FRACTURE TOUGHNESS OF SKIN/CORE INTERFACES IN SANDWICH MATERIALS UNDER MIXED MODE LOADINGS

Defects in sandwich structures are often inevitable and can originate both from manufacture and use. An important defect type is debonding, i.e. areas with weak or no adhesion between skin and core. In a sandwich structure debondings can arise in production when an area between skin and core has not been wetted properly resulting in a lack of adhesion. In use, damages from loads e.g. impact might result in formation of a debond crack. With debonds present the structure might fail under loads significantly lower than those for the intact sandwich structure. In order to improve the damage tolerance of sandwich structures an increase in the toughness between skin and core is necessary [1]. It is believed that the microstructure of the interface between skin and core can have a large effect on the fracture toughness. It has been shown that the presence of a mat with random oriented and loosely packed fibers between the core and the laminate facilitates the formation of large scale fiber bridging, resulting in a significant increase in crack resistance [2]. The formation of large scale fiber bridging results in a fracture process zone with length several times the skin height. To characterize the crack growth resistance the J integral is deployed since it is valid for handling fracture with large fracture process zone. By analytical derivation the J integral is determined for the special sandwich specimens used in the tests [3]