Object X rams a wire. Does the wire break?
Object X rams a wire. Does the wire break?
(OP)
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 Ui = ∫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.
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 Ui = ∫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.





RE: Object X rams a wire. Does the wire break?
RE: Object X rams a wire. Does the wire break?
The greater the angle the less the tension in the cable and end-points.
I wonder if there's a military manual (army core of engineering) on standard calcs for exactly this. They have to do pontoon bridges and harbors etc.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Object X rams a wire. Does the wire break?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Object X rams a wire. Does the wire break?
RE: Object X rams a wire. Does the wire break?
You could assume some sort of loss of energy to e.g. the floats so only say 75% of the boat energy gets transferred, but start with 100% as the worst case - the impact might lift the floats out of the water so they might not lose much energy there.
what's "modulus of toughness?" The key number you need is Youngs modulus which is 200 x 10^9Pa
you seem to be neglecting the force of any propeller??
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RE: Object X rams a wire. Does the wire break?
In which case, I'd expect the prop would probably do more damage.
If the boat doesn't ride over the cable, I'd expect it'd behave more like an icebreaker ... bow up over the cable, boat down onto cable, maybe breaking cable ...
another day in paradise, or is paradise one day closer ?
RE: Object X rams a wire. Does the wire break?
Hi Little Inch,
Yes that sounds like the right way I want to calculate this problem. I have the energy in Joules, and I'm taking the worst case that all energy is transferred into the wire.
The "modulus of toughness" is the area under a material's stress-strain curve until failure. I figured if I know this then I can figure out if the wire rope can absorb the boat's energy without failing. With this in mind, does this mean the longer my wire rope is the less likely it will fail?
Using Youngs modulus, how do I use this to determine failure? Youngs modulus is characterized as a force over an area, whereas the boat is impacting it with an energy of 40,000 J. I know that if I had an impact distance I could use this to determine the impact force, but I don't have this value. Is there a way for me to figure out the modulus of toughness using the material properties page I found? Or should I be calculating this a different way?
Yes, I'm neglecting any force from a propeller. The specs call for "The barrier will absorb direct impact Kinetic Energy of 40,000 Kg-Meter."
RE: Object X rams a wire. Does the wire break?
if we had a dollar for every impact problem that's been posed "if I had an impact distance I could use this to determine the impact force, but I don't have this value" we'd be rich !
if we had a dollar for every problem that wanted a quick and ready (and accurate) answer without testing or simulation, we'd be rich !
In my mind this is a very complicated problem ... how does the cable drag across the water (in response to the impact) ? (this would probably be an efficient way of distributing the impact energy). Is impact energy the right measure ? (does mass trade-off with the square of speed ?) If the impact is close to an end I suspect that the cable will be more vulnerable than if impacted mid-span. How long is the cable ?
etc
another day in paradise, or is paradise one day closer ?
RE: Object X rams a wire. Does the wire break?
I can envision a scenario where the cable would break even if it were not anchored at the ends. Even if it's just floating there, the force required to accelerate that long line of barrier floats could be very large. Thus, even if the ends were anchored, they may not be subject to any force. Instead of working like a giant reverse slingshot, the cable would break somewhere between the impact and the anchor, at the point where there were simply too many floats along for the ride.
RE: Object X rams a wire. Does the wire break?
By the way, chain link fence makes a great crash barrier for this type of application, particularly when you want to minimize damage to the vehicle.
RE: Object X rams a wire. Does the wire break?
You have a cable of known (I'm assuming, as you appear to be designing the system) characteristics.
Treat the cable as a spring. Potential energy contained inside a spring is a very simple formula:
U = (kx^2)/2
Where k = spring constant x = spring displacement, and U = stored energy.
Your stored energy and spring constant are known, solve for x.
Since you know your spring constant, once you know your displacement you will know the forces going into the end fittings of the cable as well es the total strain of the cable.
You can then use the strain value and force value to determine if the cable survives.
There is a little bit of slightly trick geometry to do, but once you know the cable stretch you will also know the stopping distance of the boat. Since you are disregarding drag on the system due to water, this estimate will be very conservative.
RE: Object X rams a wire. Does the wire break?
Small boat (typical lake or pleasure boat) will ramp up over the cable and floats, then probably "trap" the cable between boat and outboard motor propeller. Which will then suddenly stop the boat and snag the boat as it slams it down to the water while pulling out the outboard and flipping up the motor to a traveling position. Cable will not likely break at all if greater than a 1/4 inch wire rope.
Large boat/pushboat/river floating industrial craft - These have a modestly shallow draft and a long flat bow with a lot of overhang. The push boat/"towboat" will go over the wire and floats very easily and not be stopped at all until the push boat's far deeper draft and propeller/shaft/rudder snag the wire many minutes later. Wire will break and eventually will wrap around the propeller shaft and cause minor problems. Barges and towboat will not stop for any realistic wire diameter and anchor design.
RE: Object X rams a wire. Does the wire break?
My theory, which might be an error, is this:
Your wire starts as straight wire but not under tension.
As the boat hits the wire it extends the wire, straining the wire elastically.
The strain (delta L) can be calculated or just initially assume say 1%.
The stress that will induce in the wire is 2 x 10^Pa (N/m2). You know the square area of your wire (I don't), but this stress will give you a force f.
Energy is then that f (divided by 2) x deltaL to take account of the increase in force linearly as the wire stretches
This gives you the potential energy in the wire which if you play around with the strain ( so long as it remains in the elastic zone) will then equal the boat energy in the most conservative case.
Also gives you your force on the end connectors.
All very simplistic and subject to many more issues, but should get you a handle on the force required and whether the wire will take the load or not. If you get beyond 5% strain then your wire is probably not in the elastic zone any more and I would limit it to 2% to start with unless you know the yield stress of your wire....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Object X rams a wire. Does the wire break?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Object X rams a wire. Does the wire break?
Not really - the wire is connected to a drum which is braked and unwinds, hence why the air craft doesn't suddenly shoot backwards off the carrier when it stops...
More like a farmer if a bull decides to run into a wire fence....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Object X rams a wire. Does the wire break?
It would be necessary for the navy to understand the likely failure mode for a fault in arrest system.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Object X rams a wire. Does the wire break?
This is just a nice theoretical physics problem.
the chance of it actually doing what it is supposed to do is nearly zero. It would either break the wire or the boat or sink the boat or slip off the front end and decapitate the crew or ....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Object X rams a wire. Does the wire break?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Object X rams a wire. Does the wire break?
Focusing on this from an energy perspective, I now see I can use the conservation of energy to solve this.
I found the equation for calculating the modulus of toughness: ut=(1/3)εfracture(σy+2σu), which is in J/V
My identified material properties of the AISI 1035 Carbon Steel to use for the weldless end links are:
E = 200 GPa
σy = 370 MPa
σu = 585 MPa
εy = 0.00185
εfracture = 0.3
Therefore, my modulus of toughness is 154x106J/m3
Now finding my forged end links cross-sectional area to use for the area of the wire rope, I have an area of 5.678x10-4m2 from a 19mm end link size.
I have assumed a length of 2m, with the boat hitting dead center resulting in a 1m L.
With these values I have
Ut = utV = (154x106 J/m3)(5.678x10-4m3 x 1m) = 87.3 KJ
which is the ultimate energy needed to break my link.
If I'm assuming the boat to hit the chain in the diagram in my opening post, then each side is 1/2 of 40,000 J, or 20,000 J.
Using geometry I have 20,000 / sin(20 degrees) = 58,476 J of energy acting against my chain. This is well below the requirement to break my chain!
Thank you all for the help. I hope this will help others who have a similar scenario.
RE: Object X rams a wire. Does the wire break?
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
RE: Object X rams a wire. Does the wire break?
RE: Object X rams a wire. Does the wire break?
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
RE: Object X rams a wire. Does the wire break?
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.
RE: Object X rams a wire. Does the wire break?
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.