PSI - Issue 13

Ermioni D. Pasiou et al. / Procedia Structural Integrity 13 (2018) 2101–2108 E. D. Pasiou, S. K. Kourkoulis , M. G. Tsousi, Ch. F. Markides/ Structural Integrity Procedia 00 (2018) 000–000

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As a next step, proper elaboration of the raw DIC data permitted determination of the polar (radial and tangential) displacement components. Initially, the eccentricity along the loading axis is considered. In this direction, typical results for the ring and the “φ0_dxx.xx” class of specimens are plotted in Fig.4. The plots are realized along two circular loci, closely surrounding the periphery of the hole (r~16 mm) (Figs.4a,b) and that of the disc (r~0.98R o ) (Figs. 4c,d). It is clearly seen that the eccentricity along the loading axis only slightly affects the displacement field along both loci considered for d-values even up to R o /4. Considerable differences appear only for d-values exceeding R o /2. Things are different concerning the eccentricity along the axis normal to the loading line (i.e., for the “φ90_dxx.xx” class of specimens). The differences observed now for the displacements close to the periphery of the hole refer not only to their magnitude but, also, to the position at which the extreme values appear (Figs.5a,b). More specifically, it is clear that for increasing d (i.e., the hole moves away from the center of the disc to the right) the minimum value of the radial displacements do not appear on the loading axis of the specimens (i.e., at θ=90 o and θ=270 o , as it happens in the case of the ring) but at values within the 90 o <θ<270 o range. On the contrary, the displacements along the locus close to the periphery of the disc are only quantitatively affected (Figs.5c,d) compared to these of the ideal ring.

y

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(b)

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u θ [mm]

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Ring φ90_d6.25 φ90_d12.50 φ90_d25.00

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θ [deg]

Fig. 4. The radial and tangential displacement components around the hole (a, b) and along the periphery of the disc (c, d) for both the ring and the “φ0_dxx.xx” class of specimens. 4. Discussion and conclusions The influence of eccentricity on the displacement field in a circular disc eccentrically drilled throughout its thickness was quantified experimentally. The study was motivated by the fact that the ring test is quite often used as a substitute of the Brazilian-disc test, mainly because at the points of interest (i.e., the points of expected fracture) the stress field developed in a circular ring includes exclusively a single tensile stress component, contrary to what happens in a compact-disc, for which at the point of interest (i.e., the center of the disc) the stress field is biaxial and the respective stress tensor contains both a tensile and a compressive component. However, preparing ring-shaped specimens is more laborious, in comparison to that of compact-discs, due to practical difficulties in the accurate determination of the center of the hole. It is, thus, reasonable to expect that eccentricities will inevitably appear. Therefore exploring the potential role of these “imperfections” on the stress field developed in “non-perfect” (from the geometrical point of view) ring-shaped specimens appears crucial, since they could invalidate the advantages of the ring test. As a first step in this direction, it was decided to study experimentally the role of eccentricities on the displacement field, in order to provide a flexible “tool” that could be used for validation/calibration of the respective analytic and numerical solutions, for the stress field (Pasiou et al. 2018), which is, in fact, the quantity of importance for most engineering applications.