ˇTable of Contents
ˇMaterials Characterization and testing
Cracks Detection by Electromagnetic and Acoustic EmissionJ. Sikula, B. Koktavy, J. Pavelka, P. Koktavy , V. Sedlakova
Department of Physics, Technical University of Brno, Czech Republic
Geophysical Institute, Academy of Science, Prague, Czech Republic
Meisei University, Hino, Tokyo
Yamanashi University, Kofu, Japan
The granite samples with dimensions 4x4x1 and 8x4x1cm3 were examined. They were loaded with a ramp increasing stress up to their failure. Electric voltage measured on parallel plate capacitor is time dependent due to vibration of crack walls. From the high frequency component, the length crack was estimated. We found crack length 0.3 - 1.5 mm. Crack position was determined in experiment with four capacitor and four acoustic emission sensors. The simultaneous investigation of EME and AE signals enable us to localise the crack position in solids and estimate crack dimension. This method can be used for samples with low electrical conductivity and was successfully applied for granite and composite materials.
The experimental results (see  to ) can be summarised as follows: EME signals occur simultaneously with acoustic emission events. EME signals with large amplitude occurs together with large AE events. EME and AE signals increases just before the main rupture. EME signals are created by the electric charges redistribution, the piezoelectric effect, the electrokinetic effect and electrification due to friction between the newly created surfaces. In previous studies reported measurements of electric potentials related to rock fracture have been carried out under the monotonously increasing compressive stress in laboratory. Mori et al.  studied electric potential signals from a rock sample under the cyclic loading and found, that electric potential and AE signals mainly occur in the stage, in which the stress is increasing and reaches the highest level in each cycle. They confirmed, that the electromagnetic signals occur primarily due to a generation of a new crack and its extension. The result, that electromagnetic signals rapidly increase just before the main rupture suggest, that the EME signals may be useful as a precursory phenomena.
|Fig 1: Experimental set-up with one capacitor antenna||Fig 2: Experimental set-up with four capacitor antennas|
This experimental set-up indicates also the electrostatic charge redistribution. The wavelength of the emitted radiation was in all cases much larger than the sample size so that near-field effects could be observed only. The conducting plates were fitted to the specimen making the parallel plate capacitor. The electric voltage measured on this plate capacitor C is time dependent due to the vibration of crack walls, its damping and sample electrical conductivity. The time dependence of electrical voltage is determined by the charge relaxation and phonon field interaction.
|Fig 3: The time dependence of AE and EME signals for the event 7123|
Time dependencies of recorded signals are depicted in Fig. 3. In this event EME signals start approximately at the same time, while AE signals are delayed. The time difference between AE and EME signals depends on the crack position and the wave velocity both of EME and AE wave propagation.
In Fig.4. the time dependence of AE and EME signals for the event 7130 is depicted. In this case we found that two cracks are created approximately at the same time near sample edges. The position of this cracks is schematically shown in Fig.5., points A and B.
|Fig 4: Time dependence of AE and EME signals for event 7130||Fig 5: Crack position|
The time dependence of EME signal in Fig.4. shows that when the crack is created near the top or the bottom edge of the sample, then electric signal in capacitor antenna is inversely proportional to the cube of the distance. In this case signal in antenna No.7 and 8. is on the background noise level for the crack B.
|Fig 6: Electromagnetic emission signal||Fig 7: Acoustic emission signal|
Stimulated creation of cracks was observed for event 8118 (Fig.6,7), where first crack appears at the time t = 10 ms probably on surface near AE sensor No.3. After this follow volume crack. In next step the corner near AE sensor No.3. was split off.
This describe DC signal with t = RC constant and AC signal with damping constant , g =d + b ,d due to the mechanical damping and b the sample electrical conductivity. Here V0 is integration constant, wo=g- t /t is cut off angular frequency,w crack wall vibration angular frequency and factor g is given by
where v0 is velocity amplitude, d is parallel plate capacitor thickness and a is the angle between electric field intensity and electric charge velocity .
|Fig 8: Time dependence of EME and AE signals|
The crack walls are mainly oriented according to a rock crystalline structure. Samples with crystalline layers parallel to area of capacitor antennas generate EME signal given by superposition of damped DC and AC signal, as in Fig.8. This was measured on experimental set-up according to Fig.2.
We found, that the frequencies of the EME signals for granite samples were fC = 1 to 6 MHz. The crack length lC can be estimated from this frequency and for granite sample with velocity of mechanical wave propagation 3 km/s we find crack length lC = 1.5 - 0.25 mm.
Crack's eigen vibrations are attenuated due to mechanical damping and electrical sample conductivity. In our experiment this high frequency crack vibration was observed for a time interval of the order 10ľs. After this initial time interval, the main source of crack wall vibration is given by the all sample vibration due to acoustic emission energy. This frequencies are given by sample geometry and frequency of EME signal was about one order lower than for initial crack walls vibration.
We suppose, that electric charge redistribution is a main effect, which is manifested in our experiments. Electric charge redistribution is related to time dependent electric dipole moment and then electromagnetic field must be generated. Measured voltage has two components, DC and AC given by crack walls vibration and mechanical parameters of measured sample. High frequency component is of the order form 1 to 6 MHz and give information on crack length. We estimate the length of the crack of the order of 1 mm. Sensitivity of the value of electrical potential on crack orientation with respect to parallel plate capacitor can be used to study orientation of sample's layers.
Low frequency component below 500 kHz give information on mechanical vibration spectrum and this component is correlated to acoustic emission signals. For our samples, this frequency was of the order of 100 kHz. The second time constant, which is given by parameters of mechanical vibrations and electrical conductivity is dependent on humidity.
This research has been partially supported by the MSMT of the Czech Republic under grant ME285 and also as a part of a research project CEZ J22/98:261 100007 .
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