PSI - Issue 33

Dimos Triantis et al. / Procedia Structural Integrity 33 (2021) 330–336 Dimos Triantis et al. / Structural Integrity Procedia 00 (2021) 000 – 000

332

3

F

6.0

6.0

200 mm

5.5

4.0

Acoustic sensors

5.0

SG1

S1

S2

2.0

SG2

a o

a o

4.5

Nominal tensile stress [MPa]

400 mm

4.0

0.0

0.00 0.20 0.40 0.60 0.80 Nominal tensile fracture stress [MPa] Normalized (over the semi-width of the plate) notch length

0

75

150

225

Axial strain [ μ strain]

F

(a) (c) Fig. 1. (a) Sketch of the specimens; (b) The tensile strength vs. the normalized notch length; (c) Typical stress-strain curves drawn with the aid of data provided by electric strain gauges attached at the crown of the notch (dotted line) and at the center of the specimen (continuous line). indicating that it is the presence of the notch itself rather than its length that governs the load carrying capacity of marble. In Fig. 1c the nominal axial stress, for a typical specimen of the protocol, is plotted versus the axial strain, as it was recorded by two electrical strain gauges attached at the crown of the notch from which fracture initiated (dotted line) (SG1) and at the central vertical section of the specimen (continuous line) (SG2). As it is expected, the area around the notch abandons linearity much earlier due to the intense local stress field, which leads to the formation of the familiar process zone characterizing fracture of cracked brittle rock-like materials. Concerning the acoustic activity, the raw data gathered during a typical experiment of the protocol are plotted in Fig. 2, in terms of the AE hits recorded per second (Fig. 2a) and, also, in terms of the cumulative AE hits (Fig. 2b). Both parameters are plotted against the normalized (over the duration of the experiment) time, and the temporal evolution of the load imposed is, also, plotted. As it is expected, the specimen is initially almost “silent” , for more than one half of the test ’s duration. Then, weak acoustic activity starts being detected, intensified gradually until the two thirds of the test’ s duration, and becoming, finally, “ explosive ” as the load induced approaches the fracture load. (b)

8.0

75

8.0

2

load cum AE hits

load hits per sec

6.0

6.0

1.5

Cum AE hits [x1000]

50

Hits per sec

4.0

1

4.0

25

Axial load [kN]

2.0 Axial load [kN]

0.5

2.0

0.0

0

0.0

0

0.00 0.25 0.50 0.75 1.00 Normalized (over the duration of the test) time

0.00 0.25 0.50 0.75 1.00 Normalized (over the duration of the test) time

(a) (b) Fig. 2. The time variation of (a) the hits-per-second and (b) the cumulative-AE-hits, for a typical specimen, in juxtaposition to the respective evolution of the axial load imposed.