Maximum Strain For Samples of Different Cross-Sectional Areas

[person123]

TL;DR

Is the maximum strain of a sample undergoing a tensile test proportional to its cross-sectional area? I think the answer is no, but my data say the answer is yes.

I would assume that because the samples are made of the same material they would fail at the same stress and so the same strain. However, the data shows that the sample with a greater cross-sectional area fails at a greater strain, and the two are roughly proportional. Does anyone know what might be going on there? (For more context, it's two brass samples and they underwent significant plastic deformation).

 

[Chestermiller]

What are the dimensions of the samples?

 

[person123]

The samples have rectangular cross-sections. The widths are .15 in. The thicknesses are .003 and .005 in.

The initial lengths are 4 in.

 

[Chestermiller]

How do the stress-strain curves compare?

 

[person123]

The stress-strain curves appear similar and have similar values of E and tensile strength. The only significant difference is that the flatter region of plastic deformation is about 5/3 as long for the sample with 5/3 the thickness.

 

[Chestermiller]

Hmmm. For the dimensions you reported, it doesn't seem like an aspect ratio effect. Maybe it's a clamping effect? Does the 3 mil sample slip a little in the clamp?

 

[person123]

Because this is from an online lab, I did not get to see the actual testing procedure. The samples do apparently occasionally slip slightly. However, looking at other group data, the maximum strain seems to consistently be greater for the thicker sample, suggesting that it's likely not due to random error.

 

[jrmichler]

If the brass samples are the result of cold rolling, and the thinner sample was cold rolled from the thicker material without any annealing, then the thinner sample would have more total cold work. If so, the thinner sample would fail at higher stress and lower strain.

 

[person123]

If the brass samples are the result of cold rolling, and the thinner sample was cold rolled from the thicker material without any annealing, then the thinner sample would have more total cold work. If so, the thinner sample would fail at higher stress and lower strain.

Thank you! I can't be sure how the samples were produced, but this definitely could be a possible explanation. (It did fail at a higher stress as well, although that change was far less significant).