Crack Creation Kinetics Characterization by Electro- Magnetic and Acoustic Emission

Generation of crack is accompanied by generation of electric charges and mechanical vibrations. Electric charges on the crack walls are sources of electric voltage on electrodes (EME signal) and mechanical vibrations generate AE signal. The electric charges located on the cracks walls constitute an electric dipole or quadrupole system. Experimentally we have observed two different type of EME signals like damped harmonic motion with: (i) exponentially time dependent transient value and (ii) constant average value. We suppose that first one is generated by dipole structure and second one is generated by quadrupole one. We found that voltage on measuring capacitor is directly proportional to the electric charge distribution. The recorded electric signal is superposition of crack-wall “self”-vibration given by crack geometry and vibration due to an ultrasonic wave given by sample dimensions. The EME signal precedes the AE response and time delay corresponds to the distance between AE sensor and crack position due to the difference of propagation velocities of sound and electromagnetic field in the sample. The relationship between the signal amplitudes of EME and AE suggests the existence of the correlation between AE and EME in the signal amplitude. The EME signals with the larger amplitude correspond with the AE signals of larger amplitude. Dispersion is related to crack orientation with respect to EME electrodes. AC component of EME signal is inversely proportional to the distance between EME sensor plates d. Thin samples are more suitable for this experiment.