Успехи физики металлов (Dec 2022)
Nature of the Brittleness of Metals
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 (σ0.2 is yield strength), but as a specific indicator, namely, the deformation resistance (break resistance) Br, which comprehensively reflects the degree of ductility and strain hardening of the metal within the SR zone. The critical value Brс corresponds to the strength σ0.2C, at which the strength stability switches over a state of instability at temperature TC, where the fracture of the specimen with SR occurs at a nominal stress σNF, is less than σ0.2C: σNF ≤ σ0.2C. We analyse the experimental findings of different authors for samples with SR (cracks), which enable us to estimate the critical values of σ0.2C and Brс depending on the existent level of Br in steels. A regular permanent dependence of Brс on Br for different types of SR is revealed, which 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 σ0.2 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 by determining the maximum allowable critical characteristic of the strength in the alloy, σ0.2C, that guarantees a non-occurrence of brittle fracture for a product with this type of SR, if σNF ≤ σ0.2.
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