Back-calculating strain in tooling

[davepenney]

I am currently testing a composite coupon in tension. I am using the crosshead displacement for the extension - I have no means to attach extensometers or strain gauges.

The problem is, I know that a part of the tooling is adding to the strain. If I can work out the elastic modulus of the tooling, is there an equation to deduct its strain from the total extension measured by the tensile testing machine?

Cheers,

Dave

 

[CoryPad]

What you are describing (compliance correction) is done routinely with compression testing. Sometimes you just have the tooling apply forces against each other, sometimes you use a sample with much higher elastic modulus than what you usually test (e.g. WC cermet instead of Al or Fe). Can you connect your tooling to each other? What material are you testing? Can you use a WC specimen?

 

[davepenney]

Yes, compliance correction is what I want to achieve. I am pulling an M4 insert out of a piece of carbon fibre using a length of M4 threaded bar/studding. Unfortunately I haven't any other stronger M4-threaded equipment to pull it with, and I know it's straining.

I just wondered if there was an equation you could apply if you knew the material characteristics of the tooling (or studding in this case).

Cheers,

Dave

 

[CoryPad]

Please don't think I am being difficult, but your entire system is straining, not just the threaded bar. Are you trying to say that the threaded bar is plastically straining?

You could come up with free body diagrams and constitutive equations for each member of the load train and create an equation. Or, you can eliminate (or substitute) the specimen and run a test to measure the combined elastic and plastic strains in the load train, then use those results to create the equation. Both have difficulties.

 

[davepenney]

No worries Corypad, I don't think you're being difficult at all!

I appreciate that the whole system is straining, but the load is about 3Kn, not much, and the threaded bar is the obvious weak point. The other parts of the tooling are massive in comparison.

I have pulled a piece of the threaded bar on it's own. It yielded at around 3.5Kn, so I'm pretty certain it's in an elastic phase.

Can I use those results to create an equation? And if so, what's the equation!?

Thanks for your help so far,

Dave

 

[CoryPad]

If everything else is massive, then you can assume they are infinitely rigid and all of the strain is from your bar. If you loaded the bar by itself, then you should have force vs. displacement. I imagine there is a mostly linear region up to 3 kN. Measure/calculate/estimate the slope m_bar in kN/mm. The following part is best done using a spreadsheet or mathematics software. Have columns for displacement_total, force, displacement_bar, and displacement_sample. The displacement_total and force data are from your test with your sample. Displacement_bar = force/m_bar. Displacement_sample = Displacement_total - displacement_bar. Now you have force vs. displacement_sample, which can you use to calculate stress and strain if necessary using cross-sectional area and grip length.

 

[davepenney]

Thanks CoryPad,

I didn't know if you could just do a straight subtraction of the bar's force/displacement over 3Kn from the total force/displacement of the bar and sample over 3Kn, but that seems to be what your equation is inferring. Sometimes I'm wary over the obvious, simple solutions, but if you're backing it then I'll go with it!

Cheers, take it easy.

Dave