Investigations of Crack Initiation and Crack Propagation in Plain Carbon Steels by Means of Acoustic Emission.

Crack initiation and propagation has been studied during fatigue tests with ferritic, ferritic-pearlitic and pearlitic specimens by means of acoustic emission (A. E. ) instrumentation. The ferrite and the ferrite-pearlite showed acoustic emission only in connection with fast yielding of large volumes. Early steps of crack initiation showed no detectable acoustic emission because of very small yielded volumes and small amounts of crack surface friction. Gapping of the cracks inhibited A. E. due to friction during the early steps of crack propagation. Pearlitic specimens showed strong A. E. prior to microscopic detection of cracks, ascribed to fast fracture modes. Cracks in ferritic and ferritic-pearlitic CT-specimens are only detectable by crack surface friction. The increase in yielded volume with the propagating crack is too small to be distinguished from friction noise. Pearlitic specimens showed strong burst-signal activity ascribed to cleavage fracture and fast fracture modes.