# The Shear Behavior of the Curved Interface in Polyurethane-Concrete Composite Structures
## 3 Results and Discussion  
### Comparison of Shear Behavior in Curved and Planar Composite Specimens  
#### 3.1.1 Shear Stress-Displacement Curves of Curved and Planar Specimens  
Figure 5(a),b, respectively, illustrate the shear stress-displacement curves at 1.0 MPa normal stress level for polyurethane-concrete composite specimens with curved diameters of 500 mm (R250) and planar interfaces at different polyurethane densities. It can be observed that the shear stress-displacement curves exhibit similar trends regardless of the interface shape. The shear stress-displacement curve of the composite specimen indicates that its shear failure process can be divided into five stages. Stage I is the initial growth stage, corresponding to the 0-a section of the curve. Stage II is the linear elastic stage, corresponding to the a-b section. Stage III is the yield strengthening stage, corresponding to the b-c section. Stage IV is the fracture failure stage, corresponding to the c-d section. Stage V is the residual deformation stage, corresponding to the d-e section. These stages illustrate the progressive failure process of the composite material under shear loading. By comparing the shear stress-displacement curves of curved and flat composite specimens, it is evident that during the initial growth stage (0 to a), the displacement of the curved specimen is smaller than that of the flat specimen. This observation suggests that the curved specimen transitions to the linear growth stage more rapidly. At this stage, the foam structure of the polyurethane material becomes denser under the influence of external shear loading. As the shear progresses, the composite specimen enters the linear elastic stage (a to b), during which both curved and flat interfaces exhibit a linear increase in shear stress with shear displacement. The displacement range associated with the linear growth stage is larger for the curved specimen than for the flat specimen. When the shear stress reaches its peak and enters the fracture failure stage (from c to d), a significant drop in the shear stress-displacement curve of the curved specimens with a density greater than 0.42 g/cm\({}^{3}\) can be observed, whereas the shear stress-displacement curve of the planar specimens shows a relatively stable decline during this stage. This phenomenon indicates that after reaching the peak shear strength, shear failure in polyurethane-concrete composite specimens with curved interfaces is abrupt, characterized by instantaneous interface fracture and a sudden decrease in shear stress. In contrast, for planar specimens, the shear stress-displacement curve exhibits a gradual decline due to a more stable failure process. Therefore, when the polyurethane density is higher, there is a significant difference in the fracture failure stage between curved and planar specimens. For \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Interface Type** & **Diameter (mm)** & **Specimen Size** & **Normal Stress** & **Polyurethane Density (g/cm\({}^{3}\))** \\  & & **(mm)** & **(MPa)** & **Polyurethane Density (g/cm\({}^{3}\))** \\ \hline  & 400 & 150 & 1.0 & 0.33, 0.42, 0.51, 0.58, 0.66 \\ Curved & 500 & 150 & 1.0 & 0.25, 0.33, 0.42, 0.51, 0.58, 0.66 \\  & 700 & 150 & 1.0 & 0.33, 0.42, 0.51, 0.58, 0.66 \\ Planar & 2000 & 150 & 1.0 & 0.25, 0.33, 0.42, 0.51, 0.58, 0.66 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental design for shear behavior of the polyurethane–concrete interface. Figure 5: Interface forms of polyurethane–concrete composite specimens. specimens with a density not exceeding 0.42 g/cm\({}^{3}\), both curved and planar specimens show a smooth decline during the fracture failure stage. After experiencing the fracture failure stage, the shear stress gradually decreases and stabilizes, entering the residual deformation stage (from d to e). During this stage, both curved and planar specimens exhibit similar patterns in their stress-displacement curves, with shear stress maintaining a stable trend as shear displacement increases.