Maximum Strain For Samples of Different Cross-Sectional Areas

In summary, the conversation discusses the results of testing two brass samples with different cross-sectional areas. It was initially assumed that the samples would fail at the same stress and strain since they were made of the same material. However, the data showed that the sample with a greater cross-sectional area failed at a greater strain. The two stress-strain curves were similar, except for a slightly longer plastic deformation region in the thicker sample. The group then speculates about possible explanations, including a clamping effect or differences in cold rolling and annealing. It is noted that the thinner sample may have undergone more cold work, leading to a higher failure stress and lower strain.
  • #1
person123
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TL;DR Summary
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).
 
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  • #2
What are the dimensions of the samples?
 
  • #3
The samples have rectangular cross-sections. The widths are .15 in. The thicknesses are .003 and .005 in.

The initial lengths are 4 in.
 
  • #4
How do the stress-strain curves compare?
 
  • #5
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.
 
  • #6
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?
 
  • #7
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.
 
  • #8
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.
 
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  • #9
jrmichler said:
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).
 

1. What is maximum strain?

Maximum strain refers to the maximum amount of deformation or change in shape that a material can undergo before it breaks or fails.

2. How is maximum strain calculated?

Maximum strain is calculated by dividing the change in length of a material by its original length. This can be expressed as a percentage or a decimal value.

3. How does cross-sectional area affect maximum strain?

The cross-sectional area of a material plays a significant role in determining its maximum strain. Materials with larger cross-sectional areas have a higher maximum strain as they have more space to accommodate deformation.

4. Can different materials have the same maximum strain?

Yes, it is possible for different materials to have the same maximum strain. This is because the maximum strain is dependent on the material's properties, such as its elasticity and strength, rather than its type.

5. How does maximum strain affect the strength of a material?

Maximum strain is directly related to the strength of a material. A material with a higher maximum strain can withstand more deformation before breaking, making it stronger than a material with a lower maximum strain.

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