RPstress]

The bending load-deflection curves all had an almost horizontal (very low stiffness) area of almost 1 mm after the initial linear part. The authors mention 'after the plateau' in 3.1. but don't mention it otherwise, that I can see. The initial very linear 1/2 mm looks to be proportional to the reported fibre stiffness and the curves 'after the plateau' look quite reasonable. Can anyone explain the 'plateaus'?

They needed a lot of Kevlar to get much increase in impact strength and that gave a big reduction in flexural strength. It would be interesting to see the same work with glass instead of Kevlar.

 

[Compositepro]

The specimens were +/-16 degree braids with a 30% fiber volume. The flats would probably be due to plastic deformation of the matrix resin as the fibers scissored.

 

[RPstress]

Maybe. If so, that's a very nice neat resin failure and it seems a bit odd that the fibres suddenly start working abruptly at 1¼ to 1½ mm deflection. And you don't get any flat bits of a load deflection curve with ±45° cloth laminate tension tests...just an increase in load in a pretty smooth curve, albeit nonlinearly with markedly decreasing stiffness.

While I've got no data for braided laminates, let alone 3D braided ones, and no result data for anything other than ±45° or ±30° (or ±60°) laminates, I suspect something else is going on. But I don't know what!

 

[Compositepro]

Most aerospace resins are very stiff relative to the plastics used for prosthesis(acrylic), and do not exhibit much plastic deformation beyond the elastic range. But plateaus in stress-strain curves due to plastic deformation are common for most plastics (thus the term plastic). This type of material would never be used in a high performance aerospace application and is why we rarely see curves like this.