Direct-write Ultrasonic Sensors for the Application of Aircraft Diagnostics and Health Management

Ultrasonic sensors are one of the important technologies for structural monitoring and assessing the structural integrity for aircraft diagnostics and health management. Discrete ultrasonic sensors based on piezoelectric ceramics manually installed on structures have been evaluated for aircraft diagnostics and health management. However, it is challenging and time-consuming to install a large number of bulky and heavy discrete ultrasonic sensors in aircraft. In addition, positioning inaccuracy and bonding quality from the manual sensor installation may result in inconsistency. Direct-write ultrasonic sensors are batch fabricated on the structures using scalable processing method. The sensors have low profile and light weight as they are made of thin layer of coatings (typical total thickness <; 100 μm), By eliminating the adhesive layers required for installing discrete sensors, the direct-write sensors offer excellent acoustic coupling with the structure, and thus enable improved reliability and consistency. In this work, two case studies of using direct-write sensors made of piezoelectric polymer coating to detect cracks near fastener holes in aluminum structures are reported. Notches cut from the holes simulated fatigue cracks in Case Study I, and fatigue cracks extended from the notches were involved in Case Study II. The presence of these defects around the fastener holes resulted in reduced signal amplitude of different modes of ultrasonic waves. A computer-controlled testing system was developed to collect time-domain signals at various frequencies and to plot the data into 2D graphs, which can be potentially used as an “ultrasonic signature” of the structure. In addition, damage indexes based on the residual ultrasonic energy were calculated, and could be used as an indicator of severity of damage of the structure.