Mechanical Engineering

# Stronger or Stiffer ?

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I have some serious confusion regarding the title for the following issue .

I have read on plethora of forums and sites, giving the explanation of why hollow tubing is stronger than solid rod of the same dia or weight by justifying as to why it bends the least .

This makes me ponder ! Stronger or stiffer ? Isn’t that least bending property under a given load ( of hollow tubing ) attributed to stiffness ?

I would also like to clear if these conclusions of mine are right ?

1.       Of the same dia – Hollow tubing is stiffer but Solid rod is stronger (owing to the more material present in it )

2.       Of the same weight – Hollow tubing is stiffer but both have the same strength (owing to the same amount of material in it  - assuming the yield strength is independent of the geometry )

Would be very helpful if someone could kindly throw some light on the same !

Thanks

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Good Day, rameshsarana...

It is important to keep in mind that section modulus, Z, is a section property used in determining the unit stress on the outside fibers, sigma = M/Z, where M is the applied moment. The sections Moment of Inertia, I, is used when determining the sections deflection under a particular load. The section modulus, Z = I/r, where r = the distance to the extreme fiber of the section.

Gerald

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Considering the section modulus makes me conclude that the hollow one would be stronger in both the cases - same dia and same weight ! That's quite fascinating.

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Also remember it will be stronger in bending but not in tension.

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When considering any loading condition, bending moment, axial loading, and torsional loading, all contribute to the stress analysis of the particular section. It is important to do your best to identify those loads before you begin your analysis.

This is a sample of some analysis I did many years ago. It shows the sections of this cast steel section that were important to the analysis.

Please take a look at this link: https://fought-engineering-consulting.com/2012/05/05/strain-gages-and-the-mechanical-engineer/

It discusses strain gage work, and determining what the expected stresses will be under load.

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