PSI - Issue 10

D. Triantis / Procedia Structural Integrity 10 (2018) 11–17 D. Triantis / Structural Integrity Procedia 00 (2018) 000 – 000

13

3

75

4

(C)

(D)

(B)

3

50

2

25

(A)

Ib - value

stress (MPa)

1

stress Ib-value

0

0

0 25 50 75 100 125 150 175 200 225 250 275 300 325 time (s)

Fig. 1. Temporal recording of the applied stress and the I b -values during all four stages.

(a)

(b) Fig. 2. Indicative photos of the specimen prior to loading (a) and following to failure (b).

The number of the AE hits, as well as the mean rate of occurrence of the ΑΕ hits (mean value of hits per second) during the four stages previously described are presented in Table 1. The third column of Table 1 includes the average hit rate. The values in the parentheses in stages B and D, correspond to the values observed in the first 10 seconds following to the stabilisation of stress at 60.5 MPa and 63.5 MPa, respectively.

Table 1. AE hits and average hits rate during four distinct stages. stage Number AE hits Average hit rate A 893 7.2 B 730 5.8 (8.8) C 174 33.7 D 1538 25.1 (17.5)

3.1. The variability of the I b -value Provided that the ratio of weak to strong ΑΕ hits can be clearly described through the b-value analysis, it is quite common for the engineers working in the area of rock mechanics to use AE based b-value analysis in order to study the damage and fracture process in rocks (Main (1989); Rao and Lakschmi (2005)). In 2001, Shiotani et al. proposed the “improved b - value” (I b -value). The I b -value uses statistical values as mean and standard deviation of AE ampli tude that varies during the test. The I b -value defines as: