The structural characteristics (the volume fraction, size, and shape of and the distance among hardening-phase particles) of aging alloys and steels, which define the behavior of the critical stress intensity factor during thermal hardening, are determined using the structural-mechanical approach we have developed. It is experimentally demonstrated for maraging steels that our approach is capable of proving the correlations of strength, plasticity, and crack-resistance with the structural characteristics, which were varied by changing the chemical composition of steel and thermokinetic aging conditions Introduction. The authors of [3, 10] analyzed and systematized the approaches, based on deformation failure criteria, to the evaluation of the characteristic K IC of elastoplastic metallic materials from uniaxial tension data for samples without cracks and the structural characteristics of undeformed metal that determine the size of the crack tip region.Our structural-mechanical approach is based on the results of experimental validation of the Hahn-Rosenfield relation for aging metallic materials [3,9]