Centroid vs Neutral Axis

[P1ENG]

I'm going to ask a question that may make you wonder how I got my SE.

Is the centroid synonymous with the neutral axis, which is not (might not) be the same as the plastic neutral axis? Every time I hear neutral axis, I think plastic neutral axis. However, I am in the Aluminum Design Manual looking at illustrative examples, and they are using the centroid dimensions when the term is defined using "neutral axis". So now I think there are (2) terms: Neutral Axis and Plastic Neutral Axis. Unfortunately, I have a non-symmetric part, so it matters.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[MER3]

The plastic neutral axis will be set at wherever the area is perfectly bisected, not necessarily true for the elastic neutral axis (I suspect your book is calling the E.N.A. the centroid). The sum of forces on the cross section must always equal zero. The location of neutral axis (elastic or plastic) will always be set such that the sum of the forces above and below add up to zero.

This is for bending only. The same rules apply for axial loading but additional tweaking is needed.

 

[CANPRO]

I'm going to ask a question that may make you wonder how I got my SE

I wouldn't sweat it, I think we all have to break out some old text books to review a fundamental concept or two now and again. I had to the same recently for aluminum design as well.

I think jtb587 answered your question, I'll just expand on it a bit:

Your centroidal axis occurs where your cross-sectional area is balanced above and below the line.

The neutral axis occurs where the sum of the forces acting on the cross-section are balanced, based on a linear varying strain - zero at the neutral axis, and at its maximum at the extremities of the section. If your cross-section is symmetrical, the strains at the extreme ends are equal, and your neutral axis is located at the centroid.

The plastic neutral axis is based on the entire cross section reaching its yield value, in which case the strain value is no longer relevant. In this case since the entire cross section is stressed equally, the plastic neutral axis will occur at the centroid.

 

[P1ENG]

Maybe I should chalk it up to Monday. I guess I just haven't looked at my section properties calculation in a while. I am doing things correctly, I just had a nomenclature problem. I was calling the "neutral axis" the "centroid" and defining the "plastic neutral axis" as its own thing; but using them correctly .

Had to change the highlighted values below.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant