27 Center crack tension (CCT) tests were used to demonstrate the design method and resulting laminate configuration for the thin E-glass epoxy patch repairs performed on U.S. Navy ships. Though reinforcement length relative to the crack plane does not affect crack growth, initial CCT specimens had composite patches that extended 15 in. from the crack plane in each direction to represent the configuration of repair patches installed on ship structure. Minimum recommended patch length perpendicular to the crack is 40 cm for sensitized plate and often extend further past the nearest structural detail, such as a stiffener or frame below the deck being repaired. The CCT configuration shown in Fig. 3.6 was tested for three far-field stress levels; working stress, above the Eurocode 9 fatigue design stress limit, and a stress level resulting in plastic deformation at the crack tip. For two of three applied stress levels tested, 34.5 and 75.2 MPa, the composite patch continued to carry the applied load with only minor delamination adjacent to the crack, even after the aluminum crack extended the full width of the plate. Composite patch disbond failures, when they occurred, were limited to a small area adjacent to the crack plane and extended less than 12 mm beyond the crack tip. When the load was increased to 100.3 MPa the composite patch disbonded and delaminated from the base panel as shown in Fig. 3.6. Typical failure surfaces showed resin bonded to the aluminum with fiber tearing and inter-laminar delamination between various plies through the thickness of the composite patch. Fig. 3.5 Material testing specimens. Clockwise from upper right: mode I fracture, compressive notch shear, lap shear, and mode II fracture Disbonded Laminate Lamina Bonded to Aluminum Resin Bonded to Aluminum Fig. 3.6 Load frame with CCT test specimen ( left ). Failure surfaces typical of the 100.3 MPa far-field stress ( right ) 3 5xxx Aluminum Sensitization and Application of Laminated Composite Patch Repairs
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