# Calculating Young's modulus from an experiment - values are all inaccurate?Watch

#1
As part of a practical experiment I used a tensometer to record the fracture force of several materials (steel, aluminium, polypropylene)

I measured the thickness, width, and length of each sample before testing it and used this to calculate the area. From this I calculated the stress and I also calculated strain by measuring the new "length" after fracture and by doing extension/length.

However, now that I have calculated a value for Young's modulus for each material, I notice that they're all rather inaccurate. For example, for steel I got a value of something like 1.3x10^9, however the value for steel is apparently usually 2x10^11 (according to https://www.engineeringtoolbox.com/y...nsile_strength)

Is this because Young's modulus is only linear up to a certain point (limit of proportionality), and I got inaccurate values because I'm measuring at the fracture point? If so is there a way around this?

Thanks!
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1 year ago
#2
I believe you're correct, The stress-strain curve for mild steel is shown below, if you're measuring the extension at the fracture point then you're essentially treating the curve as a straight line from the origin to point of fracture, which has a lower gradient than the straight line portion of the curve, hence lower Young's modulus value. Presumably you can work around this by recording the length at discrete load values?
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