Electromagnetic and acoustic emission fine spectra

A new measuring method for the detection of fine spectra of electromagnetic and acoustic emission (EME and AE) signals from small cracks is described. It requires wide band ultra-low noise amplifiers, analogue filters, the optimization of signal to noise ratio of sensors and the application of noise elimination methods. Analyses of noise sources in sensors and preamplifiers are given. They are thermal noise, polarization noise and low frequency 1/f noise. Measuring set-up background noise suppression involving also the electromagnetic shielding allows us to detect signals in the range of 1 to 100 nV. This measuring set-up was used to observe crack initiation in granite samples, AE and EME signals show different behaviour in the first interval of about 20 mu s just after crack initiation. For the first stage of crack initiation the frequency spectrum of EME signal is given by the eigen vibrations of crack walls, by the internal friction and the sample electrical conductivity. We observed that the crack opening and crack wall vibration create the high frequency signal in the frequency band up to 10 MHz. These signals were observed in the first time interval of about 20 mu s. After that the frequency spectrum is given by the sample eigen vibration or the sample boundary conditions, and we have observed the spectra in the frequency range 100 kHz to 2 MHz.