Object X rams a wire. Does the wire break?

[kand16]

Hi,

I'm trying to figure out how to use an object's defined kinetic energy to determine if its collision with a wire will break it, assuming the wire is weaker along its length than at its mounted ends. The real scenario is a motor boat with a KE of 40,000 Kg-Meter (shouldn't the doc's specs have this in kg-m²/s²?) is ramming a marine security barrier, and I need to figure out if the components in the marine barrier system will not fail. See image below of the security barrier system for concept.

To simplify my scenario I'm assuming my 19mm weldless end links are the weakest components in the system. They are made of AISI 1035 carbon steel (link for a materials property sheet I found:

http://www.azom.com/article.aspx?ArticleID=6540).

I'm also assuming the boat to be a "blob" that sticks to and combines with my system at impact, so that it doesn't skip and just graze the system. The barrier is not a taught line and I'm assuming there's enough slack for there to be a 140 degree angle in the system before it becomes "taught" by the boat (see image to clarify).

My initial path was to find the strain energy required to break my end links, but I'm uncertain how to do this as I can't find the AISI 1035 carbon steel's modulus of toughness and I don't know how to apply the referenced material properties into my strain energy in terms of the normal strain energy equation U_i = ∫^V(σ²/(2E))dV.

Another path I was thinking of, which seems improper to me due to the principles in the mechanics of materials, is finding the boat's impact force and seeing if this force is great enough to exceed my end link's UTS. Here, I'm uncertain of the slow down distance and not feeling comfortable to assume this figure.

Am I on the right path and can you help me? Should I be attacking this problem from a different angle?

Thank you for any help. I took my undergrad with classes in materials and mechanical engineering. It's become quite rusty.

 

[drawoh]

kand16,

How do you figure your boat will cope with all this? I suspect that a fibreglass hull will be weaker than your cable. If your bow is anything other than vertical, there will be all sorts of interesting dynamic behaviour.

--
JHG

 

[kand16]

Thank you all for the pointers.

 

[gruntguru]

Not sure if LittleInch was saying this in an earlier post but:

If you are neglecting losses to the water and you want to compare boat KE to maximum energy absorption by the "wire", the latter becomes a simple calculation (strain energy to failure) dependent only on the length of wire - geometry does not come into the problem. Put simply, you could use the wire to tether the boat and the energy absorbed would be the same (forces would be different).

je suis charlie

 

[dhengr]

Kand16:
It seems to me that there are enough unknowns and the problem is complex enough, that I would want to talk to a few outfits that make these security barrier systems. They should have a wealth of real world testing results to prove their products and systems. In good part because this problem is pretty difficult to solve, expect in an over simplified form. They should also have design values and literature for their products. It is also worth noting that any stretch in wire rope/cable consists of two parts, elastic stretch and a mechanical stretch component which is a function of the lay of the rope and the way it is manufactured. It is not as simple as just knowing the ASTM number and the mech. props. of the wire.

 

[LittleInch]

I don't get the same answer, but then I still don't really know the diameter of the wire.

As dhengr says, wire rope is not quite the same thing as wire bar / single stranded wire, which was my way of getting an answer.

I don't understand the 1m bit and I don't understand the 20 degrees bit either.

You appear to be concentrating on the link at the end which is not the thing absorbing the energy - that's the rope/chain.

also "ultimate energy needed to break my link" to me is incorrect. It's not energy which breaks anything, its force and stress, neither of which you calculate as far as I can see.

A very long, high yield stress, but thin wire will exert a lot less force on the end connection than a thick wire (at least in my head it does).

Also question the 40,000 kg-meter and it should be Joules or different units to get 0.5MV^2 If its actually been converted to potential energy then its 400,000 (approx.) Joules..40,000 is a 40 ton boat travelling at 1m/sec. Sounds about right.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.