Estimation of elastic–plastic fracture toughness by numerical simulation based on a stress-based criterion for ductile crack initiation

When estimating fracture strength of cracked components, it is important to know the fracture toughness of the material. However, in many cases, it is difficult to get the fracture toughness of in-service components. In this study, the elastic–plastic fracture toughness was obtained using numerical simulation. A stress based-criterion was adopted for determining onset of a ductile crack at the notch root, and the fracture toughness was derived as the J-integral value when a ductile crack was initiated. A detailed stress–strain curve was used in the simulation in order to obtain the precise stress near the notch root of the test specimens. Carbon steel specimens with a machined notch were subjected to fracture toughness tests. The radius of the machined notch root was changed and the fracture toughness was found to increase with the notch root radius. The simulation reasonably predicted the change in fracture toughness due to notch root radius, and the fracture toughness of the fatigue notched (sharp notch) specimen was determined by extrapolating the notch root radius dependency. The stress-based criterion obtained using notched bar specimens was shown to be valid for predicting ductile crack initiation at the notch root. It was concluded that, if the stress-based criterion could be identified, the fracture toughness could be obtained by the simulation.