Investigation of the applicability of single cantilever beam test for the evaluation of fracture toughness of sandwich composites

Abstract

Sandwich composites are widely used in load-carrying structures and there is a need for a testing method that will allow for qualitative and quantitative assessment of the bond between the facesheets and the core. The presented research investigates the applicability of the single cantilever beam (SCB) test to the evaluation of the mode I dominated fracture toughness of such structures under both quasi-static and fatigue loading.The SCB method uses a relatively simple fixture, the debond front loading conditions are independent of the debond length and the propagation takes place near the facesheet-to-core interface. It allows for the determination of the initiation and the propagation fracture toughness and can also be used for fatigue testing.A sandwich composite, made of carbon-epoxy facesheets and an aramid honeycomb core, was tested in quasi-static and fatigue loading conditions. The initiation values of the Energy Release Rate (ERR) were determined using the Modified Beam Theory (MBT) approach for different definitions of the initiation point. The propagation tests were conducted along with the visual observation of the debond growth. The propagation fracture toughness was determined using two approaches – MBT and the Area Method. The fatigue testing was realized, using the same setup, by cycling the specimens through a range of ERR values. The debond length was measured during the test which allowed for the determination of a relationship between the ERR and the debond length growth rate.The research confirmed that the SCB method can be used for the quantitative measurement of the fracture toughness of sandwich structures.