Now day’s embedded sensors are used widely in many applications. It covers almost all fields of applications and makes them to work concisely. These embedded sensors are preferred to ordinary sensors because of their special characteristics
o Fixed functionality
o Reactive nature (real time response)
o Heterogeneous implementation (software + hardware implementation)
This helps to perform the operation very accurately and correctly. Embedded sensors within the structure material add intelligence to structure and enable real time monitoring at some critical location, which is not accessible to ordinary sensors.
This paper explains one of the applications of embedded sensors i.e. embedded piezoelectric wafer active sensor. These are specially used in aerospace applications. Any problem related to aircraft panel is very cumbersome to detect by using ordinary sensors. There is very large area to scan which is very difficult and time consuming. Hence these ordinary sensors are replaced by piezoelectric wafer active sensor (PWAS). This PWAS waves arrays can scan large area of aircraft and used to create the embedded ultrasonic structural RADAR.
Embedded PWAS operates on the piezoelectric principle. That couples the electrical and mechanical variables, in the material. Embedded PWAS are more advanced due to following characteristics:
• These are light in weight (68 mg).
• Minimum invasive
• Has affordable costs ($7)
• Small thickness (0.2 mm)
• Non-intrusive
One more important characteristic is ability of self-testing using electromechanical impedance.
PWAS waves are used to perform lamb wave transmission and reception, crack detection in aircraft panel. It has ability to perform in-site non-destructive evaluation (NDE) by interacting PWAS waves with lamb modes. Embedded NDE is an emerging technology. By using this technology one can change conventional ultrasonic method to embedded system.
Now the use of PWAS wave in lamb wave transmission is explain further. These wave are used for inspection of thin wall structure (e.g. aircraft shell storage tanks, large pipes, etc) .Sensors used for this application should be able to scan the structure and identify the presence of defects and incipient currently. We use ultrasonic inspection for this purpose that is very time consuming and complex.
To overcome this problem the guided waves (lamb waves) are introduced instead of conventional pressure waves. Guided wave can travel relatively large area in very small time and with very small amplitude lost .so offer the advantage of large area coverage with minimum of installed sensors. Guided lamb waves have opened new opportunities for cast effective detection of damage in aircraft structure.
In case of thin plates mode conversion occurs at the interface hence combustion of pressure and shear waves are simultaneously generated in to the thin plate. However conventional lamb wave probes are too heavy and expensive to be considered. Recently, it is possible to generate lamb waves with PWAS due to their various char. As mentioned above. PWAS probes can act as both exciters and sensors of the lamb waves traveling in material PWAS operation is very different than that of conventional ultrasonic probes. With the lamb wave partial motion on the material surface lamb waves can be either quasiaxial (s0, s1, s2...) or quasiflexural (A0, S1, S2...)
Fig 1. PWAS interaction with (a) S0 and (b) A0 Lamb modes
There is operational difference between conventional ultrasonic probes and PWAS. This can be explained as follows:
1. PWAS are strongly coupled with structure and follow the structural dynamics, while conventional ultrasonic probes are relatively free from the structure and follow their own dynamics.
2. PWAS achieve lamb wave excitation and sensing through surface pinching (in plane strain), while conventional ultrasonic probes excite through surface tapping (normal stress)
3. PWAS are non-resonant wide band devices, while conventional ultrasonic probes are narrow band resonators.
Piezoelectric-Wafer Active Sensors (PWAS)
Sensors Piezoelectric-wafer active sensors (PWAS) are small, non-intrusive, inexpensive, wide-band elastic wave generators/receptors, which can be intimately affixed to a structure and can actively interrogate it PWAS are commonly manufactured from thin wafers of the piezoceramic Pb(Zr –Ti)O3 (a.k.a.PZT).Unlike conventional ultrasonic transducer, PWAS are non-resonant devices with wide frequency-band capabilities. They can be wired into sensor arrays that can be connected to data concentrators and wireless communicators. PWAS have captured the interest of academic and industrial communities due to their low cost and small non-intrusive nature .
Generation of Lamb waves
The basic principle of Lamb wave generation and detection by embedded PWAS probes can be verified in simple laboratory experiments. A1.6 mm thick, 2024 Al alloy. Rectangular plate (914mm*504mm*1.6mm) was instrumented with 117 mm square, .2 mm thick PWAS that were placed on rectangular grid. With this set up Lamb waves can be satisfactorily generated and detected with PWAS. with PWAS omni directional transmission can be achieved and signals are strong enough and attenuation is sufficiently low for echoes to be detected. The proof of these attributes is especially important for PWAS, which are at least an order of magnitude smaller and lighter than conventional ultrasonic transducers, and, hence, utilize much lower power.
To prove that the Lamb waves excited by PWAS are omni directional, one PWAS (11) was used as a transmitter and the other PWAS (1–10) as receivers. The signals observed in this investigation are shown in Figure 2a. In each row, the electromagnetic coupling of the initial bang is shown around the origin. Then, the first wave package corresponding to the wave received from the transmitter PWAS is seen followed by other wave packages corresponding to reflections from the plate edges. The time difference between the initial bang and the wave-package arrival represents the time-of-flight (TOF). The TOF is consistent with the distance traveled by the wave. Figure 2b shows the straight-line correlation between TOF and
distance. The slope of this line is the experimental group velocity, cg = 5.446 km/s, while the theoretical value should be 5.440 km/s. Very good accuracy is observed (99.99% correlation; 0.1% speed detection error), proving that PWAS-generated Lamb waves are loud and clear, propagate omni directionally, and correlate well with the theory.
(a) Reception signals on active sensors one through ten;
(b) the correlation between radial distance and time of flight.
PWAS crack detection
Wave-propagation experiments were conducted on an aircraft panel to illustrate crack detection through the pulse-echo method. The panel has a typical aircraft construction, featuring a vertical splice joint and horizontal stiffener. Figures 3a, 3b, and 3c show three photographs of PWAS installation on increasingly more complex structural regions of the panel. Figures 3d, 3e, 3f, and 3g show the PWAS signals. All the experiments used only one PWAS, operated in pulse-echo mode. The PWAS was placed in the same relative location (i.e., at 200 mm to the right of the vertical row of rivets). Figure 3a shows the situation with the lowest complexity, in which only the vertical row of rivets is present in the far left. Figure 3d shows the initial bang (centered at around 5.3 microseconds) and multiple reflections from the panel edges and the splice joint. The echoes start to arrive at approximately 60 mm. Figure 3b shows the vertical row of rivets in the far left and, in addition, a horizontal double row of rivets stretching toward the PWAS. Figure 3e shows that, in addition to the multiple echoes from the panel edges and the splice, the PWAS also receives backscatter echoes from the rivets located at the beginning of the horizontal row. These backscatter echoes are visible at around 42 mm. Figure 3c shows a region of the panel similar to that presented in the previous row, but having an additional feature: a simulated crack (12.7 mm EDM hairline slit) emanating from the first rivet hole in the top horizontal row. Figure3g shows features similar to those of the
previous signal, but somehow stronger at the 42 mm position. The features at 42 mm correspond to the superposed reflections from the rivets and from the crack. The detection of the crack seems particularly difficult because the echoes from the crack and from the rivets are superposed.
This difficulty was resolved by using the differential signal method (i.e., subtracting the signal presented in Figure 4e from the signal presented in Figure3f). In practice, such a situation would correspond to subtracting a signal previously recorded on the undamaged structure from the signal recorded now on the damaged structure. Such a situation of using archived signals is typical of health monitoring systems. When the two signals were subtracted, the result presented in Figure3g was obtained. This differential signal shows a loud and clear echo due entirely to the crack. The echo, marked "reflection from the crack" is centered at 42 mm (i.e., TOF = 37 mm), which correlates very well with a 5.4 km/s 200 mm total travel from the PWAS to the crack placed at 100 mm. The cleanness of the crack-detection feature and the quietness of the signal ahead of the crack-detection feature are remarkable. Thus, PWAS were determined to be capable of clean and unambiguous detection of structural cracks. A manual sweep of the beam angle can be also performed with the turn knob; the signal reconstructed at the particular beam angle (here, f0 = 136°) is shown in the lower picture.
E/M Impedance Method
SHM (Structure Health Monitoring)
Figure - General concept of a sensor-array structural integrity monitoring
system suggested installation on an aging aircraft.
A novel sensor-self-diagnostics method was developed and experimentally verified. It was shown that, for a disbonded sensor, the imaginary part of the E/M impedance displays a clear resonance pattern that was not present in the perfectly bonded sensor. This sensor self-diagnostic method is essential for reliable in-field implementation of the active-sensor structural health monitoring concepts.
PWAS self test
Since the PWAS probes are adhesively bonded to the structure, the bond durability and the possibility of the probe becoming detached are of concern. To address this, a PWAS self-test procedure has been identified that can reliably determine if the sensor is still perfectly attached to the structure. The procedure is based on PWAS in-suit electromechanical impedance.14, 15,16
Fig 4. A PWAS self test: when sensor is disbanded, a clear free-vibration resonance appears at ~267 kHz.
PWAS resonance and the disappearance of structural resonance constitute features that can unambiguously discern when the PWAS has become disbonded and can be used for an automated PWAS self-test. For a partially disbonded PWAS, a mixture of PWAS and structure vibration was recorded.
Conclusion
Embedded NDE piezoelectric wafer active s can be structurally embedded as both individual probes and phased arrays. They can be placed even inside closed cavities during fabrication/overhaul (such as wing structures), and then be left in place for the life of the structure. The embedded NDE concept opens new horizons for performing in-situ damage detection and structural health monitoring of a multitude of thin-wall structures such as aircraft, missiles, pressure vessels, oil tanks, and pipelines.
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non destructive test....!!
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