Spectral Analysis of EME Signal

• Published in: E-journal of Nondestructive Testing Vol. 11; no. 11
• Main Authors: Majzner, Jiri, Sedlak, Petr, Sikula, Josef
• Format: Journal Article
• Language: English
• Published: 01.11.2006
• ISSN: 1435-4934; 1435-4934

The new measuring method for detection fine spectra of small cracks, electromagnetic and acoustic emission (EME and AE) signals, is described. It requires wide band ultra-low noise amplifiers, analogy filters, optimization of signal to noise ratio of sensors and the application of noise elimination methods. The 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 also involving the electromagnetic shielding allows us to detect signals in the range of 1 to 100 nV. This measuring set-up was used to observe the crack creation in the granite samples. AE and EME signals show different behaviour in the first interval of about 20 mus, just after the crack creation. For the first stage of crack creation, 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 mus. After that the frequency spectrum is given by the sample eigen vibration or the sample boundary conditions. We have observed the spectra in the frequency range 100 kHz to 2 MHz.