PSI- Issue 9

F. Moroni et al. / Procedia Structural Integrity 9 (2018) 86–91 Author name / Structural Integrity Procedia 00 (2018) 000–000

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- ENF: mode II delamination/debonding test For the sake of brevity, only fracture tests are described and the results used for comparison between bonded solutions and directly cured composite laminate. The nominal geometry of DCB and ENF specimens is shown in Fig. 1. Each adherent is 19 plies thick, all of them aligned along 0° direction of the fiber.

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Fig. 1. Nominal dimensions of DCB specimens. *W = 150 (DCB) or 180 mm (ENF). Dashed-dotted circles represent the ENF supports and load application points.

The specimens are prepared by inserting a sheet of non-stick material to create the initial defect. In this way, however, the tip radius of the crack depends on the thickness of the non-stick sheet. In order to create a natural crack, fatigue precracking is performed in mode I to propagate the defect a few millimeters. The fracture tests are performed in displacement control at a loading speed of 2.5 mm/min. The test is conducted with partial unloadings to allow for crack length monitoring from the value of unloading compliance and, in turn, the value of G I , G II (strain energy release rate under mode I or mode II loading) as a function of crack length (R-curve). The value of G I , G II are evaluated at each unloading point by FEM analysis. Also the crack-length vs. compliance relationship is evaluated numerically by FEM (Finite Element Modelling). Since in ENF test the crack is not open and the compliance is less sensitive to crack length changes than in DCB tests, DIC (Digital Image Correlation) was used to detect the crack tip position on the specimen side at unloading points during the test. The crack length was then evaluated by matching DIC values with and the values of crack length evaluated by inverse FE analysis of the unloading compliance. However, in ENF no R curve was detected due to unstable crack propagation from the initial length to below the loading point. Directly cured composite specimens are manufactured with the same dimensions and methodologies but without using the film adhesive and except the total thickness that is 13 mm instead of 5.7x2=11.4 mm due to a small excess of resin in the laminate. Test conditions for mode I or mode II delamination are the same as in the case of bonded joints. At least three repetitions were done for each kind of test. 3. Results and discussion Results will be presented scaled with respect to the mode I or mode II fracture toughness, respectively, of the directly cured CFRP for the sake of confidentiality. Therefore, diagrams show normalized values denoted as G I_norm and G II_norm . For analogous reasons, the value of force in force-opening diagrams is normalized with respect to the maximum force of the same test, F max . 3.1. Co-cured CFRP delamination An example of Force vs. opening behavior and fracture surface of DCB tests is shown in Fig. 2. In some specimens it is possible to identify also a marked bridging phenomenon with the initiation and propagation of more delaminations. This phenomenon requires a greater amount of strain energy release, as visible in the diagram of Fig. 3: for all three specimens propagation begins at about the same G I .