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INTRODUCTION

Emedded nondestructive evaluation (NDE) is an emerging technology that will allow for the transitioning from conventional ultrasonics methods to embedded systems structural health monitoring (SHM), such as those envisioned for the Integrated Vehicle Health Management (IVHM). Structural health monitoring for IVHM requires the development of small, lightweight, inexpensive, unobtrusive, minimally invasive sensors to be embedded in the airframe with minimum weight penalty and at affordable costs.¹ Such sensors should be able to scan the structure and identify the presence of defects and incipient damage.

Current ultrasonic inspection of thin-wall structures (e.g., aircraft shells, storage tanks, large pipes, etc.) is a time-consuming operation that requires meticulous through-the-thickness C-scans over large areas. One method to increase the efficiency of thin-wall structures inspection is to utilize guided waves (e.g., Lamb waves) instead of the conventional pressure waves.^2–4 Guided waves propagate along the mid-surface of thin-wall plates and shallow shells. They can travel at relatively large distances with very little amplitude loss and offer the advantage of large-area coverage with a minimum of installed sensors.^5,6 Guided Lamb waves have opened new opportunities for the cost-effective detection of damage in aircraft structures,⁷ and a large number of papers have recently been published on this subject.⁸ Traditionally, guided waves have been generated by impinging the plate obliquely with a tone-burst from a relatively large ultrasonic transducer.⁹ Snell’s law ensures mode conversion at the interface, hence, a combination of pressure and shear waves are simultaneously generated into the thin plate. However, conventional Lamb-wave probes (wedge and comb transducers) are too heavy and expensive to be considered for widespread deployment on an aircraft structure as part of a SHM system. Therefore, a different type of sensors than the conventional ultrasonic transducers are required for the SHM systems.

Figure 1. Piezoelectric wafer active sensors (PWAS) mounted on an aircraft panel.

Several investigators^10–12 have recently explored the generation of Lamb-waves with piezoelectric wafer-active sensors (PWAS).¹³ Piezoelectric wafer-active sensors are inexpensive, non-intrusive, unobtrusive, and minimally invasive devices that can be surface-mounted on existing structures inserted between the layers of lap joints or inside composite materials. Figure 1 shows an array of 7 mm square PWAS mounted on an aircraft panel, adjacent to rivet heads and an electric-discharge machined (EDM) simulated crack. The minimally invasive nature of the PWAS devices is apparent. These PWAS weigh around 68 mg, are 0.2 mm thick, and cost $7. They operate on the piezoelectric principle that couples the electrical and mechanical variables in the material (mechanical strain, S_ij, mechanical stress, T_kl, electrical field, E_k, and electrical displacement D_j) in the form:

(1)

where is the mechanical compliance of the material measured at zero electric field (E = 0), is the dielectric permittivity measured at zero mechanical stress (T = 0), and d_kij represents the piezoelectric coupling effect. For embedded NDE applications, PWAS couple their in-plane motion, excited by the applied oscillatory voltage through the piezoelectric effect, with the Lamb-wave-particle motion on the material surface. Lamb waves can be either quasi-axial (S0, S1, S2, . . . ) or quasi-flexural (A0, S1, S2, . . . ) as shown in Animation 1a and 1b. Piezoelectric wafer-active sensor probes can act as both exciters and sensors of the elastic Lamb waves traveling in the material.

a		b
Animation 1. PWAS interaction with (a) SO and (b) AO Lamb modes.
Click Here to view Animation 1a as an .rm file using RealPlayer (~1.03 Mb).		Click Here to view Animation 1b as an .rm file using RealPlayer (~1.1 Mb).
Click Here to view Animation 1a as an .avi file using Windows Media Player (~1.82 Mb).		Click Here to view Animation 1b as an .avi file using Windows Media Player (~1.55 Mb).

For non-destructive evaluation, PWAS can be used as both active and passive probes. Thus, they address four IVHM-SHM needs:^14,15

• Far-field damage detection using pulse-echo and pitch-catch methods
• Near-field damage detection using a high-frequency impedance method
• Acoustic emission monitoring of crack initiation and growth
• Low-velocity impact detection

Piezoelectric wafer-active sensors operation is different than that of conventional ultrasonic probes. For example, PWAS achieve Lamb-wave excitation and sensing through surface “pinching” (in-plane strains), while conventional ultrasonic probes excite through surface “tapping” (normal stress). In addition, PWAS are strongly coupled with the structure and follow the structural dynamics, while conventional ultrasonic probes are relatively free from the structure and follow their own dynamics. Finally, PWAS are non-resonant wide-band devices, while conventional ultrasonic probes are narrow-band resonators.
The main advantage of PWAS over conventional ultrasonic probes lies in their small size, light weight, low profile, and low cost. In spite of their size, these novel devices are able to replicate many of the functions of the conventional ultrasonic probes, as proven by the proof-of-concept laboratory demonstrations described.

PWAS-GENERATED LAMB WAVES

The basic principles of Lamb-wave generation and detection by PWAS probes were first verified in simple laboratory experiments. A 1.6-mm-thick, 2024-aluminum alloy rectangular plate (914 mm × 504 mm × 1.6 mm) was instrumented with 11 7-mm-square, 0.2-mm thick PWAS (American Piezo Ceramics Inc., APC-850) that were placed on a rectangular grid. With this setup, the authors verified that Lamb waves can be satisfactorily generated and detected with PWAS. Omnidirectional transmission is 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 omnidirectional, 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, c_g = 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 omnidirectionally, and correlate well with the theory.

a		b
Figure 2. (a) Reception signals on active sensors one through ten; (b) the correlation between radial distance and time of flight.

PULSE-ECHO WITH PWAS

Piezoelectric wafer-active sensor 11 was used to demonstrate pulse-echo capabilities. Figure 3a shows that the sensor 11 signal has two distinct zones: the initial bang, during which the PWAS 11 acts as transmitter, and the echoes zone, containing wave packs reflected by the plate boundaries and sent back to PWAS 11. These echoes were processed to evaluate the pulse-echo capabilities of the method. Since the wave generated by the initial bang underwent multiple reflections from the plate edges, each of these reflections had a different path length, as shown in Figure 3b. It is interesting to note that the path lengths for reflections R₁ and R₂ are approximately equal. Hence, the echoes R₁ and R₂ in the pulse-echo signal of Figure 3a are almost superposed.

Also interesting to note is that the reflection R₄ has two possible paths, R_4a and R_4b, of the same length. Hence, the echoes corresponding to these two reflection paths arrive simultaneously and form a single but stronger echo signal, which has roughly twice the intensity of the other echoes. A plot of the TOF of each echo vs. its path length is given in Figure 3c. The straight-line fit has a very good correlation (R² = 99.99%). The corresponding wave speed is 5.389 km/s (i.e., within 1% of the theoretical value of 5.440 km/s). The echoes were recorded from over 2 m distance, which is remarkable for such small ultrasonic devices. Thus, it was proven that the PWAS are fully capable of transmitting and receiving pulse-echo signals of remarkable strength and clarity.

Figure 3. The pulse-echo method applied to active sensor 11: (a) the excitation signal and the echo signals on active sensor 11, (b) a schematic of the wave paths for each wave pack, and (c) correlation of path length with 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 stiffeners. Figures 4a, 4b, and 4c show three photographs of PWAS installation on increasingly more complex structural regions of the panel. Figures 4d, 4e, 4f, and 4g 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 4a shows the situation with the lowest complexity, in which only the vertical row of rivets is present in the far left. Figure 4d 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 4b shows the vertical row of rivets in the far left and, in addition, a horizontal double row of rivets stretching toward the PWAS. Figure 4e 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 4c 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. Figure 4g 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 Figure 4f). 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 Figure 4g 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.

a		d
b		e
c		f
Figure 4. Crack-detection laboratory experiments on an aircraft panel: 4a-4c are specimens (1 mm 2025 T3) with increasing complexity. 4d-4g represent the pulse-echo signals; 4g shows the crack detection through the differential signal method.
		g

PWAS PHASED ARRAYS

The advantages of phased-array transducers for ultrasonic testing are multiple.^17,18 Krautkramer, Inc.¹⁹ produces a line of phased-array transducers for the inspection of very thick specimens and for the sidewise inspection of thick slabs, etc. These transducers employ pressure waves generated through normal impingement on the material surface. In this research,²⁰ a phased-array technology was developed for thin-wall structures (e.g., aircraft shells, storage tanks, large pipes, etc.) that uses Lamb waves to cover a large surface area through beam steering from a central location. The authors called this concept embedded ultrasonics structural radar (EUSR) and constructed a simple proof-of-concept experiment (Figures 5 and Animation 2). A PWAS array was made up of a number of identical 7 mm square elements aligned at uniform 9 mm pitch. The PWAS phased array was placed at the center of a 1.2 m square thin aluminum plate (Figure 5). The wave pattern generated by the phased array is the result of the superposition of the waves generated by each individual element. By sequentially firing the individual elements of an array transducer at slightly different times, the ultrasonic wave front can be focused or steered in a specific direction. Thus, electronic sweeping and/or refocusing of the beam was achieved without physically manipulating the transducers. In addition, inspection of a wide zone was possible by creating a sweeping beam of ultrasonic Lamb waves covering the whole plate. Once the beam steering and focusing was established, crack detection was done with the pulse-echo method. During these proof-of-concept experiments, the EUSR methodology was used to detect cracks in two typical situations: a 19-mm broadside crack placed at 305 mm from the array in the 90° direction, and a 19 mm broadside crack placed 409 mm from the array in the 136° direction. Of these two, the latter was more challenging because the ultrasonic beam is not reflected back to the source but rather deflected sideways. Hence, the echo received from the offside crack is merely the backscatter signal generated at the crack tips. Animation 2, which visualizes the crack detection methodology, presents the front panel of the embedded ultrasonic structural radar graphical user interface (EUSR-GUI) displaying the offside signals. The sweep is performed automatically to produce the structural defect image in the right pane. Manual sweep can be performed with the turn knob. The lower pane shows the signal reconstructed at the beam angle f₀ = 136° corresponding to the crack location.

Figure 5. A proof-of-concept EUSR experiment: thin-plate specimen nine-element PWAS array and 19-mm offside crack.		Animation 2. The graphical user interface (EUSR-GUI) front panel. The angle sweep is performed automatically to produce the structure/defect imaging picture on the right. Click the figure above to view animation. Click here to view a larger, unanimated, image of the console.

PWAS SELF-TEST

Figure 6. A PWAS self test: when sensor is disbonded, a clear free-vibration resonance appears at ~267 kHz.

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.

This emerging technology requires a sustained R&D effort to achieve its full developmental potential for applicability to full-scale aerospace vehicles.

ACKNOWLEDGEMENT

The collaboration with the U.S. Air Force Research Laboratory NDE Branch during the Summer Faculty Fellowship Program 2002 is acknowledged.

References

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14. V. Giurgiutiu, In-situ Structural Health Monitoring, Diagnostics, and Prognostics System Utilizing Thin Piezoelectric Sensors¸ USC-IPMO Disclosure #00284, 1/24/2001, U.S. patent application 10–072,644 (8 February 2002), Attorney Docket No. 16139/09021 (in process).
15. V. Giurgiutiu and A.N. Zagrai, “Embedded Self-Sensing Piezoelectric Active Sensors for Online Structural Identification,” ASME J. Vibration and Acoustics, 124 (January 2001), pp. 116–125.
16. V. Giurgiutiu, A.N. Zagrai, and J. Bao, “Embedded Active Sensors For In-Situ Structural Health Monitoring of Thin-Wall Structures,” ASME J. Pressure Vessel Technology, 124 (3) (August 2002), pp. 293–302.
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18. D. Lines and K. Dickson, “Optimization of High-Frequency Array Technology for Lap-Joint Inspection,” Proceedings of the 3rd Joint Conference on Aging Aircraft (1999) www.galaxyscientific.com/agingaircraft/index2.htm.
19. Krautkramer Products Catalog (2002), http://www.krautkramer.com/arrayweb/default.htm.
20. V. Giurgiutiu and J.J. Bao, Embedded-Ultrasonics Structural Radar (EUSR) with Piezoelectric-Wafer Active Sensors (PWAS) for Wide-Area Nondestructive Evaluation of Thin-Wall Structures, USC-IPMO, Disclosure ID No. 00327 (13 February 2002).

For more information contact V. Giurgiutiu, University of South Carolina, Department of Mechanical Engineering, 300 S. Main St., Columbia, SC 29208, email victorg@sc.edu