Abstract
We propose a new method for estimating the AE (or elastic wave) source wave in the delayed fracture of a finite specimen (NACE-TM0177) by using the experimental transfer functions of medium and/or AE transducer. A laser induced dielectric break-down of silicon grease was utilized to examine the frequency resolution of the AE system employed. Due to the limited frequency resolution of the system and the reflected waves, the accurate source analysis of micro-fracture was limited to a relatively slower crack generation with the rise time longer than 0.5μs. Crack generation rate in the delayed fracture of QT-treated SCM 440 alloy with tensile strength of 1323 and 1500 MPa was estimated to change from 50 m/s ( effective rate : 145m/s) to 170 m/s (490 m/s), depending on both the strength of material and hydrogen concentration in front of notch.
