Delamination fracture and core crushing in composite sandwich beams

The paper deals with the nonlinear response of composite sandwich beams subject to static and dynamic out-of-plane loading. The internal structure of such composite systems gives rise to complex damage and failure mechanisms, which include multiple delamination of the skin, interfacial fracture at the skin-core interface and core crushing. The objective of the study is to analyse how different damage mechanisms interact and how these interactions affect the mechanical behavior of the system. The study refers to the model system of a sandwich beam, continuously supported by a rigid plane and subject to out-of-plane loading; the problem is studied using the schematic of a multiply delaminated beam on a nonlinear Winkler foundation. Static and dynamic analyses are performed and the influence on damage propagation of the initial position and size of delaminations in the skin and the constitutive parameters of skin and core are analysed. The skin-core interaction strongly affects the fracture parameters of the delaminations. When the foundation is elastic, a shielding phenomenon is observed on increasing the ratio between the stiffness of the core and the flexural stiffness of the skin. The same ratio controls phenomena of localized amplification, which occur when the delamination tips come close to each other, as well as phenomena of localized shielding. These local effects can induce accelerated growth of the delaminations or crack arrests due to the presence of energy barriers. Under dynamic loading conditions, inertial effects enrich the response with new regimes of behavior leading for instance to oscillations in the arms of the delaminated beam which are characterized by different frequencies and amplitudes that favour damage propagation.