Potential causes of Young's (Elastic) Modulus to be half what is expected for alloy?

[DuncanGibbons]

Hi All,

I recently received tensile test data from a certified testing facility and the young's modulus is almost exactly half of what is expected for this particular metal alloy. I even double-check this with the raw data. Any ideas what could cause this?

Ti64 Grade 23 standard is between 112 to 120 GPa and the results received are giving us 58 to 60 GPa.

 

[Jim Wynne]

Is it half of what's expected or half of what's required? What does the material supplier say?

 

[chris1price]

Check how they measured extension. Did they use a correctly attached extensometer of did they base it on the movement of the cross-head? If the latter, then a significant portion of the extension will go into machine system and not all in the gauge-length, giving you a reduced elastic modulus.

 

[DuncanGibbons]

Is it half of what's expected or half of what's required? What does the material supplier say?

Half of what is expected.

This was for a research project so the material was produced in house, but I would like to make sure I am not doing anything silly before querying the testing facility's procedures.

I have been told that heat treatments won't affect the young's modulus of the alloy either, and our data corroborates this as we have samples there were stress relieved and others that were duplex annealed, all exhibiting a young's modulus of between 58 and 60 GPa.

The fact that all the samples are in the same range and that they are almost exactly half of what is expected for this alloy makes me think there is a simple mistake in the data or calculations. The tensile tests were performed at a strain rate of 0.005 mm/mm/min up until yielding and then at 3.5 mm/min (crosshead speed) until fracture.

 

[DuncanGibbons]

Check how they measured extension. Did they use a correctly attached extensometer of did they base it on the movement of the cross-head? If the latter, then a significant portion of the extension will go into machine system and not all in the gauge-length, giving you a reduced elastic modulus.

Hi Chris, they did have the extensometer attached to the crosshead.

Am I able to calculate these losses to then calculate the true Young's Modulus of the samples?

 

[chris1price]

I would certainly advocate getting the extensometer as close as possible the gauge length.

You may be able to start a calculation, in the case I was working on, I had very long upper and lower extensions either side of the sample to clear heaters, together with multiple screw threads which also extended. I couldn't come up with a correction, but they certainly contributed to the inaccuracy.

One other point, if you are looking at a fully annealed material such as stainless steel, then the modulus will continuously change as it work hardens. This could also lead to getting unexpected answers.

 

[DuncanGibbons]

Thanks for the tips Chris. The material was a Ti64 ELI with quite low oxygen, hydrogen and nitrogen content.
I found it a bit strange that all 48 of my samples has Young's Modulus calculated to almost exactly half of what is expected for the material. Still waiting for the broken samples back from the test lab, so will remeasure them when received.