How to model Spring element in Radioss

Umehr Beebeejaun

Hello everyone, 

i'm modelling a spring between a floor and a cube which needs to be permanently destroyed after the loading case. I am having some difficulty in modelling the spring. 

The spring seems to be decelerating the cube but at t = 0.05s it accelerates in the opposite direction, which i want to prevent. I'm using Spring Type 32 and the settings are listed below..

I've tried increasing the Stiffness Stif0 but it just blocks the cube at a certain point and it vibrates on itself till the end of the simulation. Also with the setting Ilock = 1 penetrates the cube into the floor... Any ideas what i am doing wrong or how to tackle this problem in a better way? 

Thank you very much for your responses ! 
Umehr 

PaulAltair

I'm not sure what you are trying to do here exactly, what is the initial condition? You are trying to decelerate a moving cube with the spring?

What is the purpose of using a pretension spring here rather than a 'normal' spring (TYPE4, TYPE8 or TYPE13)?

You have set a Stif1 stiffness of -10000 (I'm not sure what this would do exactly, stiffness would ordinarily be positive and define the spring preload along with D1, i.e. increasing force with increasing tensile load)

Your plotted curve is velocity magnitude (i.e. it will always be positive), so acceleration isn't occurring the 'other way' at 0.05, (the deceleration looks fairly constant between approx 0.01 and 0.09 passing through 0 velocity, so the cube reverses direction at 0.05, but deceleration doesn't change there) at 0.1 I guess the cube will be more or less back where it started? And the spring looks to be undamped with a constant stiffness.

What type of sensor are you using? Is it being triggered? It sounds like possibly not if changing Stif0 is changing the result. My guess is you have effectively a linear elastic spring with stiffness 100 in your image.

If setting Ilock=1 causes the cube to be 'pulled' to the floor, I am guessing then that your negative stiffness Stif1 is coming into play causing the spring Tensile Force to actually increase as the spring shortens (thereby accelerating the cube rather than decelerating it)

If you can share a little more about what you are trying to represent/achieve, or share the model, then we may be able to specify a better spring set up.

 

 

Umehr Beebeejaun

Paul Sharp_21301 said:

I'm not sure what you are trying to do here exactly, what is the initial condition? You are trying to decelerate a moving cube with the spring?

What is the purpose of using a pretension spring here rather than a 'normal' spring (TYPE4, TYPE8 or TYPE13)?

You have set a Stif1 stiffness of -10000 (I'm not sure what this would do exactly, stiffness would ordinarily be positive and define the spring preload along with D1, i.e. increasing force with increasing tensile load)

Your plotted curve is velocity magnitude (i.e. it will always be positive), so acceleration isn't occurring the 'other way' at 0.05, (the deceleration looks fairly constant between approx 0.01 and 0.09 passing through 0 velocity, so the cube reverses direction at 0.05, but deceleration doesn't change there) at 0.1 I guess the cube will be more or less back where it started? And the spring looks to be undamped with a constant stiffness.

What type of sensor are you using? Is it being triggered? It sounds like possibly not if changing Stif0 is changing the result. My guess is you have effectively a linear elastic spring with stiffness 100 in your image.

If setting Ilock=1 causes the cube to be 'pulled' to the floor, I am guessing then that your negative stiffness Stif1 is coming into play causing the spring Tensile Force to actually increase as the spring shortens (thereby accelerating the cube rather than decelerating it)

If you can share a little more about what you are trying to represent/achieve, or share the model, then we may be able to specify a better spring set up.

 

 

Thank you very much for your response Paul.

I'm actually modelling a seat which have an energy absorption function. Since it was too complicated to master all the parameters on my seat model, i decided to switch to the cube model to understand how the spring is actually working. The scenario is described as the cube having an initial velocity and will be decelerated using the spring , hence turning kinetic energy into deformation energy. 

However the spring is not allowed to 'spring back'. since Spring Type 32 can be locked at any given moment, it was my preferred choice of spring. Furthermore when the dummy sits on the seat, the spring should not be activated. Hence i thought this could be avoided using the Pretensioner.

 

With Stiffness -10000 i had two ideas actually. Firstly, how does Hyperworks interprete my curve if i want elongation or compression? 

I based myself on this curve actually. Since at elongation = 0, the force = 0 in this model. And at the end of the test i have like the maximum force at max compression. When you plot this graph on paper, it has a negative gradient. Maybe i am wrong there.  But it just did not work with positive stiffness. 

on the curve Velocity/time the gradient of the curve is negative, hence describing a deceleration. 

It was a time sensor starting at t=0 which should have activated the loading part of the spring. 

In the end i went for a spring type 4 which seems to work okay till now. But i need to figure out how to add the pretension force on the spring (Due to weight of dummy) so that it does not activate direct after the dummy sits on it. 

I hope i explained my case well. In case of further question, please do contact me  

Thanks and kind regards,

Umehr

PaulAltair

Umehr Beebeejaun said:

Thank you very much for your response Paul.

I'm actually modelling a seat which have an energy absorption function. Since it was too complicated to master all the parameters on my seat model, i decided to switch to the cube model to understand how the spring is actually working. The scenario is described as the cube having an initial velocity and will be decelerated using the spring , hence turning kinetic energy into deformation energy. 

However the spring is not allowed to 'spring back'. since Spring Type 32 can be locked at any given moment, it was my preferred choice of spring. Furthermore when the dummy sits on the seat, the spring should not be activated. Hence i thought this could be avoided using the Pretensioner.

 

With Stiffness -10000 i had two ideas actually. Firstly, how does Hyperworks interprete my curve if i want elongation or compression? 

I based myself on this curve actually. Since at elongation = 0, the force = 0 in this model. And at the end of the test i have like the maximum force at max compression. When you plot this graph on paper, it has a negative gradient. Maybe i am wrong there.  But it just did not work with positive stiffness. 

on the curve Velocity/time the gradient of the curve is negative, hence describing a deceleration. 

It was a time sensor starting at t=0 which should have activated the loading part of the spring. 

In the end i went for a spring type 4 which seems to work okay till now. But i need to figure out how to add the pretension force on the spring (Due to weight of dummy) so that it does not activate direct after the dummy sits on it. 

I hope i explained my case well. In case of further question, please do contact me  

Thanks and kind regards,

Umehr

Yes, I think maybe TYPE4 is a better choice. The Pretension spring is for a specific use case that I am not sure fits your model here. (When it 'locks' it is switching from Stif1 to Stif0).

 

For Spring TYPE4, for preload (and non linear behaviour) you may define a non-linear function for Force/Displacement, that function does not have to pass through 0,0 so you can use that to start it off with an initial compressive (-ve) or tensile (+ve) force. e.g. {-5,0 ; 0,10 ; 5,20} would describe a spring with a linear stiffness of 2 (10/5), that in its initial condition at t0 would have a tensile force of 10. This would cause the spring to attempt to shorten to reach equilibrium. Similarly a negative force at t0 would cause the spring to try to lengthen. You need to be careful in the spring definition that there is 'enough' spring to account for a tensile preload. e.g. in my tensile example, it needs to shorten by 5mm to reach 0 force (unloaded condition) so if the spring was only 3mm long, this would result in the spring reaching 0 length, this then causes numerical instabilities.

Umehr Beebeejaun

Paul Sharp_21301 said:

Yes, I think maybe TYPE4 is a better choice. The Pretension spring is for a specific use case that I am not sure fits your model here. (When it 'locks' it is switching from Stif1 to Stif0).

 

For Spring TYPE4, for preload (and non linear behaviour) you may define a non-linear function for Force/Displacement, that function does not have to pass through 0,0 so you can use that to start it off with an initial compressive (-ve) or tensile (+ve) force. e.g. {-5,0 ; 0,10 ; 5,20} would describe a spring with a linear stiffness of 2 (10/5), that in its initial condition at t0 would have a tensile force of 10. This would cause the spring to attempt to shorten to reach equilibrium. Similarly a negative force at t0 would cause the spring to try to lengthen. You need to be careful in the spring definition that there is 'enough' spring to account for a tensile preload. e.g. in my tensile example, it needs to shorten by 5mm to reach 0 force (unloaded condition) so if the spring was only 3mm long, this would result in the spring reaching 0 length, this then causes numerical instabilities.

Hi Paul, thanks for the response. I still need help with my spring model. Is it possible to contact you directly? 

 

Thanks,
Umehr

PaulAltair

Umehr Beebeejaun said:

Hi Paul, thanks for the response. I still need help with my spring model. Is it possible to contact you directly? 

 

Thanks,
Umehr

Yes, you should go through support channels though, I'm not in technical support, but if you send an email to your local altair technical support email and mention you have been discussing with me in community forum and that I can help, it will make it back to me! (if they cannot help directly)