Problem with simulation, change of rotation after helmet impact
Problem with simulation, change of rotation after helmet impact
(OP)
I am doing a finite element analysis using LS-DYNA of a helmeted head form being struck on an inclined plate.
The helmet and head is given an initial speed of 5.3 ms in the Z direction. As a result of the impact to the inclined plate, the helmeted head starts to rotate in the Y axis. The problem I have is that after a while, the helmeted head starts to rotate in the opposite direction for some reason. I can not understand the cause of this.
When I run the same simulation without the helmet I do not see this problem.
The helmet in these pictures is made up of elements that have been extended from the head form. It is 2cm thick and of the material crushable_foam. The outer part of the helmet is a linear elastic shell.
I have checked so that the centre of gravity of the head form is located in the centre of the head as it should be and that the inertia properties of the head are set.



The helmet and head is given an initial speed of 5.3 ms in the Z direction. As a result of the impact to the inclined plate, the helmeted head starts to rotate in the Y axis. The problem I have is that after a while, the helmeted head starts to rotate in the opposite direction for some reason. I can not understand the cause of this.
When I run the same simulation without the helmet I do not see this problem.
The helmet in these pictures is made up of elements that have been extended from the head form. It is 2cm thick and of the material crushable_foam. The outer part of the helmet is a linear elastic shell.
I have checked so that the centre of gravity of the head form is located in the centre of the head as it should be and that the inertia properties of the head are set.








RE: Problem with simulation, change of rotation after helmet impact
is the inclined plate fixed ?
RE: Problem with simulation, change of rotation after helmet impact
How do i check the line of action in LS-DYNA? Can't find how to display the direction of the reactionforce at the plate.
The inclined plate is fixed.
RE: Problem with simulation, change of rotation after helmet impact
RE: Problem with simulation, change of rotation after helmet impact
corus
RE: Problem with simulation, change of rotation after helmet impact
RE: Problem with simulation, change of rotation after helmet impact
Once the head starts rotating widdershins then it really should carry on doing that, in the absence of external forces. Mr Newton says so.
Cheers
Greg Locock
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RE: Problem with simulation, change of rotation after helmet impact
This is under a very short period of time so the gravity effect should be negligible. I can confirm this as I have done experimental testing of this exact situation.
I turned the gravity off just in case the gravity was set wrong and somehow effected the rotation abnormally. The results remain the same.
Frustrating problem this.
RE: Problem with simulation, change of rotation after helmet impact
RE: Problem with simulation, change of rotation after helmet impact
corus
RE: Problem with simulation, change of rotation after helmet impact
Rob Stupplebeen
RE: Problem with simulation, change of rotation after helmet impact
Good quiz question anyway.
corus
RE: Problem with simulation, change of rotation after helmet impact
I have also tried using a linear elastic material instead of the crushable foam as padding in case there was something wrong with the material model.
As I was using a more advanced model for the helmet I would get warning messages when I used a "tied surface to surface" connection between two shells separated too far from each other, the warning would say that there could be rotational constraints.
This model however does not need any defined contacts as the helmet shares the nodes with the head.
The inner part of the head is made of defined as rigid whereas the outer surrounding part is defined as viscoelastic. There would be no moving parts inside the head.
RE: Problem with simulation, change of rotation after helmet impact
I remodelled the helmet and somehow the "ghostforce" is gone =)
I thought this "ghostforce" was the cause of my original problem but it seems it wasn't. I will post a new thread about that as it is a different problem.
Thanks for the help on this guys!
RE: Problem with simulation, change of rotation after helmet impact
I could only imagine that it would be the british Health and Safety Executive (HSE) that would insist that the condemned man wore a safety helmet.
corus
RE: Problem with simulation, change of rotation after helmet impact