# Experimental Study on the Behavior of Polyurethane Springs for Compression Members
## 4 Experimental Results and Discussion  
### Type 1 Specim Size Control  
The Type 1 specimens had equal compressive stiffness but consisted of five different sizes, and there were twenty cases for each maximum strain rate of 25%, 30%, 35%, and 40%. The compressive behavior of the five different size specimens for the 30% maximum strain case is presented in Figure 5. The compression force (F) of the experimental specimens considerably decreased from the second loading after the first loading, and the unloading curve displays behavioral characteristics along a relatively similar path. In addition, residual displacement occurred as the compression force reached zero before the end of each cycle period. After the displacement also reached zero, it could be restored to a certain level. In the next cycle, the compression force increased again at the point where the corresponding displacement was not zero. This phenomenon may be attributed to the delay when polyurethane rubber springs are compressed and restored. Unlike other cases, the width of the curve at which the compression force decreased in the first cycle and the second cycle became very narrow (see Figure 5). This behavior took place because of the repeated experimental tests performed on the same specimen, which shows that when polyurethane rubber springs are subjected to a certain level of compression at least once, the same behavior cannot be obtained subsequently. In general, strength degradation takes place after the initial compression, and subsequently, the rate of decrease declines and thus induces an equal compression force at the same displacement. Figure 6 presents the difference between the compressive behavior of the first cycle and the subsequent cycle. After the first cycle, the closed curve area of the force versus displacement graph greatly reduced, resulting in a significant decrease in the energy dissipation capacity. However, the area of the closed curve slightly decreased from the second cycle, and there was no significant difference between the loading and unloading paths. By analyzing the compressive hysteresis behavior of the same specimen for each cycle separately, the difference between the first and the subsequent cycles can be observed more clearly. Figure 6 presents the clear difference in strength degradation and energy dissipation capacity between the first and the subsequent cycles. Because the compression behavior of the polyurethane rubber springs under cyclic loading demonstrated a large difference between the first cycle and the subsequent cycles, for the purpose of evaluating the performance of the polyurethane rubber springs, such as the energy dissipation capacity, it was classified into the first compression performance and the subsequent performance. The applied performance points were different depending on the application purpose and environment. Figure 4: Loading protocol of the cyclic loading test. Figure 5: Compressive behavior without precompression (Type 1 specimens). (**a**) 60L-72D-0P-30%; (**b**) 70L-77D-0P-30%; (**c**) 80L-82D-0P-30%; (**d**) 90L-88D-0P-30%; (**e**) 100L-92D-0P-30%. Figure 6: Force-displacement curve cycle separation. (**a**) Behavior of 1 st cycle; (**b**) behavior of 2 nd and 3 rd cycle. As can be seen in Figure 7, within the same maximum strain, the maximum force (F\({}_{\text{max}}\)) tended to increase when the length of the specimen increased. The length conditions of the experimental specimens were designed to feature the same stiffness by adjusting the length and cross-sectional area of the Type 1 specimens, and therefore the change in length can reflect the variables of the specimen sizes. At 25% and 40% maximum strains, specimens with a length of 90 mm demonstrated a greater maximum force than those that were 100 mm in length. On the other hand, at 30% and 35% strains, both the specimen length and maximum force demonstrated an almost perfectly linear relationship.