Single cantilever beam test for honeycomb sandwich materials with very thin facesheets - effects of test conditions and material properties

Abstract

This paper deals with the facesheet/core disbonding characterization of CFRP/Honeycomb sandwich structures using the Single Cantilever Beam (SCB) test. Facesheet/core disbonding is characterized by determination of the corresponding critical strain energy release rate Gc. Motivated by the high relevance of damage tolerance analysis of complex shaped and loaded lightweight sandwich structures, e.g. for aerospace application, the SCB test has been widely investigated over the last years and guidelines are available to properly apply the SCB test to various kinds of sandwich materials. Standardization of the test method is currently being prepared. Nevertheless, particularly in the case of lightweight honeycomb sandwich materials with very thin facesheets, various effects are observed, which are not yet entirely understood. The proposed test conditions are still under discussion. In aerospace applications, often low density aramid paper honeycomb core material is combined with thin CFRP facings. In the case of thin facesheets with a thickness of 1 mm or less and a high disbond toughness, the deflections and rotations of the loaded upper skin cantilever beam of the SCB specimen is considerably high – test evaluation based on small deformation theory, like the Compliance Calibration Method (CC) or the Modified Beam Theory (MBT), are no longer applicable. In this case, based on a strain energy concept, the Area Method (AM) is under consideration for fracture toughness determination. Alternatively, to avoid improper nonlinearities during loading and crack propagation, the use of doublers, applied to the upper facesheet, is often recommended. Further investigation is presented here to understand specifics of thin facesheet sandwich SCB testing and to assess the applicability of the simple and robust SCB test set-up for facesheet/core disbonding fracture mechanical characterization. Different test modifications and situations are numerically analysed by means of Finite Elements (FE) and compared to each other. In addition, mesoscopic in-situ observation of the fracture region during the test was carried out using X-ray CT measurements