Study of fatigue crack characteristics by acoustic emission

Acoustic emission (AE) technique was used to investigate fatigue crack characteristics such as initiation closure and propagation on smooth specimens of Incoloy 901 at room temperature, over the range of stress ratios -1.0 ≤ R ≤ 0.2. AE technique was applied to determine when and where a microcrack initiated on the specimens. The threshold stress intensity ranges were determined by combining AE tests and microscopic examination for crack initiation points. The AE technique revealed the crack closure process in smooth specimens, and crack-opening/closing stress was easily determined from the AE signature. Several published formulae for crack-opening stress were examined. During crack propagation, AE count rate and total counts, associated with the crack growth state, were stress- and lifetime-dependent. The results showed a nonlinear relation between the AE count rate and the crack propagation rate.