Littérature scientifique sur le sujet « Piezopolymer film transducers »

Interdigital transducers fabricated with piezopolymer film have been realized to excite ultrasonic Lamb waves in a composite laminate subjected to pure bending stresses. Lamb waves were generated and detected in a cross-ply [0°/90°] 4 mm thick carbon-fiber composite, by using two interdigital transducers in pitch-catch configuration. We demonstrate that the choice of the piezopolymer transducer technology is suitable for this type of investigation and the advantages of the proposed transducer assembly and bonding are described. A full set-up is described to determine the relationship between the time of flight of the recorded signals and the applied bending moment. Interdigital transducers were designed according to simulations of the dispersion curves, in order to operate at a central frequency of 450 kHz. This frequency corresponds to a central wavelength of 16 mm and to a group velocity of about 6000 m/s for the first symmetric guided wave mode. The variations in the time of flight of ultrasonic recorded signals were measured as a function of the variations in the bending moment. The static and dynamic load tests were in good agreement with strain gage measurements performed in the micro deformation range (0–1400 µm/m).

The design of interdigital transducers (IDT) for active structural health monitoring (SHM) systems often requires a tuning of their characteristics for specific applications. IDTs are generally preferred for the selectivity of Lamb’s guided modes, but the directivity of the radiation pattern is a design parameter that is often difficult to customize for complex mechanical structures. This work proposes a comprehensive experimental study of the IDT with regular geometry, highlighting the dimensional parameters that can optimize the overall performance. From this study, a scaled electrode geometry emerged as a possible solution to shape the directivity diagram while maintaining the selectivity of the guided wave modes. This study based on FEM simulators led to a more versatile design of IDTs built with piezopolymer films of polyvinylidene fluoride (PVDF). The experimental validation showed the directivity diagrams and the ultrasonic guided mode selection were in very good agreement with the simulations. Another outcome of the investigation was the off axis propagation due to the contribution of the bus bars for connecting the IDT fingers to the excitation electronic circuit.

This dissertation covers the design of the transducers and electronics of a structural health monitoring (SHM) testbench system targeted at plate-like structures. The health inspection principle behind the system is the transmission and reception of guided-wave ultrasound along the structure under test, using piezoelectric transducers made of poled polyvinylidene fluoride (PVDF) film. The aim of this work is the creation of a system with custom electronics that can serve as a versatile testbench for the research activities in the field of SHM of complex material, such as carbon and glass-fiber composites, and will eventually bridge the gap between research and the development of highly-integrated sensor networks to be used in industrial, automotive, and aerospace applications. While many guided-wave SHM techniques base their operation on monolithic elements, the proposed system moves in the direction of providing multichannel transmit/receive capabilities to each transducer, transforming them in small-scale phased arrays. Since different SHM applications require different topologies and number of transducers to effectively cover a structure, the system architecture is designed around the vision of a (wired) sensor network: each transducer is connected to its own dedicated electronics, emulating a sensor node. Multiple identical nodes can thus be placed on the target structure and interact to perform the required health monitoring functions in a distributed fashion. The transducers designed for this system are an improvement of the well-known interdigital transducer (IDT), where a few novelties are added: a circular sensor (intended for isotropic guided-wave reception) and a resistive temperature device. A different version of the IDT is also presented where every electrode (finger) has an independent connection that can be attached to different transmitters and receivers, thus creating an array. The electronics are designed to include multichannel transmission and data acquisition tailored to the proposed transducers. Guided-wave generation is performed by high-voltage, 5-level, differential class D amplifiers that can generate arbitrary signals up to 1MHz with inter-channel synchronization. The signal reception circuitry includes two swappable pre-amplifier stages (charge-mode and voltage-mode) in addition to a standard data acquisition chain. The electronics are completed by a system-on-chip (FPGA plus ARM processor) that operates the various components, performs signal analysis, and exchanges data with other nodes. The core contents of this dissertation include the development and testing of the transducers and a subset of the system electronics: the ultrasound transmission and reception modules. The remainder of the system is presented at the architectural level.