The creation of cracks is accompanied by electric charge redistribution due to loosened chemical bonds. Electric charges on crack walls create dipole moments. Vibrations of crack walls produce time-dependent dipole moments and, consequently, electric and magnetic fields are generated. An electric signal is induced on metal electrodes. Information about the vibration of crack walls was obtained from this signal analysis. For crack lengths below 1 mm the electrical signal has a frequency of over 2 MHz. In this paper the frequency analysis was performed in a frequency band of up to 5 MHz.

The formation of a crack is accompanied by the generation of an electromagnetic field. A differential equation was derived and solved to describe the transformation of the field parameters into an electric signal, provided capacitive sensors are used. In the next stage, an analysis of spurious external electromagnetic and acoustic fields was carried out, and measures to suppress their effects were designed. The issue of amplifying extremely low electromagnetic signals generated during the crack formation was dealt with, too. The mentioned analyses resulted in the design of a method to detect and evaluate electromagnetic emission (EME) signals and acoustic emission (AE) signals, which accompany mechanical stressing of materials and structures. Based on the results of such analyses, an automated double-channel measuring apparatus was used for signal data acquisition, saving and processing. The operation of the apparatus was successfully tested by processing a large set of measurement results obtained from a composite material called ‘extren’. Our experimental results confirmed the correctness of both the EME and AE measurement methods, and measuring apparatus design. Our measurement results make it possible to track the behaviour of cracks, localize them and evaluate some of their basic parameters.

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