Guided Lamb waves in the ultrasonic range have potential for structural health monitoring of thin structures, e.g. for the
detection of impact damages which may cause delamination in carbon fiber reinforced materials. For the emission of
guided waves piezoelectric transducers can be used which are applied to the surface of the structure.
By using a phased array of transducers a directivity pattern for the inspection of a limited area on the structure can be
created with common beam forming algorithms. Line arrays require only a small number of transducers but the main
lobe is generated on both sides of the array which means an excitation towards an unwanted direction is produced.
In this contribution a 2D array design is introduced which tends to emit only one main lobe towards the direction of
interest. The concept basically utilizes two parallel line arrays. Both arrays emit signals with a single burst. The signal
emitted by the second line array is meant to suppress the unwanted lobe of the main array by out-of-phase superposition.
This requires an appropriate timing of the emission of the signals of the single transducers.
The feasibility of the concept has been studied by simulation. Practical experiments on CFRP (carbon fiber reinforced
polymer) sheets have been carried out with an array layout with eight single piezoelectric transducers. A PC-controlled
electronics system has been used for the actuation of the transducers. Emission and directional behavior of the Lamb
waves on the structure has been monitored with a Laser scanning vibrometer.
This contribution is concerned with the hardware design of a structural health monitoring (SHM) system for
continuous delamination detection in carbon fiber reinforced polymer components. The component is equipped
with an integrated actuator array of eight piezoelectric patches which are driven by miniaturized high frequency
power amplifiers. The phased line array is capable of emitting directional guided Lamb waves with a frequency
of several hundreds of kilohertz and with user-selectable waveform pattern and directivity angle into the continuum.
The directivity of the Lamb wave depends on the phase difference between the individual actuator signals.
A special milling technique allows to create phased array stripes of arbitrary size and shape of the electrodes
with high precision and reduce the placing and time complexity. A laser scanning Doppler vibrometer is used to
visualize the propagation of the corresponding Lamb waves, as well as reflections which are caused by delamination
defects. The results of the measurements can be evaluated to characterize the damage and the material
properties. The hardware platform provides a portable system for the investigation of real world components,
e.g. aircraft CFRP structures.
This contribution deals with the implementation of a PC-controlled structural health monitoring system for continuous
damage detection. The system is implemented in a real size demonstrator component made of carbon fiber reinforced
polymer (CFRP). The component is equipped with an actuator array of piezoelectric patches which are driven by power
amplifiers. With the appropriate test signals elastic Lamb waves are emitted into the continuum in a specific direction.
Vibrometer measurements of reflections which are caused by delaminations make it possible to observe the size and
position of the defect areas.
A hybrid approach involving both analytical computations and measurement data is proposed for designing
optimal actuators for piezoelastic structures. The analytical part comprises the linear mathematical structure of
models of beams, plates or membranes which are appropriately described by mode shapes and modal paramters,
i.e. eigenfrequencies, modal gains and modal damping. Also, the governing differential equation for perfect compensation
of external loads due to piezoelectric actuation is applied. The measurement part relies on identification
of mode shapes and parameters under an external load using experimental modal analysis. The real-world data
is fed into the analyical part with the ultimate goal of achieving a match between the modal gains due to external
loading and piezoelectric actuation. The algorithm which can be extended to various mathematical models of
piezoelastic structures, is evaluated on a beam structure which is clamped at both ends.
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