Abstract

Smart materials based on carbon fibre-reinforced plastics with embedded PZT sensors and actuators are expected to be a favourite composite for vibration damping and noise reduction. Comprehensive non-destructive characterization and integral health monitoring help to optimise the structure and its manufacturing and are essential prerequisites to ensure performance and availability of smart components during their life time. The first part of the paper presents high resolution non-destructive imaging methods including microfocus X-rays, ultrasonics and eddy currents. These methods are used to characterize damages resulting from non-optimal manufacturing and external load. The second part is dedicated to newly developed imaging techniques using the active piezoceramics as transmitters of acoustic, electromagnetic and thermal fields. The third part focuses on health monitoring by impedance spectroscopy using the same piezoceramics as for vibration damping. Electromechanical finite-element-modelling and experimental investigations at strip-shaped specimens have shown the close connection between mechanical properties and electrical impedance.

Introduction

Figure 1 presents the material system manufactured by the German Aerospace Centre DLR (1). The ceramic plate is embedded between thin layers of a non-conductive material (polyester fibre mats) for electrical insulation. This insulation is necessary to avoid short circuits between the electrodes of the piezoceramics caused by the conductive CFRP layers. One part of the active composite plates was prepreg manufactured (figure 1b) and the other part was made in DP-RTM technology (Differential Pressure - Resin Transfer Moulding, figure 1c), that guarantees for an extreme high quality and reproducibility of the structure. Here, the fibre material is laid out in dry state which facilitates the positioning of the piezoceramic plates and the electrical leads. The DP-RTM technology becomes especially interesting as it is not necessary to provide massive moulds since the clamping forces are created by the internal pressure in an autoclave.

a)	b)	c)
Fig 1: The smart material system, a) stacking sequence, b) microsection of prepreg laminate, c) microsection of DP-RTM material.

Significant differences between mechanical and thermal properties of the ceramic patches and the matrix demand sophisticated manufacturing techniques. The major challenge is to create a suitable damage tolerance concept based on advanced non-destructive diagnostics (2). In extension to conventional non-destructive evaluation (NDE) after manufacturing and during inspection breaks, a smart material can be used in a self diagnostic manner to detect early damage stages. This approach results in more intelligent NDE procedures. Its successful application requires fundamental knowledge of nature, size and location of damage as well as extensive data acquisition and processing (3). The real-time health monitoring techniques should provide reduced maintenance costs and offer many unique opportunities to assess the structural integrity (4).

The scope of properties to be evaluated includes the internal structure of the composite (fibre orientation, ply sequence, sensor and actuator position, local fibre or epoxy concentrations), planar flaws oriented parallel to the surface (delamination, ceramics debonding), perpendicular oriented flaws (cracks of the matrix or of the patches) and volume imperfections (pores, voids, inclusions).

Steps from NDE to health monitoring

Figure 2 presents three stages of NDE methodology from the signal generating point of view. At the first stage (figure 2a), a transmitter-receiver probe scans the object. The object may be either passive or active. That is the classic NDE method. At the second stage, the internal actuators are used as transmitters for the test signal (figure 2b). An external scanning sensor (or sensor array) receives the signal characterizing the transmitter (actuator) as well as the channel (structure). Of course, this principle may be inverted. Both stages create pictures as the inspection result. Finally, the third stage avoids external sensors or transmitters. Here, the internal sensors and actuators perform as transmitters and receivers (figure 2c). To get information about local distribution of structural properties the transmitting function may be switched to various actuators. Unfortunately, no pictures can be expected and it is a matter of current investigations to find out the chances and limitations of this approach.

Fig 2: a. Classic NDE with external transmitter and external receiver. b. Active NDE with internal transmitter and external receiver. c. Health monitoring with internal transmitter and internal receiver.

High resolution imaging NDE methods

As stage 1 methods have been applied radioscopy (X), ultrasonics (US) and eddy currents (EC). Radioscopy is capable to study both surface and internal damage of the laminate as well as of the piezoceramics. Digital imaging was used in a differential mode to detect cracks in the piezoceramic. Ultrasonics is based on the reflection and transmission behaviour of elasto-mechanic waves at internal boundaries (5). The perpendicular incidence of longitudinal waves corresponds to the existence of pure longitudinal wave mode and admits the detection of most structural flaws. For testing of smart CFRP structures, the problems of the materials anisotropy, heterogeneity and layered structure have to be kept in mind (6).

Eddy current method uses the anisotropic conductivity of the material. Along the carbon fibres much better conductivity is found than across the fibres. Additionally, the piezoceramic patches are coated with thin copper-nickel layers for electrical contacting. This layer is able to carry eddy current. In case of patch cracking the coating also cracks thus interrupting current paths. The performance of these methods is demonstrated in figure 3 at a cracked actuator in the laminate. The patch was broken before embedding it into the laminate.

Fig 3: NDE images of a broken PZT patch in the laminate.

The eddy current image (EC) reflects the metallic coating of the actuator and the CFRP layers. Since the metallic layer in the reported sample does not cover the ceramic plate completely, crack areas in the right hand part remain unnoticed. The ultrasonic image (US) shows rectangular areas near the edges of the piezoceramic patch from adhesive tapes for fixing the broken actuator. Furthermore, the crack-network becomes partly visible. The clear crack lines in the X-ray image (X) result from a procedure using differential technique and special aperture to limit the radiation to the PZT area.

Flaws at contacting
A very sensitive point is the contacting of the actuators because it also may damage them during autoclave curing. One kind of contacting is a dedicated window in the insulation fleece as shown in Figure 4a. One conducting CFRP layer is the common electrode for some actuators. The other electrode is contacted by a fibre roving consisting of some thousand filaments. The contact window can bend or even crack the piezoceramics at curing. The US-C-scan indicates the deformed area due to reduced time of flight of longitudinal waves. The degree of deformation can be measured in the US-B-scan with an accuracy up to 20 µm. Additionally, US-C-scan shows some branches of a crack and the contacting carbon fibre roving. EC image clearly makes visible the cracks and indicates the deformation as dark area due to the reduced distance between the actuator and the probe. X-ray image brings up the cracks but cannot detect the deformation.

Another kind of contacting preventing shear stress during autoclave curing is a copper mesh cut to strips. Above and beneath the piezoceramics the mesh covers the whole patch area. Figure 4b shows some details. Nevertheless, in the beginning of acquiring this technique cracks occurred invisible for common X-ray equipment. Only a microfocus system brought up the cracks together with the wires of the copper mesh.

Fig 4: NDI results of flaws caused by contacting.

Damage of electrical circuitry
During the function test of multi-actuator specimens one sample was not controllable as intended. Some PZT patches acted together when they should act separately. Another patch did not work at all.

Fig 5: Detection of electrical breakdowns using a) and b) ultrasonic and c) eddy current techniques.

Fig 6: Polluting fibres in the stack.

The US-scan (figure 5a) of one of the ceramic patches (grey rectangle) brought up a bright circular area caused by the bending of the patch into the contacting window. Additionally, dark spots at the right and left edges of the patch could be observed. A US C-scan of another depth plane (figure 5b) showed bright lines starting at these points. Due to their properties these reflectors were assumed to be spliced carbon fibre rovings unintentionally brought into the material.

The EC-scan (figure 5c) let recognize the edges of the piezoceramic patch as well as the contact roving. Additionally, linear objects could be observed corresponding to the US result. These anomalies of conductivity confirm the assumption of spliced carbon fibres close to the piezoceramic patch. Plastographic investigations led to the reconstruction in figure 6 showing the polluting fibres.

When carrying current the filaments became burned causing local material degradation (dark spots in figure 5b).

Impact damage
Impact damage can be visualized by various NDE methods giving a comprehensive survey of its complicated structure. The example (figure 7) presents NDE images of a low speed impact of only 0.5 Joule caused by drop weight on a DP-RTM specimen. No damage can be recognized visually. In the lower part of the piezoceramics crack lines occur caused by two contacting rovings from both sides of the patch. The circular area results from the impact. The US scan was recorded using active US method reported in paragraph 5.1. One advantage of this method is the suppression of covering CFRP layers. It becomes obvious that the inner and outer areas of the ring remain active. The absence of the signal in the ring area is caused by delaminations interrupting the transmission of the acoustic waves to the scanning transducer. The differential X-ray in figure 7 brings up circular cracks in the piezo ceramic patch. Covered by the pattern of the woven CFPR mat, the EC scan also visualizes a dark ring resulting from the damaged metal coating of the patch. All NDE images confirm the absence of any significant damage in the centre of the impacted zone.

Fig 7: NDE images of a 0.5-Joule-impact on the embedded piezoceramic patch.

Active non-destructive techniques

Active ultrasonics
The active ultrasonic technique as presented in figure 8 uses the embedded piezoceramics to transmit ultrasonic waves. A short electrical pulse generates a broad frequency band. The conventional scanning probe receives the waves from the structure and builds up an image of the spatial distribution of the piezoceramic activity. At high frequencies, flaws in the piezoceramics and in the covering CFRP- and insulation layers become visible. Structural features like the local bending of the piezoceramic patch connected with cracks in it also become detectable. So this technique admits a very comprehensive visualization of the structure.

Fig 8: Active ultrasonics setup and results at two frequencies.

Active thermography
An infrared camera visualizes thermal fields, generated by the active piezoceramics and its wiring. Figure 9 displays the effects of resistance heating in the conductive rovings and vibration heating in the piezoceramic patches. The form and the position of the three piezoceramic patches become clearly visible at a frequency of 50 Hz. With increasing frequency the capacitive resistance of the piezoceramics decreases relocating the energy conversion to the current leading wiring. The fibre rovings become visible as well as the bright shining contacting points. The temperature is a measure for the potential danger of material degradation.

Fig 9: Principle and results of active thermography.

Active electromagnetics
For active electromagnetic methods, the actuator is driven by a harmonic electric current producing an electromagnetic field detectable by external sensors. Here, the amplified output voltage of a commercial eddy current device at 2 MHz was used to feed the piezoceramics and a differential EC probe served as a scanning sensor. So, the normal component of the magnetic field was captured visualizing the current distribution in the specimen. The resultant image in Figure 10 shows the current leading rovings, the left-hand edge and a perpendicular crack of the actuator. A conventional X-ray image confirms this result.

Fig 10: Principle and result of active electromagnetics.

Health monitoring

For high responsible applications, e. g. in aeronautics, a permanent and integral supervision of structural and functional integrity is required. Smart CFRP-structures provide different approaches for this aim (Figure 11). The piezoceramic patches can act

• as acoustic emission sensors for the detection of developing damages,
• as transmitters and receivers of ultrasonic
• LAMB-waves or as transducers for impedance spectroscopy basing on the coupling of the structures’ mechanical and electrical behaviour (7).

Fig 11: Different approaches to health monitoring.

Electric impedance spectroscopy
Due to the coupling of mechanical and electrical properties in the examined smart structures it should be possible to conclude from measurements of electric impedance to the existence of flaws. The demonstrated investigations ranged from low kHz-frequencies to high frequencies of some MHz, as usual in non-destructive evaluation. Figure 12 shows the result of an experimental setup, using a network analyser for electric control and measurement and a strip of CFRP-material as specimen. The impedance-frequency- diagram displays the broad band characteristic of the integrated piezoceramics. Resonance peaks in the lower kHz-range, resulting from vibrations of the specimen, become visible. These peaks are clearly correlated with special forms of vibration modes in length or width direction. Further experiments showed changes of the impedance peaks caused by impact damages. The small inlaid picture in figure 12 enlarges the denounced frequency range bringing up shape details of the resonance peak. After a 1-Joule-impact the peak escaped (dashed line).

Fig 12: Impedance spectrogram with resonances of certain eigenmodes.

LAMB-wave-techniques
The integrated piezoceramic patches are able to generate and to receive ultrasonic LAMB- waves which propagate in the plane of the plate-like structure. Depending on the thickness- to-wavelength-ratio many wave modes exist differing e. g. in their shape and dispersion behaviour. In fact, at least two modes occur at a distinct frequency. Additionally to frequency and thickness the propagation characteristics of these waves are governed by the (anisotropic) material properties of the CFRP-structure.

For application in a health monitoring system two ways are promising:

• generation of LAMB-waves by the piezoceramics and reception by distributed sensors and
• sensing of LAMB-waves generated by an damaging event (impact) with following localization and evaluation (impact sensor).

For these purposes basic experiments referring the generation, the reception and the propagation characteristics were performed. The results showed that the piezoceramics mainly generates the first symmetric and asymmetric LAMB-wave modes, dominated by the asymmetric mode. The edges of the piezoceramic patch are identified as sources so, that the complete signal results from the superposition of different waves.

Fig 13: Lamb-wave generation and visualization.

To confirm the detectability of flaws by LAMB-waves, a delamination in the CFRP- material was produced by an impact and the LAMB-wave-field in this region was scanned by an external transducer. In figure 13 the field distribution of the LAMB-wave, launched by the edge of the rectangular piezoceramic patch becomes visible and the area of the impact is marked by a lower wave amplitude.

Acknowledgements

The presented work is a part of the project “Adaptive mechanische Systeme – ADAMES” of the Otto-von-Guericke-Universität Magdeburg, sponsored by the Deutsche Forschungsgemeinschaft (DFG). The support is gratefully acknowledged. Furthermore, we would like to thank S. Herold, A. Hilbig and L. Beindorf for their assistance.

References

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