Crack Detection with Wireless Inductively-Coupled Transducers

Peng Zhenga, David W. Greveb, and Irving J. Oppenheimc*

a Dept. of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 b Dept. of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 c Dept. of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

ABSTRACT In earlier work we developed inductive coupling for surface-mounted Lamb wave transducers operating at relatively low frequencies, such as 300 kHz. We now report on similar inductively-coupled transducers at higher (multiple MHz) frequencies. Our investigation was motivated by a particular application, to examine a box girder top flange, using transducers mounted along the flange edge. We employ a pair of transducers in pitch-catch mode, offset to create a diagonal path, and show that a shadow is detected when the path is intercepted by a through-thickness crack. We compare results obtained using conventionally wired transducers and using inductively-coupled transducers, showing that effective performance can be achieved with wireless (inductively-coupled) operation. Superior performance is obtained if plexiglas wedges are used to direct the beam along the diagonal path. Reflection from the crack is evident, as is the shadow effect along the direct diagonal path. Keywords: Box girder, fatigue crack, Lamb waves, longitudinal waves, plate girders, ultrasonics.

1. INTRODUCTION In recent publications [1, 2] we introduced wireless, inductively-coupled, piezoceramic active wafer transducers to serve as transmitters and receivers of Lamb waves in plates, and in related articles [3, 4] we presented their equivalent circuits and studied their electrical behavior. In another paper [5] we extended the technology to higher frequency and observed transducer performance when exciting the edge of a plate [6], and in other publications [7, 8] we studied the longitudinal waves produced by edge excitation. In this paper we apply those findings to the detection of through-thickness flaws in the top flange of a steel box girder member. The inductively-coupled concept is pictured schematically in Figure 1. A PZT wafer transducer is wired to a coil with a ferrite core; the assembly can be encapsulated and permanently mounted to the structure. A probe (containing one coil acting both in transmission and reception, or two separate coils) is then brought into proximity with the transducer. Figure 1 shows the probe coil excited by a pulse generator with internal resistance Rs. The pulse is inductively coupled to the PZT wafer and consequently an ultrasonic wave is excited in the structure. Echoes reflected from boundaries or flaws will then be coupled back into the probe coil resulting in a signal vs(t) appearing across the generator resistance Rs. Alternately, the received signal vr(t) can measured using a separate winding, possibly with a different turns ratio. Another version of the transducer technology using a planar coil is shown in Figure 2, where the transducer is a three-part assembly: a double-sided printed circuit board forming a flat coil of 32 turns; a ferrite sheet; and a PZT wafer with a patterned top surface. The ferrite sheet is Steward MP1040-100, 1 mm thick and 26 mm square, and the PZT wafer is type 5A4E, 1 mm thick. The probe uses a ferrite core (18 mm diameter and 5 mm thick, CWS Bytemark) with 20-turn coils for transmission and reception. Figure 3 shows a laboratory assembly representing a steel plate girder at roughly one-third scale, with a web thickness of 3.2 mm and a flange width of 10 cm. A single transducer is mounted roughly 2.5 cm from a stiffener, approximately 19.5 cm from the web-flange joint, and approximately 28 cm from a free edge.

* Contact author: ijo@cmu.edu; 412-268-2950; Carnegie Mellon University, Department of Civil and Environmental Engineering, Pittsburgh, PA, 15213.

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PZT

)(tvr

pulsev)(tvs

sR

Figure 1. Wireless, inductively-coupled PZT transducer

Figure 2. Planar coil transducer and probe

61 cm

91 cm

19.5 cm

28 cm

transducer

10 cm

crack

Figure 3. Laboratory assembly representative of plate girder geometry

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Testing was performed with a National Instruments PCI-6110 DAQ board controlled by LabVIEW, using a five-cycle windowed sinusoid as the excitation for pulse-echo measurements. Figure 4 shows the signals obtained at center frequencies between 250 and 351 kHz, after averaging of ten transients. Test results in our first paper [1] showed the signal to be comparable to one obtained in a wired configuration, and undistorted as the gap between the probe and the transducer was changed. Figure 4 shows reflections detected from various boundaries in Figure 3, and arrival time calculations identify different Lamb wave modes (S0 and A0) and different signal paths as noted in Figure 4.

Figure 4. Pulse-echo signals obtained at four different excitation center frequencies In our first studies operated the transducers at relatively low frequencies, in the range from 100 kHz to 400 kHz, surface-mounted on thin plates. For a typical web in a steel plate girder, operation in this frequency range would limit the frequency-thickness product, fd, to values below 1.6 MHz-mm where only the lowest-order Lamb wave modes are generated. One advantage to generating only low-order Lamb modes is that limiting the number of modes makes it easier to interpret reflections. However, operation at low fd also causes the wavelength to be relatively long, typically several cm, which reduces the scattering from small defects. For that reason, we also studied and tested the function of transducers at higher frequencies with correspondingly higher fd products [5] with operation at shorter wavelengths. Figure 5 shows a detailed sketch and a photograph of an inductively-coupled high frequency transducer edge-mounted on a relatively thick plate. The piezoelectric element is 5A4E PZT from Piezo Systems, 15 mm square and 1 mm thick, bonded to the plate edge with cyanoacrylate adhesive, along with an inertial backing consisting of lead sheet 3 mm thick. The coil consists of a ferrite E-core (Magnetics product FR-42216-EC) with three turns of wire. (The insulated conductors visible in Figure 5b attach to the PZT element but do not act as part of the inductively-coupled configuration; they permit operation of the same PZT element as a conventional wired transducer for comparison purposes.)

PZTleadferritecoil

Figure 5a. Sketch of transducer for edge-mounting on a thick plate; 5b. Transducer edge-mounted on plate 19 mm thick

E-core inductor

PZTand lead inertial backing

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2. TESTING OF EDGE-MOUNTED INDUCTIVELY-COUPLED TRANSDUCERS To take advantage of inductive coupling, it is necessary that sufficient signal strength be achieved and that the signal not be excessively distorted through the coupling. We first studied these questions, for the low frequency Lamb wave transducer, by finite element simulation of the electromagnetic field and by analysis of the equivalent circuit [3]; we determined that effective coupling and good fidelity would be achieved, and confirmed those results experimentally [4]. Operation of an edge-mounted transducer at much higher frequencies, in the multiple MHz range, raised those questions anew, and we again achieved successful performance [5]. Transducers shown in Figure 5 were tested on a wide steel plate 19 mm thick and 30.5 cm long. An Agilent 33120A waveform generator was used to produce a five-cycle windowed sinusoid for excitation, and a National Instruments PXI-6115 data acquisition card was used for data collection. Tests were conducted at different excitation center frequencies and with different gaps between the probe coil and the inductor. Maximum response was observed at a center frequency near 2.2 MHz, reflecting a through-thickness resonance of the PZT wafer, and that excitation frequency was used in our subsequent experiments. The frequency and plate thickness correspond to an fd product of 42 MHz-mm, well into the range at which longitudinal waves with trailing pulses are observed.

Figure 6. Pitch-catch test results, steel plate 19 mm thick and 30.5 cm long

Figure 6 shows the results of a typical pitch-catch experiment, using a gap of 3 mm between the probe coil and the transducer. The time of first arrival corresponds to the longitudinal bulk wave in steel, and trailing pulses are plainly evident. The train of pulses arriving at approximately 155 μs represents the original disturbance after two boundary reflections, for a total path length of 91.5 cm.

Figure 7. Pulse-echo test results using inductive coupling to a commercial transducer, showing reflection from part-thickness slot

Figure 7 shows the results of a pulse-echo test on the same steel plate using inductive coupling to a 1 MHz commercial transducer, after machining a part-thickness slot at the middle of the plate, 5.7 mm deep and 12.6 cm long in the transverse direction. The pulse centered at approximately 57 μs is a reflection from that crack, and the pulse train arriving at roughly 110 represents the reflection from the far edge, with a total path length of 61.0 cm. In this experiment a Krautkramer 1 MHz gamma series probe was used, connected to a coil consisting of a ferrite E-core (Magnetics product FR-43208-EC) with 18 turns of wire. The probe was driven by a five-cycle windowed sinusoidal voltage at 15.6 VPP, with a 1 MHz center frequency, produced an Agilent 33120A waveform generator.

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3. BOX GIRDER APPLICATION The edge-mounted (high-frequency, inductively-coupled) transducer was developed in the course of a research project to detect through-thickness cracks in the top flange of a steel box girder. As depicted in Figure 8, the box girder is a hollow section with a wearing surface on the top flange that precludes visual inspection. (The opening in the end diaphragm pictured in Figure 8 is not present in the subject structure, where it is impossible to access the interior.) The box girder is 94 cm deep, the flange is 94 cm wide and 18 mm thick, and the wearing surface is 40 mm thick. The goal of the research was to detect through-thickness cracks in the top flange before such cracks exceeded a length of, say, 15 cm.

BA

Figure 8. Sketch of full-size box girder specimen (94 cm deep, 94 cm wide) with wearing surface on top flange

Initial ultrasonic pitch-catch experiments were performed using wired transducers mounted at four different locations: opposite positions on the web plates, opposite soffit positions on the overhanging segments of the top flange, opposite positions on the edges of the top flange as labeled A and B in Figure 8, and opposite positions on the edges of the bottom flange. The experiments were first performed driving the transducers at low frequencies, in the 100-400 kHz range, and in all cases Lamb waves propagated through the flange; in these first experiments we observed that reflections from the web-flange joints were not significant in comparison to the Lamb waves pulses in the flange, and that the wearing surface had little influence on propagation of the Lamb wave pulse. Ultrasonic wave propagation through the flange was then tested using commercial ultrasonic equipment (Krautkramer Gamma series probes and a Krautkramer USPC-2100 system) with transducers mounted on the flange edges. Tests were performed using 1 MHz, 3.5 MHz, and 5 MHz probes and the results were comparable across that range of frequencies. A typical pulse-echo record is shown in Figure 9; the times of first arrival yield a wave speed of 5.9 km/s, corresponding to the longitudinal bulk wave velocity in steel.

Figure 9. Pulse-echo results, 5 MHz commercial transducer; longitudinal wave first arrival at 318 μs

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Figure 9 displays trailing pulses associated with boundary reflections from the two plate surfaces, which are termed “secondary echoes produced by split-off transverse waves” in Krautkramer [6]. Examining any pulse in the train, it becomes evident that energy is transferred to trailing pulses, and therefore the pulses do not propagate indefinitely. Using both simulation studies and experiments, in recent papers [7, 8] we describe the transition from Lamb waves at relatively low frequencies (low fd products) to longitudinal waves with trailing pulses at higher fd. One major advantage of working at higher frequencies is to shorten the wavelength. For example, a 1 MHz longitudinal wave has a wavelength less than 6 mm, which is small compared to the flaw size of interest, whereas a 100 kHz Lamb wave has a wavelength that is a substantial fraction of the flaw size. Another major advantage is that the bulk wave (in this case the longitudinal wave with trailing pulses) dominates the response, making it relatively easy to interpret reflections, as contrasted to the case of multiple Lamb wave modes. For these reasons, edge-mounted transducers operating at relatively high frequency were chosen for further development.

4. CRACK DETECTION USING WEDGES FOR OFF-NORMAL BEAMING, WIRED In the subject box girder, under primary bending stress in a region of negative moment, a fatigue crack in the top flange would grow in the transverse (A-B) direction in Figure 8. The radiation pattern of edge-mounted transducers is at its maximum normal to the edge, and a transverse crack would produce little scattering of such a wavefront. Therefore, crack detection requires a wave oriented obliquely, not normally, to the flange edge. We noted earlier that waves excited in the top flange showed little influence of the box girder webs and little influence of the wearing surface, in effect observing the top flange to behave in a manner largely indistinguishable from a long rectangular plate with free edges. This allowed us first to investigate the behavior in an experiment on a plate at 55% scale, 10 mm thick and 50 cm wide. That plate size permitted us to machine slots into the plate as a physical simulation of transverse through-thickness cracks, as pictured in Figure 10.

Figure 10. Steel plate (55% scale) with a machined slot to simulate a through-thickness transverse crack

We use angle-beam (wedge) transducers to excite or detect ultrasonic pulses oblique to the flange edge. Figure 11 shows a schematic set of three transducers: one transmitter, one receiver on the direct path to detect a crack shadow, and one receiver to detect a crack reflection (echo). The set can be envisioned to advance along the plate with fluid coupling to the plate edges. Alternately, a series of inductively-coupled transducers can be envisioned along each edge, operated in sets of three in the same manner shown in Figure 11. We show each of the receivers as a single transducer, for simplicity, but we can also implement each receiver as an array of 2 or 3 closely-spaced transducers to provide redundancy and to characterize crack length.

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Figure 11. Sketch showing a set of three transducers, translating along plate, to detect shadow and reflection evidence

Our wired tests were performed with 1.0 MHz Krautkramer Gamma series probes with 30o Krautkramer plexiglass wedges, driven by a Krautkramer USPC-2100 system. A longitudinal plane wave in plexiglass at an incidence angle (measured with respect to the normal) of 30o into steel is above the critical angle for transmitting a longitudinal wave, and transmits only a transverse wave at an angle of 36o. The transverse wave is polarized in the plane of the steel plate (SH polarization) and therefore will be free of the boundary reflections that produced the trailing pulses seen in Figure 9. Figures 12 and 13 show waveforms received, respectively, by the shadow detector and by the reflection (echo) detector as the set of transducers is advanced over a distance of 12 cm centered about the crack position, for a crack 3.2 cm long in the steel plate at 55% of full size. A relative null is plainly evident in Figure 12, and a strong reflection from the crack is clearly evident in Figure 13. The high signal-noise ratio achieved by commercial ultrasonic transducers and wedges is very effective at detecting the slot that is the physical simulation of a through-thickness crack. We note, as predicted, that the transverse wave polarized in the plane of the plate arrives with no trailing pulses.

Figure 12. Waveforms showing shadow (relative null) when transmission path is intercepted by crack, 55%-scale plate experiment

Wired experiments were then conducted on the full-size specimen. Figures 14 and 15 show waveforms received, respectively, by the shadow detector and by the reflection detector for a crack 6 cm long in the full size specimen. A relative null is plainly evident at approximately 370 μs in Figure 14, top, and a strong reflection from the crack is evident in Figure 15, top. Again, the high signal-noise ratio of commercial ultrasonic transducers and wedges is effective at detecting the through-thickness crack. Small, premature arrivals are seen in many records in Figures 12 through 15, which we interpret to result from mode conversion when the primary shear wave pulse converts to a quicker longitudinal wave when scattered from the crack, or upon its oblique incidence at the web-flange joint.

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Figure 13. Waveforms showing reflections from crack, 55%-scale plate experiment

Figure 14. Waveforms comparing shadow effect at crack (top) to pulse arrival elsewhere (bottom), full size experiment

Figure 15. Waveforms comparing reflection from crack (top) to no reflection elsewhere (bottom), full size experiment

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5. CRACK DETECTION USING WEDGES, INDUCTIVELY-COUPLED We conducted one preliminary set of experiments operating commercial angle-beam (wedge) transducers on the full-size specimen with inductive coupling. We used the same transducers and wedges that had been employed in the wired tests, connected to a coil consisting of a ferrite E-core (Magnetics product FR-43208-EC) with 18 turns of wire. The Agilent 33120A waveform generator produced a five-cycle windowed sinusoidal voltage waveform at 15.6 VPP with a center frequency of 1 MHz for excitation, and a National Instruments PXI-6115 data acquisition card was used for data collection. Figures 16 and 17 record representative signals obtained in these preliminary experiments for the case of a transverse crack 12 cm long. Figure 16 (top) shows the arrival of a pulse in pitch-catch mode near 370 μs for typical locations where the crack does not intercept the ultrasonic beam, and Figure 16 (bottom) shows the relative null when the crack intercepts the beam and casts a shadow. Figure 17 shows the reflection (echo) from that crack.

Figure 16. Preliminary results, angle-beam transducers (Krautkramer 1 MHz gamma series) with inductive coupling; Top: Typical direct pulse arrival near 370 μs, for locations away from crack; Bottom: Shadow effect (relative null) at crack location

Figure 17. Preliminary results, angle-beam transducers (Krautkramer 1 MHz gamma series) with inductive coupling; Reflection from crack arriving near 370 μs

The results shown in Figures 16 and 17 are presented as evidence that inductive coupling can be used with commercial angle-beam transducers for purposes of flaw detection. The signal strength and signal-noise ratio (SNR) associated with these results are inferior to those observed in Figures 12 through 15, but we note at least three conditions in our preliminary experiments that can be improved in future work. The signals in Figures 12 through 15 were excited by the high-voltage pulser circuit in the Krautkramer USPC-2100, whereas the signals in Figures 16 and 17 were excited, through inductive coupling, by a 15.6 VPP windowed sinusoid. Moreover, the inductor developed for these tests was a first, non-optimized, attempt at tuning the system for coupling. Finally, the noise observed in these preliminary results is a candidate for reduction in future experiments.

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6. DISCUSSION AND CONCLUSIONS We described our recent experience developing inductively-coupled ultrasonic transducers, which can be subdivided into two major categories. We first described surface-mounted transducers operating at relatively low frequencies on relatively thin plates, with fd products below (say) 2 MHz-mm, to function as active Lamb wave transducers. We next described edge-mounted transducers operating at higher frequencies on thicker plates, with fd products greater than (say) 20 MHz-mm, to generate waves traveling at the velocity of a longitudinal wave, with trailing pulses. We showed that edge-mounted transducers, coupled inductively, performed successfully in pitch-catch and in pulse-echo testing, including detection of a part-thickness flaw in a steel plate. Our purpose was to detect through-thickness transverse cracks in the flange (94 cm wide, 18 mm thick) of a steel box girder, and we first conducted studies on a steel plate at 55% scale with machined slots as physical simulations of through-thickness cracks. The crack orientation, transverse to the longitudinal axis of the beam, precluded detection by a wave propagating normal to the plate edge because little scattering would occur in that geometry. We detected cracks using commercial angle-beam (wedge) transducers, in wired experiments, and observed them to be highly effective at illuminating the cracks and permitting unambiguous crack detection. The performance of the transducers was ample to detect a crack 3.2 cm long in the steel plate, which would be equivalent to a crack 6 cm long in the full-size prototype, well below the target range. Those results were then confirmed, in the wired case, on the full-size box girder specimen with a crack 6 cm long. Finally, successful preliminary experiments were conducted using inductively coupled angle-beam (wedge) transducers on the full-size box girder specimen containing a crack 12 cm long.

ACKNOWLEDGEMENTS The authors wish to acknowledge support from Bombardier Total Transit Systems, from the Pennsylvania Office of Economic and Community Development under the Pennsylvania Infrastructure Technology Alliance program, and from the National Science Foundation under grant CMS-0329880. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the other sponsors.

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