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

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based on small deformation theory, like the Compliance Calibration Method (CC) or the Modified Beam Theory (MBT), are no longer applicable.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

Comparing unreinforced and pin-reinforced CFRP/PMI foam core sandwich structures regarding their damage tolerance behaviour

Foam Core Sandwich Structures are offering a good ratio of bending stiffness- and strength-to-weight. By using closed-cell rigid foam cores of Polymethacrylimid (PMI), low priced and highly integral structures could be built in a vacuum infusion process. Therefore the investigated sandwich structures are suited for primary structure applications in commercial aircrafts. Foam core sandwich structures consist of two CFRP-face sheets and a PMI foam core. Structures without foam core reinforcement were compared to structures with CFRP pin-reinforcement. The CFRP-pins can be used to increase the out-of-plane properties of the sandwich structure and particularly the Damage Tolerance (DT) behaviour. One of the important specific values concerning the Damage Tolerance is the critical Energy Release Rate (ERR) GIC, which has to be determined. In case of very thick and stiff face sheets the climbing drum peel test is not suited for such structures. Currently there is no other standardised test available to evaluate the GIC value for sandwich structures. That is why the Single Cantilever Beam (SCB)-test is used to determine GIC here. The SCB-test was optimised to be used for sandwich structures. Two different methods of GIC evaluation from the SCB-test data, using force, deflection and crack length, are analysed. Afterwards the pros and cons of the used Compliance Calibration Method (CCM) and of the Area Method (AM) are discussed. Beside the method validation, the structures without reinforcement and with pins of two different patterns are compared to each other. It becomes apparent that the Energy Release Rate (ERR) can be increased threefold by special CFRP-pin configuration

RVE modelling of deformation and failure behaviour of closed cell rigid polymer foams

Closed cell rigid polymer foams are used as light core material in high loadable lightweight sandwich structures. Their mechanical behaviour depends on both the mechanical properties of the constituent solid polymer and the cellular structure on the mesoscopic scale which has therefore to be taken into account for the mechanical characterization of foam materials. This paper deals with the investigation of the deformation and failure behaviour of closed cell rigid polymer foams via numerical representative volume element (RVE) modelling approach. For this purpose the cellular structure of the foam was modelled by a 3d random Laguerre tessellation which was adapted to the real cellular structure of a closed cell Polymethacrylimide (PMI) foam in terms of morphological properties (e.g. cell size distribution) obtained from X-Ray computed tomography (X-Ray CT) data and subsequent 3d image analysis. In addition to the effective linear elastic material properties the strength of the foam model was calculated via finite element analysis (FEA) and consideration of nonlinear failure modes of the cellular structure. Parametric studies revealed the correlation between structural parameters (e.g. foam density, material content in the cell walls etc.) and the effective mechanical properties of the foam. Evaluation of the numerical results was done by standard mechanical testing of foam specimens on the one hand and X-Ray CT in situ deformation experiments with stepwise loading and scanning of foam specimens in a deformed state on the other hand. Both showed the potential of mesoscopic foam modelling with realistic consideration of the cellular structure as well as load case dependent deformation and failure mechanisms for exact prediction of material properties of closed cell foams

Characterization of a polyurethane adhesive and comparative calibration of different material models in LS-DYNA

A rubber-like polyurethane adhesive was experimentally characterized and parametrized for use of different material models in LS-Dyna. Due to the characteristically large elastic deformations of the material and adhesive layer thicknesses of several mm, established modelling strategies for the simulation of structural adhesive bonding in automotive applications have to be carefully evaluated and necessary adaptations or alternative modelling possibilities have to be considered