10.9 grade bolt means ultimate strength = 981 MPa and Yield strength = 883 MPa. The Ramberg–Osgood equation can be used to describe the non linear relationship between stress and strain curve—in materials near their yield points. It is especially useful for metals that harden with plastic deformation.
using E=206010 MPa, uts=981 MPa and YS = 883 MPa use Ramberg-osgood equation to describe the non-linear relationship between stress-strain curve- in material.