Second Moment of Area

[Anna]

Dear all,

I'm trying to get the second moment of area (SMA) so that I can calculate the stress from bending tests on rat tibiae. I run the test using Slice Geometry. I also use "Orientation" to determine my principal axis based on my experimental work-bending test. I've got the results: Imax, Imin, ICrCa and IML. I only interested in the I where the load was applied (compression&tension), which results should I take Imax or ICrCa?. 

Further, I want to calculate the stress. I measure the distance,y (distance from the neutral axis to the periosteal) for each slice of images-take the average. Then, I calculate the stress (FLy/4I).Am I do run the algorithm correctly?

Greatly appreciate any help!

Ann

[Michael Doube]

Dear Ann,

> I also use "Orientation" to determine my principal axis based on my experimental work-bending test.

Great! That's exactly what Orientation is meant to help with.

>I only interested in the I where the load was applied (compression&tension), which results should I take Imax or ICrCa?.

ICrCa (or IML): you want the I calculated around the axis that represents your neutral plane of bending, which is what you set with Orientation. Imax and Imin are calculated from the cross-sectional geometry alone, so they can give an estimate in the common case where you don't know anything about loading conditions, neutral axes, etc.

Would it help if the crosshairs in Orientation could be moved to a specific location, so that you could set the centroid? I ask because the neutral plane of bending does not necessarily cross the geometric centroid of the cross section. At the moment I is always calculated from axes that cross at the centroid. (I would also note that in life the neutral plane of bending is highly unlikely to be static and more likely varies as a function of gait kinematics: ground reaction forces, muscle contraction dynamics, etc., etc.)

> Further, I want to calculate the stress. I measure the distance,y (distance from the neutral axis to the periosteal) for each slice of images-take the average. Then, I calculate the stress (FLy/4I).Am I do run the algorithm correctly?

Slice Geometry returns the maximum chord distance (R) for precisely this analysis, and to calculate section modulus (Z). So, you could use your equation for every slice in the image and get a slice-specific result. There might be interesting variation along the length of your tibiae.

Hope it helps,

Michael

[Shafini Mohamad]

Dear Michael,

Thanks for your help! 

best regards

Ann

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