Experimental Study on the Effect of Slip on the Flexural Performance of Composite Sandwich Wall Panels

Under out-of-plane loading, composite sandwich wall panels may develop relative slip between the wythes and end slip at the intermediate-layer interface, weakening composite action and flexural stiffness. Previous studies have mainly focused on bearing capacity and connector performance, while the complete slip-development process, parameter effects, and quantitative slip-warning indicators remain insufficiently investigated. To investigate the flexural slip mechanism and design control method, three one-way composite sandwich wall panels with different wythe thicknesses, reinforcements, and stiffness ratios were tested under four-point bending. The load–deflection response, crack development, relative slip, and end slip were recorded. The measured slip values were normalized by the midspan yield deflection to obtain the absolute slip ratio, relative slip ratio, and end slip ratio. The results show that both the relative slip between the inner and outer wythes and the end slip exhibit a three-stage evolution with increasing load: almost no slip before cracking, approximately linear development after cracking, and rapid increase after yielding. The stiffness matching of the inner and outer wythes and the thickness of the intermediate layer are important factors affecting slip development. Based on the test results and comparison with existing experimental data, the yielding stage is recommended as the slip-warning control point, with warning values of 0.03 for the relative slip ratio and 0.06 for the end slip ratio. Finally, a simplified model based on partial composite action theory was established using binary linear regression and the least-squares method. The model achieved a centered R2 of 0.937, while the slip influence coefficient increased from 2.4–8.8% at yielding to 30.3–66.0% at the peak stage, quantitatively supporting yielding-stage warning control.