Experimental bending fatigue data of additive-manufactured PLA biomaterial fabricated by different 3D printing parameters

Additive manufacturing (AM) is used in several industries, such as automotive, aerospace, and medical sciences. One of the most common devices used in additive manufacturing is fused deposition modeling (FDM) 3D printers. This fabrication method has different inputs that affect the quality of the parts. In this research, the bending fatigue properties of polylactic acid (PLA) biomaterial made with a 3D printer are investigated. To demonstrate the influence of printing parameters on fatigue lifetime, standard specimens with nozzle diameters of 0.2–0.6 mm, extruder temperature of 180–240 °C, and print speed of 5–15 mm/s were printed. After performing fully reversed bending fatigue tests, it was found that printed specimens at 180 °C have the best fatigue lifetime in most cases. Accordingly, the fatigue behavior improved by reducing the nozzle diameter. Printing at lower temperatures also improved the fatigue lifetime. The printing speed affected the slope of the stress–lifetime (S–N) diagram, known as the fatigue strength exponent. Valuable experimental fatigue raw data (not analyzed) were presented for additively manufactured PLA biomaterials fabricated by different 3D printing parameters.


Introduction
With the development of various processes in additive manufacturing (AM), the study of parameters affecting the material properties of parts produced by these techniques has become important [1]. For example, Colton et al. [2] investigated the effect of print parameters and environmental conditions on two-dimensional and three-dimensional parts produced by the binder jetting (BJ) process. In addition, Azadi et al. [3] compared the fatigue properties of acrylonitrile butadiene styrene (ABS) polymers, manufactured by the fused deposition modeling (FDM), compared to the extruded ABS material.
Polylactic acid (PLA) has been widely utilized in biomechanical applications besides other industries. Therefore, researchers have focused on the properties of the structure, fabricated by the AM. Aghareb Parast et al. [4] studied the fatigue behavior of similar and dissimilar polymer welded joints created after an AM process. It was found that nonwelded PLA had lower fatigue resistance than PLA/PLA joints. In addition, PLA/PLA, ABS/ABS, and PLA/ABS had the highest fatigue resistance, respectively.
Abeykoon et al. [5] investigated the effects of the filling pattern, the density, and the speed on the mechanical, thermal, and morphological properties of the printed parts. Their results indicated that 215 °C and 90 mm/s were the most appropriate processing temperature and the infill speed for pure PLA filament, respectively. Alafaghani and Qattawi [6] used the design of experiments for the influences of the filling percentage, the infill pattern, the layer thickness, and the extrusion temperature on the mechanical properties of the parts by the FDM. Based on the results, a lower extrusion temperature, a smaller layer thickness, a lower infill percentage, and a hexagonal infill pattern generated a better dimensional accuracy. On the other hand, a higher extrusion temperature, an optimized layer thickness, a triangular infill pattern, and a higher infill percentage, increased the strength of FDM parts. Sandhu et al. evaluated the influence of slicing parameters on the surface roughness [7] and mechanical properties [8] of FDM prints. Travieso-Rodriguez et al. [9] demonstrated the fatigue behavior of PLA-wood composites produced by FDM. It was concluded that wood fibers had a negative effect on the PLA matrix in terms of mechanical behavior. However, the nozzle diameter was introduced as the most influential parameter in the fatigue lifetime, the printing speed did not affect the results. Moreover, a honeycomb pattern with 75% of the filling density, 3D-printed with a nozzle diameter of 0.7 mm and a layer height of 0.4 mm was recommended for making samples. Patil et al. [10] provided a paradigm of multi-objective functions for optimizing FDM process parameters for printing PLA components. The optimum parameters for this investigation were reported as the triangles pattern, the infill percentage of 70%, the printing speed of 100 mm/h, and the layer thickness of 0.2 mm.
It is necessary to design and optimize the polymeric materials to fully comply with the mechanical requirements. That is due to the effect of different 3D printing parameters on the parts fabricated by the FDM. Studies on the fatigue behavior of polymer materials are not sufficient and it is difficult to understand the role of the fatigue phenomenon in FDM 3D-printed polymers [11]. Therefore, in this study, the effect of printing parameters, including the printing speed, the nozzle diameter, and the extruder temperature, on the bending fatigue properties of standard PLA specimens were illustrated. The nozzle diameters were considered as 0.2-0.6 mm, the extruder temperatures were selected as 180-240 °C, and the printing speed was 5-15 mm/s.

Materials and tests
The standard dog-bone samples were fabricated according to ISO-1143 [12], as shown in Fig. 1a, as the CAD file and the 3D-printed specimen. It should be noted that to save material consumption and printing time, the least length of the cylindrical part in the samples is considered based on the standard value, as shown in Fig. 1b. To prepare the fatigue test samples, an FDM printer was used (Fig. 2). 3D printing was performed by transparent PLA filaments with a diameter of 1.75 mm, made by the YouSu Company.
Due to a large number of these parameters, the effect of three parameters, including print temperature, print speed, and nozzle diameter on fatigue lifetime, had been experimentally investigated at three different levels, based on Table 1, and other parameters were constant, according to Table 2. A three-level full factorial approach in the design of experiments was considered to study the interactions between input variables. Therefore, 27 samples were fabricated in a similar 3D printing condition. Moreover, for each 3D printing condition, various specimens were considered under fatigue testing at different stress levels. Finally, the total number of standard test samples was 175.
The printing speed affects the quality and also the production time of specimens. On the other hand, the physical specifications of the printer must be considered. According to the structure of the introduced 3D printer, speeds of 5, 10, and 15 mm/s were selected based on the possibility. Moreover, at high temperatures, materials do not retain their shapes. In addition, at low temperatures, the material is not soft enough. Therefore, to deposit layers of the filament, it is necessary to heat it to a temperature between the melting temperature and the glass transition temperature [13]. Based on this issue, the nozzle temperature between 180 and 240 °C was selected to find the appropriate printing temperature. In addition, according to studies [14,15] and the nozzles available in the commercial market, the nozzle diameter of 0.2, 0.4, and 0.6 mm has been used to 3D print the samples.
Bending fatigue tests were performed under fully reversed cyclic loads through a high-cycle fatigue regime with the rotating bending fatigue testing machine at room temperature with a frequency of 100 Hz. This test device, made by the Santam Company, is shown in Fig. 3. Moreover, Fig. 3 illustrates the 3D-printed PLA specimen under testing conditions.  Table 3. These data (based on Table 3) show that the fatigue tests were performed correctly and the results were also comparable and reliable. Moreover, the trend of all data and also the scatterband of obtained results could be compared accordingly.
The results of the high-cycle fatigue tests showed that the samples printed at a temperature of 180 °C had a higher number of cycles to failure, in most cases. Therefore, the samples 3D-printed with a nozzle diameter of 0.2 mm with a printing speed of 5 mm/s at a temperature of 180 °C had the highest fatigue lifetime compared to other specimens. In contrast, the lifetime of printed samples with a nozzle diameter of 0.4 mm with a printing speed of 5 mm/s and an extruder temperature of 240 °C was about 34,000% shorter and these samples had the shortest lifetime. The smaller nozzle diameter improved the fatigue properties. The lifetime of the specimens decreased as the print temperature increased. In addition, increasing the speed of printing reduced the fatigue strength exponent. As a result, in samples 3D-printed with a 0.2 mm nozzle, the slope of the stress-lifetime (S-N) plot at 240 °C decreased by 61%. Depending on the materials and conditions of use, the effect of the parameters varies and must be considered at the design stage. For example, higher printing speed improves tensile properties, while temperature has been reported to not affect the tensile properties [16].
In Table 4, all experimental data are reported including the averaged value and the standard deviation for the fatigue lifetime of various PLA samples, which were 3D-printed under different conditions. As it could be seen, the standard deviation of fatigue testing was in an appropriate mode with an averaged scatter-band ratio of 23%, for the ratio of the standard deviation to the averaged fatigue lifetime. In other words, unless one value (50.33%) for the R 2 value, all other coefficients of determination were more than 78%, which illustrated a proper scatter-band of experimental data.

Conclusions
Additive manufacturing (AM) is a technology that has the potential to change conventional industrial manufacturing processes. However, the parameters used for the development of components with AM have a significant impact on their behavior and properties. Accordingly, in the present    article, the experimental bending fatigue data of additivemanufactured PLA biomaterials were presented under different 3D printing conditions, which highlighted results are obtained as follows, • The influence of input parameters for 3D printing was checked on PLA. The nozzle diameters were 0.2-0.6 mm, the extruder temperature was 180-240 °C, and the printing speed was 5-15 mm/s. • The decrease in printing temperature led to an increasing trend in the fatigue lifetime, under fully reversed bending loads. • The smaller the nozzle diameter caused to have the longer fatigue lifetime. • Increasing the printing speed reduced the slope of the stress-lifetime (S-N) diagram and thus, this printing variable affected the material behavior through the low-cycle fatigue regime. • 3D printing parameters could have different influences on the material behavior, depending on the materials and the conditions of the use.