Some codes of practice (i.e. BS5400, Eurocode 2) present some equations describing stress strain relation relation for either steel or concrete. One can also obtain them from experimental or any other discreet data by using "genfit" or "pwfit" functions. Check Help for these functions.

Success

I have used the Ramberg-Osgood stress
strain model.  It only requires the root
finder of MathCAD when looking for the
stress at a given strain because the
stress is implicitly expressed in terms
of a known or assumed strain.

Best regards,

The MatheMagician

I'm in the middle of doing the same thing myself.  I broke the problem down into the 3 pieces:  the elastic part is an easy linear relationship, and the tensile and compressive plastic deformation zones are curve-fit using "linfit".
I use an "if" statement to put the 3 parts back together, and now have a continuous function from UTS to UCS that is expressed as:

stress(strain)=_*ksi

so I can type:

stress(0.001*in/in)=20*ksi

Generate the chart with a series of strains in the x-axis, and the function solved for the strains in the y-axis.

Have fun!

STF

Give an Email and I will send you a mathcad file for calculating MU in accordance with BS5400.
ZMEI

Zliazmei, my e-mail is meskridge@dextragroup.com, thanks for your help.

Cheers Big Bars