Tangent Modulus

[snoop02]

Hey Guys,
Let me know if I've got this correct. When performing and FEA I can do a non-linear analysis to determine the stress after passing the elastic limit of the material. This should tell me if the material will catastrophically fail. I've read the tangent modulus is simply the slope of the stress-strain curve after the elastic limit. So, if my material has a yield strength of 29,700 psi, u=70,300 psi, elongation of 35% and Young's modulus of 2.76 x 10^7 then my tangent modulus is equal to 116,330 psi. Correct?

Thanks

 

[Compositepro]

The tangent modulus is the slope of the the stress-strain curve an any given point. After yield the slope of the stress strain curve falls drastically. In the plastic region the slope may be zero.

 

[davidmoul]

In fact the tangent modulus is E(epsilon). It changes for every value of epsilon. If you want to do a significant simulation, you need to get the practical tensile curves with the right parameters of your material.

 

[Wrenchbender]

Ditto above on what tangent modulus is. I don’t know much about FEA but I believe searching these key words may help in the definition you are seeking: Considere’s criterion, Levy-Mises, Holloman equation, strain hardening exponent.

In the plastic range, the work hardening ability of the material has to be characterized. The Holloman Equation (and others similar) is an empirical fit to the shape of the stress-strain curve, which is determined experimentally. With any luck you won’t have to break any tensile bars, but can look up the required parameters ‘K’ and ‘n’, then somehow plug them into the FEA software.

Also beware of the differences in ‘engineering’ vs ‘true’ stress and strain. Mechanical engineers prefer the former for practicality; the latter is more useful to materials people or for those doing the math. Note the slope of the 'true' curve never decreases; never goes to zero.