Determine the elongation of the square hollow bar when it is subjected to the axial force P - 3-9

3–9. Determine the elongation of the square hollow bar when it is subjected to the axial force P=100 kN. If this force is increased to P=360 kN and released, find the permanent elongation of the bar. The bar is made of a metal alloy having a stress–strain diagram which can be approximated as shown.

All problems from Mechanics of Materials Eleventh Edition by Russell C. Hibbeler.

[thelegitpilot]

I got a totally different value for slope (5.13*10^3) compared to yours. How did you get 200*10^3?

EDIT: Figured it out, you took the slope from the elastic example! That would explain the discrepancy.