# Linking the relationship between a body with another

Member Posts: 3

I'm simulating a child car seat impact test. I created the cube to indicate the baby. How can I create a link between the cube and the seat so that it will receive the same impact force as I don't have baby model. Thank.

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• Posts: 11,372Member
edited March 2019

Baby seats for infants face backward so that in a frontal impact of the car, the baby's back is pressed into the seat back, which is facing forward. This would be the easiest model to simulate, but you are going to need a baby model.

Baby seats for larger children face forward, and in that case there are straps and sometimes chest plates that the baby's front is going to press into. You are going to have to model all the straps/plates and the baby.

• Posts: 31Member
edited March 2019
I will soon done my rear and isofix base model but about the baby model, what's level of detail I should model on the infant? Because I'm thinking how to attach with the chair and also inserting the mass.
Peter, the way I simulate the impact test is right?? Frontal impact is about 50km/hr. I put my initial velocity on the 2 baby and the child seat bang a wall.. I read some sled tests where they using a rigid seat, but I couldn't figure out how to set up it nicely. Hope you can help me on this
• Posts: 11,372Member
edited March 2019

I don't believe an impact between a seat and a wall is the best way to model this. A car seat is strapped to the car frame. The car has crumple zones that dissipate the energy of the crash over a long time period and lower the peak acceleration experienced by the occupants of the car.

Look for some literature on the acceleration-time history on the frame of the car, after the crumple zone has reduced the peak acceleration. Apply an acceleration-time profile to the model. You don't have to do this in Explicit Dynamics. It may be easier to do this in Transient Structural.

• Posts: 31Member
edited March 2019
You mean I can apply acceleration-time profile in transient structural ansys Workbench? I have no experience using trasient analysis what's the major different between these? I want to study about the difference between rear and forward facing child car seat in a frontal impact crash, I have read some journals that their simulation is without a car frame. I wondering how they set up
• Posts: 11,372Member
edited March 2019

Yes, you can apply an acceleration-time load in Transient Structural, where you can also define contact between bodies. Transient structural is easier to get solutions from compared with Explicit Dynamics.  What do the journal articles use for the impact load?

• Posts: 31Member
edited March 2019
• Posts: 31Member
edited March 2019
Page 27 of the journal have the profile of an frontal crash impact. I also want insert joint at the isofix hinge to get the reaction force. Basically I still need to model the baby?
• Posts: 11,372Member
edited March 2019

You can digitize the coordinates from the graph on page 27 and use those as an acceleration load in a Transient Structural model, along with the 30 mph initial velocity.

I don't know what the isofix hinge is, please show an image. You can get a reaction force through any joint in your model or any fixed support boundary condition.

Yes, you still need to model the baby.

• Posts: 31Member
edited March 2019

Below image is the hinge point of a isofix base. What kind of joint function i should use as it 's purpose is to fix with car back seat in real. Am I going to applied on both child car seat, isofix base and the baby model?

• Posts: 11,372Member
edited March 2019

You could use two spherical joints to ground, one for each end. That will leave one last degree of freedom, the ability to rotate around those two end points. It looks like there is a third leg. Does that rest on the seat?  You could apply a Remote Displacement and set all components to Free except the vertical displacement (whichever axis that is, Y-axis perhaps?).

• Posts: 31Member
edited March 2019

Answer toward the third leg which called as isofix leg. It used to put on the ground of the back seat as you said, rest on the seat . Applying remote displacement? Doesn't it will make the body move if I input any number on (Y-axis)?

• Posts: 11,372Member
edited March 2019

The number you put on the Y displacement is zero. That will hold it fixed where it is.

• Posts: 31Member
edited March 2019

I get your point of view, setting zero and other is free, so that y axis don't have displacement.

i got the files below , I still need the baby and the straps? since this is front facing

• Posts: 11,372Member
edited March 2019

Yes, you will always need the baby. You can make a prolate sphereoid shaped blob that fits nicely into the seat back. Make the Center of Gravity of that blob the correct height as a baby and adjust the density so the mass is equal to the baby. This is very easy to create in SpaceClaim. Start with a sphere and use the Scale tool to elongate the sphere. You need to have the blob in contact with the seat back.

You might not need the straps when the child seat back is at the front so the baby would be pushed into the child seat back during the acceleration of a frontal car impact, unless the baby slides up the seat back and falls out the top of the seat during the impact, then you will need straps. Or you could use bonded contact to glue the baby to the seat back if you don't want to model the straps. This will transfer the load to the seat back and allow you to see the high stress area somewhere else in the model.

• Posts: 31Member
edited March 2019
I'm using Maya for modeling. Seat belt will be modeled soon. I wonder if the seat is attached with the seat cushion like in the photo above, what Boundary condition should I set up? also about the neck of the baby? This morning I tried meshing, I have 20mils of nodes and 15mils of elements. It takes alot of time, any way to reduce it? By increase mesh size? I used default back then
• Posts: 31Member
edited March 2019

I have model it. When i try to fixed the support I can't use body as i circle in the image, any solution for that as if I just pick faces, it could not be highlight the whole thing

• Posts: 31Member
edited March 2019

Try to upload my .wbpj file but failed, I don't know why. It's just 40kb.

• Posts: 11,372Member
edited March 2019

The *.wbpj file is useless without the *_files folder of the same name. That is why this site does not let you attach the .wbpj file.

Use File > Archive to create a .wbpz file that combines those two things into a single file, which can be attached if the file size is < 120 MB.

• Posts: 31Member
edited March 2019

oh ya my bad oops hahaha

• Posts: 11,372Member
edited March 2019

I opened your IGES file. Below is one solid.

This should be two separate solids. One is the thin-walled hard plastic shell that is the seat form, the other is the cushion, which is a soft foam. You need two solids to apply to different materials.