Ultrasonic guided wave inspection of Inconel 625 brazed lap joints: Simulation and experimentation

The aerospace industry has been investigating the use of structural brazed joints. As a result of this effort, there is now a need for a rapid, robust, and cost-effective non-destructive testing method to evaluate the structural integrity of the joints. The mechanical strength of brazed joints is known to depend on the microstructural quantity of brittle phases. Ultrasonic guided waves offer the possibility of detecting brittle phases in joints using spatio-temporal measurements. This study focused on the development of a technique based on ultrasonic guided waves for the inspection of Inconel 625 lap joints brazed with BNi-2 filler metal. A finite element model of a lap joint was used to optimize the inspection parameters and assess the feasibility of detecting the quantity of brittle phases in the joint. A finite element parametric study simulating the input signal shape, the center frequency, and the excitation direction was performed. The simulations showed that the ultrasonic guided wave energy transmitted through, and reflected from, the joints was proportional to the amount of brittle phases in the joint. Experimental validations were then performed on three distinctive samples with variable amount of brittle phases in the joint. The samples had three different brazing times (1, 60 and 180 min). Finally, experimental results were in accordance with simulations and demonstrate the potential of the inspection method to estimate the quantity of brittle phases in a lap joint.