Akira Nozue1, Teruo Kishi2 and Ryo Horiuchi2
1Graduate School, University of Tokyo, Tokyo
During stable crack growth, crack extension is assumed to be discontinuous and initiation, growth and coalescence of microcracks ahead of a pre-fatigue crack occur. The plane-strain fracture toughness KIC (or GIC) obtained by the ASTM standard corresponds to a 2% increment of crack extension. These criteria are all lacking in physical basis. Fracture criteria should be based upon physical models from the viewpoint of microscopic fracture mechanisms.
In this paper an investigation is made with the aid of AE signal analysis and fractographic observation of quenched and tempered AISI 4340 steel, especially with an attempt to make clear the relationship between AE energy and plane-strain fracture toughness GIC. The cracking modes for stable crack growth of the specimens are intergranular and dimple fracture. The AE energy increases with decreasing GIC, and the dimple fracture is accoustically quiet. From the experimental results it is concluded that the dominant source of detectable AE during the crack propagation is intergranular fracture.
A new concept of GmicroIG (critical value of energy release rate for intergranular micro-fracture), and GmicroD (critical value of energy release rate for dimple fracture) has been proposed as characterizing parameter of a finite crack growth step from the microscopic viewpoint, as well as the experimental evaluation of these micro-fracture toughness terms. The mechanical energies for the crack extension can be obtained by the summation of energies of intergranular and dimple fracture. Therefore the plane-strain fracture toughness GIC (ASTM) can be interpreted in terms of micro-fracture toughness characteristics according to the following relation.
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