Abstract: Specimens of sandwich plate with viscoelastic core for 3-points bending test appear delay debonding at interface after unloading. Theoretical and experimental approaches for sandwich plate with steel faceplates and polyurethane elastomer core were utilized here to study this phenomenon. Double Cantilever Beam(DCB) and Single Cantilever Beam(SLB) tests were conducted here to obtain strain energy release rate (SERR) between face plates and core. And the expression of critical SERR was deduced on condition of unchanged crack tip opening angle. Furthermore, critical crack length for two types of sandwich plate with different core was calculated. The results show that despite of some limitations, crack tip opening angle can illustrate the propagation of interface crack and reveal the characteristic of delay debonding of sandwich plate with polyurethane core and steel faceplate.