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 passive vibration damping via a digital shunting device. This device is
capable to simulate arbitrary R-L input / output-phase behaviour and is able to adjust these parameters in real
time to fulfil the underlying structural demands. The virtual resonator is coupled with a capacitive piezoelectric
patch bonded on the vibrating structure. The software implementation of the R-L circuit offers the possibility
of adaptive adjustment and frequency tracking in case of eigenfrequency shifts on the vibrating structure that
can happen due to temperature and structural stress changes. The multi band damping is realized by increasing
the number of virtual R-L shunts connected in parallel. In order to separate the individual resonators and
decrease the mutual influence, an additional virtual capacitor for each R-L section is needed. The algorithms
are evaluated on a simple mathematical example equipped with a piezoelectric element. To demonstrate the
capabilities of the system tests were carried out on a steel plate and a mechanical harmonic oscillator. By
placing the patch on a common anti-node of different frequencies the digital shunting device is able to damp
selected eigenfrequencies. The effectiveness of the passive shunting device was demonstrated during tests, where
a reduction of the vibration level up to 15 dB was achieved.
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.
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