PSI - Issue 31

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 31 (2021) 105–110 Mohammad Reza Khosravani et al. / Procedia Structural Integrity 00 (2020) 000–000

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Although 3D printing was initially used for prototypes, due to its advantages and the increasing demand for the fabrication of components, currently it has been used for fabrication of end-use products. Therefore, several engineer ing aspects such as mechanical strength, durability, fatigue resistance, and physical appearance of 3D-printed parts have become important issues. Among mentioned 3D printing methods, FDM is the extrusion-based method utilized for fabrication of polymeric structural components. Extrusion-based processes are one of the most widely used 3D printing technologies. In di ff erent printing processes, several printing parameters need to be considered. For instance, in the FDM process, nozzle temperature, raster direction, layer thickness, extrusion temperature, and printing speed must be defined. Although documented results showed influence of parameters on the mechanical strength of poly meric printed parts, the failure mechanism of di ff erent polymeric and metallic 3D-printed parts is still unclear. In this context, printing factors, utilized material, powder morphology, bonding feature, reinforcement, and manufacturing defect play crucial roles. 3D printing technology indicated several advantages and benefits compared to conventional manufacturing pro cesses, but challenges exist in this manufacturing method. In detail, mismatched material properties and imperfection in additively manufactured parts are examples of inherent challenges in this field. Therefore, experimental practices are required to determine and observe influence of di ff erent parameters in structural performance of 3D-printed parts.

3. Experimental setup and procedure

3.1. Design and printing of specimens

In this work, polylactic acid (PLA) material was used to prepare specimens based on FDM process. In detail, dog-bone shaped specimens were designed and printed according to ASTM D638 (ASTM D638, 2014). Here, the specimens with two contours and raster directions of 0 ◦ and 90 ◦ were printed. The raster direction shows the printing direction with respect to loading direction. Hence, 90 ◦ refers to the specimens that would be subjected to the loading transverse to the raster direction. Here, the specimens were printed with unidirectional layup and infill density of 100%. It is noteworthy that in fabrication of all specimens, printing parameters such as extruder width, printing speed, and layer thickness were kept constant to ensure consistent printing quality. In Fig. 2, defected dog-bone shaped specimens and FDM printing parameters are schematically illustrated. As it shown, there are missing extrudates in the middle of the specimens which are considered as a manufacturing defect. In this study, at least six specimens for each above-mentioned raster direction were fabricated ans examined.

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Contour width

Contour to contour air gap

Raster angle

Air gap

Number of contours

Raster width

Fig. 2. Schematics of printed specimens and FDM printing parameters (Dim. in mm).

Di ff erent manufacturing defects such as overlaps, gaps, and o ff set can be occurred during 3D printing process. In order to determine influence of defect on the mechanical behavior of 3D-printed parts, gaps are intentionally placed into the specimens. Here, missing extrudates are along the length and the width in 0 ◦ and 90 ◦ specimens, respectively. It is noteworthy that the missing extrudates are oriented with the raster direction and they designed in a way to provide the same defect area percentage in all defected specimens. Both groups of intact and defected specimens experienced accelerated thermal ageing conditions which is explained in the following subsection. In this context, accelerated thermal ageing test is an e ffi cient experimental practice to evaluate the thermostability of structural components.