A hypothetical engineering stress-strain curve shown in the figure has three straight lines PQ, QR,
RS with coordinates P(0,0), Q(0.2,100), R(0.6, 140) and S(0.8, 130), ‘Q’ is the yield point, ‘R’ is
the UTS point and ‘S’ the fracture point
The toughness of the material (in MJ/$^{m^3}$ ) is ____________