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Practical and Robust Bonded Joint Analysis Methodology: Application to Durable Redundant Joints
Abstract
Adhesively bonded joints offer weight savings over mechanical joint designs. However, a disadvantage of bonded joints is that their structural performance is highly sensitive to manufacturing parameters. This poses a challenge when assessing joint strength. Since failure is most likely to initiate near the free edges of the joint, significant effort has been expended to better predict the stress field near those regions. This paper recommends that the design be shown to be robust by assuming flaws in regions of the joint where non-destructive inspection (NDI) is ineffective. This requires characterization of the joint’s fracture toughness using coupon level tests, which should use the same manufacturing method as service hardware. The fracture toughness is then used to predict the joint strength and to adjust the design as needed, while keeping the limitations of NDI in mind. This methodology is illustrated with a durable redundant joint and a secondary structure bonded to a primary structure. Known limitations of NDI are used to place flaws in joint locations where they may not be detected, and the capability of the joint is estimated. While, theoretically, a joint design may be defect tolerant, it is important to recognize that the structure may still not be reliable because of the potential for weak bonds, a separate phenomenon. A methodology composed of proof testing combined with pre- and post-proof NDI will be discussed as an effective workmanship screen for mitigating concerns with weak bonds.