Relationships between KIC and CVN at the Lower Shelf of the Transition Curve CVN-T

M. Habashi, M. Tvrdy
2015 Journal of Materials Science and Engineering: A  
The objective of this work is to verify the validity of the Spink's model at the lower shelf of the transition curve CVN-T. Knowing that in this field of temperatures, previous results have shown that CVN is se nsitive to the steel micro-structure of the steel, heat treatments and the existence of defects such as those caused by internal hydrogen. Mild steel, with and without internal hydrogen and a metastable austenitic 2404 alloyed steel transformed at − 196 °C to about 90% martensite are
more » ... ied. Standard charpy specimens with different notch root radii varying from 0 to 1 mm are used to measure fracture toughness by applying j-integral and also to measure the impact energy CVN. For all, bending tests were performed and the tests temperature was − 196 °C . For mild steel without internal hydrogen, the changes in both fracture toughness and impact toughness as a function of notch acuity coincide perfectly and are, also, in good agreement with those obtained at the upper shelf by Ritchie et al in AISI 4340 steel in two different heat treatments. However; in the case of mild steel severely charged with internal hydrogen and containing more than 10 ppm H 2 , which promotes high density of defects in the grain boundaries, the two linear relations are not similar but for the two cases of zero notch radii are the same and equal to 0 mm. The bi-phases 2404 alloyed steel shows that the slopes and the critical notch root radii of the linear relations are also different. The strain induced martensite from the residual austenite γ during the fracture toughness measurements at − 196 °C , with low strain rate is assumed to be inhibited. Only the strain rate sensitivity is responsible for this difference. However; for all three cases studied at − 196 °C and for the results obtained at the upper shelf by Rithie et al, the effective notch root radii whether measured by fracture toughness or by impact energy tests are the same. The fracture type in mild steel free from internal hydrogen is by macro-cleavage, while in the presence of internal hydrogen, macro-cleavage and inter-granular feature, with large cracks are observed. After fracture toughness tests, the fracture surface of the aged martensite 2404 alloyed steel is by fine dimples "ductile-fragile" feature. The main conclusion is that by applying the Spink's model described above, large dimension specimens satisfying the standard LFEM criterion (ASTM E23-01, 2001) are not necessary. specimens. The most commonly used is CVN (Charpy-V) notch at the upper shelf of the transition curve CVN-T. Several results have shown in Refs. [1] [2] [3] [4] [5] , the existence of different relationships between the mechanical toughness K IC measured either by the LEFM (Linear elastic fracture mechanisms) or by applying J-integral J IC = K 2 IC (1-υ 2 )/E and the impact energy CVN. At the upper shelf, the most used correlation is that of Barsom-Rolfe: (K IC /σ y) 2 = 5 (CVN/σ y − 0.05) [6] . The prediction of K IC is within 6 to 18% of the measured value. On the other hand, two analytical equations have been derived by a) Rithie, Francise and Server [7] and b) Rithie and Horn [8]
doi:10.17265/2161-6213/2015.5-6.004 fatcat:jfeatptk3vci3bgm22ewthogma