Measurement of Interface Fracture Toughness of Sandwich Structures under Mixed Mode Loadings

Fracture of sandwich structures loaded with axial forces and bending moments is analyzed in the context of linear elastic fracture mechanics. A closed form expression for the energy release rate of interface cracking of a sandwich specimen is found by analytical evaluation of the J-integral. A method for determining the mode mixity is described and applied. Expressions are presented whereby the mode mixity can be calculated analytically for any load combination when the mode mixity is known for just one load case. The theory presented is applied to a new test method based on double cantilever beam sandwich specimens loaded with uneven bending moments. The interface fracture toughness of two sandwich types are measured as function of the mode mixity. The sandwich structures that are tested consist of glass fiber reinforced polyester skins and PCV core. The tests show that the interface fracture toughness depends strongly on the mode mixity. Under dominated normal crack opening, the crack grows just below the interface in the core at a constant fracture toughness. Under dominated tangential crack deformation, the crack grows into the laminate resulting in extensive fiber bridging and an increase in fracture toughness. As a result of the development of a large process zone due to fiber bridging, the analysis by linear elastic fracture mechanics becomes invalid and modeling with cohesive zones is proposed.