Attached is a simple code that I wrote a while back to convert a force-deflection/engineering-stress_strain/true_stress_strain curve to effective stress vs effective strain curve. The input is a simple LS-DYNA valid keyword file with *define_curve keyword. Its not an elegant code but works in most cases. A sample curve is also included.
I am working on extending this to cover all materials with a HTML front-end. You should see this in the next release of d3View.
Other related post on a similar topic is here