Transition Behaviour and Evaluation of Fracture Toughness in Charpy Impact Test

Abstract

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 relationshill between load and time during impact. Besides the standard V-notch sj)ecimens, the brittle boundary layer specimens that were prepared by carburiz 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 temj)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. * ** Yield loads at this low temperature reg ion in Fig. 2 were determined by Charpy specimens with their n otch on th e compressed, rather than on the tensile, side to preven t fracture before general yielding. *** * Some ductility, such as , for example, the plastic hinge shown in Fig. 1, was still observed even at temperatures as low as this; thi s is clue probably to the effect of strike of the hammer.

Effect of Delamination and Grain Refinement on Fracture Energy of Ultrafine-Grained Steel Determined Using an Instrumented Charpy Impact Test

“…The P-u curves for the 015C sample shown in Figure 6 exhibited a typical curve for each fracture type: ductile fracture, DBT fracture, and brittle fracture [24,25]. In particular, the curve at −40 • C near the DBTT reveals the change in fracture type during crack propagation.…”

Ductile to brittle transition of an A508 steel characterized by Charpy impact test

“…The specimens were then broken at liquid nitrogen temperature in order to avoid any further macroscopic plastic deformation and ductile growth. Usually, the load-line displacement curve is used to separate the initiation and growth energy [46] and it is generally considered that ductile crack propagation roughly initiates at maximum load [47].…”

“…Further propagation occurs with formation of shear lips at the outer surface. It is worth noting than the energy absorbed for shear lips formation is not negligible compared to the total CVN energy [46].…”

Progress in the Instrumented Charpy Impact Test

“…The analysis of deformation and fracture behaviors, particularly the ductile-brittle transition behavior of steel, by recording the load-time or load-deflection curve during impact has been attempted since the 1920s [22]. Figure 6 shows a representative load-time curve and fracture surface in the transition region [23]. Properties such as the general yield load (Py), maximum load (Pm), brittle fracture load (Pf), and brittle fracture arrest load (Pa) are clearly recorded.…”

“…Kobayashi et al classified the transition behavior of steel into 6 temperature regions based on the results of the analysis of the load-deflection curve and fracture surface morphology (Fig. 7) [23]. The outline of the definition of transition temperatures and the correlation with the results of a large-scale test are also shown in this figure.…”