I'm getting approximations within 5% of the actual values. Does that sound about right?
Depends on the values, really.
5% of 10Pa is 10 +/- .5 Pa, that's pretty close.
5% of 10MPa is 10MPa plus or minus 500000 Pa. That's a pretty big swing in precision.
I don't see that 5% is too horribly bad though, for estimation purposes.
What do you mean by "actual values"?
Mohr's circle is mathematically "perfectly accurate". Whether your real life situation exactly matches the mathematical assumptions, or whether your phyiscal measurements are accurate to within 5%, are different questions.
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