Brittle fracture initiation in low carbon steels at the ductile-brittle transition temperature region

Conformity between Mechanics and Microscopic Functions of Hydrogen in Failure

“…(7) and (8) was performed using the experimentally obtained flow stress and the nucleation void density. Figure 5 46) shows calculated J-integrals and the experimental R-curve at crack advances of less than 200 μm at testing temperatures of 153 K and 303 K. The agreement is quite satisfactory, supporting also the finding 44) that Rcurves are almost insensitive to the testing temperatures.…”

“…The decrease in fracture toughness at lower testing temperatures is due to earlier termination of the R-curve while its shape is hardly affected. 44) On the other hand, microstructural effects on fracture toughness appear as an alteration of the R-curve shape. 44) An example is the effect of second phases along grain boundaries in low carbon steels as demonstrated by Yagi et al.…”

“…44) On the other hand, microstructural effects on fracture toughness appear as an alteration of the R-curve shape. 44) An example is the effect of second phases along grain boundaries in low carbon steels as demonstrated by Yagi et al. 45) The boundary phases act as a constraint against slip extension across boundaries, and crack growth resistance substantially decreases as the fractions of constraint phases along boundaries increase.…”

Microstructures controlling the ductile crack growth resistance of low carbon steels

“…Figure 7 shows examples for each steel at the early stage of the ductile crack growth, say, at the positions of about 500 m when tested at the transition temperatures. [5] With the advance of the crack, the number of secondary dimples showed a slight decrease. Corresponding to Figure 6, the number of secondary dimples in steel B was substantially more than in steel A, while that of steel C was between steels A and B.…”

“…[12] They represent the R curves for the respective steels, although the data are from different test temperatures since the temperature dependence of the R curve is insignificant. [5] The resistance against the ductile crack growth increases in the order of steels A Ͻ C Ͻ B.…”