PSI - Issue 61

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Shadi Salamatian Hosseini et al. / Procedia Structural Integrity 61 (2024) 20–25 Salamatian Hosseini et al. / Structual Integrity Procedia 00 (2024) 000-000

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performed by Ramesh and Panneerselvam (2021), the infill parameters including infill pattern and infill density were introduced as the main effective factors in improving the ultimate tensile strength. Also, Liu et al. (2023) showed that the triangle infill pattern provided a better performance from a compressive strength point of view. It is worth noting that the mechanical behavior of FDM products varies in different temperatures relative to the glass transition temperature of polymers (Hsueh et al. (2021)). Recently, some researchers have focused on the fracture behavior of the FDM specimens under different loading conditions. To this end, Hart et al. (2018) investigated the influence of layer orientation on the mode I fracture behavior of FDM-ABS specimens. The results confirmed that the specimens printed horizontally had higher fracture resistance compared to those printed vertically. Razavi et al. (2022) investigated the mixed-mode I/II fracture performance of FDM-ABS specimens printed with different layer and raster configurations. It was observed that when the initial pre-crack was perpendicular to the printed layers, higher fracture properties were obtained in both mode I and mixed-mode I/II loading conditions. According to the literature, most of the research papers were about the effects of limited manufacturing parameters including layer orientation, raster angle, nozzle temperature, etc. on the mechanical and fracture behaviors but there is a lack of information about how the nozzle diameter affects the mechanical properties and fracture resistance of the FDM specimens. Thus, the current research study aims to explore the effects of different nozzle diameters on the basic mechanical properties and mode I fracture behavior of FDM-PLA samples. 2. Theoretical background Based on the linear elastic fracture mechanics, brittle fracture occurs when the value of K I in a cracked body reaches its critical value K Ic (i.e., fracture toughness). But for the materials with limited plasticity around the crack tip, another parameter named J (J-integral) is introduced that is related to the rate of change in the net potential energy for non-linear elastic solids. In the cases of crack problems with elastic and elastic-plastic behavior, fracture initiates when the value of J reaches the J c (i.e., the critical value of J-integral) (Anderson (2017)). 3. Material and experimental procedure 3.1 Material Polylactic Acid (PLA) filament (1.75 mm in diameter) with the mechanical properties illustrated in Table 1 was used for printing the samples. Some manufacturing parameters were selected based on the parent material data sheet, for instance, nozzle and bed temperatures were 210 o and 60 o C, respectively. Also, 60 mm/s printing speed was utilized for the printing procedure. Layer thickness and raster width were 0.2 and 0.42 mm, respectively. Four different nozzle diameters of 0.4, 0.6, 0.8, and 1 mm were considered to print the tensile and fracture samples.

Table 1. Basic mechanical properties of PLA filament (Wittbrodt and Pearce (2015)). E , ( GPa ) Y  , ( MPa ) UTS  , ( MPa ) Maximum elongation ( % ) 2.3-3.2 40-50 54-60 2.2-5.1

3.2 Tensile tests For conducting the tensile experiments, dog-bone-shaped samples were considered according to the ASTM D638 standard (2014) (see Fig. 1) and printed through a FDM machine by using the manufacturing parameters mentioned above. It should be noted that two raster angles of 0/90 o and 45/-45 o were used in the printing procedure. Then, tensile tests were conducted using a universal testing machine Santam STM-150 under displacement control at the rate of 1 mm/min. To obtain stress-strain curves, stresses were evaluated by dividing the external loads by the cross-section of the dog bones. Besides, Digital Image Correlation (DIC) technique was implemented to calculate the strains. Table 2 provides the average tensile properties for the FDM-PLA samples printed with four different nozzle diameters of 0.4, 0.6, 0.8, and 1 mm (each tensile experiment was repeated three times). Based on Table 2, the highest ultimate tensile strength belonged to the FDM-PLA dog bones printed with a nozzle diameter of 1 mm and raster angle of 0/90 o . Also, the lowest one was related to the sample with a 0.6 mm nozzle diameter and raster angle of 0/90 o . Comparing the raster orientations in each case of nozzle diameter revealed that the 45/-45 o raster angle resulted in higher elongation and ultimate tensile strength.