Poisson ratio appears to be changing based on application/relaxation of load?
Poisson ratio appears to be changing based on application/relaxation of load?
(OP)
I was wondering if anyone could explain this behavior to me. The setup was I had a custom made fixture which was more or less a 3.75" high by 3" dia 6061 aluminum block put onto our compression testing machine here at work. I loaded it up to 14000lbs, and then unloaded it. Initially the ramp rate of the stand was rather low I want to say it was .05"/min going up and going down.
I had always thought that the poisons ratio would have governed the fact that given a particular force one would expect a particular deflection. Thus I would expect to see a single line.
However here’s a view of my initial run.

So thinking of it a little more the only thing that I could think of that may explain such behavior was perhaps the rate of load application would have some affect here. Perhaps it took some time for the molecules to recover or something. But as one can see on the plots of different rates of application that wasn’t the case.

So I was wondering does anyone know the cause for such behavior? I can’t be the first to have come across this.
I am pretty sure the machine itself is telling me the truth but you never know.
I had always thought that the poisons ratio would have governed the fact that given a particular force one would expect a particular deflection. Thus I would expect to see a single line.
However here’s a view of my initial run.
So thinking of it a little more the only thing that I could think of that may explain such behavior was perhaps the rate of load application would have some affect here. Perhaps it took some time for the molecules to recover or something. But as one can see on the plots of different rates of application that wasn’t the case.
So I was wondering does anyone know the cause for such behavior? I can’t be the first to have come across this.
I am pretty sure the machine itself is telling me the truth but you never know.





RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
I have done this with strain gages on four sides of the block under testing. If they don't match you have issues.
If you want this to be highly accurate you will need to first load to 20%, then unload to 5%, and then reload and measure. The play in the system will be a big factor if you actually relax the load.
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Plymouth Tube
RE: Poisson ratio appears to be changing based on application/relaxation of load?
In the second case poisons ratio would come in to play.
Either way, until you have cycled the load a few times and gotten the exact same curves you don't have reliable data.
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Plymouth Tube
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Poisson ratio appears to be changing based on application/relaxation of load?
Does the loop stay the same?
How are you measuring the displacement?
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Plymouth Tube
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
http://files.engineering.com/getfile.aspx?folder=b...
The way the fixture is actually loaded gives me a contact pressure of 6900psi, so I am still quite a bit below yield.
Strain is being measured right now solely by cross head travel by the machine, I would have to do more research on the machine as to how it is actually measuring this but I would suspect they have a glass scale much like a Digital Readout on a machine tool? Friction was one of my first thoughts too as to what may cause this behavior however I would have thought the effects of friction would be more along the lines of slowing the movement of the material, and thus I would have expected a far greater dependence on crosshead speed to the curve shape than I see.
Ed, I haven't tried cycling the load yet with out actually taking it back to zero, what would cycling it back and forth without going to zero accomplish and what load should I take it down to?
RE: Poisson ratio appears to be changing based on application/relaxation of load?
RE: Poisson ratio appears to be changing based on application/relaxation of load?