7 Crack Jumping in Fabric Composite Fracture Testing 53 Fig. 7.2 Toughened GFRP panel; interfaces near the crack are colored due to presence of PL7000 film adhesive Fig. 7.3 Crack bridge resulting from crack jumping to an adjacent ply in experiment (left) and cohesive zone modeling simulation (right) 7.3 Analysis During the spaced crack experiment, the main crack was influenced by the release ply before it fully reached it. The crack then jumped into the adjacent release film and extended along the adjacent ply interface until it neared the release ply on the main crack interface and arrested. The presence of the ply bridge allowed the DCB to load up significantly more before the crack grew forwards from the initial growth plane and backwards from the release ply to eliminate the ply bridge. At this point, the main single crack reformed, was overdriven, and shot forward a significant distance until another bridge stopped it. These results helped determine the shape of the traction-separation law and calibrate the mesh density for the analysis. A comparison between the analysis and experiment is shown in Fig. 7.3. Initial attempts at a co-cured panel were laid up improperly. The precracked interface is typically made on a plane of symmetry with warp fibers on warp fibers running in the direction of crack growth. For the first attempt, the Teflon precrack was placed on the wrong side, which led to the fibers being perpendicular to the direction of crack growth. This produced a panel where the crack would jump to the weaker warp-weft interface one ply away from the main precracked interface. When the panel was manufactured properly though, the crack was restricted from jumping as designed and the results are shown in Fig. 7.4. Additional work should be done to ensure that the film adhesive does not produce root rotation as the crack grows, which would explain the positive R-curve response. A properly designed experiment can now lead to proper characterization of toughened composite/film adhesive systems.
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