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

2104

4

Table 1. The specimens used in the study. Code

φ ( deg )

d ( mm )

Ring

-

0

φ0_d6.25 φ0_d12.50 φ0_d25.00 φ90_d6.25 φ90_d12.50 φ90_d25.00

0 0 0

R o /8 = 6.25 R o /4 = 12.50 R o /2 = 25.00 R o /8 = 6.25 R o /4 = 12.50 R o /2 = 25.00

90 90 90

2.2. The Digital Image Correlation (DIC) technique DIC is a non-contact optical sensing technique. For its application, the surfaces monitored are covered by a dense grid of dots (speckles). Either one or two cameras capture the surface of the specimen before loading (“undeformed” state) and during the loading procedure (“deformed” states). Using specially developed software, the technique corre lates the position of each speckle with respect to a reference speckle in every photo taken, providing finally the dis placement field developed all over the surface of the specimen. The theoretical background of the technique is dated more than thirty years ago (Sutton et al. 1986), and the technique is since then continuously developed. Initially the technique used a single-camera system providing only two dimensional displacement fields. This limitation was over come later using two cameras (3D DIC) (Sutton et al. 2000) monitoring the specimen from different angles. Nowadays, DIC is considered as a powerful and flexible sensing tool and it is worldwide used in a wide variety of engineering problems, ranging from the determination of crack tip opening displacement and fracture toughness (Brynk et al. 2012) to the assessment of the efficiency of fixation techniques for acetabular fractures (Kourkoulis et al. 2017). In the present protocol, a 3D DIC system by LIMESS (Messtechnik & Software GmbH, Germany) was used, including two digital cameras (resolution 1234×1624 pixels), two sources of strong white light and the respective software. The size of the “optical window” is adjustable ranging from 10 mm 2 to 1000 mm 2 . The accuracy of the system is equal to 0.01 pixel and 0.02% for displacement and strain measurements, respectively. 2.3. The experimental set-up For the needs of the main experimental protocol, the device suggested by ISRM for the standardized Brazilian-disc test was used. All experiments were carried out with the aid of an electromechanical ΜΤS Insight loading frame of capacity equal to 10 kN. A semi-spherical rigid metallic head is placed between the moving traverse of the frame and the upper jaw of the apparatus ensuring normality of the applied load (see Fig.2a). The dimensions of the “optical window” of the DIC technique were adjusted to 135x105 mm 2 , in order to monitor the whole specimen. As a result, the speckle’s size was calculated equal to about 0.2-0.3 mm. Before the onset of each test the undeformed state of the specimen (ignoring the own weight of the upper jaw) was photographed by the cameras of the DIC system. During loading each specimen was photographed at a rate equal to one photo every four seconds. All experiments were carried out under quasi-static conditions and displacement-control mode at a rate of 0.3 mm/ min. The purely mechanical quantities recorded during each experiment were the load induced by the frame and the displacement imposed on the upper jaw of the loading fixture. The overall experimental set-up is shown in Fig.2. 3. Results Taking into account that the tests were to be considered comparatively the fracture load of the weakest specimen was used as reference load for all experiments. Therefore the experiments were terminated either when the specimens were fractured or when the load-displacement curve deviated from linearity or when the capacity of the frame was exhausted. Typical load-displacement plots are shown in Fig.3a. Ignoring inevitable initial bedding errors, the plots are almost perfectly linear during the whole duration of the tests, as it was expected for the specific material.