Poisson's Ratio 1

[RvL04]

Hi!

I'm pretty much new to metals. We had two materials that we had tested in a quasi-static tensile test. The axial strain results look fine, but the transverse strain results, I'm having some trouble interpreting.

The first material decreased in thickness by 11% (before the test vs. after the test). Its width also decreased by 11%.

The second material saw it's thickness decrease by about 10%, but its width was reduced by 22%.

Transverse strain during the test was only measured in the 'width' direction, based on the assumption that the change in the width and thickness direction is parallel.

Is this assumption incorrect? Have any of you metals folks seen this, can you explain it to me?

By the way, the materials are both some kind of steel (don't know anything more than that).

Thanks very much!

Ron

 

[metengr]

Ron;
What is a quasi-static tensile test?
There is an ASTM Standard for performing tensile testing of materials (ASTM E8). Using this standard would provide for a more meaningful comparison of mechanical properties between materials.

http://www.astm.org/cgi-bin/SoftCart.exe/index.shtml?L+mystore+gyhm8818

 

[redpicker]

metengr has a point in that the a common tensile test standard (ASTM E8) does not describe a quasi-static tensile test. Without having a standard method of conducting the test, it is difficult for others to answer your questions since we really don't know what you are doing.

However, in a general sense, it seems you have discovered ansitropic behavior in your samples. In general, you would expect the width and thickness of the specimen to be reduced by the same porportion (although geometric constraints can affect this). Because the geometry of the specimen is important, I have to wonder if both your test specimens had the same dimensions. If they do, there are other factors that can contribute to what you have observed

1) Cast or wrought?
While cast materials are often considered to have a uniaxial structure, directional solidification patterns are often encountered (either by accident or design). Wrought materials have a definate directionality in their structure.

2) Heat treatment
Knowing that the material is "some kind of steel" does not help much. Differences in hardness (as well as ductility, yield strength, etc) across the cross-section (or along the length) of the specimen can also cause the behavior you have noticed.

There are certianly other factors (such as chemical composition) that I'm not addressing (mostly because I can't think of them...), but geometry, hot work, and heat treatment are the biggest ones.

RP

 

[Maui]

Another important factor is transverse versus longitudinal orientation. Were these samples cut from a plate, or were they sectioned from a forged bar or a rolled bar? When they were sectioned from your stock material, was any attention paid to the manner in which they were sectioned, i.e. longitudinal versus transverse orientations? This is information that you should have obtained prior to the execution of your tests, and could have a measurable effect on the strain measurements that you're making.

Maui