Learning Forum

[plum0721]

Hello everyone,

I did a simple experiment using Explicit Dynamics and LS-Dyna.

I fixed each base side of the cylindrical bar and gave force(z axis, 0s : 5000N to 0.1s : 15000N)to side of the bar.

I reviewed the normal stress(z axis), and it doesn't seem to be reliable. As I applied linear force(F=100000t+5000(N)), I think the result should show linear stress. But the normal stress fluctuates as I attached below.

I want to know why this happens, and how to resolve this phenomenon. Can someone please help me with this?

Thanks in advance.

[Chris Quan]

Both Explicit Dynamics and LS-DYNA systems are doing dynamic simulations. When a loading is applied to a geometry, the loading is a dynamic load. The stress incurred is dynamic in nature and thus subject to oscillations due to stress wave propagation and interactions.

In your model, 5000 N is applied at the time = 0. This means that an impact force directly hits the structure at the initial time. A stress wave is thus created at the impact end and propagates towards the other end, then is reflected from the other end, travels back towards the impact end, and finally interacts with the new stress wave from the impact  end due to the increasing external force.

You didn't mention what boundary condition on the other end is in your post. If it is the stress-free boundary condition, the reflection wave is rarefaction wave and thus the resulting stress is decreased while the rarefaction wave travels through the model. Furthermore, the new stress wave from the impact end due to the increasing external force will increase the magnitude of the stress.  

[Ram Gopisetti]

Hi DO HYEON LEE, 

I agree with my colleague's response on this issue, if you tried your model on the implicit setting for the force wave propagation your expectation of linearised stress output is given as follows

but this does not address the dynamic response as the implicit dynamics card is ignored by default. 

Cheers, Ram 

[plum0721]

I really appreciate your kind replies and they were so helpful.

I have to use Implicit solver to remove reflection wave effect.

Thanks again for your advice.