Effects of notch root radius on both crack initiation sites and fracture toughness of ceramics and steel

In this study, fracture toughness tests using three-point bend specimens of Al2O3, Si3N4 and mild steel with a notch were carried out to study the effects of notch root radius on fracture toughness and crack initiation sites. The notch radius was varied in size ranging from 0.1 to 1.0mm. The tests were also conducted at room temperature(RT) and high temperature(1000 degrees C) to examine the influence of temperature. The test temperature for mild steel was -170 degrees C. The results were discussed based on the process-zone-size failure criterion. The relationships between notch root radius and fracture toughness measured at both RT and 1000 degrees C were in good agreement with the theoretical relations deduced from the criterion. In mild steel, the crack initiation sites are distributed in the vicinity of the eleastic-plastic boundary.