New Test Method for Mode I Dominant Facesheet-to-Core Fracture Toughness of Sandwich Constructions
1. Scope
This test method is used to determine the Mode I dominant fracture toughness, GIc, associated with facesheet-to-core separation of sandwich constructions using the single cantilever beam (SCB) specimen. 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.
The SCB specimen consists of a sandwich construction containing an existing crack within the facesheet-to-core interface region produced using a non-adhesive insert or machined into the specimen. Opening displacements are applied to the SCB specimen by means of a hinge or loading block bonded to the end of the separated facesheet. The opposing facesheet of the specimen is fixed to the base of the test fixture. The facesheet-to-core separation in the SCB specimen is opened by controlling either the opening displacement or the crosshead movement, while the force and crack length are recorded. Instantaneous crack front locations are marked on the force-displacement record at intervals of crack growth during the crack initiation fracture toughness test procedure. However, only the final crack front location is marked during the crack propagation fracture toughness test procedure. The Mode I fracture toughness for crack initiation is calculated using a modified beam theory method. The Mode I fracture toughness for crack propagation is calculated using an area method.
Acceptable failure modes for GIc are those resulting in crack growth occurring either at the facesheet-to-core interface or within one-fourth of the core height from the facesheet-to-core interface, and parallel to the plane of the facesheet-to-core interface. The measured fracture toughness of the facesheet-to-core interface region is a Mode I dominant structural property that is a function of the dimensions and constituent materials of the sandwich construction and the method of manufacturing.
Considerable interest exists in the sandwich composites community, particularly in the CMH-17 (Composite Materials Handbook) Sandwich Disbond Working Group, for a Mode I dominant fracture toughness test method. The proposed standard test method will be used by the sandwich composites community for measuring the resistance to disbond growth under mode I dominant loading conditions.
