Stress test using a nylon rope

[niall.smyth]

I am carrying out a simulation to find out the axial forces in a nylon rope when a load is applied.

I am using non linear MES with truss elements (sub-divided) to define the rope and the analysis with large deformation switched on.

I need to find out the force which results in the rope when the weight is applied.  One load case is when the rope is vertical and held by a pin constraint at the top, the other is when the load is applied to the midpoint of the rope with two pinned constraints at either end.

My question is how do I relate the Youngs modulus of the rope to specify the dashpot coefficient for the element and how does the dashpot coefficient affect the stiffness of the rope?

Also how do I specify the damping in the material definition when specifying curved elements?

Many thanks in advance and any help is greatly appreciated.

Niall

[John_Holtz]

Hi Niall,

This sounds like a homework assignment, so I hope these answers are not directly related to the grading.

There is no correlation between the Youngs modulus and any type of damping, whether it is the dashpot coefficient or the material damping. Unless some other information was given from which the damping can be determined, you can leave the values at 0.

Regardless of what values you choose for damping (0 or anything else), damping does not affect the force in the rope once it reaches its static equilibrium. Once the motion stops, there is no damping. (damping * velocity = force = 0 when velocity = 0)

[niall.smyth]

Hi John, Many thanks for your response, looking back my question does seem like a homework assignment! Unfortunately not, my goal is to simulate a drop test to test the integrity of a lanyard for fall restraint from various heights so I would have to incorporate impact loading. Your response is very helpful, would I be correct then in saying that I have two options- Option (1) Carry out a static load test using non linear truss elements and input a force that incorporates impact loading worked out from the height fell and the max static deflection. or Option (2) Carry out a Mechanical Event Simulation in which I would have to know the damping properties in order to see how much force is transmitted through the rope due to impact/rope elasticy i.e. draw a spline to simulate the rope (subdivided) in the pre impact position and then apply a force and see how the rope arrests the fall? cheers, Niall

[JohnMcCrory]

From what I understand, because it is dificult to calculate the stiffness of most systems, it's not very accurate to calculate the impact force and apply it as a static load (since the impact force is dependant on the impulse which is dependant on the stiffness). I've been adviced to always model the entire impact scenario where possible and use MES.

Can I ask what you did with your model in the end? Were you able to calculate the stress in the truss elements during your chosen loading scenario?

Cheers,

John

[arizawilmer]

Hi if I understand correctly you want to simulate the behavior of the rope under tension when a mass  at the end is dropped.

I saw a few years ago a similar simulation If I am right they add two elements a beam and a damping element . The combination of this two elements get them the respective behavior. You can try to simulate with this elements. You can try a search in google scholar for similar simulations.