change in stress location

[Lucy A]

A spine is instrumented with rods and screws and then simulated for a particular application. Why does the location of stress on rods change when varying the modulus of elasticity of the intervertebral disc of the spine?

 

[IceBreakerSours]

You need to gain some mechanics intuition.

Imagine a 2D x-section of an FSU with the bottom vertebral body fixed and a load applied at the top. Instead of a 3D disc, think of the role of the disc being just a uniaxial spring. Now, extend this abstraction, think what will happen if the spring were stiff vs compliant, and come up with a hypothesis. You could even make a super simple FE model and test your hypothesis in that setting.

*********************************************************
Are you new to this forum? If so, please read these FAQs:

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[Lucy A]

Thank you. The reason I was thinking is the change in center of rotation causing to change the stress locations.

 

[IceBreakerSours]

Two comments about your statements so far:

a) You are using your intuition to guess what might be happening. While intuition can be powerful at times, often times it can lead to catastrophic failure. The only way to be sure is to hypothesize, test, and learn (as I suggested above). Minimize complexity (simple geometry/small number of elements/simple materials/simple loading/BC/..), make mistakes fast to learn fast. This will be the empirical approach to learning (experimental computational mechanics!). Alternatively, and perhaps more valuable in the long run, you could grab a good book on structural mechanics and take the deductive approach to learning.

b) As soon as an object accelerates or has a load applied to it .., a stress field arises everywhere. What you are probably thinking of is the default stress field output (von Mises stress) and I imagine the location of the max/min of this stress field is changing.

*********************************************************
Are you new to this forum? If so, please read these FAQs:

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[Lucy A]

Yes you are true. And sorry that I was not quite clear. I was talking about the maximum von Mises stress on rods and change in its location after varying the modulus of elasticity of the disc with all other parameters remaining the same. Thank you for your suggestion.