Strain energy U, in the case of axially loaded bar is expressed as. (where P is axial load, L is bar length, A is cross-section area of the bar, E is Young’s Modulus of the bar, σ is axial stress, δL is elongation due to axial load)
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\(\frac{{{P^2}{L^2}}}{{2AE\left( {\delta L} \right)}}\)
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Area under stress versus strain diagram of axially loaded bar
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\(\frac{\sigma }{2}\frac{{\left( {\delta L} \right)}}{L}\)
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\(\frac{{{P^2}L}}{{2AE}}\)