Transition Behaviour and Evaluation of Fracture Toughness in Charpy Impact Test

Abstract

Synopsis Ana~sis of the transition behaviour of mild steel in Charj)y impact test was made over a temperature range of 100°-196°C, observing Ihe relation-shill between load and time during impact. Besides the standard V-notch sj)ecimens, the brittle boundary layer specimens that were prepared by car-buriz ing the free surface layers of the standard sp ecimens were used. As a result, the transition in Charpy impact test was found to be divided into six tem/)erature regions. The fracture structure characteristics for each of those transition regions were examined by means of electron microscopic observation. Of the usual transition temperatures, Charpy-V 15 ft-lb energy, Charj)y-V 0% shear, and Charpy-V 15 mil lateral expansion were found to be the yield point fracture criteria; CharjJy-V 30-35ft-lb energy or CharjJY-V 25% shear, on the other hand, was identified as the initiation (ductility) transition temjJerature, at which, no ductile crack being able to j"o-(lagate after the maximum bending load has bee/l attained, raj)id transition to brittle fracture occurs,' this temp erature was seen to correspond to the NDT of NRL drol) weight test. Another important transition temj)erature foun d was the cleavage initiation temj)erature at which cleavage frac ture aj)pears first in the load/time curve; this temj)erature coincided with the near~ 100% cleavage ji-acture apj)earance teml)erature in the brittle boundary layer specimCl7s. This tem-j)erature gave a measure of estimating the brittle crack jJrojJagation arresting temperature of large sp ecimen test. Furthermore, an aitemj)t was made to obtain fracture toughness value, G e , by the brittle boundary layer s/Jecimens. Some agreement with the results from the temperature gradient type ESSO test was obtained. Ge's relative to the onset and the arrest of the brittle crack jJrojJagation were determined and their significances discussed.