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