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Carbon fiber-reinforced polymer (CFRP) laminates, a key composite material, are widely used in aircraft structures and are susceptible to low-velocity impact (LVI) damage from bird strikes, lightning strikes, hail impacts and other situations. Therefore, finding a method that repairs the damaged structure and detects the effect of these repairs under LVI is a very important goal. In this work, the repair effect of LVI damage in CFRP laminates repaired with patches of various sizes is investigated via experimental and numerical nonlinear Lamb wave analyses. An integrated numerical procedure that combines LVI with nonlinear Lamb wave detection is developed to predict the nonlinear Lamb wave behavior in LVI-damaged patch-repaired CFRP laminates. The CFRP laminate damage in the nonlinear Lamb wave simulation is evaluated based on relative acoustic nonlinearity parameters (RANPs). As a result, the integrated numerical procedure is validated with drop-weight impact tests and RAM-5000 SNAP nonlinear ultrasonic detection system. An optimal patch design is established via interpolation to optimize the absorbed energy, delamination surface area, second RANP and third RANP with different patch repair sizes. These parameters exhibit consistent curve fitting trends, indicating that they can be used as important indicators of impact damage. The optimal circular patch design with a radius of 2.5 r has better impact resistance behavior and repair performance.

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Compared with the traditional connection (bonding and bolting), the hybrid connection has attracted increasing attention in various fields for its better mechanical properties. Some research demonstrates that providing preload, a necessary condition for bolted connection increases the strength of the conventional bolted joint. However, those research depends on the test procedure, and there are few studies on the mixed joint. In this paper, we studied the tensile behavior of CFRP（carbon‐fiber‐reinforced‐polymer)/titanium alloy multi‐bolt hybrid bonded/bolted (HBB) joints under preload with multiple parameters, such as the size of preload, the number of bolts, and the way of connection. A finite element model of joint strength prediction under preload based on the 3D Hashin failure criterion was established and combined with experimental verification. This work reveals that: The preload can effectively reduce the stress concentration around the bolt hole of CFRP laminate and enhance the strength of the mechanically fastened joints. As for the HBB joint, the simulation results of the scalar stiffness degradation (SDEG) of the adhesive layer indicate that the application of preload can also effectively slow down the failure process of the joint and has a significant impact on the strength of the joint. However, for the single‐bearing full‐thread bolted joint, the sizeable preloading force will lead to the extrusion deformation of the bolt thread and screw, and excessive preloading may even lead to the shear failure of the bolt.

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