This test method is used to determine the fracture toughness, Gc, associated with the facesheet-to-core separation of sandwich constructions at various Mode I to Mode II loading ratios. This separation can consist of a disbond of the facesheet-to-core interface or a fracture of the core near the facesheet-to-core interface region. Since disbonds can be subjected to and extended by loadings with a wide range of mode mixtures, it is important that the fracture toughness be measured at various mode mixtures. This test procedure consists of subjecting the composite sandwich specimen to loading forces applied via tabs that are applied near the ends of the disbonded section of the specimen and through rollers that bear against the specimen in the non-disbonded region. The test method can be used to determine one or both of the two mixed-mode fracture toughness values: the crack initiation fracture toughness and the crack propagation fracture toughness. The sandwich specimens used for mixed-mode testing are of the same dimensions as specified in the Mode I dominant single cantilever beam test method for sandwich composites (ASTM WK56166) and contain an existing disbond within the facesheet/core interface region at one end of the specimen. Note that this proposed test method is analogous to ASTM D6671, an existing standard test method for determining the mixed Mode I Mode II fracture toughness of polymer matrix composite laminates. Additionally, the sandwich mixed-mode bend test fixture is similar to that used in ASTM D 6671 for polymer matrix composite specimens, but enlarged to test longer and wider sandwich specimens. The mixed-mode fracture toughness values obtained from this test may be used to develop facesheet/core disbond failure criteria for sandwich composite damage tolerance and durability analyses. Test results may also be used to compare quantitatively the relative values of Gc versus mode mixture for sandwich composites with different facesheets, core materials, and bonding methods.
composite materials; sandwich; sandwich construction; core; facesheet; disbond; crack; separation; mixed-mode; fracture toughness.
Considerable interest exists in the sandwich composites community, particularly in the CMH-17 (Composite Materials Handbook) Sandwich Disbond Working Group, for a mixed-mode bend test method similar to that in ASTM D6671 for laminated composites. The proposed standard test method will be used by the sandwich composites community for measuring the resistance to disbond initiation and growth under commonly occurring mixed-mode loading conditions.
The title and scope are in draft form and are under development within this