The following results were obtained through a tensile test on a mild-steel specimen with an original diameter of 30 mm and a length between the gauge points of 60 mm. The load at the yield points was 86 kN and the limit of proportionality was 78 kN with a corresponding extension of 0,052 mm. The maximum load recorded was 158 kN. After fracture, the length of the test piece was 70,2 mm and the neck diameter was 19,5 mm.
Calculate the following:
1.1 The stress at the limit of proportionality