I change the stiffness of the spring, the displacement does not change

Karshenas Reza

hi, 

In my simulation I want to use a 1d spring which does not depend on the stiffness of the matrial. Therefore I have chosen clas2 as the element type and the property PELAS. Now the problem is that if I change the stiffness k of the spring, the displacement does not change and remains the same as before. Can someone help me where I made a mistake? I thank you for any help, advice and tips. 

PaulAltair

Karshenas Reza said:

thanks for the reply. I have changed the element type but it still doesn't seem to work. Can you please check it yourself? I have tried entering different stiffness each time, but the results are the same each time. 

Please consider that the problem is a symmetry model in y-direction, i.e. the top edge should be fixed in y and should not be able to rotate around the z- and x-axis. 

see attached: this 'works' now but I'm not sure it makes much sense in terms of set up

1st issue your 'left spring' property wasn't applied to anything, so changing it doesn't change anything, I know you had assigned it to the left spring component, but that component only contains the left spring, which itself was assigned the right spring property directly (as well as the right spring having that assignment), I assume you want the left spring to have the left spring property, so I removed the direct assignment (so now if you change the properties, you will get different behaviours)

2nd issue is that your springs have lengths of 5 and stiffnesses of 1 and 100, but you are applying a load of 10k, so the softer right spring shortens 10000 (effectively lengthening 9995) and your stiffer one, even if it saw the full load, would shorten by 100.. as a result, your model is not well constrained and turns inside out (because you have set a stress limit of infinity practically it does this). I changed left to 10000 and right to 5000 so it doesn't fly away now. And it doesn't make sense to reduce either of them to below 2000, or they will go past 0 length potentially.

3rd: I'm not sure what you are actually aiming for here!? do you really want to 'maximise' x displacement (make it move 'toward' the load) or do you want to minimise it (move with the load), or do you want to minimise resultant displacement (which would likely end up with no structure to the response entity at all) what is your goal at the end? To maximise the stiffness of the structure for a given amount of material?

PaulAltair

You have created a CELAS2 there? CELAS2 does not reference a property, all the spring parameters are defined directly on the element (card edit the element), if you want to reference a PELAS instead, you should use CELAS1

Also, check the DOF are correct for your application

If you can't work it out and you can share your model here, then someone may be able to give more help.

Karshenas Reza

thanks for the reply. I have changed the element type but it still doesn't seem to work. Can you please check it yourself? I have tried entering different stiffness each time, but the results are the same each time. 

Please consider that the problem is a symmetry model in y-direction, i.e. the top edge should be fixed in y and should not be able to rotate around the z- and x-axis. 

PaulAltair

Karshenas Reza said:

thanks for the reply. I have changed the element type but it still doesn't seem to work. Can you please check it yourself? I have tried entering different stiffness each time, but the results are the same each time. 

Please consider that the problem is a symmetry model in y-direction, i.e. the top edge should be fixed in y and should not be able to rotate around the z- and x-axis. 

see attached: this 'works' now but I'm not sure it makes much sense in terms of set up

1st issue your 'left spring' property wasn't applied to anything, so changing it doesn't change anything, I know you had assigned it to the left spring component, but that component only contains the left spring, which itself was assigned the right spring property directly (as well as the right spring having that assignment), I assume you want the left spring to have the left spring property, so I removed the direct assignment (so now if you change the properties, you will get different behaviours)

2nd issue is that your springs have lengths of 5 and stiffnesses of 1 and 100, but you are applying a load of 10k, so the softer right spring shortens 10000 (effectively lengthening 9995) and your stiffer one, even if it saw the full load, would shorten by 100.. as a result, your model is not well constrained and turns inside out (because you have set a stress limit of infinity practically it does this). I changed left to 10000 and right to 5000 so it doesn't fly away now. And it doesn't make sense to reduce either of them to below 2000, or they will go past 0 length potentially.

3rd: I'm not sure what you are actually aiming for here!? do you really want to 'maximise' x displacement (make it move 'toward' the load) or do you want to minimise it (move with the load), or do you want to minimise resultant displacement (which would likely end up with no structure to the response entity at all) what is your goal at the end? To maximise the stiffness of the structure for a given amount of material?

Karshenas Reza

Hi thanks for help and reply, Ya I got it,