Fracture Strength at Low Temperature in Notched Steel Specimens Preloaded at Room Temperature

Abstract

It is well known that plastic deformation at the tip of a notch or defect influences the brittle fracture in structural steels. Recently the COD (crack opening displacement) concept has come to be applied for the brittle fracture initiation of low and medium strength steels, attaching great importance to research for the influence of the condition at the tip of a notch on the brittle fracture initiation of steel. In this study, the effect of preloading on the fracture strength was studied using notched specimens of low strength structural steel (SM 50). Standard Charpy V specimens were preloaded up to a general yield load by 3 or 4-point loading and then fractured by 3 or 4-point loading at various strain rates at liquid nitrogen temperature. The influence of preloading appeared in specimens which had been preloaded so that their notched surface might be subjected to tensile stress, and their fracture loads tended to increase with preload. However, the specimens which had been preloaded so that their notched surface might be subjected to compressive stress, was not apparently influenced by preloading. The above-mentioned results indicate that the residual stress as well as the plastic deformation at the root of a notch influences the fracture strength of the notched specimens.