Experimental Critical Buckling Load

[KevCah]

Hi guys,

I was wondering could I get some help with determining the experimental critical buckling load of a pinned ended strut. I am hoping to compare predicted values from Euler buckling and Perry Robertson to experimental values. I have carried out a experiment where by I applied a axial load to the strut and recorded both the horizontal and vertical displacement. From this data can I apply a formula to give me the critical buckling load? Or is the critical buckling load simply the highest value recorded in the experiment?

Thanks for any help!

 

[OldEngr63]

Assuming that your strut was initially straight (so that it would actually buckle rather than simply bend), then make a plot of load as a function of axial displacement. You should see a pronounced "knee" in the curve. That is the buckling load. It is not the highest load recorded because most post-buckled columns continue to support the load and the load rises slowly.

 

[KevCah]

Thanks for the reply,

There is no clearly defined "knee" in some of my samples. Is there anymore accurate way to determine the "knee" rather then doing it by eye? I am just trying to keep my experimental errors to a minimum.

 

[SteamKing]

Thanks for the reply,

There is no clearly defined "knee" in some of my samples. Is there anymore accurate way to determine the "knee" rather then doing it by eye? I am just trying to keep my experimental errors to a minimum.

View attachment 77667

If you look at the test results for Sample 1, I would say the 'knee' is clearly defined when the test load is approx. 350-360 Newtons. Where the applied load is relatively constant, the test piece undergoes a measurable change in deflection, which suggests that some sort of inelastic behavior is occurring. The other two samples do not appear to exhibit the same behavior.

Of course, a lot of information about the test pieces and the test set up have not been disclosed.

 

[OldEngr63]

What is the material and what is your geometry (length, width, etc. for the test specimens)?
Did your experiments result in a permanent set, or were they fully elastic? (Buckling is an elastic phenomenon; plastic deformation puts an end to all questions of buckling.)

I don't think we can help you very much without this data.