PSI - Issue 51

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 51 (2023) 81–87 Mohammad Reza Khosravani et al. / Procedia Structural Integrity 00 (2022) 000–000

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3. Experimental setup and procedure

3.1. Design and fabrication of specimens

In the current study, two group of specimens are designed, printed, and examined: (i) dumbbell-shaped test coupons, and (ii) single edge notch bending (SENB) specimens. Here, we used biocompatible photopolymer resin which a suitable material for fabrication of dimensionally accurate implant guides. The first group of specimens are examined to determine basic mechanical properties. The dumbbell-shaped specimens are designed according to the ASTM D638 (ASTM D638, 2014). In order to determine the fracture behavior of dental resin, SENB specimens are designed according to ASTM D5045-14 (ASTM D5045, 2014). The length of the pre-crack a 0 is 0.5 times the width W . According to the standard, the length of pre-crack should be selected such that 0.45 < a / W < 0 . 55. Three-point bending tests were conducted where the span and width used in the tests are S = 80mmand D = 20 mm, respectively. The thickness of all specimens was kept at the same value of 10 mm. Geometries of dumbbell-shaped and SENB specimens are illustrated in Fig. 2. All samples are first drawn in a CAD platform and then saved as ‘.stl’ format. The specimens were printed using Formlabs Form 2 SLA printer. The slicing software was set to the highest printing quality. After printing, the supports structures were removed and the specimens were cleaned using an isopropanol bath. In addition, the specimens were subjected to as post-curing process. Multi-purpose mini razors were used in order to cut the preexisting cracks in the SENB specimens. After printing process of an SLA parts, they remain on the build platform in a green state which means the parts have reached their final form, but the polymerization reaction is not yet completed, and full mechanical properties are not yet achieved. Material properties (e.g., modulus) would be improved by post-curing printed parts. It is noteworthy that each type of resin requires unique post-cure settings which should be selected based on the manufacturers’ datasheets.

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Fig. 2. Schematics of SLA 3D-printed specimens (Dimensions in mm).

The printing process parameters (e.g., printing direction and infill density) have influence on the structural perfor mance of the parts. In this study, printing parameters were kept constant in fabrication of all test coupons. Here, six specimens in each group are printed to conduct experimental tests and check the repeatability.

3.2. Tensile and three-point bending tests

The dumbbell-shaped specimens were subjected to the tensile load using a hydraulic machine at room conditions: 23 ± 3 ◦ C and 50 ± 5%, temperature and relative humidity, respectively. The machine is fitted with 15 kN load cell, and it has cross-head speed range 0.01 mm / s to 30mm / s. Electronic control unit allows monitoring the applied load and movement of the top cross head. The series of tensile test are performed under displacement control via constant cross-head movement of 5 mm / min which provides a static loading conditions. In series of test, the shoulder portions of the specimens were clamped directly by the wedge grips of and the tensile load was applied uniaxially to the test coupons. The displacements on the specimens were measured by using a linear variable displacement transducer. In Fig. 3, load-displacement curves and corresponding stress-strain curves of dumbbell-shaped specimens are illustrated.