Echo Cancellation for Ultrasonic Flaw Detection behind Reverberating Layers

The evaluation of structures behind reverberating layers is a challenge for ultrasonic nondestructive testing. Metal-laminated polymer structures, used in a variety of applications such as automotive, aerospace or electronics industry, are examples of such structures. Depending on the geometry of the structure or its subsequent installation, failure assessment of such composites can be challenging. In addition to the outer metal layer and the adhesion between the layers, the evaluation of the underlying layer is also critical. To solve the latter challenge, a novel coded excitation approach inspired by noise cancellation in audio processing is presented for pulse-echo ultrasonic testing. To apply the principle of cancellation to ultrasound, the ultrasonic echoes in the reverberating layer are evaluated using a full period sinusoidal pulse. To cancel ultrasonic echoes from the backwall of the reverberating layer, a second inverted, scaled and delayed wave is added to the excitation signal. The approach is tested on a specimen consisting of a 30 mm polystyrene block with several test flaws covered by a 3 mm metal plate. The specimen is scanned in contact mode using a piezoelectric transducer and a robot. A-, B- and C-scans generated with the base stimulus and the echo cancellation stimulus are compared. In addition, post-processing with background subtraction and time-gain compensation is tested to increase the detection range. Defects at 4-18 mm depths could be reliably detected by combining our novel echo cancellation approach with background subtraction. Echo cancellation with time-gain compensation and only one delayed time interval for evaluation reliably detects all defects, demonstrating the potential of this technique for detecting defects behind reverberating layers.