the brittleness of material is considered as a specific manifestation of mechanical behaviour under load, which appears in the instability of the strength characteristic value under conditions of non-uniformity of the stress-strain state (SSS). for the naturally ductile metals, such a mechanical behaviour is possible under conditions of non-uniform SSS under the action of stress raisers (Sr), cracks, etc. the existent ductility of material counteracts the strength instability (σ 0.2 is yield strength), but as a specific indicator, namely, the deformation resistance (break resistance) B r , which comprehensively reflects the degree of ductility and strain hardening of the metal within the Sr zone. the critical value B rс corresponds to the strength σ 0.2C , at which the strength stability switches over a state of instability at temperature T C , where the fracture of specimen with Sr occurs at a nominal stress σ NF , is less than σ 0.2C : σ NF ≤ σ 0.2C . We analyse the experimental findings of different authors for samples with Sr (cracks), which enable to estimate the critical values of σ 0.2C and B rс depending on the existent level of B r in steels. A regular permanent dependence of B rс on B r for different types of Sr is revealed, that allows predetermining the predisposition of the investigated steel to the strength instability under the action of Sr according to the known values of standard mechanical characteristics of stretched samples σ 0.2 and S K (true failure stress in the specimen neck). the concept of the metals' brittleness, as a manifestation of the strength instability under the conditions of Sr, may become a foundation for the development of innovative methodology for engineering calculation of force reliability of products containing Sr or known cracks. this is possible through determining the maximum allowable critical characteristic of the strength in alloy, σ 0.2C , that guarantees a non-occurrence of brittle fracture for a product with this type of Sr, if σ NF ≤ σ 0.2 .
