Study of dynamic crack formation in nodular cast iron using the acoustic emission technique

In this study, acoustic emissions (AE) were analyzed to detect the onset of crack growth in nodular cast iron during dynamic loading at room temperature, 50 °C and 65 °C. To determine the AE signal from the crack growth, two types of specimens were used: Charpy V-notch specimens without a fatigue crack (CVN), and precracked Charpy V-notch specimens (PCVN). While the uncracked specimens showed only elastic and plastic deformation, the precracked specimens exhibited crack formation and extension, additionally. Damage at the notch/crack tip was analyzed by means of scanning electron microscopy. Investigations revealed that local cleavage fracture on the notch of the fatigue precrack occurred before the start of ductile stable crack. Acoustic emission measurements on Charpy impact test were complemented with the AE measurements on quasi-static bend test. The acoustic emissions, emitted from plastic deformation and the ductile crack growth during the impact test were not measurable. In contrast, local cleavage fracture was determined by means of AE even if it had a small area. The amplitude of AE signal correlates with the size of the cleavage areas.