Yield Strain
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By oscillation at low frequency with increasing strain, the yield strain can be measured.
Yield Strain
The app illustrates the sorts of behaviours you might find with a strain that varies from 0 to 10% and where G' and G'' are constant up to your chosen yield strain. After that, the standard formula gives a reduction of G' with a power law of 2, and G'' has a power law of 1, so tandδ will increase from its starting value.
The product of G* and the yield strain provides a good approximation for the yield stress, so is yet another way to get at this sometimes elusive value.