The tensile ductility of seven structural steels was substantially reduced when the plastic strain state was changed from axisymmetric (round specimen) to plane strain. The ratio of plane-strain-tension ductility divided by axisymmetric-tension ductility declined from 72 percent for a mild steel to 17 percent for a steel with 248 ksi yield strength.
The ductility (fracture strain) in the Charpy V-notch specimen was equal to the ductility in the plane-strain-tension specimen. Therefore, the Charpy toughness correlated with plane-strain-tension ductility, not with the ordinary axiSymmetric-tension ductility. The reduction in ductility when the plastic strain state is changed from axisymmetric to plane strain is here defined as plane strain sensitivity. The decreased toughness that accompanies increased strength is caused by the increase in plane-strain sensitivity that occurs as the strength increases. * The tensile properties were measured on a 0.252-inch-diameter tension specimen at 70 F at a crosshead speed of 0.005 inch/minute. ** The first three values are lower yield stresses. The last four values are 0.2~ offset yield strengths. *** Strain at which fracture instability occurred. This value is less than the strain measured on the two broken pieces.