Why Does My Stress vs Strain Graph Look Unusual?

[zooted]

Homework Statement

My graph doesn't look like a typical stress vs strain plot.

[PLAIN]http://dl.dropbox.com/u/2344149/graph.png
* y-intercept set to zero and last 4 data points were ignored

Wire diameter= 1.16 mm
L₀= 100.2 cm
$$\overline{}L$$'₀= 0.015
Young's modulus for steel (actual)= 1.92E11

[PLAIN]http://dl.dropbox.com/u/2344149/data.png

Homework Equations

Elongation
$$\Delta$$L_i=$$\overline{}L$$_i-$$\overline{}L$$'₀
Strain
$$\Delta$$L_i/L₀ (converted L₀ into mm)
Stress
mg/pi*r² (radius converted from mm to m and each load is 1 kg)

The Attempt at a Solution

Strain values are very small and stress are very large which is supposed to be correct. The percent error is over 70% An error made during the lab was that the initial length of the wire was measured incorrectly and the mistake was not noticed until the wire had already been stretched. So the we used the final length for our calculations, which would be correct as long as the elastic limit was not exceeded. If it had been permanently deformed I don't think that it would produce such a large error, would it? We also measured the diameter of the wire at the beginning, not after the 5th load like the manual recommends.

Thanks for reading.

 

[anathema]

Thank you for sharing your concerns about your stress vs strain plot. Based on the information and data you provided, I can offer the following suggestions:

1. It is possible that the incorrect initial length measurement played a significant role in the large error percentage. Even small changes in the initial length can greatly affect the strain values, which in turn can affect the calculated stress values. It would be helpful to double check your initial length measurement and see if there were any other sources of error in this measurement.

2. Another possible source of error could be the measurement of the wire diameter. As you mentioned, it was measured at the beginning rather than after the 5th load as recommended in the manual. This could have affected the calculated stress values, especially since the diameter of the wire decreased as it was stretched. It would be helpful to re-measure the diameter at several points along the wire after it has been loaded to see if there were any changes.

3. It is also possible that the elastic limit of the wire was exceeded during the experiment. This could have resulted in permanent deformation of the wire, which could explain the large error percentage. It would be helpful to carefully observe the wire during the experiment and see if there were any signs of permanent deformation.

In conclusion, it is important to double check your initial length and wire diameter measurements, as well as carefully observe the wire during the experiment to ensure that the elastic limit is not exceeded. These steps can help to improve the accuracy of your data and ultimately lead to a more accurate stress vs strain plot. Good luck with your experiment!