Defect resistance as a measure of the resistance of steels containing stress raisers to brittle fracture

Conclusions

• 1. Fundamental mechanical characteristics of steel — resistance to microcleavage R_mc and the ductility coefficient K_d=R_mc/σ _t − together with certain standard mechanical characteristics make it possible to determine the physically substantiated defect-resistance parameter of the steel D₂=log K_d-n_t.

• 2. The defect-resistance parameter D₂, when compared with the logarithm of the “severity” of the expected defect log J_o, makes it possible to determine the suitability of the chosen steel for use under given conditions if D₂>log j_o.

• 3. At D₂>log j_o, the greatest nominal stress in the product can safely be near the yield point. Otherwise, the service loads should be reduced in accordance with the value of D₂.

• 4. In the case of uniformly distributed tensile stresses, small concentrators do not endanger the load-carrying capacity of mechanical elements even for very low values of residual ductility (D₂≈0.1) if the nominal loads have not reached the yield, point any-where in the material.

• 5. Small defects in steels present a real danger only in structural elements having zones with nonuniformly distributed stresses (at recesses and fillets and in the outermost fibers of bent elements). Assuming that the steel is not sufficiently ductile, small defects can be active sources of fracture in a structure only in those zones where a moderate amount (about 2% or more) of plastic strain has occurred locally. The criterion of adequate ductility in the steel is the relation D₂>log j.