Effect of Mechanical Condition on Acoustic Emission during Fracture Toughness Tests in High Strength Steel

The acoustic emission technique was applied to the elastic-plastic fracture toughness tests of the compact tension specimens with various thicknesses in the 60 kg grade high tensile strength steel in order to investigate effects of the mechanical condition on the microscopic fracture processes. Since the high AE activity was confined to the fracture process before the macroscopic crack growth point, JIc, these AE events can be related with the deformation or the microscopic fracture events within the crack tip plastic zone. However, AE event counts did not directly depend on the specimen thickness or the plastic zone size. Then the volume of plane strain plastic zone was estimated by the three-dimensional plastic zone model, and AE event counts were shown to be proportional to this volume independent of the specimen thickness.Furthermore, change in the AE activity is given by a dimensionless parameter, α=E′J⁄Bσy2, and corresponds well to the transition of deformation mode from the hinge type to the shear type based on the Dugdale-Muskhelishvili model.From these results, the emission source can be attributed to the initiation of microcrack such as cracking or decohesion of the second phase particles which predominantly occurs within the plane strain plastic zone prior to the macroscopic crack growth.