Capability Study of Embedded Ultrasonic Transducer Microsystems for SHM Applications in Airplane Composite Structures
An emerging trend in modern structure design is the combination of structures and sensors in order to measure environmental conditions and to evaluate structural integrity. One possible approach of this Structural Health Monitoring (SHM) paradigm is based on ultrasonic guided waves. These elastic waves interact with damages inside the structure and the evaluation of their echo response permits damage identification and localization. Typical applications are rotor blades of wind turbines made of GFRP and aircraft components made of CFRP. The sensor nodes consist of small piezo transducers and sensor near electronics for signal processing, power supply, and wireless communication. The high demands for lifetime and reliability of the structure are directly transferred to the electronic microsystem. The authors are working on a novel approach to embed the sensor nodes into CFRP structures. Functionality, manufacturability and reliability were experimentally investigated and supported by numerical simulations. For this purpose material characterization of the layered composite structures has been conducted to provide material data for the calculations. Finite Element Simulations help to understand the structural mechanics during simultaneous sensor embedding and CFRP lamination and were also applied to risk estimation in terms of sensor and electronics reliability. Elastodynamic Finite Integration Technique (EFIT) was applied to study guided wave propagation inside multilayer CFRP structures and to determine the directivity pattern of sensors laminated inside or on the surface of CFRP panels. Various sensor integration concepts were modeled to study their influence on guided wave performance and sensitivity. Finally, the numerical results were compared to experimental wave field measurements based on non contact laser vibrometry. MOTIVATION AND TARGET This work is based on a SHM system, which has been developed and continuously improved for years. It operates by the ultrasonic guided wave technique to identify damages in ridged structures. Based on this separate sensor system further developments have been conducted to increase the acceptance of SHM system as structure orientated condition _________________ 1 Fraunhofer Institute for Non Destructive Testing, Dresden Branch (IZFP D) 2 Dresden University of Technology, Electronics Packaging Laboratory (IAVT) 3 Cotesa GmbH, Mittweida, Germany 6th European Workshop on Structural Health Monitoring Tu.4.A.2 Licence: http:/creativecommons.org/licenses/by nd/3.0 1 M or e in fo a bo ut th is a rt ic le ht tp w w w .n dt .n et id 14 11 5 monitoring system. In this particularly application the sensor system is supposed to be integrated in a CFRP constructed fuselage of the new Airbus A350. The general approach is to improve the structure integration of a SHM Sensor system by focusing the following targets: High efficiency of ultrasonic wave coupling and wave propagation in the CFRPbody structure, CFRP Lamination process integration and Long lasting field operation (improve reliability) All upper factors are equally respected during the development process. Taking airplane structure condition, flight conditions and the conditions for the CFRPlamination process into account several steps of development must be conducted to obtain design rules for sensor integration in CFRP body structure. To obtain optimal design rules for integrating the sensor in the CFRP body structure several rounds of development will be conducted incorporating requirements for flight conditions, airplane structures and the CFRP lamination process. Figure 1. flow chart for sensor integration development. State of the art technology of sensor assembly is the external attachment on the structure surface. The adhesive layer is one potential weak point in the design [1, 2] mainly due to material degradation of the a dhesive layer [3, 4] Improved integration concepts are required. 1. INTEGRATION CONCEPTS AND LOADING CONDITIONS At first the loading conditions for the CFRP manufacturing process and the in flight operation conditions were determined. The CFRP lamination conditions were provided by the manufacturer. The in flight conditions were provided by a test company for Airbus structure components and additionally determined from the RTCA/DO 160F standard as a guideline. Related to the airplane fuselage structure the deduced loading condition for embedded/integrated US transducer and electronics were determined. Table 1 is an excerpt of the main factors of the loading list.