In the present tests, observations of EMR generation in rock samples were performed under constant strain rate conditions. However, it is conceivable that the generation of EMR events may be affected by the experimental conditions. For example, abrupt rock failure can be better controlled through the use of the following control relation (Okubo and Nishimatsu, 1985):
â├ - â┐ âđ / E = C tü@ü@ (1)
where e is strain, s is stress (Pa), E is the Young's modulus (Pa), C is the loading rate (s-1), t is time (s), and a is a control parameter for adjusting the feedback gain of stress. The effects of changing the test conditions (loading rate, linear combination control of stress and strain by equation (1), ground or unground specimen ends) on emissions are investigated below.
4.1 Loading Rate (Strain Rate)
Figure 3 shows the results of a test on the Inada granite at 10-5 s-1, one order of magnitude slower than that applied in the first series of tests (10-4 s-1). EMR events were observed at final failure, as observed in the first test. The stress drop after 730 s in this case was much less abrupt than in the 10-4 s-1 test, although EMR was still generated during these minor failures accompanied by an audible cracking noise. A sharp final stress drop was also seen as in the previous case, accompanied by a spike in EMR generation. At both 10-4 s-1 and 10-5 s-1, the peak electric field intensity occurred in association with failure at a stress of about 150 MPa, after which the stress reached approximately zero with additional loading. The electric field intensity of this main event was 85 dBâ╩V/m at 10-4 s-1 and 80 dBâ╩V/m at 10-5 s-1, indicating that the intensity of the EMR event was higher at the higher strain rate.
4.2 Linear Combination Control of Stress and Strain
More attention has been paid recently to the behavior of rock and rock mass in the post-failure region, which is considered to be closely related to the stability of many underground structures. One difficulty with research on the post failure region is that samples are often in an unstable state. In the present study, to investigate the post-failure region without inducing the sudden breakdown of specimens when the peak strength is reached, additional uniaxial compression tests were conducted using a linear combination of stress and strain as the control variable (Okubo and Nishimatsu, 1985). The higher the value of a in equation (1), the slower the rate of failure. In this study, a was set at 0.6.
Figure 4(a) shows the stress-strain curve, and 4(b) shows the temporal response of the electromagnetic field and stress for the Inada granite at C = 10-4 s-1. While the specimen exhibited Class II characteristics in the post-failure region in Fig. 4(a), the failure can also be seen to have proceeded slowly. The stress reached a peak value of 210 MPa at around 38 s in Fig. 4(b), thereafter decreasing to 160 MPa accompanied by peak electric fields of 72-73 dBâ╩V/m. The stress decreased to 50 MPa with further loading, and four additional EMR events with electric field intensities of around 70 dBâ╩V/m were observed in this extended gradual failure process.
Figure 4(c) shows the corresponding results for C = 10-5 s-1. The stress can be seen to drop abruptly at five points, with each drop occurring within 2 s. Although the failure process was relatively slow, large EMR events were generated at the five failure points. Thus, EMR events occur even under conditions of relatively slow failure. Furthermore, from Figs. 4(b) and (c), the loading rate did not appear to affect the number of events or the electric field strength. The analysis of EMR events in the post-failure region may prove highly useful for monitoring of the cataclastic zone around underground structures and more research is strongly recommended.
4.3 Specimen End Faces
The effect of grinding the specimen ends on the EMR behavior was also examined. Generally, when a specimen with unfinished ends is used, inconsistent local stress conditions tend to arise at the ends soon after initial loading. One of the purposes of this study was to verify whether electromagnetic waves are emitted from specimens with unfinished ends.
Figure 5 presents the results for unfinished specimens of the Inada granite (from the cutter, end flatness within 0.5 mm) under uniaxial compressive loading at a constant strain rate of 10-4 s-1. The uniaxial strength of the specimens was about 70 MPa, much lower than that measured for specimens with ground ends. The first EMR event (over 70 dBâ╩V/m) was observed when the stress reached 35 MPa (after 6 s). This even was marked by a clearly audible cracking noise from the specimen. EMR events are clearly visible before failure in the figure, but the strongest event occurred during the sharp drop in stress in the post-failure region, as in the specimens with ground ends. Thus, EMR is generated during micro-failure processes, even outside of the post-failure region.