PSI - Issue 10

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

16

6

(a)

(c)

(a1)

(a2)

(b)

(b)

200

200

200

87%

AF (kHz)

47%

94%

13%

100

100

100

53%

6%

AF (kHz)

AF (kHz)

0

0

0

0

200

400

0

200

400

0

200

400

RA (μs/mV)

RA (μs/mV)

RA (μs/mV)

(d2) (d2)

(d1)

(c)

(d1)

(c)

200

200

200

60%

67%

47%

33%

100

100

40%

100

53%

AF (kHz)

AF (kHz)

AF (kHz)

0

0

0

0

200

400

0

200

400

0

200

400

RA (μs/mV)

RA (μs/mV)

RA (μs/mV)

Fig. 5. Classification of crack types for m≤0.5 kHz∙mV/μ s, into the distinct stages of loading as follows stage A: 70% to 90% failure stres (a1) and 90% to 96% failure stress (a2), stage B (b), stage C (c), stage D for total AE hits (d1) and for the hits during the last 3sec prior to the failure of the specimen (d2). 4. Conclusions The AE recorded during an experimental protocol including a specific compressive loading protocol of prismatic marble specimens were investigated using the b-value analysis. The cracks formed during the various stages of loading were classified according to RILEM TC 212-ACD. The results obtained in the laboratory constitute a valuable tool for understanding the various stages of micro-cracking activity during the fracture process in quasi-brittle material. The I b -value is a parameter which consistently provides warning signals well before macroscopic failure. In the present work, a sudden drop of the I b -value was observed, especially while approaching values close to 1, when the stress becomes higher than 96% of the compressive strength of the specimen. Under constant stress level, approaching the strength limit of the specimen, the I b -value starts decreasing rapidly indicating that the final collapse is impending. The widely adopted RA-AF classification process was used to determine the proportion of each type of damage (Mode I and Mode II). The results show an AF shift to lower values and an RA shift to higher ones, when the I b values show an intense decrease at values approaching 1. During these time periods, the percentage of the AE hits that seems to be related with Mode II type of damages increases. Thus, it appears that, at moments close to the sample fracture, shear mode AE prevail. References Aggelis, D.G., 2011. Classification of cracking mode in concrete by acoustic emission parameters. Mech. Res. Commun. 38, 153-157. Aggelis, D.G., Soulioti, D.V., Sapouridis, N., Barkoula, N.M., Paipetis, A.S., Matikas, T.E., 2011. Acoustic emission characterization of the fracture process in fibre reinforced concrete. Construction and Building Materials 25, 4126-4131. Calabrese, L., Campanella, G., Proverbio, E., 2012. Noise removal by cluster analysis after long time AE corrosion monitoring of steel reinforcement in concrete. Construction and Building Materials 34, 362-371. Colombo, I.S., Main, I., Forde, M.C., 2003. Assessing damage of reinforced concrete beam using „„ b- value‟‟ analysis of acoustic emission signals. Journal of Materials in Civil Engineering 15, 280-286. Cox, S.J.D., Meredith, P.G., 1993. Microcrack formation and material softening in rock measured by monitoring acoustic emissions. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstract 30, 11-24.