In Situ Fracture Toughness Testing of Core Materials in Sandwich Panels

Abstract

The in situ fracture toughness of six core materials was measured using a new test method. The materials tested were seven composite sandwich panels fabricated using vacuum-assisted resin transfer molding (VARTM). The core materials were PVC or balsa of different densities. All of the core materials were tested with E-glass–vinylester face-sheets, while one was tested with carbon–epoxy face-sheets. The fracture tests were conducted using a modified Cracked Sandwich Beam (CSB) test configuration. The cracks in the foam-core panels grew in the subinterface region and paralleled the interface. The distance from the interface to the location of the sub-interface crack was related to the mismatch in the material properties between the face-sheet and core. The fracture toughness of the PVC core materials ranged from 367 to 1350 J/m2. The results for the sandwich specimens with PVC cores were compared to the Mode I fracture toughness of the neat PVC foam-core materials and values obtained from in situ testing of foam-cored sandwich panels using the Tilted Sandwich Debond (TSD) specimen with no tilt angle. The in situ fracture toughness of the core materials in a sandwich panel was found to be significantly higher than the Mode I fracture toughness of core material even though the cracks grew completely within the core. The crack growth in the balsa-cored panels was significantly different than that of the cracks in the foam panels. The cracks in the balsa panels grew in one of the three distinct fashions: as an interface crack when bonding between face-sheet and core was weak, or when the bonding was strong, as a through-the-thickness crack, or a sub-interface crack. The fracture toughness of the sandwich specimens with balsa-core materials ranged from 693 to 1008 J/m2. The results from the balsa-cored panels were compared to results obtained from similar balsa-cored sandwich materials using the Single Cantilever Beam (SCB) specimen.