Abstract
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 SK (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.