Concept:
True strain:
\({\epsilon_T} = \ln \left( {\frac{{{l_f}}}{{{l_i}}}} \right)\)
Where, lf = final length, li = original length
Calculation:
Given, lf = li/2
\({\epsilon_T} = \ln \left( {\frac{{{l_f}}}{{{l_i}}}} \right) = \ln \left( {\frac{{l/2}}{l}} \right) = \ln \left( {\frac{1}{2}} \right) = - 0.693\;\)
The magnitude of true strain is 0.693