## 3 Results and Discussion  
### Bolt Preload  
The preload variation of the 3D-printed bolt under cyclical temperature changes from 10 \({}^{\circ}\)C to 40 \({}^{\circ}\)C is depicted in Figure 4. From the figure, the preload of the ABS-2 bolt increased slowly over the heating process and was 1025 N at the end of the heating process (40 \({}^{\circ}\)C) of the first cycle of cyclical temperature changes. Following this, the clamp force decreased sharply at 720 N, while the temperature of the joint was 25 \({}^{\circ}\)C. In this interval, the bolt softened by the effect of temperature. Therefore, the preload of the joint decreased dramatically. Following this, the preload went up significantly to 877 N, which means that the bolt started to shrink until the end of the cooling process (10 \({}^{\circ}\)C). The point that should be noticed here is that the loosening of the ABS-2 bolt was 123 N after the first cycle when the preload at the end of the first cycle is compared to the initial preload. Then, the preload increased slightly to 935 N at the peak point of temperature changes in the heating process of the second cycle and went down tremendously to 685 N when the temperature of the joint was 20 \({}^{\circ}\)C. The preload went up significantly to 861 N at the end of the cooling process (10 \({}^{\circ}\)C). If the preload after the second cycle is compared to the preload after first cycle, the bolted joint loosened by 16 N. In the rest of the cycles, the bolted joint loosened by 1 N and 4 N. Furthermore, it can be assumed that after the second cycle, the ABS-2 bolt did not loosen.  
The glass bolt loosened to 898 N in the beginning cycle. However, the preload kept loosening in the rest of the cycles and was 814 N after the final cycle, which means that the glass bolted joint loosened by 42 N more than the ABS-2 bolted joint. When it comes to the PLA bolt, the preload of the PLA bolted joint increased slowly up to 1024 N when the temperature was 30 \({}^{\circ}\)C in the heating process, then the preload started to go down and was 1005 N when the temperature hit the peak point. The reason for this is that the glass transition temperature of PLA material is much lower than that of ABS-2 and glass materials [24,25]. Hence, the loosening in the PLA bolt started earlier compared to the other materials. Following that, the preload kept decreasing until the temperature reached 25 \({}^{\circ}\)C in the cooling process, and it measured 466 N. Then, the preload went up to 686 N at the end of the beginning cycle of temperature changes. In the rest of the cycles, the bolted joint loosened to 641 N, 597 N, and 584 N. From the results of the experiments, it can be assumed that there was a similar periodic result after the second cycle. Therefore, four cycles of temperature changes were considered satisfactory for analysis of all 3D-printed bolts.  
Figure 5 depicts the preload changes of three different 3D-printed bolts under cyclical temperature changes from 10 \({}^{\circ}\)C to 80 \({}^{\circ}\)C. It can be noticed from the figure that there was a similar scenario with the lower temperature-change test condition. The starting point of loosening of the bolted joints, however, is an exception since the bolts began to loosen earlier, before the temperature reached the peak point of 80 \({}^{\circ}\)C, than that of a lower temperature-change condition. The preload of the ABS-2 bolt was 708 N, which loosened by 292 N after the first cycle of temperature changes, and in the rest of cycles, it kept loosening  
Figure 3: Experimental set up.  
by 690 N, 668 N, and 664 N, respectively. The glass and PLA bolts loosen by 254 N and 359 N, respectively after the first cycle, and then they did not loosen any more by the end of the test. It can be assumed from the results that glass and PLA bolts reached the maximum loosening that can be caused by the amplitude of the temperature changes after the first cycle.