Micro- and macrozone of plastic strain as criteria of limiting state of material under fracture

The paper presents data on fractures under static failure of 15Kh2MFA steel and impact failure of steel 45 obtained by X-ray structure analysis. Comparison of depths of microzone h(yh) and h(y) of plastic deformation under the fracture surface with the plastic zone radius estimates for plane deformation (PD) and plane stressed state (PS) gave reasons to suppose that presence or absence of the slightly deformed macrozone h(y) under the fracture surface may be indicative of the PD and PS state realization at the crack tip, respectively. It is shown that the stressed state of material at the crack tip is related to critical brittle temperatures. The interval of ductile-brittle transition of the materials includes temperature ranges, each having a particular (as to fractographic and structural criteria) failure pattern