Some cases of different structural sensitivity of impact strength and fracture toughness

Abstract

Two cases of practical importance are observed of a breakdown in the symbatic correlation of impact strength and fracture toughness concerning the influence of overheating before hardening and also of irreversible temper brittleness of high-strength steels. In both cases the breakdown in the correlation is related to differences in the structural and mechanical situations occurring at the opening of stress concentrators differing in the sharpness of the notch and, consequently, in the dimensions of the zones of plastic deformation and the degree of triaxiality of the stressed and deformed state. Differences in the behavior of KIc anda n with overheating of high-strength low-temperature-tempered steels reflect the fact of a decrease in the work for origin of a brittle crack with an increase in austenitic grain size. In some steels this is the overwhelming portion of the work for failure of Mesnager samples. A necessary condition for the appearance of temper brittleness in fracture-toughness tests is a shift in the temperature range of its appearance of the mechanism of propagation of a crack from intragranular to intergranular. The above-described breakdown in the correlation between impact strength and fracture toughness will appare: γ be characteristic of other cases of treatment of materials leading to the appearance of appreciable differe:; s in the strength of layers near the boundary and of the body of the grain with a low level of material plasticity. The cases considered are an indication of the incorrectness of choosing fracture toughness as a universal index of resistance to brittle failure of alloys without taking into consideration their structural state and the specific elastoplastic situation in mechanical tests determining the conditions of formation and growth of a brittle crack. With an increase in tempering temperature and also with a drop in carbon content in the steel there is a decrease in the share of the work for origin of a crack ina n. In connection with this for high-temperature-tempered and low-carbon steels fracture toughness becomes a more representative characteristic of resistance to brittle failure of alloys and maintains a symbatic correlation with impact strength.