Acoustic monitoring of percussion drilled holes for gas turbine engine components

An acoustic monitoring data acquisition system was developed, calibrated and tested over a range of percussion laser drilling conditions. The raw acoustic emissions were used to calculate the Fast Fourier Transform (FFT) and the Power Spectral Density (PSD) as functions of emission amplitude and acoustic frequency. The amplitudes of the acoustic measurements were used to determine when breakthrough occurs during percussion laser drilling of materials representative of gas turbine engine airfoils. The acoustic emission frequency range associated with the percussion laser drilling process was found to be unique from that associated with the gas jet assist and was found to be between 500 to 5500 Hz. The amplitude of the acoustic signature was found to be the Key Process Output (KPO) indicating partial and complete breakthrough, i.e., complete breakthrough was found to occur when the acoustic amplitude dropped into the acoustic signature of background noise.