PSI - Issue 51

Maja Dundović et al. / Procedia Structural Integrity 51 (2023) 192 – 198 M. Dundovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. (a) Schematics of bottom-up DLP technology; (b) Batch of printed corner-filleted hinge specimens.

2. Experimental method The material properties have been determined by three-point bending tests on rectangular beam samples with dimensions in accordance to ASTM D 790 standard. A rough estimate of fatigue strength and a qualitative assessment of photoelastic methods in dynamic tests on DLP printed corner-filleted flexure hinge samples have been performed by dynamic cantilever beam bending experiments. Three groups of specimens were printed separately to prevent unwanted additional curing from light exposure due to different sample heights. Namely, the vertically and horizontally printed rectangular beams (for flexural tests), and corner-filleted flexure hinge samples. All test samples have been printed from transparent photoreactive DentaCLEAR resin on an ASIGA MAX UV 3D printer. The layer height has been set to 0.1 mm and all other printing parameters have been set according to material defaults. After printing, each part has been individually cleaned in alcohol in order to remove any uncured resin residue. Afterwards, the specimens have been left in a dry and dark place for 72 hours at room temperature. 2.1. Flexural tests Three-point bending tests have been performed in order to determine flexural properties for vertical and horizontal printing orientations, namely the stress-strain curves and flexural modulus. A rectangular cross section bar 3.5 mm in depth, 14 mm wide and 70 mm long, shown in Fig. 1a, resting on two supports has been loaded with loading nose placed halfway between the supports until rupture. A STEP Labs electro-mechanical testing machine for static and dynamic testing with a maximal static force of 16 kN has been used. The loading nose and supports were configured to 5 mm and 3.2 mm radius cylindrical surfaces, respectively. The length of the support span was 16 times the depth of the beam, that is 56 mm. The rate of testing machine crosshead motion has been 1.5 mm/s. The tests have been conducted in environmental conditions of 23 ± 2 °C temperature and 50 ± 5 % relative humidity. The load-deflection data has been measured by the 25 kN load cell and the motor encoder data, used to derive relative motion of the loading nose in regard to the supports. The width and thickness of the test specimens used for flexural modulus calculations were measured before the testing with a ratcheting hand micrometer to an incremental discrimination of 0.025. 2.2. Corner-filleted flexure hinges experiments Corner-filleted flexure hinge test samples, shown in Fig. 2b, have been manufactured for dynamic testing. The cross section of the hinge compliant part has been exactly 3,5 mm deep and 14 mm wide, according to ASTM D790 standard test sample. The vertically printed flexure hinges were clamped as a cantilever beam with the load applied to the free end. Displacement controlled cyclic tests have been conducted at two stress levels at a frequency of 1 Hz.