PSI - Issue 37

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 37 (2022) 97–104 Mohammad Reza Khosravani et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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3.2. Experimental tests This section provides information regarding performed tests on intact and defected specimens. A series of uniaxial tensile tests was conducted on a hydraulic machine at room temperature. The machine was fitted with 15kN load cell with a cross-head speed range of 0.01 mm/s to 30 mm/s. The tensile tests were performed in the displacement control mode at a displacement rate of 5 mm/min according to ASTM D638 (ASTM D638 (2014)) which provides a static loading conditions. In the experimental practice, no external extensometer was used for strain measurement. The displacements on the specimens were measured by using a linear variable displacement transducer. Fig. 2 shows an intact specimen prior and after the loading.

Fig. 2. An intact 0 o specimen; prior to loading (left) and after applied tensile load (left).

It is noteworthy that the tests were repeated six times for each raster orientation to confirm that the obtained results are reliable. All specimens were tested under the same conditions and the experimental findings are presented in the following section.

4. Results and discussion Experimental tests showed a brittle fracture in both intact and defected PLA-wood specimens. In both groups of the examined specimens, the fracture occurs in the gauge section. The weakest points inside 3D-printed specimens must be paid attention when taking practical applications into account. In the FDM printed parts, the molecular thermal diffusion at the interface play a crucial role in the bonding strength between two adjacent deposition roads. Indeed, more molecular entanglement leads to a better bonding strength and therefore higher mechanical properties. Based on the experimental observation, different phases like initial loading, deformation, crack initiation, and final rupture can be considered for the examined specimens. According to the crack propagation mechanism, after loading, the cracks propagate along with the weakest point. Therefore, in the defected specimens, fracture was occurred at the place of intentional defect. Experimental observation confirmed that defect in the 3D-printed specimens leads to decrease in the failure load. Fig. 3 shows force-displacement curves of intact and defected specimens printed with different raster orientations. The applied tensile load is increased almost linearly with displacement according to the specimen stiffness until the peak load is reached. Experimental results indicated that the fracture load is decreased due to the defect in all cases (different raster directions). Experimental findings demonstrated relatively a large reduction in fracture load was