# Interfacial Shear Properties of Red Clay and Polyurethane with Different Densities
## 3 Results and Discussion  
### Effect of Polyurethane Density on Shear Stress-Shear Displacement Curve  
Figure 3 illustrate the shear stress-strain curves of the composite interface at different polyurethane densities under normal stresses of 300 kPa and 400 kPa, respectively. As  
Figure 1: Polyurethane-red clay composite sample. Figure 2: Schematic diagram of direct shear test. shown in the figures, the addition of polyurethane significantly influences the shape of the shear stress-strain curve of red clay, transforming the curve from shear hardening to shear softening. The shear stress-shear displacement curve after the addition of polyurethane exhibits distinct stages: a rising stage, a falling stage, and a gentle stage. These correspond to the stress rise stage, interface failure stage, and residual stress stage during the specimen testing process. The same phenomenon was also observed in polyurethane-modified sandy soil experiments [13; 19; 31], which indicates that the addition of polyurethane enhances the ductility of red clay to a certain degree [32]. This behavior is attributed to the infiltration of polyurethane, which fills the gaps between the red clay particles, adheres to them, and wraps around them. Additionally, polyurethane and red clay form hydrogen bonds, generating favorable interactions [33]. As the polyurethane density increases, the maximum shear stress of the interface also increases, although there is no significant change in the horizontal displacement during composite failure. When comparing the shear stress-shear displacement curves of composites with polyurethane densities of 0.5 g/cm\({}^{3}\) and 0.8 g/cm\({}^{3}\), it is observed that, after reaching the maximum shear stress, the stress drop is more pronounced in the composite with a polyurethane density of 0.8 g/cm\({}^{3}\). The final residual stress of this composite is lower than that of the composite with a polyurethane density of 0.5 g/cm\({}^{3}\). This indicates that a higher polyurethane density is not always better; the optimal density lies between 0.5 g/cm\({}^{3}\) and 0.8 g/cm\({}^{3}\), which was similarly corroborated by Study [24]. Furthermore, as shown in Figure 3a, when the polyurethane density is 0.2 g/cm\({}^{3}\) or 0.5 g/cm\({}^{3}\), the shear stress of the composite specimen decreases gradually after failure. However, when the polyurethane density reaches 0.8 g/cm\({}^{3}\), the shear stress decreases sharply, this observation aligns with results from polyurethane-fiber and zeolite modified soils [34; 35], This is due to the increased density of polyurethane, where the shear softening behavior of PU becomes more pronounced [36], ultimately leading to polymer fracture under such conditions [23]. These findings further confirm the existence of an optimal density range for PU. Excessive density induces a drastic reduction in composite shear strength, with this phenomenon becoming increasingly evident under elevated normal stress levels, extending the effective density range to 0.2 g/cm\({}^{3}\)-0.8 g/cm\({}^{3}\), as illustrated in Figure 3b. Figure 3: Shear Stress Shear Displacement Curve of Composite Interface under Different Polyurethane Density: (**a**) normal stresses of 300 kPa; (**b**) normal stresses of 400 kPa. Considering the peak of the curve as the shear strength of the polyurethane-red clay interface, the relationship between shear strength and polyurethane density is shown in Figure 4. From the graph, it can be concluded that the incorporation of polyurethane significantly improves shear strength. Under the same normal stress conditions, as the polyurethane density increases, the shear strength also increases, the experimental results exhibit similar patterns to those reported by [PERSON] and [PERSON] [31; 37].