# Preparation and Properties of Waterborne Polyurethane and SBS Composite-Modified Emulsified Asphalt
## 3 Results and Discussion
### Fatigue Performance Analysis  
During the LAS test, asphalt undergoes shear stress as the shear strain increases, making the stress-strain curve indicative of the material's dependence on strain and the damage characteristics during the shear process [35]. Figure 11 presents the stress-strain curves obtained from the LAS test for four modified asphalt evaporation residues. All three dosages of PU+ exhibited stress-strain curves with significant yield stress and yield strain. Initially, the shear stress increased rapidly with shear strain, then gradually decreased after reaching its maximum value. The maximum shear stress is defined as the yield stress, and the corresponding shear strain is defined as the yield strain.  
PU+9 demonstrated the highest yield stress and the fastest increase in shear stress with strain before reaching the yield strain, indicating a strong resistance to deformation. Fluorescence microscopy experiments revealed that the large-particle polymers crosslinked by SBS and PU+ in PU+9 exhibited excellent shear resistance, consistent with previous analysis results. Beyond a strain level of 15%, PU+3 had the highest shear stress, and its yield strain was larger than those of PU+9 and PU+6. Given that the yield stress of PU+3 was close to that of PU+9, further analysis of the material damage characteristic curve (DCC) and fatigue life is required to compare the fatigue performance of PU+3 and PU+9 conclusively.  
The stress-strain curve of SEA differed significantly from those of the other three. The SEA curve showed a lower rate of shear stress increase throughout the shear strain range of 0% to 30%, without a distinct yield stress point. Since the maximum shear strain  
Figure 11: Stress–strain curves of four types of asphalt.  
in the standard LAS test is 30%, this study takes 30% as the yield strain of SEA. This characteristic is likely due to the denser and richer SBS polymer network in SEA that remains unaffected by stirring. The strain of the SBS polymer network in SEA increased slowly with stress, reflecting a lower shear stress value. Thus, within the strain range of 10-15%, SEA exhibited greater deformation under the same stress conditions, indicating poor resistance to deformation.  
The damage characteristic curve (DCC) based on PSE (Schapery's elastic-viscoelastic correspondence principle) is shown in Figure 12. It is evident that the DCC of SEA is lower than those of the three dosages of PU+, indicating that SEA had the lowest material integrity at the same damage level and the worst fatigue resistance. PU+3 has the highest DCC, meaning it maintained the highest material integrity at the same damage level, showcasing its superior fatigue resistance. From this analysis, PU+9's fatigue resistance is better than that of PU+6.  
A 35% failure level was used as the material failure standard to calculate and analyze the fatigue life of the four types of asphalt evaporation residues, as shown in Figure 13. PU+3 exhibited the highest fatigue life and the best anti-fatigue performance, while SEA had the lowest fatigue life, nearly an order of magnitude lower than those of the three dosages of PU+, indicating the worst anti-fatigue performance. PU+9 had a higher fatigue life than PU+6, suggesting better anti-fatigue performance. The results of the fatigue life analysis are highly consistent with those of the DCC analysis.