Tangent modulus need yield stress, yield strain, ultimate stress and ultimate strain. ASTM give ultimate stress, yield stress and Modulus elasticity. I can get yield strain value from Modulus Elasticity and yield stress. The problem is how to get ultimate strain. Is Ramberg-Osgood equation can be used to determine the value of ultimate strain ?
You may use power law expression for stress-strain relationship. Set dF=0 for necking and determine the strain at necking. For a simple power law expression s=Ke^n, the strain is simply the exponent n
Hello,
One way to use the Ramberg-Osgood equation is to fit this relation with the data values you have. By this way you will get all the parameters. However, you need more information from the material to get a good model, like the tangent modulus or the ultimate strain to address the plastic zone
Best Regards,
Paulo
Not sure of what analysis your intended to use the Ramberg-Osgood law but it is just an idealization of a give material stress-strain curve and not the other way around. Also, it represents the actual stress-strain curve only approximately, either at low strains or high strains, not both. You need a typical material test stress-strain curve or tensile test data with the percent elongation at temperature.
It sounds like you would like to find a quantity related to the uniform elongation, the true strain at which the material begins to undergo plastic instability (necking). You can simply determine the derivative depsilon/dsigma from the Ramberg-Osgood equation and then invert it. This will be the strain hardening rate, as a function of the flow stress. The Conside're criterion states that the material will begin to neck when the flow stress equals the strain hardening rate. If you plot the flow stress and strain hardening rates vs. strain, you will see that the two intersect at the true stress and true strain condition associated with necking.
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