Structural integration of piezoceramic fibers in deep drawn sheet metal for material-integrated health monitoring

The integration of active piezoceramic sensors and actuators into passive sheet metal assemblies produces smart structures. Smart structures are capable of sensing the condition of parts and systems as well as interacting with the environment. Today, smart sheet metal parts are manufactured by surface bonding of sensors and actuators on shaped structural parts. This paper presents a technology for the direct structural integration of lead-zirconate-titanate (PZT) fibers. The fibers are integrated in flat sheet metal, which is subsequently shaped by forming. Microcavities are formed into the surface of the flat metal sheet in which the piezoceramic fibers are inserted and joined. After that, the composite is shaped into a complex 3D surface by deep drawing. The production technology is discussed in detail with regard to the limits of each process step.Functional prototypes with ten interconnected parallel piezoceramic fibers were fabricated. The function as actuator and sensor for ultrasonic elastic waves is demonstrated and the degradation due to the shaping is compared for different parameters of the deep drawing process. The composite shapes are cups with a double curvature radius of 100 mm and 250 mm respectively.As a result, degradation of the piezoelectric function varies with the process parameters. In the case of a double curvature of 250 mm, a degradation of the transducer function of 20% was shown in the experiments. The cause of the decreased performance after shaping is discussed and explained by debonding mechanisms. Results of numerical simulations show a good agreement with the experimental findings of the optimum process parameters.Furthermore, a setup of a wing assembly segment is used to demonstrate the potential of integrated PZT fibers in shaped metal structures. The integrated fibers are used for health monitoring of a multiple bolted joint. It is shown that loosening of a single screw can accurately be detected by the directly integrated PZT fibers.