Impact Resistant Application 1

[redeyerighley]

Hi Guys,

This is my first eng-tips post, have had a read through all over and am enjoying the site - its great! So a big hi from South Africa!!!!

I am a mech eng but have no experience in composites and this is why the post....

I am looking to replace aluminium sheet metal in an impact resistant application in order to reduce mass. The item to be manufactured is a guard to take impact and prevent damage to components behind/below it.

Things to consider:
1. Impact resistance, this will take place many times and the guard should not fail.
2. Stiffness, the guard cannot be too far from the "item" due to space constraints and if the guard comes into contact with the "item" it will cause it to be damaged.
3. Ease of manufacture and cost: I am looking at a prototype in my personal capacity at the moment and would prefer relatively simple methods for manufacture.

I have done a fair bit of reading online, but as I said I have very little knowledge of composites to begin with; so far this is what I have come up with:

A sandwich structure to improve stiffness (is this a bad idea as it would be better to load a sandwich structure on an axis parallel to the plane of the "skins" and because it could compress the core material impacting on the performance????) I am considering this material:

http://www.amtcomposites.co.za/data_sheets/R63_data_sheet.pdf

, any other suggestions? (I must admit I havent checked the cost of this material).

I would like to use a combination of Carbon and Aramid (Kevlar) materials to obtain both stiffness and impact reistance/toughness). Any thoughts on this and which materials to use?

Also, in terms of analysing this system: where would a good place be to start? The load that will be placed on the material will be on a variety of planes as the load can come from anywhere and the form of the guard is a complex 3D shape. The load will also be repeated and occur randomly. Also, I do not really know about where to begin with in terms of determining load (how do I measure the force due to impact?) - thus far I have just compared it to aluminium plate of 4mm thickness.

Finally, regarding mounting: The guard will be mounted using fasteners. Should the core be replaced in these sections using aluminium and then insulating the aluminium from the rest of the material to prevent galvanic corrosion (From what I have read this seems to be a good idea)? What is commonly done during the layup to achieve this?

Also, it says that these core materials are vacuum formed, has anyone got experience with doing this and how easy is it to achieve as opposed to vacuum forming of plastics? (I am looking at doing this to achieve the desired shape and then vacuum moulding the CF/Aramid to this form); I would look at doing this in a simple home setup vacuum forming system - any thoughts on this? Remember, the prototype will be built with cost constraints and an expensive manufacturing facility is not available.

Thanks in advance for any comments, advice and critisism. Also, I find this very interestiung and have had a look/read at the resources linked to on this forum, but if there are any more resources (especially regarding the layup and design of sandwich structures) it would be greatly appreciated.

 

[MikeHalloran]

You apparently haven't really quantized your loading case(s). I'd start by making an article out of polished aluminum, and measuring the scars after it's been in service for a while. From that, you can estimate what hit it and how hard.

I don't associate composites with resistance to repeated impact. Looking at it simplistically, above some minimum impulse, every impact breaks a fiber. The remaining fibers provide redundant load paths, until there aren't any more unbroken fibers. Then you've got confetti.

Whereas with solid metal, at a microscopic level, you have more redundant load paths, and something you don't get with composites; plastic deformation, where the material continues to take abuse even after it's yielded.


For about the same effort as making a vacuum form mold for a core, you could make a hardwood or concrete die and get some parts hydroformed from thin steel sheet.







Mike Halloran
Pembroke Pines, FL, USA

 

[redeyerighley]

Thanks for the reply Mike!
Manufacturing a guard from metal isnt really a problem in this case, so it is really the easier option.
In terms of quantisizing the load: how would one achieve that for, say, a canoe/kayak used to paddle down rivers and over rapids? The load could come from any angle at any point. But I do see that this would help point out the majority of cases or over a long enough period it would be a good representative of what would happen. Sticking with the example of the kayak, these take a fair amount of impact damage repeatedly and are built very lightly - I have seen people hitting canoes with a 4 pound hammer to "test" them. I know that isnt a very scientific statement, and your statement does make a lot of sense to me.
Also, regarding quantisizing the load, how would one determine the load by the damage done to the aluminium article? I ask because I am currently building an aluminium article but am considering replacing that at a later stage with the composite one.
Cheers

I still would like to explore this option however and am still wondering where I can get some more information.

 

[MikeHalloran]

Simplistic experiential way: Isolate each scar and try to duplicate it, e.g. with a 4lb hammer dropped from varying heights.

Analytical way: See "Brinell".



Mike Halloran
Pembroke Pines, FL, USA