For a tensile structure, should reinforcement be uniform or patterned?

[Charles West]

Hello!

A friend and I were discussing how to make the best material for solar sails. The most important properties of a sail material are that it be as light as possible, smooth surface, shiny and not tear when you unfold it in space. The current state of the art 2.5 micron fluorinated polyamide sheet with a thin deposited aluminum coating.

I proposed that you could do thicker depositions in a fractal pattern to create a support structure to strengthen the film and prevent tearing (make it easier to deploy or reduce thickness accordingly). The idea being you have some lines of thicker/stiffer material, more numerous/shorter lines of thinner material/etc and that any tensile stress would tend to concentrate on the stronger/stiffer line (resulting in more tear resistance for the same amount of weight).

My friend asserted that uniform distribution of support material would work as well or better than a patterned reinforcement given the same amount of reinforcement material.

If I may ask, which of us is right? Would using a non-uniform 2d reinforcement pattern help prevent tearing or increase the 2d sheets effective tensile strength?

Thanks!

 

[Ron]

In my opinion, fractal patterns are too random for consistent stress distribution, which would likely lead to problems.

 

[Charles West]

That is fair. I suppose it would be more accurate to ask:
Is it better for a tensile film to have some sort of grid structure of stronger reinforcement lines (triangles, hexes or something else) or have all of the reinforcement material evenly spread out?

 

[Ron]

Hex patterns give better stress distribution

 

[HTURKAK]

Consider rubber tire construction ..the filament arrangement with angle 50-55 degr. You may choose the angle within reasonable range , acc. to force distribution that you are looking for...

Just write ( car tire construction ) and search the web... one of the outcomes explaining the concept, ( four ply car tire )

 

[SwinnyGG]

Tires are made that way because it's an inexpensive way to get a structure that is strong 'enough' while providing the required level of compliance - not because it's the absolute strongest way to make a sheet good strong. The plies of a multi-ply tire are also not loaded in pure tension - they are constantly alternating between tension and compression as the tire rotates, and the stress state in the sidewall around the contact patch is, to say the least, complicated.

Woven fabrics are made the way they are because organic thread can't be made into a continuous sheet easily while maintaining strength in multiple directions; a continuous sheet of organic material is basically what felt is, and it's not very strong.

In short, looking at applications which require different properties in service are going to guide you down the wrong path. The first step would be to identify what performance characteristics the sail needs to have - does it need flexibility, or is a high level of in plane stiffness better? Does it matter if localized areas are less stiff? Where are tears most likely to start - mounting points?

Basically you'd want to determine where load is applied to the sail and how the current sail design deals with that load, and then determine if it's possible to optimize further.

Not to dump on you but this is a relatively simple idea; I'd be surprised if it hasn't already been evaluated by NASA or satellite manufacturers. If it's not in use, I'd assume it was discarded for any number of reasons.

 

[Compositepro]

"Tires are made that way because it's an inexpensive way to get a structure that is strong 'enough' while providing the required level of compliance - not because it's the absolute strongest way to make a sheet good strong."

I have to disagree with that statement. Tires are made that way because it is the best possible way (or only way) to meet the requirements. There are very high tensile loads in tires due to air pressure, but they have to be flexible in every other way. Wire rope compared to steel bars is another example showing how structural design is used to alter strength vs flexibility of a material.

Solar sails have much in common with boat sails, which used to be made from panels of woven fabric sewn together with the fabric in each panel oriented for the load in that panel. The degree to which the fiber orientation could match the load direction was very limited, but still far superior to using a single sheet of fabric. Modern high performance sails use plastic films to support carbon tows that can be oriented perfectly with the loads. The plastic film catches the wind, the carbon fiber transfers the loads to the boat.

https://www.uksailmakers.com/sails-overview-racing/x-drive-carbon-racing-genoa

 

[SwinnyGG]

Tires have been made with embedded/overmolded/woven material - in various stages over time, literally since they were invented; I'd argue that it is very likely possible to R&D a tire design that is 'better' in some arbitrary tire performance metric, while using fundamentally different construction (i.e. something other than a couple of layers of woven material in the carcass) but no one does that because the current technology is just fine, and there's no motivation to spend 8 or 9 figures developing something new that might only be marginally better.

A solar sail is a fundamentally different design problem than a tire. A sail for a sailboat is a little closer, but still fundamentally different.

However - the concept explained in the sail design in the above link - continuous reinforcing fibers placed corner to corner along the points of known highest stress - to me seems to be very applicable.

A generic pattern of hexes or a random fractal pattern is, in my opinion at least, not very likely to be the most structurally efficient method of reinforcement.

 

[dik]

Ron's correct, but with equal strength at right angles, the material is considered isotropic reinforced and has similar properties at any angle.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik