This paper is dedicated to experimental and atomistic study of the influence of so called T-stress (acting along the
crack plane) on fracture processes in bcc iron. We analyze experimental results from fracture tests performed at
room temperature on bcc iron-silicon single crystals with a long edge crack (1¯ 1 0)[1 1 0] (crack plane/crack
front). The specimens were loaded in tension mode I under different border conditions inducing different sign of
the T-stress. The brittle-ductile behavior at the crack front was monitored on-line via optical microscopy together
with external force and prolongation of the specimens. Topology of the specimens has been investigated before
and after the fracture tests via the white light interferometer. The microscopic processes produced by the crack
itself were studied at 300 K via 3D molecular dynamic (MD) simulations in bcc iron under equivalent boundary
conditions and the T-stress was examined by means of stress calculations on the atomistic level. The experimental and atomistic results show that the sign of the T-stress affects the fracture behavior. MD simulations
reveal that positive T-stress makes the emission of blunting dislocations ⟨1 1 1⟩{1 1 2} from the crack front more
difficult. As a consequence, higher external loading is needed for crack blunting in the experimental specimens
with T > 0 in comparison with the specimen having T < 0.
