26 is performed to remove paint and coatings (a). The O-temper plates are easily formed to the shape of the deck (b) and the MMA adhesive is applied to the deck (c) and then cures with simple fixturing methods (d). The MMA achieves a durable bond with minimal surface preparation. The BAR method achieved similar fatigue performance benefits to the composite repairs; However upon base plate failure, the BAR plate immediately cracked and did not carry additional load. As will be discussed below, the composite patches remained intact after the base structure aluminum failure, and continued to carry load. Though BAR is meant to be temporary, there are several repairs that have been in service for over 3 years (e). 3.4.2 Composite Patch Repair In cases where welded repairs are not practical for immediate accomplishment such as fuel tank repairs, which must be first emptied and gas-freed, or the case of highly sensitized plate, alternative repair methods are necessary. Other potentially dangerous conditions which would preclude immediate welded repairs include ammunition spaces, locations with equipment that must be temporarily removed to permit access to the back side of the structure, or the lack of trained personnel. Deck repair using a composite patch offers a significant repair cost and safety benefit from the fact that applying the patch does not require “hot work”, and can be applied from one side. Hot work refers to torch cutting and welding tasks that require a fire watch and a clear area on both sides of the repair, and when in the vicinity of a flammable substance requires the spaces to be gas free. Strength and fatigue life benefits of the composite patch repair method have been well documented. Static testing of edge notch panels repaired with a composite patch demonstrated a significant increase in panel strength with a thin composite patch across the crack tip. Testing results showed that a single layer of unidirectional E-Glass increased ultimate strength of 6.35 mm thick specimens by 33 and 37 % for 3.18 mm inch thick aluminum plate [30]. Testing 6.35 mm thick aluminum center crack tension specimens with stiffness-matched bonded unidirectional boron patches to study effects of a composite patch dimensions on the aluminum plate fatigue life. Results showed no benefit of increased patch length perpendicular to the crack plane. A critical finding was that an elliptical crack front was developed during fatigue loading which lagged by up to 10 mm from the patched to un-patched surfaces [13]. Fatigue performance benefits were also demonstrated for three configurations of 3-ply unidirectional boron-patched, stiffened 1 mm. thick aluminum plates were tested to study the tensiontension fatigue crack growth rate: cracked plate, cracked stiffened plate with 101.6 mm spacing, and cracked stiffened plate with 152.4 spacing. FEA was utilized to calculate the Stress Intensity Factor (K) and modeled the crack growth using Paris Law coefficients for the underlying plate. Test and analysis results showed that the composite patches decreased the crack growth rate and increased the fatigue life of the repaired panels [12]. Over the past 5 years, the Navy has utilized thin low modulus E-glass epoxy composite patch repairs on 12 ships. Patches have demonstrated their ability to mitigate crack growth, inhibit water ingress, and maintain a weather tight compartment. These patches were installed in-situ with vacuum consolidated hand laminations [5, 25]. Laminate design and installation procedures were supported by laboratory lamination trials, ASTM standard material testing, environmental conditioning, large center crack tension fracture mechanics specimens, and experience with a full scale plate ductile tearing test. Patch repairs increased the fatigue life of the center crack tension (CCT) plates 4×, 6×, and 10× for three far-field stress levels [31]. Additional performance capability was demonstrated on a full scale ductile tearing specimen subjected to high stress and durability was demonstrated on patched aluminum plates subjected to dynamic impact. 3.5 Composite Patch Strength and Durability Surface preparation, lamination procedure, and material selection were supported by laboratory lamination trials and material testing. The surface preparation method that achieved the highest lap shear strength involves mechanical abrasion of the surface followed by a phosphoric acid scrub and application of an acetic acid based coupling agent. Vacuum schedule, peel ply, perforated film, and bleeder cloth selection were based on post lamination quality assurance testing, such as fiber volume fraction and void content, and limited in-plane mechanical testing. Additionally, resin gel time studies were performed to understand behavior at various temperatures experienced at repair facilities around the world. ASTM standard tests, performed on material coupons pictured in Fig. 3.5, were used to measure elastic-plastic resin behavior, lamina and laminate in-plane mechanical properties, interface ply fracture behavior, and long term durability of environmentally exposed samples. Baseline composite patch laminates are nominally 6.44 kg/m 2 (1.32 lbs/ft 2 ) with a fiber volume fraction (FVF) of 32.5 %, an elastic modulus of 11.7 GPa, tensile strength of 181.3 MPa, and compressive strength of 214.4 MPa. D.C. Hart
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