**Concept:**

Engineering stress-strain is lower than true stress-strain because we consider the initial area only throughout the process while in true stress-strain curve we consider the **instantaneous** area.

**Mathematically,**

\(\rm{Engineering~Stress}\;(σ_o)={Applied~Force\over Initial~Area}\)

\(\rm{True~Stress}\;(σ_t)={Applied~Force\over Instantaneous~Area}\)

Mathematically, true stress (σT) and engineering stress (σ_{o}) relation is represented as

**σ**_{t} = σ_{o}(1 + ϵ_{o})

ϵt = ln (1 + ϵo)

where σ_{o} = engineering stress, σ_{t} = true stress, and ϵ_{o} = engineering strain and ϵ_{t} = true strain.

**Calculation:**

**Given:**

ϵo = 0.25, σ_{o} = 400 MPa

**We know that**

σ_{t} = σ_{o}(1 + ϵ_{o})

σt = 400(1 + 0.25)

σt = 500 MPa.