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Fracture toughness studies of line-pipe steels are extremely crucial as line-pipes are exposed to extreme conditions in application. The effects of crystallographic texture and constraint on fracture toughness of API X70 line pipe steel were investigated in this study. The J-integral method was considered to quantify the fracture toughness across orientations and thicknesses of the investigated steel. The difference in the crack initiation toughness clearly demonstrated the presence of strong anisotropy. Further the counteracting effects of thickness and delamination on constraint were found to govern fracture mechanism as well as the fracture toughness of the specimens. The anisotropy in toughness was investigated with macro-texture measurements of the specimens. The major crystallographic textures and their influences on fracture toughness were analyzed. Amongst the observed texture components, strong intensities of {113}<110>component of α-fiber was identified as the major reason for anisotropy in fracture toughness. The distribution of volume fractions of {112}<110> component also showed a strong correlation with the fracture toughness across orientations. [ABSTRACT FROM AUTHOR]

Conventional testing of fracture toughness measurement for thin metallic sheets is not possible owing to the large plastic zone ahead of the crack tip. Hence, the critical-crack tip opening angle $$(\hbox {CTOA}_\mathrm{c})$$ has been considered to be the key parameter to evaluate resistance of a thin sheet against stable crack propagation. In the present work, attempt has been made to measure the $$\hbox {CTOA}_\mathrm{c}$$ of interstitial free high strength steel (IFHS) with 1 mm thickness using compact tension C(T) specimen. A high speed camera has been used for monitoring and imaging the progress of the crack length and crack tip angle during loading. The CTOA $$_\mathrm{c}$$ measured from these photographs found to be $$12^{\circ }\,\pm \,1.5^{\circ }$$ . In these experiments, the crack tip velocity was also varied from 1.9 to 960 mm/min and no changes in the $$\hbox {CTOA}_\mathrm{c}$$ value were observed. The high ductility of IFHS steel along with prevailing plane stress condition might be the reason of appreciably large $$\hbox {CTOA}_\mathrm{c}$$ value. [ABSTRACT FROM AUTHOR]