Homework 7 Problem 4
Monday, April 11th, 2016
For the cases e=0 and e=1 mm:
4a) Calculate the bending moment at the cross-section AA.
4b) Determine the neutral axis (should be easy for this case).
4c) Calculate the peak stresses in the trabecular and cortical regions.
4d) Determine the fracture risk.
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Any suggestions on how to calculate the neutral axis? I feel like I am over complicating it since the problem statement says it should be easy…
I think you need the density of the respective bones to calculate the neutral axis.
Are we supposed to calculate a numerical answer for the bending moment and peak stress, or provide an answer in parameter variables?
I have this question as well. I’m thinking we need to assume some values in order to calculate fracture risk.
Is there a score or scale for fracture risk?
Does anyone know a good source for the values?
I’m assuming in order to find the peak stresses we need to do something similar to what was covered at the end of the April 4th lecture.
The equation for how to superimpose bending and axial loads looks like it fits this situation well. He used in during that lecture with the example of a spine with rods on it.
Does anyone understand what each of the variables are in the image? Specifically, what Dt and Dc are?
I believe it is supposed to represent the diameters.
Yes, I would assume those are the diameters for the trabecular and cortical
I am working the problems in variables only i didn’t apply values to the variables. I think there are too many variables so any answer we would get wouldn’t be similar depending on all of the values each person selects.
I agree. This has been a common theme in this homework 7. We either need to make assumptions for certain values or leave variables in the answer
I think the reason it says “should be easy for this case in 4 first part is because the neutral axis is at the geometric center of the section A-A. For part II we have to perform the area integrals
How do you calculate the neutral axis of a composite material?
Best way to calculate the neutral axis for this is to evaluate a pair of offset concentric circles.
(not sure why the time stamp for posts is 4 hours off?)
You can use 0 = E1A1y1 + E2A2y2 to solve for the neutral axis when e=1mm!
So would we plug in the value for one of the y’s (use an assumed value for radius minus 1mm) and then solve for the other y?
Does anyone have the E values for the two types of bone?
I believe this has the E’s : http://wings.buffalo.edu/eng/mae/courses/417-517/Orthopaedic%20Biomechanics/Lecture%201%20support%20materials/Orthbonemech.htm
is the paper available. I don’t see it on the HMK main page?
Where should the “joint contact force” be applied that “occurs at some angle q”?
When we look at finding the moment around point O what forces did you include? I was thinking J and F (not sure about HOW for F) but was confused about what the A’s represented (it seems like they are the “equilibrating loads”)??