# Linking the relationship between a body with another

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tungfw posted this 18 March 2019

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.

peteroznewman posted this 18 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.

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tungfw posted this 19 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

peteroznewman posted this 19 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.

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tungfw posted this 19 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 BTW thanks for your fast reply.

peteroznewman posted this 19 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?

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tungfw posted this 19 March 2019
tungfw posted this 19 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?

peteroznewman posted this 19 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.

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tungfw posted this 19 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?

peteroznewman posted this 19 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?).

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tungfw posted this 19 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)?

peteroznewman posted this 5 weeks ago

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

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tungfw posted this 5 weeks ago

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

peteroznewman posted this 5 weeks ago

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.

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tungfw posted this 5 weeks ago

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

tungfw posted this 4 weeks ago

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

tungfw posted this 4 weeks ago

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

peteroznewman posted this 4 weeks ago

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.

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tungfw posted this 4 weeks ago

oh ya my bad oops hahaha

peteroznewman posted this 4 weeks ago

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.

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tungfw posted this 4 weeks ago

OK will modify it soon, but I need try whether this model can be used, let's assume both are cushion material. When I just want to try mesh the baby only it already took me about 10min for a default meshing. My laptop has 4 core and 920 Nvidia, I scare it may took a long time and the simulation output is error😅

peteroznewman posted this 4 weeks ago

Maya creates too many long thin triangular faces which makes the meshing slow down. Do some SpaceClaim tutorials on this site and learn SC.  Either build the car seat and baby from scratch in SC or learn how to replace Maya's collection of faces that represent one smooth face with one smooth face in SC.

Many of the parts of a car seat are thin-walled parts. For those parts, you want a surface, not a solid body. For example, below is the back face (approx) of the chair part which could represent the midsurface of a thin-walled part that is the hard-plastic molded shape of the chair. You can easily mesh this in mechanical and assign the thickness to that surface.

Here I have used the Merge Faces tool on the Repair tab of SC to combine a row of triangles into a single face.

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tungfw posted this 4 weeks ago

Understood. Merge faces tools is great! Gonna merge the faces. Hi Peter, how much mesh sizing you use to mesh? The above seat back you create on your own? I will look on it. Actually I'm bad at using either solid work or space claim to do surfacing, that's why I'm using Maya 😅which I still OK with. Tonight gonna upload an archive file here. Brb

peteroznewman posted this 4 weeks ago

The seat back above was made by copying those faces off the back of your model.

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tungfw posted this 4 weeks ago

Hi Peter,

I have modify the faces and the back seat. I'm currently gathering the properties of the all bodies (material etc.), I've success done the meshing and it's about 130k elements. What I gonna do next is applying material properties, applying profiles and boundary condition.

*I have uploaded my current progress

Attached Files

tungfw posted this 4 weeks ago

Hi Peter, I don't know why can't add post through that so I screenshot.

peteroznewman posted this 4 weeks ago

Looks good.  This site interprets the text typed in the post as code so it can render special characters, but if some code is found and it's not valid, then you can't post.

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tungfw posted this 4 weeks ago

How can I know estimated time finished like explicit does? I have already run for 2 hours and looks like not even 5% overall process. I'm using time step and I see most of the tutorial used substeps, will it decrease the simulation time? Just now I encountered a error which sounds ' a random element has highly distorted, offecing bla bla bla, I changed mesh size and BC and its gone, I don't know am I doing it right haha

peteroznewman posted this 4 weeks ago