The paper presents guided elastic waves and their identification and damage interaction in a CFRP plate. After the excitation of a fiber transducer, different elastic waves emerge in a plate. By using specially developed 3D laser scanning software it was possible to specify the different wave modes. These wave modes have been described concerning their propagating velocities and different motion components. The interaction of different wave modes with introduced impact damage (7J) is shown. In some expts., it was proven that impact locations can be derived from the detected Lamb waves. This work is continued to develop structural health monitoring systems (SHM) for selected aircraft components (e. g. stringer elements, panels)
Structural health monitoring systems are increasingly used for comprehensive fatigue tests and surveillance of large scale structures. In this paper we describe the development and validation of a wireless system for SHM application based on Lamb-waves. The system is based on a wireless sensor network and focuses especially on low power measurement, signal processing and communication. The sensor nodes were realized by compact, sensor near signal processing structures containing components for analog preprocessing of acoustic signals, their digitization and network communication. The core component is a digital microprocessor ARM Cortex-M3 von STMicroelectronics, which performs the basic algorithms necessary for data acquisition synchronization and filtering. The system provides network discovery and multi-hop and self-healing mechanisms. If the distance between two communicating devices is too big for direct radio transmission, packets are routed over intermediate devi ces automatically. The system represents a low-power and low-cost active structural health monitoring solution. As a first application, the system was installed on a CFRP structure
Conventionally, modal monitoring of Wind Turbine Rotor Blades is primarily based on the evaluation of eigenfrequencies. Beyond this, combining a sensor network with the Operational Modal Analysis (OMA) method, mode shape and parallely a local component are utilized here. In addition it is expected that the damping, which is also determined by the OMA method, will give a lead on damage development at the rotor already at an early stage. Modal monitoring by means of measurement is combined with FEM simulation and with the comparison of results obtained from measurement and simulation. Moreover, this will establish a connection between the engineer and the design data of a rotor blade, which also are based on FEM analyzes. A further significant increase regarding error resolution is possible by combining the global modal methods with locally sensitive monitoring methods, based on guided elastic waves. These assume plate-like structures through which elastic waves propagate in the low-frequency ultrasonic range (10 - 100 kHz) in certain modes. These different wave modes interact distinctively with inner structural damages such as web fractures and delaminations. It is differentiated between piezoelectrically excited waves (acousto ultrasonics), and waves produced by energy released at fractures, delamination etc. (acoustic emission). Applying a moderate number of sensors, the combination of both methods can allow an effective monitoring of the global structure
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