# Experimental Study on the Behavior of Polyurethane Springs for Compression Members
## 2 Design of Polyurethane Springs  
Polyurethane rubber springs capable of restoring their original shape were used as compression members; they are manufactured in a cylindrical shape and are used in various fields that require restoration force from external loads. Therefore, as shown in Figure 1, the polyurethane rubber spring specimen used in this study was manufactured in a cylindrical shape with a hole in the center. The shore hardness of the polyurethane rubber springs was composed of 95A, which was significantly larger than that of typical rubbers; hence, it could secure a high resistance with a relatively small area [32]. In addition, polyurethane rubber can develop desired properties and performance according to the polymer blending, and its microstructure can be confirmed using a scanning electron microscope (SEM) [33; 34; 35; 36]. The outer diameter (D) varied from 100 mm to 72 mm, according to the experimental conditions, and the inner diameter (D\({}_{\text{in}}\)) was uniformly fixed at 20 mm. The 20 mm central holes were drilled in order to prevent the separation of the specimen from the jig during cyclic loading tests by fastening experimental specimens to separation-preventing pins installed on the jig. In addition, these holes provided spaces into which the separation-preventing pins could be inserted to prevent the release of the polyurethane rubber springs from the device when applied to an earthquake damage reduction device, such as an actual damper or a seismic base isolator. The length of the experimental specimens was designed to decrease by 10 mm from 100 mm to 60 mm, depending on the experimental conditions. Moreover, the compressive stiffness (k\({}_{\text{c}}\)) of the specimen could be adjusted by varying both length and outer diameter. A total of 40m of specimens were produced in this study, and the names of the specimens comprised the variables for each case. Thus, the experimental specimens were named in the order of length (L, mm), outer diameter (D, mm), precompression strain (\(\varepsilon_{P}\), %), and maximum strain (\(\varepsilon_{max}\), %). as presented in Figure 1. However, since precompression is not considered in this study, precompression was 0. For example, the specimen 100 mm in length, 92 mm in outer diameter, with 0% precompression strain, and a 390% maximum strain was designed as the 100L-92D-0P-30% model. The experiments were mainly divided into two types. Because polyurethane springs are affected by the shape factor, the compressive stiffness varies depending on the shape. Therefore, in this study, the size and compressive stiffness of the specimens were set as variables and classified into two types. The Type 1 specimens had the same compressive stiffness, but were designed to feature five different sizes. The Type 2 specimens were designed to possess five different compressive stiffness values, in which the outer diameter of the specimen was fixed to 100 mm and the length decreased from 100 mm to 60 mm in steps of 10 mm, respectively. The actual specimens for each type produced for the experimental tests are presented in Figure 2. Ten specimens were designed for their own compressive stiffness by controlling the shape factor (S). In other words, the Type 1 specimens demonstrated almost similar compressive stiffness values because each case had a different length but similar shape factors. The Type 2 specimens had different compressive stiffness values due to an equal outer diameter but different shape factors for each case. The compressive stiffness (k\({}_{\text{c}}\)) of each specimen was calculated in accordance with Equation (1):  
\[\text{k}_{\text{c}}=\frac{\text{E}_{\text{a}}\text{A}}{\text{L}} \tag{1}\]  
where E\({}_{\text{a}}\) indicates the apparent elastic modulus of the polyurethane rubber spring, A is the cross-sectional area, and L stands for the length of the specimen. The apparent elastic modulus was calculated using Equation (2), which is a function of the shape factor and  
Figure 1: Design of experimental specimens. (**a**) Isometic view.