kevlar and compression 2

[mendadent]

hello gentlemen,
knowing that kevlar has similar charecteristics to metal(i assume someone may know the answer to my question) can anyone answer why kevlar has a high impact resistance but low compression.

let me rephrase that. first briefly: i'm considering using kevlar for a safety measure to rest the butt of a tool on in order to leverage off an automobile side window while pedrforming my craft. i'm not sure of the psi i put on the window while working my craft, but it has to be substantial considering at times i use every ounce of energy my 180 lb body can muster.
so understanding that in body armor a bullets impact is stopped by kevlar, i'm wondering if by compression they(dupont) means steady pressure in one area when they are talking about compression. as in continuing increased pressure by me and my tool in this instance.

 

[TVP]

The reason Kevlar has poor properties in compression has to do with the arrangement of the polymer chains during fiber production. Dry jet-wet spinning is the process used for producing aramid fibers such as Kevlar, which means that the polymer is mixed with a strong acid, extruded through spinnerets and into cold water. This process causes rotation and alignment of the liquid crystals. The highly oriented polymer chains are quite linear, which allows for a high packing efficiency along the axis of the fiber. The polymer chain consists of strong covalent bonds, which gives the fiber very high strength and modulus. The bonding between the chains is rather weak, though, because the bonds are hydrogen bonds not covalent bonds. All of this adds up to a fiber structure that can be visualized as a group of radially arranged, axially pleated crystalline sheets. The pleats cause the poor compression properties.

 

[MikeHalloran]

What wasn't quite said is that it's available only in fiber or fabric form.

The fibers are surprisingly brittle. The butt of your tool will chew through a Kevlar pad and scratch the glass while you're not paying attention.

A sheet of nylon would probably be a lot more satisfactory.



Mike Halloran
Pembroke Pines, FL, USA

 

[mendadent]

thank you both.i do use a lexan,or plexi glass piece now and this method is fine for preventing scrathes.my intent is to come up with a solution to prevent busting tempered glass in higher end cars. too much pressure, and boom,it's gone.

 

[mendadent]

excuse me, i meant scratches.

 

[EdStainless]

So you are trying to prevent scratching and spread the load to minimize flexing?
You really need to try and match the stiffness (flexural modulus) of the glass. Your Lexan (polycarbonate) if too flexible. What about an steel plate with plastic coating on it? Cut a plate the right size, (thickness will dictate stiffness), and have someone in your area that does spray-on pickup truck bed liners coat it for you.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[CoryPad]

I have to disagree with MikeHalloran. Fibers can have high or low compression strength, just like bulk materials. Carbon fibers, in contrast to Kevlar, have good compression strength.

Also, Kevlar is not brittle. Rather, it has good ductility, in contrast to other fibers (like glass or high modulus carbon).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

Kevlar has excellent numbers. That's why I bought some very small Kevlar cable for a special application. Bought the custom ferrules and custom crimping tool, too. Provided strain relief, wrapped the cable around custom machined grooved bushings, etc.

Two problems:

- The ferrules couldn't maintain a grip on the cable. _I_ didn't engineer the connection; this was all commercial stuff, alleged to work together, etc. Think of the cable stays on a power pole, except the cable is 1/32" diameter. Damn stuff just oozed through the ferrules. Even multiple ferrules. Even every damn ferrule in the bag. Even crimped extra hard.

- Despite being very careful about bend radii, the fibers fractured when the cable was bent, even just once, even over a controlled radius mandrel, at room temperature.

The broken fibers leave a fairly nasty puncture wound. They are stiff, and the broken ends are sharp.

Nylon dial cord worked much better.



Mike Halloran
Pembroke Pines, FL, USA