PSI - Issue 26

Andronikos Loukidis et al. / Procedia Structural Integrity 26 (2020) 277–284 Loukidis et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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(Emmermann and Lauterjung, 1997). Amphibolite is a coarsed-grained rock and has a mineral composition of 58% amphibole, 25% plagio-clase, 5% garnet, 2% biotite, and 7% minor opaque minerals. Its porosity ranges between 0.1-0.34%. Its density is equal to about 2960kg/m 3 and its absorption coefficient by weight 0.30% (Nover et al., 1995; Möller et al. , 1997; Chang and Haimson, 2000). The marble specimens were prepared out from rocks quarried from Dionysos Mountain in Attica region. Dionysos marble is an extremely fine-graded white marble consisting of 98% calcite and 0.2% of quartz and other minerals such as muscovite, sericite, quartz and chlorite. Its porosity is very low about 0.37%. Its density is equal to about 2730kg/m 3 and its absorption coefficient by weight varies around 0.11% (Perdikatsis et al., 2006; Kourkoulis et al., 2018). The amphibolite specimen was cylindrical with dimensions of 25mm in diameter and 50mm in height while the marble specimen was prismatic with square cross-section of 40mmX40mm and 100mm length. 3. Loading protocol and experimental set-up The specimens were subjected to a constant uniaxial stress, interrupted by an abrupt step-wise stress increase while concurrently the PSC emission was recorded. The loading protocol involved eight sequential steps of mechanical stress at a gradually higher level until the failure of the specimens. After each step, the stress was maintained constant until the PSC signal was restored to a final low value. After the PSC restoration, the next stress step was applied. In the case of the amphibolite specimens the load was applied parallel to the main axis of the cylindrical specimen, the limits of the loading – unloading process were approximately within the limits 8MPa to 45MPa, at a constants rate of approximately 1MPa/s, while for the Dionysos marble specimens the load was applied along the strong direction of Dionysos marble anisotropy, the limits of the loading – unloading process were approximately between 5MPa to 48 MPa at a constant rate of approximately 0.4MPa/s. The experimental set-up used for the application of the loading protocol can been seen in Fig.3. A pair of copper electrodes was attached to each specimen using conductive paint. In the case of the cylindrical amphibolite specimen, the dimensions of the copper electrodes were chosen to have half of the specimens’ height and to be narrow enough, i.e. practically flat when attached to the cylindrical surface of the sample. The electrodes were placed in a direction perpendicular to the axis of the applied stress in order to minimize friction effects, with the loading system. Teflon sheets 2mm thick were placed on both the upper and the lowe r surfaces of the specimens as electric insulators. In order to perform electrical measurements a sensitive programmable electrometer (Keithley 6514) was used. The data output from the electrometer was stored in a computer using a GPIB interface. The experiment was conducted in a Faraday shield to prevent measurements from being affected by electric noise.

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b)

DAQ (h)

DAQ (h)

Load cell (i)

Load cell (i)

(c)

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PC/storage (g)

PC/storage (g)

(f)

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(b)

(b)

(b)

(b)

Sensitive Electrometer (e)

Sensitive Electrometer (e)

(a)

(a)

(c) (d)

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Fig. 2. The experimental set-up for a) amphibolite and b) marble specimen. The experimental set-up included: (a) specimen, (b) copper electrodes, (c) Teflon sheets, (d)steel base, (e)Keithley 6514, (f) GPIB bus, (g) PC/storage unit, (h) Data Acquisition module (Keithley KUSB 3108), (i) Load Cell.