# Experimental Study on the Behavior of Polyurethane Springs for Compression Members
## 5 Conclusions  
In this study, the compression behavior characteristics of polyurethane springs that can be used as compression members in seismic devices, such as dampers and seismic isolators, were experimentally verified. A cyclic loading test was performed on 40 specimens with different conditions, and size and compressive stiffness were set as the design variables. The maximum force, residual strain, and energy dissipation were used as performance indicators, the relationship with each design variable was identified, and the possibility of using a polyurethane spring as a compression member and considerations were reviewed. The main conclusions are as follows:  
The polyurethane springs underwent stress relaxation after the first loading owing to the Mullins effect under cyclic loading conditions, and they were relatively stabilized in the second cycle. Under the same stiffness conditions, the size and maximum force of the specimens were almost proportional, and at 35% and 40% of the maximum strain, the compressive stiffness and maximum force demonstrated a linear relationship.  
At the maximum strain of 25% to 40%, the maximum force typically presented a linear relationship with the maximum strain. This trend can be applied by idealizing the linear  
Figure 15: Energy dissipation reduction ratio of all Types.  
relationship with the maximum force within a specific strain range and introducing it into the design formula of a polyurethane spring.  
The unloading, residual, and recovered strain could not confirm a significant trend according to compressive stiffness and size. However, except for some cases, the unloading strain and residual strain tended to increase in proportion to the maximum strain. Furthermore, they appeared at a similar level irrespective of the two variables. Therefore, it is concluded that it is necessary to study other variables that affect unloading, residual, and recovered strain.  
The energy dissipation demonstrated a decrease rate of about 61.81% after the first cycle under cyclic loading conditions. The reduction ratio of the energy dissipation displayed a tendency to be concentrated near the average value, although there was a slight deviation. It is concluded that the two variables set in this study did not significantly affect the reduction of energy dissipation and were determined by the intrinsic properties of the material. In addition, under cyclic loading conditions, the polyurethane spring lost its original performance significantly by reducing its initial energy dissipation ability by more than 60%. Therefore, the energy dissipation of polyurethane springs is not expected to significantly contribute to the energy dissipation capacity of the system.  
Finally, as a result of analyzing the effect of the compressive stiffness and size established in this study on the performance index of the polyurethane spring, the relationship with the maximum force could be clearly verified. If additional research is carried out, the possibility of deriving a simple design formula by linearly simplifying the nonlinear compression behavior of polyurethane springs can be confirmed. However, in the case of residual displacement and energy dissipation capacity, significant results were not obtained, so studies for deriving additional design variables should be conducted.  
Conceptualization, [PERSON] and J.W.H.; methodology, [PERSON] and J.W.H.; validation, [PERSON] and J.W.H.; investigation, [PERSON] and J.W.H.; writing--original draft preparation, [PERSON] and J.W.H.; writing--review and editing, [PERSON] and J.W.H.; visualization, [PERSON] and J.W.H.; supervision, J.W.H.; project administration, [PERSON] and J.W.H.; funding acquisition, J.W.H. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2021R1A2B5B02002599).  
Not applicable.  
The authors declare no conflict of interest.