·Table of Contents
·Materials Characterization and testing
Non-destructive Inspection of Smart MaterialsGerhard Mook, Juergen Pohl, Fritz Michel, Thomas Benziger
Institute of Materials Engineering and Materials Testing (IWW)
GPO Box 4120, D-39016 Magdeburg
|Fig 1: : Design of the adaptive structure|
The scope of properties to be evaluated includes the internal structure of the composite (fiber orientation, ply sequence, sensor and actuator position, local fiber or epoxy concentrations), planar flaws oriented parallel to the surface (delamination, ceramic debonding), perpendicular oriented flaws (cracks of the matrix or of the patches) and volume imperfections (pores, voids, inclusions). Some typical imperfections in the smart material system are shown in Figure 2.
|Fig 2: Typical imperfections after manufacturing of a CFRP laminate with embedded piezoceramic patch|
|Fig 3: Three stages to health monitoring|
Fig. 3 presents these stages from the signal generating point of view. At the first stage, a transmitting and receiving probe scans the object. The object may be either passive or active. At the second stage, the internal actuators are used as transmitters for the test signal. 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 images as the inspection result. Finally, the third stage is aimed to avoid any external sensor or transmitter. Here, the internal sensors and actuators perform as transmitters and receivers. To get information about local distribution of structural properties the transmitting function may be switched to various actuators. Unfortunately, no images can be expected and it is a matter of further investigations to find out the chances and limitations of this approach.
Results of the stages 2 and 3 are outlined in paper 250 of the present proceedings .
|Fig 4: Load-displacement curve and AE cumulative energy-displacement curve|
The bending test yields information on the behavior of the piezoceramics and the surrounding interface under tension and compression load. From the SEM images in Figure 5 the damage progress with increasing load (see marks in Figure 4) can be described as follows:
||Fig 5: SEM images of damage propagation within CFRP contai-ning two embedded piezoceramic patches during in-situ bending
|a) crack initiation in the tension stressed piezoceramic patch accompanied by first acoustic emission (AE) signals,
b) delamination without cracking in the compression zone with increasing AE,
c) crack initiation in the insulation fleece and propagation into the matrix and the carbon fibers,
d) crack propagation in the compression stressed piezoceramic patch.
Eddy current (EC) method:
The essential idea is to use the anisotropic conductivity of the material. Along the carbon fibers much better conductivity is found than across the fibers. Additionally, the piezoceramic patches are coated with thin copper-nickel layers providing electrical contacting. This layer is able to carry eddy current. In case of patch cracking the coating also cracks thus interrupting current paths [6,9].
Ultrasonic (US) method:
Ultrasonic testing is based on the reflection and transmission behavior of elastomechanic waves at internal structures and so different reflection and transmission testing techniques are applicable . 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 [8,9].
X-radiation is suitable to study both surface and internal damage of the laminate as well as of the piezoceramic. Digital X-ray imaging using a special differential technique was used to detect cracks in the piezoceramic.
Fig 6: Ceramic cracking caused by structural inhomogeneities. |
a) US C-scan, b) EC scan, c) micrograph at the indicated position
A very sensitive point is the contacting of actuators because it also may damage them during autoclave curing . One kind of contacting are dedicated windows in the insulation fleece as shown in figure 7. The conducting CFRP layer acts as the common electrode for some actuators. The contact window causes an actuator deformation at curing.
|Fig 7: : Actuator deformation and cracking caused by window- roving contacting|
The resulting stresses deform the actuator and even may induce cracks. US-C-scan indicates this 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 fiber 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.
Fig. 8 presents results of a multi-actuator specimen with some patches remaining passive at function test. The reason were unidentified short-circuits and breakdowns causing local material degradation. The US-scan in figure 8a) of one of the ceramic patches (grey rectangle) brings up a bright circular area caused by the bending of the patch into the contacting window. The dark spots at the right and left edge of the piezoceramic patch are the points of short-ciruits between the current carrying rovings and the metallic coating of the piezoceramic patch.
A US C-scan of another depth plane in figure 9b) shows bright lines starting at these points. Due to their reflection properties they could be identified as carbon fibre rovings unintentionally brought into the material.
|Fig 8: Detection of electrical breakdowns using ultrasonic (a and b) and eddy current techniques (c)|
The EC scan in figure 8c) brings up the edges of the piezoceramic patch as well as the contact roving on the front side of the patch. Additionally, linear objects can be recognised corresponding to the US result. These anomalies of conductivity confirm the assumption of spliced carbon fibres close to the piezoceramic patch. Plastographic investigations confirmed this result, which gave explanation of the short-circuits.
|Fig 9: Joule low speed impact damage in the embedded piezoceramic actuator|
The second example in figure 10 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.
|Fig 10: : NDE images of a 0.5 Joule impact on the embedded piezoceramic patch|
In the lower part of the piezoceramic patch, the NDE images bring up cracks caused by two contacting rovings from both sides of the patch during autoclave curing. The circular area results from the impact. The US scan in fig. 10a) was recorded using active US method reported in paper 250 of the present proceedings. Here, the piezoceramic patch acts as the US transmitter and the scanning transducer only receives the acoustic waves. One advantage of this method is the suppression of covering CFRP layers. This special technique shows that the piezoceramic in the inner and outer areas of the ring remains 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 X-ray in figure 10b) brings up circular cracks in the piezoceramic patch. In the EC scan of figure 10c) also a dark ring can be seen resulting from the damaged metal coating of the patch. All NDE images confirm the fact that no significant damage has occurred in the centre of the damaged zone.
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