# Experimental Study on the Behavior of Polyurethane Springs for Compression Members
## 4 Experimental Results and Discussion
### Type 2 Specimen Compressive Stiffness Control  
In Type 2, a constant outer diameter of 100 mm was maintained, and the lengths of individual specimens were different at 60, 70, 80, 90, and 100 mm, so that the compressive stiffness of each case was designed to feature different values. The compressive behavior of each case demonstrated similar behavior without a significant difference from Type 1, and after the first cycle, compressive behavior stabilized with stress reduction. The compressive stiffness decreased as the length of the specimen increased (see Figure 11). In other words, the change in the length of the specimen can be regarded  
Figure 10: Behavior of three types of strain corresponding to each maximum strain in Type1. (**a**) Unloading strain; (**b**) residual strain; (**c**) recover strain. as a change in the compressive stiffness of the specimen. Therefore, the relationships between the compressive stiffness and the maximum force were almost similar between the length and the maximum force. Because the cross-sectional area was constant, as the length increased in steps of 10 mm (from 60 mm to 100 mm) each, the compressive stiffness decreased in inverse proportion and the two variables had a negative linear relationship. The relationships between the length and the maximum force under the same maximum strain condition were also observed between the compressive stiffness and the maximum force, considering the variation in the length of the specimen as a change in the compressive stiffness. As the compressive stiffness of each specimen decreased under the maximum strain conditions of 25%, 30%, 35%, and 40%, a decrease in maximum force was also observed, as presented in Figure 12, and the two variables displayed a relatively linear relationship. The R\({}^{2}\) values obtained through the linear regression analysis for the 35% and 40% maximum strain conditions were 0.858 and 0.840, respectively, and the relationships between the two variables were relatively clear. However, the 25% and 30% maximum strain conditions yielded the R\({}^{2}\) values of 0.1673 and 0.628, respectively, demonstrating weaker relationships between the two variables than the other strain cases. The relationships between the maximum force, maximum strain and compressive stiffness of the specimens are depicted in Figure 13. As mentioned above, the maximum force increased as the compressive stiffness decreased, and the maximum force increased proportionally with the maximum strain. The first-order linear regression equations for the maximum strain and maximum force for each specimen are listed in Table 4. For all the specimens, the R\({}^{2}\) values are close to 1, and thus the regression equation accurately reflects the measured data. As can be observed in the Type 1 specimens, the relationships between the strain and maximum force of the specimen in a fixed compressive stiffness state exhibited a linear relationship. The relationships between the two variables can be simplified through a first-order linear regression analysis. Figure 11: Relationships between compressive stiffness and length in Type 2 specimens. Figure 12: Relationships between stiffness, length, and maximum force. (**a**) Type 2-25%; (**b**) Type 2-30%; (**c**) Type 2-35%; (**d**) Type 2-40%. Figure 13: Relationships between maximum strain and maximum force. The unloading strain slightly increased or decreased as the length of the specimen varied, but the increase and decrease were small and occurred at the same level, respectively (see Figure 14a). Under the conditions of 25%, 30%, 35%, and 40% of the maximum strain, the average unloading strains were 2.54%, 4.13%, 4.18% and 5.42%, respectively. It can be inferred that the stiffness of the specimen did not significantly influence the unloading strain. In addition, when the unloading strain had the same length as the specimen, the specimens with the lengths of 80, 90, and 100 mm displayed a tendency to increase as the maximum strain increased.