The generalized fracture mechanics approach is applied to two ductile steels, namely mild steel and 18/8 stainless steel in plane stress. The theory defines a fracture parameter\(\mathcal{T}\), which is a truly plastic analogue of theJ contour integral and, for an edge crack specimen, is given by $$\mathcal{T} = k_1 ( \in _0 )cW_{0_c } $$ wherek1 is an explicit function,c is the crack length ande0, W0c are respectively the strain and input energy density at fracture, remote from the crack. The functionk1(eo) is derived experimentally and the constancy of\(\mathcal{T}\) with respect to crack length and applied load is demonstrated. The variation of\(\mathcal{T}\) with crack extension during slow growth is investigated, as is the rate dependence of\(\mathcal{T}\) in mild steel.