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Impact experiments were performed with Charpy specimens and with large bend specimens made from a ferritic pressure vessel steel. All specimens were side-grooved and fatigue-precracked. Different amounts of ductile crack extension up to 16 mm were achieved by varying the available impact energy. The measured hammer load signals were evaluated to calculate J integral values.
The determined dynamic J-resistance curves of the bend specimens are significantly above the Charpy data. Examinations of the fracture surfaces and metallographic sectioning perpendicular to the crack front showed that behind the stretched zone the ductile main crack is accompanied by side cracks with a length up to about 3 mm for the large specimens, whereas such side cracks are hardly found in Charpy specimens. The additional energy needed for the formation of side cracks might explain higher J-resistance curves for larger specimens.
Further results indicate that with increasing loading rate and specimen size the tendency to develop side cracks increases in correlation with higher J-resistance curves. Finally, an attempt is made to discuss the relevance of this effect for the behavior of components.
ductile fracture, J, -integral, crack resistance curve, multiple cracks, side cracks, crack branching, impact test, high loading rate
Fraunhofer-Institut für Werkstoffmechanik, Freiburg,