7 Replies Latest reply on Jan 8, 2015 7:09 PM by Jared Conway

Simulating g forces on impact on a helmet

Ankit Parikh Jan 5, 2015 11:26 AM

I have an assembly of a helmet with two polycarbonate shells separated by rubber spacers. I want to measure the 'g's' inside the helmet under impact forces on the outer shell. However, both the shells need to be free to move in that case. I can't run a simulation without a fixed BC, so is there any other way I can do this ? I am applying linear and torsional forces on the outer shell, and all I can do right now is observe the stresses on the two shells and the rubber spacers (nonlinear static). Since the inner shell is fixed, the 'g' value is zero in this case.

Re: Simulating g forces on impact on a helmet

Jared Conway Jan 6, 2015 6:42 PM (in response to Ankit Parikh)

how are you measuring the "g's" in a nonlinear static analysis?

how would you test this in the "real world"?
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- Re: Simulating g forces on impact on a helmet
  
  Ankit Parikh Jan 7, 2015 10:05 AM (in response to Jared Conway)
  
  Right now I'm just calculating the difference in stresses between the two shells, which shows how much energy is being absorbed by the spacers. In the real world, the helmet would be mounted on a dummy head and dropped on an anvil from a specific height. An accelerometer inside the head measures the g's experienced.
  
  I guess this means I have to do a drop test. I tried it but the simulation keeps solving at 0%. No errors or warnings, the convergence plot is blank.
  
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Re: Simulating g forces on impact on a helmet

Michael Kalin Jan 7, 2015 1:22 PM (in response to Ankit Parikh)

You will have to fix it in some way. Consider modeling a simple 'head' for it to rest on with a neck portion, which is fixed at the bottom but made of a durable material the allows for displacements. Also keep in mind that to do accelerations, vibrations, and shock, linear dynamic analysis is required. If you know the force and time of impact, a simple modal time history with force curve, like this one, will be the most proper way to simulate it.
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- Re: Simulating g forces on impact on a helmet
  
  Ankit Parikh Jan 7, 2015 2:17 PM (in response to Michael Kalin)
  
  Would something close to the properties of Aluminum work ? A hollow head with a neck, about 6 mm thick ? I thought about replicating the crash test dummy heads, but the head and neck is an assembly of various plastic and metal parts. Using that would put a huge load on the solver and CPU.
  
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  - Re: Simulating g forces on impact on a helmet
    
    Michael Kalin Jan 7, 2015 2:23 PM (in response to Ankit Parikh)
    
    A simple body for the head is all that is needed, and aluminum would suffice. Again you need to use a linear dynamic and not the drop test as you are testing for an object hitting another, not an object hitting the floor. In linear dynamic there are more material options available so you can use your rubber parts.
    
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    - Re: Simulating g forces on impact on a helmet
      
      Ankit Parikh Jan 7, 2015 2:44 PM (in response to Michael Kalin)
      
      Got it. I'll try this out. Thanks Michael !
      
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  - Re: Simulating g forces on impact on a helmet
    
    Jared Conway Jan 8, 2015 7:09 PM (in response to Ankit Parikh)
    
    you could remove the dummy head and just fix the inside faces. basically a worst case scenario.
    
    and drop test could work as well but it does have a long compute time.
    
    linear dynamic, time history is your best bet to get started with a shock test.
    
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