Fatigue Crack Detection in a Plate Girder Using Lamb...

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Fatigue Crack Detection in a Plate Girder Using Lamb Waves I. J. Oppenheim a* , D. W. Greve b , Wei Wu b , and Peng Zhen c a Dept. of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 b Dept. of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 c Dept. of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT We report on the application of wafer-type PZT transducers to the detection of flaws in steel plate girders. In these experiments one transducer is used to emit a pulse and the second receives the pulse and reflections from nearby boundaries, flaws, or discontinuities (pitch-catch mode). In this application there will typically be numerous reflections observed in the undamaged structure. A major challenge is to recognize new reflections caused by fatigue cracks in the presence of these background reflections. A laboratory specimen plate girder was fabricated at approximately half scale, 910 mm deep with an h/t ratio of 280 for the web and a b/t ratio of 16 for the flanges, and with transverse stiffeners fabricated with a web gap at the tension flange. Two wafer-type transducers were mounted on the web approximately 175 mm from the crack location, one on each side of the stiffener. The transducers were operated in pitch-catch mode, excited by a windowed sinuosoid to create a narrowband transient excitation. The transducer location relative to the crack corresponded to a total included angle of roughly 30 degrees in the path reflecting from the crack. Cyclic loading was applied to develop a distortion-induced fatigue crack in the web at the web gap location. After appearance of the crack, ultrasonic measurements were performed at a range of center frequencies below the cutoff frequency of the A1 Lamb wave mode. Subsequently the crack was extended mechanically to simulate crack growth under primary longitudinal (bending) stress and the measurements were repeated. Direct differencing of the signals showed arrivals at times corresponding to reflection from the crack location, growing in amplitude as the crack was lengthened mechanically. These results demonstrate the utility of Lamb waves for crack detection even in the presence of numerous background reflections. Keywords: Lamb waves, plate girders, ultrasonics. 1. INTRODUCTION Lamb waves have been extensively investigated for flaw detection in plate-like structures. Attractive features of Lamb waves include straightforward techniques for selective generation of particular modes from wafer-type transducers [1] and low attenuation. Flaw detection has been demonstrated by many investigators; however, even when the specimen is a plate of uniform thickness this has not been completely straightforward. For example, the presence of edge reflections motivated one group to use a phased array [2]; Lu et al. used the wavelet transform as an aid in signal interpretation [3]; and Lee and Staszewski examined carefully the effect of sensor placement [4]. Detection becomes even more challenging when the specimen exhibits reflections from joints and welds in the undamaged state [5]. In this paper we examine the detection of fatigue cracks in a specimen that exhibits the complications of multiple welded joints. We induced a fatigue crack in a particular location and recorded Lamb wave reflections during the development of the fatigue crack and also later after mechanical enlargement of the fatigue crack. This work provides an indication of the crack size that can be detected in specimens of this type. We also gain insight into problems associated with the reproducibility of transducer bonding. 2. TEST SPECIMEN AND PROCEDURE * Contact author: [email protected]; 412-268-2950; Carnegie Mellon University, Department of Civil and Environmental Engineering, Pittsburgh, PA, 15213.

Transcript of Fatigue Crack Detection in a Plate Girder Using Lamb...

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Fatigue Crack Detection in a Plate Girder Using Lamb Waves

I. J. Oppenheima*, D. W. Greveb, Wei Wub, and Peng Zhenc a Dept. of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 b Dept. of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

c Dept. of Physics, Carnegie Mellon University, Pittsburgh, PA 15213

ABSTRACT We report on the application of wafer-type PZT transducers to the detection of flaws in steel plate girders. In these experiments one transducer is used to emit a pulse and the second receives the pulse and reflections from nearby boundaries, flaws, or discontinuities (pitch-catch mode). In this application there will typically be numerous reflections observed in the undamaged structure. A major challenge is to recognize new reflections caused by fatigue cracks in the presence of these background reflections. A laboratory specimen plate girder was fabricated at approximately half scale, 910 mm deep with an h/t ratio of 280 for the web and a b/t ratio of 16 for the flanges, and with transverse stiffeners fabricated with a web gap at the tension flange. Two wafer-type transducers were mounted on the web approximately 175 mm from the crack location, one on each side of the stiffener. The transducers were operated in pitch-catch mode, excited by a windowed sinuosoid to create a narrowband transient excitation. The transducer location relative to the crack corresponded to a total included angle of roughly 30 degrees in the path reflecting from the crack. Cyclic loading was applied to develop a distortion-induced fatigue crack in the web at the web gap location. After appearance of the crack, ultrasonic measurements were performed at a range of center frequencies below the cutoff frequency of the A1 Lamb wave mode. Subsequently the crack was extended mechanically to simulate crack growth under primary longitudinal (bending) stress and the measurements were repeated. Direct differencing of the signals showed arrivals at times corresponding to reflection from the crack location, growing in amplitude as the crack was lengthened mechanically. These results demonstrate the utility of Lamb waves for crack detection even in the presence of numerous background reflections. Keywords: Lamb waves, plate girders, ultrasonics.

1. INTRODUCTION Lamb waves have been extensively investigated for flaw detection in plate-like structures. Attractive features of Lamb waves include straightforward techniques for selective generation of particular modes from wafer-type transducers [1] and low attenuation. Flaw detection has been demonstrated by many investigators; however, even when the specimen is a plate of uniform thickness this has not been completely straightforward. For example, the presence of edge reflections motivated one group to use a phased array [2]; Lu et al. used the wavelet transform as an aid in signal interpretation [3]; and Lee and Staszewski examined carefully the effect of sensor placement [4]. Detection becomes even more challenging when the specimen exhibits reflections from joints and welds in the undamaged state [5]. In this paper we examine the detection of fatigue cracks in a specimen that exhibits the complications of multiple welded joints. We induced a fatigue crack in a particular location and recorded Lamb wave reflections during the development of the fatigue crack and also later after mechanical enlargement of the fatigue crack. This work provides an indication of the crack size that can be detected in specimens of this type. We also gain insight into problems associated with the reproducibility of transducer bonding.

2. TEST SPECIMEN AND PROCEDURE * Contact author: [email protected]; 412-268-2950; Carnegie Mellon University, Department of Civil and Environmental Engineering, Pittsburgh, PA, 15213.

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Figure 1 shows the laboratory plate girder specimen that was used for this study. The specimen contains two full panels between transverse stiffeners and is 2450 mm in length with a web height of 910 mm. The flanges are 6.4 mm thick and the web and stiffeners are 3.2 mm thick. The stiffeners (one side only) are welded to the top flange but have a web gap of 20 mm at the bottom flange, a detail which is sometimes preferred because it makes precise fitting of the stiffener unnecessary. However, forces acting on stiffeners (typically through bracing members) cause localized high bending strains in the web gap material, and cyclical loading can create a fatigue crack at that location; this phenomenon has been termed distortional fatigue. When such a crack forms, it creates the danger of subsequent growth under application of primary (bending and shear) in-plane stresses.

Figure 1. Laboratory steel plate girder specimen (dimensions in mm)

In this paper we explore the detection of crack generation and growth using Lamb wave transducers. The plate girder was instrumented with wafer-type Lamb wave transducers as shown in Fig. 1. Two wired transducers were located on each side of the girder, with each pair (PZT0-PZT1 and PZT2-PZT3) placed symmetrically on either side of the stiffener. In principle one could detect a crack by using a single transducer as both an emitter and a receiver, or alternatively by using a transducer pair in a pitch-catch configuration. However, by using additional transducers we obtain insight into the robustness of the crack detection. Note that the transducers are near to but not extremely close to the anticipated crack location, making crack detection more challenging. Note also that it will be necessary to distinguish crack-induced reflections from other reflections from the various joints and the flange and stiffener ends. The transducers were type 5A4E PZT wafers 10 mm square and 0.5 mm in thickness, obtained from Piezosystems, Inc. Each transducer was attached to the web using silver epoxy adhesive. All data was collected by choosing one transducer as a transmitter and using the remaining three transducers as receivers. In addition the reflected signal at the transmitting transducer was recorded although in general this signal had a higher noise level. The exciting signal was a five-cycle waveform with amplitude vs0 = 5 volts provided by a National Instruments PXI-6110 S-series DAQ board. The same board also acquired the signals from the three receiving transducers. At each stage of the experiment received signals were collected with the four different transducers used as transmitters and for five different pulse center frequencies (200, 250, 286, 333, and 400 kHz).

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Figure 2. Laboratory excitation to induce high bending strain in web gap material

In the first part of the experiment a fatigue crack was induced by stressing the girder in an Instron 4400R testing machine. The bottom flange of the girder was anchored to the testing machine and the crosshead was used to apply cyclic loading to a bracket attached to the top flange, as shown in Figure 2. The top flange was displaced laterally approximately 40 mm to each side of the equilibrium position. This loading produced high stresses in the web gap material; 11 loading sequences were undertaken, with between 50 and 400 cycles in each sequence. The testing machine was stopped after each loading sequence and a complete set of ultrasonic measurements was collected; the mounting of the girder was not disturbed during this process. Table I outlines the number of cycles and observations related to each loading sequence.

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Table 1. Cyclical loading of laboratory specimen

Loading Sequence Nr. of cycles Observations Photo

1 50 Setup series, low amplitude displacement - 2 50 - 3 100 - 4 200 - 5 100 - 6 150 - 7 300 Wrinkling in paint - 8 270 “Pings” = audible starting at cycle 236 - 9 200 Crack 7.5 mm long observed at end of series 9 Yes

10 190 Crack 9 mm long observed at end of series 10 - 11 400 Crack 11 mm long observed at end of series 11 Yes

A crack was clearly formed by the end of loading sequence 9 and significantly enlarged during sequences 10 and 11. “Pings” became audible during sequence 8 and consequently we believe that the crack started to form during this sequence. The crack was visible on the side of the web without the stiffener; it did not appear to penetrate through the web thickness, which is as expected for a crack created by bending stress. Figure 3 shows the crack at the end of loading sequence 11.

Figure 3. Appearance of fatigue crack (11 mm long) at the end of loading sequence 11 In order to create a large and unambiguous signal, the crack was then enlarged after loading sequence 11 in two steps using a thin abrasive grinding wheel. This resulted in a crack 0.6 mm wide and 16 mm long (first step) and 30 mm long (second step). The crack was enlarged with the girder removed from the testing machine, but removal was preceded by the collection of two complete sets of reference ultrasonic measurements. Complete sets of ultrasonic measurements were also collected after each step of crack enlargement. Figure 4 shows a photograph of the enlarged crack. The crack penetrated through to the opposite side only after the second step (30 mm crack length) of enlargement.

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Figure 4. Appearance of crack after two steps of mechanical enlargement

In section 3 we first provide an overview of the experimental results and we then discuss in detail observations of the enlarged crack, because those latter experiments produced consistent and unambiguous changes to the reflected signals. In section 4 we will discuss in greater detail the experiments undertaken with the 11-mm fatigue crack, prior to mechanical enlargement.

3. OVER VIEW OF RESULTS AND DETAILS OF EXPERIMENTS WITH ENLARGED CRACK This series of measurements was performed on the girder after it was demounted from the testing machine with the fatigue crack, 11 mm long, partially penetrating the web. Figure 5 shows the measured transients received at PZT1 with PZT0 used as a transmitter, at four frequencies. Identification of reflections in complex structures is difficult because wafer-type transducers emit both S0 and A0 modes with amplitude that depend on frequency [1,6]; in addition, reflections from discontinuities are mode-dependent and frequency-dependent [7]. Nevertheless, some of the large reflections can be identified. The exciting waveform is visible at the start of the transient because of electrical coupling. The next pulse visible is a broad single pulse at low frequencies, but resolves into two distinct pulses at higher frequencies. We attribute the first pulse (t = 24 µs) to the reflection of an S0 mode pulse from the stiffener-web joint and the second to a pulse reflected from the end of the stiffener (t = 38 µs). The next large pulse, visible at all frequencies, arrives at about 162 µs. This is consistent with reflection of an A0 mode pulse that has been coupled into the flange and reflected from the flange end. Small reflections around 100 µs that are visible at low frequencies (250 and 286 kHz) may be due to an S0 mode reflecting from the flange end and the web-flange joint.

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Figure 5. Transient signals received at PZT1 from PZT0; partial thickness fatigue crack (11 mm long) at web gap

We now consider the effect of enlarging the crack. Figure 6 shows the observed transients at 286 kHz (PZT0 to PZT1) before and after enlarging the crack. Reference #1 and reference #2 were taken before enlarging the crack and should be identical. The lower two transients show the effect of enlarging the crack in two steps. There are some evident changes beginning at about t = 84 μs, particularly for the 3 cm crack.

Figure 6. Transient signals received at PZT1 from PZT0, as crack is extended to 16 mm and 30 mm in two steps

In order to see the crack-induced changes more clearly, Figure 7 shows the result of point-by-point subtraction of reference #1 from each of the other three waveforms. In order to exhibit the robustness of observed changes we have plotted the results obtained at three different frequencies. There are evidently substantial changes beginning at

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approximately 79 µs after the exciting pulse and less prominent but significant changes after this. This time difference agrees well with the round-trip time for an S0 pulse to the crack location 76 µs at 333 kHz). It is important to note that the scale has been changed in Figure 7 and that the difference signals are substantially smaller than the reflections in Figure 6. The difference signals are not large but are consistently observed.

Figure 7. Differenced signals received at PZT1 from PZT0, as crack is extended to 16 mm and 30 mm

A similar analysis of other transducer pairs and various frequencies shows that the crack-induced changes are consistently observed, although the magnitude of the changes varies and some transducer pairs show other artifacts. An example is presented in Figure 8, where the difference between a reference signal and various signals observed at 286 kHz with PZT3 as transmitter are plotted for all three receiving transducers. A strong reflection associated with the

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enlarged crack appears in all cases. However other artifacts are apparent, for example pulses that are too early to be caused by the crack (PZT1 and PZT3) and baseline drift (PZT3, caused by the unintentional use of AC coupling).

Figure 8. Differenced signals received at PZT0, PZT1, and PZT1 from PZT3

Part of the variation between transducer pairs is a consequence of the geometry as some transducer pairs have a more favorable geometry for detecting reflections from the crack. This can be seen by plotting a measure of the signal change for all transducer pairs. We define a measure of the signal change

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∑∑

==

=−=n

mii

n

mii

n

miii

gf

gfM

22

1

where fi and gi are the ith samples of two signals. We have computed this measure for all transducer pairs where m and n have been chosen to include only the first S0 reflection from the flange. Figure 9 shows this plot for 286 kHz, where we have compared the signals prior to crack enlargement (left), after enlargement to 16 mm (middle), and after enlargement to 30 mm (right.)

Figure 9. Damage Index (left-to-right) under 11 mm fatigue crack, 16 mm enlarged crack, 30 mm enlarged crack

Note that the crack causes large changes for transducer pairs 0 and 1 and to a lesser degree 0 and 2, 2 and 3, and 3 and 1. For these transducer pairs, a wave emitted from one transducer and reflected from a crack parallel to the flange with angle of incidence equal to angle of reflection will be directed at the second transducer. However there is considerable variation in the magnitude of the changes even for transducer pairs that are geometrically similar. We attribute this to variations from one transducer to another, almost certainly associated with variations in the bonding process. Difficulties with reliably mounting wafer-type transducers have been noted previously [6]. Improving reproducibility of transducer mounting is therefore a major concern. Even so, we have shown here that we can reliably detect changes associated with larger cracks even in the presence of other reflections. In the analysis described above we rely on the stability of the transducers and the electronics. Changes in either of these components will cause changes to the measured signal even in the absence of a crack. The requirements for stability are more stringent when reflections are present before a crack appears. Evidently in these experiments stability is good enough to resolve relatively large cracks. In the following section we report on similar data taken for a smaller fatigue-induced crack.

4. DETAILS OF MEASUREMENTS WITH 11-mm FATIGUE CRACK We now consider measurements taken during the initial loading sequence that induced the fatigue crack. Of the large amount of data available we focus on one transducer pair, using PZT0 as transmitter and PZT1 receiver, as this transducer pair was most effective in detecting the large crack in the experiments above. We first consider data taken after loading sequences 2 through 4. A fatigue crack was only observed visually after stress cycle #8, so we expect that there were only minimal changes to the specimen during these loading sequences. Consequently, we can use these transients to obtain an indication of the stability of the signals in the absence of a crack.

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Figure 10 shows the transient waveform at 286 kHz after loading sequence 2, and the difference waveforms between all combinations of measurements after subsequent loading sequences 3 and 4. The transients observed are similar in appearance and show the prominent reflection associated with the stiffener shown in Figure 5. However the reflections are not expected to be identical because these measurements were taken with the girder clamped to the bed of the testing machine, changing the acoustic reflections at the web- flange joint. We compare the difference waveforms in the range 50-200 µs after the exciting pulse as this is approximately the region in which the largest crack-induced changes are expected. Comparison of these waveforms suggests that transient #2 was clearly different from both #3 and #4. Possibly a minor disturbance of the connections took place after the data for stress cycle #2 was taken. Similar results (transient #2 clearly different and transients #3 and #4 nearly identical) are obtained at other center frequencies. Consequently in what follows we will use the data taken after cycle #3 as a reference.

Figure 10. Transient waveform after loading sequence 2 (top); difference waveforms after loading sequences 3 and 4

In order to follow the changes as the fatigue crack is produced, we use the previously defined damage index M during the time interval in which we expect the first return of an S0 pulse reflected from the web-flange joint. These measurements are plotted in Fig. 11 for three different pulse center frequencies. In order to clearly see changes in the damage index we have plotted log(M) on the ordinate axis, and for comparison we include the values measured in the previous section for the enlarged crack. The measurements show that the damage index is small (less than or of the order of 0.01) throughout the entire stress period. In these nine data series there is no evidence for changes correlated with the appearance of the fatigue crack after loading sequence 8. The data is much more consistent with a damage index that is constant within experimental error. We note that the damage indices measured after enlargement of the crack to 30 mm are substantially greater than the changes due to the fatigue crack. Indeed, in most cases it appears that the 16 mm enlarged crack is also easily detectable.

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Figure 11. Log(M) (damage index) plotted against loading sequence (stress cycle #) at different center frequencies

4. DISCUSSION AND CONCLUSIONS We have measured the reflected Lamb waves from defects of varying, size, ranging from small fatigue-induced cracks to enlarged cracks 16 and 30 mm in extent. These reflections were in addition to reflections from the stiffener and flange present in the undamaged structure. The enlarged cracks were clearly apparent from the reflected Lamb waves while the smaller fatigue-induced cracks were not unambiguously detected. These results were robust with respect to changes in the pulse center frequency and also were clearly observed for all transducer pairs. However, variations between transducers were evident.

ACKNOWLEDGEMENTS The authors wish to acknowledge support 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.

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