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Investigation of Thick Bondline Adhesive Joints

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In recent years, the use of polymer matrix composite materials as primary structural components has risen, especially in the general aviation GA industry. The use of composites not only results in weight savings, but also reduces part counts, joining operations, and results in significant savings in assembly, storage, and inspection. However, joining of some integral parts is still required. GA aircraft industry also uses bonded joints with bondline thicknesses much greater than the 0.01 in. that was standard in aircraft bonded joints. For composites, there are two methods of joining bonding and mechanical fastening. Of the two, adhesively bonding composite structures is the preferred method for a variety of reasons. There are several adhesive test methods that are used to determine the in situ properties of an adhesive joint for use in design. From these methods, ASTM D 1002, D 3165, and D 5656 were evaluated in this investigation with the substrate materials of 2024-T3 phosphoric anodized aluminum, carbonepoxy quasi-isotropic laminate, and fiberglassepoxy quasi-isotropic laminate. Bondline thicknesses from 0.010-0.160 inch were evaluated for three paste adhesive systems using different test methods and substrate materials. The apparent shear strength given by the test methods investigated was found to be highly dependent on adherend bending stiffness, which directly effects the peel stress distributions in the adhesive layer. Thin-adherend specimens, regardless of bondline thickness, yielded lower apparent shear strengths than the thick adherend specimens and gave misleading information when comparing the apparent shear strengths of different adhesive systems. The adhesive shear-stress behavior was characterized over the range of bond thicknesses and environmental conditions and several recommendations and correction factors were offered for the thick-adherend test method.

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  • Couplers, Fasteners and Joints

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