The rocks were classified into three groups based on the characteristics of EMR generation in the specimens under a constant uniaxial strain rate of 10-4 s-1.
a) Inada granite
Figure 2(a) shows an example of the results for the Inada granite. The electric field intensity did not rise above noise in the pre-failure region, but once failure occurred, the electric field intensity spiked accompanying an abrupt stress drop. With continued loading after complete failure, however, no further EMR events were detected.
b) Honkomatsu andesite, Kuzuu dolomite
Three tests under identical conditions were carried out on the Honkomatsu andesite, and EMR events were detected in only two of the three tests. Figure 2(b) shows the results for one of the samples in which EMR events were observed. As no emissions were detected in the pre-failure region, the figure shows only the data near failure. Failure occurred at about 141 s, at which time the stress decreased abruptly from 250 MPa accompanied by a simultaneous increased in the electric field intensity from 67 dBâ╩V/m (noise level) to 82 dBâ╩V/m, clearly indicating the occurrence of an EMR event upon failure.
EMR events were also observed in two of three tests on the Kuzuu dolomite. An example is shown in Fig. 2(c). In the figure, a drop in stress from 200 MPa to approximately zero at around 56.5 s is accompanied by an increase in electromagnetic activity, with the electric field intensity increasing from 65 dBâ╩V/m (noise level) to 73 dBâ╩V/m. The uniaxial compressive strength of the sample from which no emissions were detected was 150 MPa, somewhat lower than that of the other samples. The failure mode of the Kuzuu dolomite specimens in this constant strain rate test was approximately vertical splitting, and failure proceeded rapidly once the stress began to fall.
The intensity of the electric field began to increase roughly simultaneously with the decrease in stress in both the Honkomatsu andesite and Kuzuu dolomite. The peak in the electric field intensity lagged the abrupt decrease in stress in both specimens, similar to the observation for the Inada granite. The calculated impulse response of the RFI field intensity meter assuming an impulse simultaneous with the stress drop coincides very well with the measured curve as shown in Fig. 2(c), demonstrating that the actual electric field intensity can be approximated by an impulse or delta function simultaneous with the stress drop.
c) Akiyoshi marble, Kimachi sandstone, Sanjome andesite, mortar
No increase in the electric field intensity above the noise level (65 dBâ╩V/m) was detected for the Akiyoshi marble, Kimachi sandstone, Sanjome andesite, or mortar.