# Experimental Comparison of Fastener Implementation Approaches in Fused Deposition Modeling
## 3 Results and Discussion  
In this study, three different sample designs for the tensile strength test and four different sample designs for the three-point flexural test were produced in three infill ratios. A total of 144 samples, 12 parts in each, were tested for tensile strength. A total of 180 samples, 12 parts in each, were tested for three-point flexural with an Instron test bench. A total of 144 samples, 12 parts in each, were tested for torque. The deformed samples are shown in Figure 7. In Figure 7a,b, it is observed that the heat-set inserts came off when separating from its surface inside the type I.a samples. The insert used in type I.b does not easily separate from the surface under load and disintegrates the infill structure. During the tests of type I.b samples, minimal deformation was observed on the surface before reaching ultimate strength. The other two types of samples shown in Figure 7c,d were deformed by first disintegrating the infill structure and then tearing the upper surface of the part. The three-point flexural test samples are shown in Figure 8. Type I samples show minimal cracks, as well as blank samples. Among all the samples, type II samples have the most damage, with more crack lines, and some of the type I.b samples were cracked into two pieces. Figure 6: Tensile test bench setup. After the torque tests, no visible change was observed on the outside of the samples. In the type II samples, it was observed that the square nut first rose by breaking the infill between the nut and the bolt, and then it started to spin freely. In type III samples, it was observed that the screws were stripped in some of the 40% filled samples. The tensile test results for one of each group are given in Figure 9. Each of them becomes more durable as the filling ratio increases, but this is not a linear relationship. Additionally, with increasing infill ratios, the type III design becomes more brittle. Figure 8: Sample parts after flexural test: (**a**) type Ia, (**b**) type Ib, (**c**) type II, (**d**) type III, (**e**) blank. Figure 7: Sample parts after tensile test: (**a**) type Ia, (**b**) type Ia detailed, (**c**) type I.b, (**d**) type Ib, detailed, (**e**) type II, (**f**) type III. The average ultimate strength of all the tensile samples is shown in Figure 10. Type Ia samples with 10% infill have an average of 301 N tensile strength and 13% standard deviation. Samples with 20% infill have 370 N tensile strength and 16% standard deviation. Tested samples with 40% infill have 428 N tensile strength and 14% standard deviation. The strength of type Ia samples varies slightly depending on the infill ratio compared to the other designs. In type Ia, the force transfer between the fastener and the part body occurs through the cylindrical surface that melts during the assembly of the fastener. The surface area between the fastener and part body remains the same regardless of the infill ratio; because of this, strength is not directly related to the infill ratio. Type I.b showed the best result of the tensile test: for the samples with 40% infill, the average strength is 1080 N with a standard deviation of 6%. Samples with 10% infill have 502 N strength with 7% standard deviation, and 20% samples have 701 N strength with 6% standard deviation. Figure 10: Tensile test results of samples. Figure 9: Tensile test load/displacement results. Type II samples show significant improvement in terms of increasing the infill ratio, and the results are almost linearly related to infill. For the 10% infill ratio, the average tensile strength is 435 N, and the standard deviation is 7%. Samples with 20% infill have a tensile strength of 594 N with a standard deviation of 7%. Type II samples printed with 40% infill have a tensile strength of 885 N with a standard deviation of 3%. The results for type III samples show some improvement with increasing infill. The average tensile strength of the 10% infill samples is 547 N, and the standard deviation is 8%.