PSI - Issue 41

G. Agalianos et al. / Procedia Structural Integrity 41 (2022) 452–460 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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elasticity ranges between 2.5 and 10 GPa, its uniaxial compression strength between 15.0 MPa and 36.8 MPa and its Poisson ratio is equal to about 0.26, depending on the exact quarrying point (Kourkoulis and Dakanali, 2017). Alfas stone is among the materials considered by the scientists working for the restoration project of the Epidaurus monuments as a potential substitute of the Kenchreae porous stone, i.e., the original building stone of the monuments. Before the three-point bending tests, the porosity and the density of each specimen were measured according to the Liquid Absorption Technique (Stefanidou, 2010). The results are presented in Table 1 from which it can be seen that the specimens can be classified in two groups. One class with porosity equal to about 22% (denoted as Class A) and one with very low porosity, equal to about 6% (denoted as Class B). For the needs of the current work, beam-shaped specimens with dimensions equal to 450 x 100 x 100 mm 3 were loaded under three-point bending until fracture. The load was applied monotonically under displacement-control mode at a rate of 0.05 mm/s ensuring quasi-static conditions. The specimens were supported by two solid metallic cylinders, each one positioned at a distance of 160 mm from the specimens’ central cross section. The load was applied at the central cross-section by means of a third solid metallic cylinder (Fig.1). All experiments were conducted using an Instron 300DX Static Hydraulic Testing Machine loading frame of capacity equal to 300 kN. For the detection of the AEs, six piezoelectric sensors (model R6a) were used and they were properly attached at strategic points of the beams as it can be seen schematically in Fig.1. The sensors were mounted on the specimens’ surface using silicone, in order to achieve optimum sensor-specimen coupling. To ensure the validity of the acoustic data, the parameters of the Peak Definition Time (PDT), the Hit Definition Time (HDT) and the Hit Lockout Time (HLT) were set according to the manufacturer guide of the AE system and relevant available literature (Mohanty, 1996; Unnþórsson, 2013; Oh et al., 2020). The AE propagation velocity (U) was calculated using the Pencil Lead Break (PLB) method (Hsu, 1981) and the threshold was set at 40 dB. The numerical values of the AE parameters are shown in Table 2 for both classes of specimens tested.

Table 1. The calculated porosities and densities of the tested specimens Specimen / Class Porosity (%) Density (kg/m 3 ) 1 / A 22.86 1836 2 / A 25.07 1810 3 / A 23.89 1908 4 / A 26.61 1880 5 / B 5.85 2150 6 / B 8.79 2277

Fig. 1. The experimental setup.

Table 2. The values of the acoustic emission parameters for each class of specimens. Porosity Max duration (ms) PDT ( μ s) HDT ( μ s)

HLT (μs)

U (m/s)

Low High

10

75

150 800

300 1000

3500 3800

100

4060