| Abstrakt: |
This article considers the problem of brittle fracture of steels due to stress raisers (SRs), including cracks, at low test temperatures. The authors consider the ductile-to-brittle transition at the critical temperature of fracture of specimens with SRs, TC, as a mechanical instability of the ultimate strength, σNF, at fracture of above specimens below the yield stress of metal, σ0.2. The mechanical strength stability of metal materials is provided by ductility, which is manifested within the strain-hardening range between the yield stress σ0.2 and true fracture stress SK as the break resistance index Br = SK/σ0.2 at ductile break in specimen 'neck'. The stress raiser localises the entire margin of mechanical stability of metal, Br, in the ductility zone, which causes the macroscopic brittle behaviour of specimen at temperatures T ≤ TC, where strength of the specimen with SRs is σNF ≤ σ0.2. Dependence of the critical level, Brc, at TC on both the margin of mechanical stability of strength, Br, and the SR type is ascertained. Effectiveness of the embrittle- ment effect of various SR types (notches and cracks) is considered using the example of armco-iron (α-Fe), as well as of structural steels over both a wide strength range (σ0.2 = 150-1500 MPa) and ductility range (ψK ≈ 10-83%). It is shown that the critical parameter of mechanical stability margin, Brc, may become a key tool for ranking structural alloys in practical materials science, when certifying these alloys for their use in products containing structural stress raisers or fatigue cracks. [ABSTRACT FROM AUTHOR] |
