Feasibility of using Lamb waves for disbonds and corrosion detections in aircraft fuselage structures was investigated in recent years. Measurement performed on various laboratory-fabricated specimens as well as on panels removed from aircrafts has shown consistent results and demonstrated its potential applications for large area structural integrity evaluation [1–3]. It has been observed that structural flaws existing on the path of Lamb waves not only changed amplitude of waves but also affected their velocities as well. Amplitude change caused by a disbond of size less than 0.5 in. × 0.5 in. was significant and has been measured. Variation in phase velocity was used to quantify the corrosion-induced thickness reduction in aluminum sheets. An area of size 1 in. × 1 in. with 8% thickness loss in subsurface of an 1 mm thick aluminum plate was detected by monitoring the phase velocity increase of SO mode. While these tests made major progresses toward developing a practical and low cost flaw assessment system, effects due to the presence of certain structural elements, such as coatings and fasteners, on the propagation of Lamb waves are becoming important issues, and need to be analyzed. These effects are themselves interesting physical phenomenon and worth investigation, however, it is hoped that propagation variations of waves induced by structural defects can be separated from these effects and be quantitatively correlated to the physical properties of defects

Johnston, Patrick H.

Sun, Keun J.

English

Feasibility of using Lamb waves for disbonds and corrosion detections in aircraft fuselage structures was investigated in recent years. Measurement performed on various laboratory-fabricated specimens as well as on panels removed from aircrafts has shown consistent results and demonstrated its potential applications for large area structural integrity evaluation [1–3]. It has been observed that structural flaws existing on the path of Lamb waves not only changed amplitude of waves but also affected their velocities as well. Amplitude change caused by a disbond of size less than 0.5 in. × 0.5 in. was significant and has been measured. Variation in phase velocity was used to quantify the corrosion-induced thickness reduction in aluminum sheets. An area of size 1 in. × 1 in. with 8% thickness loss in subsurface of an 1 mm thick aluminum plate was detected by monitoring the phase velocity increase of SO mode. While these tests made major progresses toward developing a practical and low cost flaw assessment system, effects due to the presence of certain structural elements, such as coatings and fasteners, on the propagation of Lamb waves are becoming important issues, and need to be analyzed. These effects are themselves interesting physical phenomenon and worth investigation, however, it is hoped that propagation variations of waves induced by structural defects can be separated from these effects and be quantitatively correlated to the physical properties of defects.</p

Sun, Keun

Johnston, Patrick

Digital Repository @ Iowa State University (ISU)

EFFECT OF RIVET ROWS ON PROPAGATION OF LAMB WAVES IN MECHANICALLY FASTENED TWO-LAYER ALUMINUM PLATES Keun J. Sun Applied Science, Department of Physics College of William and Mary Williamsburg, VA 23187 Patrick. H. Johnston NASA-Langley Research Center MS231 Hampton, VA 23681 INTRODUCTION Feasibility of using Lamb waves for disbonds and corrosion detections in aircraft fuselage structures was investigated in recent years. Measurement performed on various laboratory-fabricated specimens as well as on panels removed from aircrafts has shown consistent results and demonstrated its potential applications for large area structural integrity evaluation [1-3]. It has been observed that structural flaws existing on the path of Lamb waves not only changed amplitude of waves but also affected their velocities as well. Amplitude change caused by a disbond of size less than 0.5 in. x 0.5 in. was significant and has been measured. Variation in phase velocity was used to quantify the corrosion-induced thickness reduction in aluminum sheets. An area of size I in. x 1 in. with 8% thickness loss in subsurface of an 1 mm thick aluminum plate was detected by monitoring the phase velocity increase of SO mode. While these tests made major progresses toward developing a practical and low cost flaw assessment system, effects due to the presence of certain structural elements, such as coatings and fasteners, on the propagation of Lamb waves are becoming important issues, and need to be analyzed. These effects are themselves interesting physical phenomenon and worth investigation, however, it is hoped that propagation variations of waves induced by structural defects can be separated from these effects and be quantitatively correlated to the physical properties of defects. When scanning Lamb waves on lap splice joints and on similar structures of a fuselage, scattering and diffraction of waves by rivets and rivet rows often add complexity in data analysis and interpretation. With our setup in pitch-catch measurement, the received waveforms were sometimes very different between with and without having rivets on the path of waves. In fact, decrease in amplitude and shift of phase, because of the rivet rows, were so predominant that changes caused by flaws became less apparent or obscure. Test results shown in Figures I and 2 may illustrate this point. Curves displayed in Figure I give the changes in phase velocity of first symmetric (SO) mode of Lamb waves when they were Review of Progress in Quantitative Nondestructive Evaluation, Vol. 14 Edited by D.O. Thompson and D.E. Chimenti. Plenum Press. New York. 1995 1569 ~ c () 1.004 ::::I ~ ..... () u c 1.002 () .. J:! ~ "E 1.000 .!:i -; § 0 0.998 z 0 5 10 15 20 25 30 D (cm) 181~ 181 - transducer Figure 1. Curves shown in the plot are phase velocity of SO mode as a function of transducers' position. When waves propagate across the thinned area, phase velocity increases. The increasing is thickness dependent. made to scan on three aluminum plates. Thickness of the plate is 2.3 mm and there is an area of 2.5 cm x 10 cm in each plate, where certain percentage of thickness was machined out to simulate the corrosion-induced thickness reduction. Three curves from top to bottom represent the change in phase velocity of SO mode at 0.7 MHz when waves were scanned across the areas with 5%,10%, and 15% thickness reduction respectively. Increase in 0 . 713 0.003 I 0.002 0.711 g 0.001 ~ " ~ 0 . 709 0 ~ CI U ::::I a [ 0 .707 ~ .... .;-g '-" ~ 0.705 ~ .B u ~ '-" 0.703 0 5 10 15 20 25 30 D (cm) Figure 2. Detection of thickness reduction in an aluminum plate using time-of-flight measurement becomes less effective when there are rivets on the path of waves (curve (b)). Subtraction of rivet-caused variation (curve (a) from test results (curve (b)), thickness decrease can still be quantified (curve (c)). 1570 nonnalized reference frequency in the curve indicates the increase in velocity. As can be seen, the phase velocity increases with the percentage of thinning, and is almost in a linear proportion in the range from 0 to 15 percent at 0.7 MHz. These results are expected when dispersion curve of SO mode is examined. As a matter of fact, an equation can be derived to correlate the velocity change and thick reduction [3]. However, when there are rivets on the path, the appearance of data curve was quite different as it is exhibited in Figure 2(b). In this measurement, there are evenly-spaced rows of rivet holes and an area with 5% thickness reduction fabricated in the aluminum plate. The presence of array of rivet rows results in periodic variations of amplitude and phase with minima located at the positions of rivet rows and up-and-down variations symmetric to these locations. The changes in both amplitude and phase of waves are so drastic that the indication of thickness reduction is not as observable. Fortunately, if the variation pattern arisen from the rivet rows (curve (c) in Figure 2) was removed from curve (b), the net is shown as curve (a), which is quite close to 5% reduction curve displayed in Figure 1. This result reveals that the raw data may appear complicated, information of defects can still be extracted if proper analysis is performed. It is assumed that interference of waves plays a major role to form the patterns of variation. For future data analysis and simulation work, several experiments have been conducted to suggest the possible parameters, which determine these variations. It was found that wave interference caused by rivets not only is an intriguing problem for investigation. but also provides additional capability of Lamb waves for defect detection. MEASUREMENTS AND TEST RESULTS The test specimens were either single layer or mechanically fastened two-layer aluminum (2024-T3) plates. Thicknesses of the plates were 1 mm, 1.6 mm, and 2.3 mm. Rows of countersunk rivet holes were made in the plates with separation of 5 cm between the rows and between the individual holes. It was set to have three holes on the path of waves when the wave propagation direction is aligned with the rivet row. The even separation of rivets simulates periodically spaced arrangement for rivets seen in many areas of a fuselage. Except the plates used for checking the effect of diameter on wave propagation, the diameter of rivet holes in the rest of test specimens is 0.6 cm. No adhesive or sealant was used in the mechanically fastened plates. To generate Lamb waves, a pair of broad band piezoelectric transducers (1.27 cm in diameter) was placed on top of the aluminum specimen [4]. Separation of transducers is 14.5 cm, which is larger than the typical width of a lap joint. Two predominant signals, the lowest-order symmetric Lamb wave (SO mode), which was the ftrst arrival, and anti symmetric mode (AO mode) can be observed at frequency in the range from 0.7 to 1.5 MHz. Pulsed pitch-catch method was utilized for amplitude and phase measurements. During the testing, an automated scanner carrying the transducer-pair was moving across the specimen. At each location of the transducer-pair, amplitude of the SO mode was monitored by a peak detector. Phase shift of SO mode was acquired through a pulsed-phase-Iocked-loop (P2L2) circuitry [4]. This instrument compares the phase of its pulsed output signal (which is sent to transmitting transducer) with that of the returned signal (from the receiving transducer). A frequency counter was connected to the output ofP2L2, which gives information of phase difference of the two signals in tenns of frequency (called reference frequency). Base on the design of the circuitry, the value of the reference frequency can be use to calculate phase shift, time-of-flight, or phase velocity of waves. A nonnalized reference frequency essentially provides the percentage of change in these quantities. In our study of wave interference phenomena, the change of reference frequency represents the shift in phase A schematic diagram for the measurement setup is depicted in Figure 3. 1571 • • rivets • · / -.,. • / K' / 'VVV'v . / / / aluminum plate Figure 3. Setup for Lamb wave measurement. One of the results for testing the rivets effects is plotted in Figure 4. Curves exhibited in the figure are the changes of amplitude and phase of SO mode at 0.706 MHz when transducer-pair was made to scan on a single plate of size 60 cm x 30 cm with three sets of rivet holes. Diameters of rivet holes are 0.63 cm, 1.27 cm, and 1.91 cm for each set respectively. As can be observed, both amplitude and phase vary periodically as a function I 0.71 0.705 > ~ 0.7 5 8 CI "0 Q 4 ~ ~ 0.695 = ~ 3 Co <' .... 0.69 3 g 2 ~ 1 .8 Q 0 c:a.: 0 10 20 30 40 50 o (em) • 1~4 " • • • • • • • • • • • • Figure 4. Phase (upper curve) and amplitude of SO mode as a function of transducers' position in a 2.3 mm thick aluminum plate. Different size of rivet holes results in changes in variation patterns. 1572 .-.. 0.712 ~ 6.5 >. 0.708 > u 8 CI 4.5 u "Q ::::I s:: r:r ~ c 0.704 Co .... (10 8 2.5 :5 CI ~ ~ 0.5 u c.:: 0 10 D (em) 20 30 • • • 0 0 0 0 • • 0 0 0 • • • 0 5 % t'!inned area 0 Figure 5. Both phase and amplitude response to the loose of rivets. Rivets located at the dark spots shown in the bottom drawing were loosened and plates were separated. of transducers' position. The periodicity is related to row separation of rivet holes, and each set has its own variation patterns. In fact, further analyses reveal that ratio of diameter of rivet holes to that of transducers is one of the major parameters determines the patterns. Total thickness of a structure is also a factor determining the shape of variation pattern. Since velocities of Lamb waves are medium thickness dependent [5-6], time-of-flight for waves propagating in a single aluminum plate is generally different from that in a properly bonded double-layer plate at a same distance. Energy distribution of wave is also different depending on what mode is propagating. Results displayed in Figure 5 have demonstrated these properties. The measurements were conducted on a mechanically fastened two-layer aluminum plate with 2.3 mm in thickness for each layer. As it is indicated in the diagram at the bottom of Figure 5, four rows of rivets on the right hand side of the plate were well-tightened and the three on the left were completely loosened. A spacer was inserted between the plates at the left edge to ensure the separation of the two layers. Variations of amplitude and phase of waves reflect the situation pretty well. The segments of curves centered at 5.5 cm are the results for waves travelling in the top layer only as a result of the separation, which is in coincidence with the data obtained for a single plate in separated measurements. Those centered at 26 cm are for a double layer plate. The rest in the curves shows the combined effects of rivets and thickness reduction. At the locations of rivets, phase of wave tends to shift to lower value for the loosened case, while to higher value for tightened areas. Furthermore, minimum amplitude is larger for the former than that for the latter. That is, more wave energy actually has been received by receiver in single layer case than the two-layer case when transducers are aligned with rivet rows. DISCUSSION Figure 6 exhibits comparison of amplitude variations caused by rivet holes with different diameters. As it shows, amplitude of SO mode picked up by the receiving transducer is largely reduced when transducers are aligned with rivet holes. The width of decrease in amplitude are closely related to the diameter. of hole. It is wider for holes with larger diameters. The decrease can be attributed to the scattering of wave into other directions by rivet holes. However, it is not quite expected that there is energy received when transducers are aligned with the holes with 1.91 cm in diameters, which is 0.63 cm larger than the 1573 3.5 ..-.. 3.0 !!l -0 2.5 2:-0) 2.0 "1:) ::3 :E 1.5 Co 8 <: 1.0 0.5 -3 -2 -1 0 1 2 3 Distance (em) Figure 6. Amplitude variations of SO mode when wave beam was moved close to and then away from the position of rivet row (located at 0 cm). Curves (a), (b), and (c) represent these variations for rivet diameters of 0.63, 1.27, and 1.91 cm respectively. TR is the diameter of transducers which is 1.27 cm. diameters of transducers. Dispersion of wave beam from the transmitter and reflection of waves from the neighboring rows could be the primary reasons. With our setup for the measurements, amplitude of the first arrived signals was measured. Time span of this pulsed signal is approximately 4-5 Ils. Therefore, reflection of waves from the neighboring rivet rows can still reaches the receiver in time and interfere with the first arrived signal, which results in distortion of the received waveform. The drastic but symmetric changes centered at the rivet rows in the phase of waves and the amplitude as well may partially evidence this interference effect. In fact, the finite size of transducers instead of a point source and a point receiver also contribute to the interference. The relatively small hump located at the trough of each curve is very possible the consequence of this finite size effect and wave diffraction. A preliminary calculation for the expected received amplitude as a function of distance of transducers relative to rivet rows, which includes diameters of transducers and rivets into consideration, agrees at least qualitatively with the observed results. Furthermore, SO mode propagating at several different frequencies ranged from 0.7 to 1.32 MHz in the same plate exhibited wavelength dependent variation pattern, which may enhance the interpretation with interference mechanism. The dependence of interference pattern on the ratio of diameters finds its application in flaw detection. To an incoming wave, flaws adjacent to the rivet holes effectively increases the dimeter of hole in the direction perpendicular to the wave propagation direction. As a consequence, both amplitude and phase shift patterns deviated from patterns of the flaw-free rivets. By comparison approach, these defects can be found. Figure 7 shows an example for this application. An intergranular corrosion occurred in an area right next to a rivet hole. As a result, the received amplitude decreases and phase has an anomalous change. The change in amplitude may not obvious but that in phase is quite substantial. A crack extended from the edge of rivet can also affect wave propagation and displayed similar results. In our tests, a long crack which connects the neighboring rivets would totally prevent waves from propagating between the two rivets. And, small cracks extends from rivet holes for 1 -2 mm would change the shape of amplitude curves and displace the position of minimum in the curve. 1574 1.355 10 > ~ ---+ 8 ~ 1.345 c::': C Q. ~ 0 CI 1.335 < u I» :;) 0 :I . ~ ~ 1.325 o· .... CI tl ..-.. CI > t:! ... .s 1.315 ?" c u E! . c.:: 1.305 !ii '-' 0 I 0 o lib 0 0 0 0 rivet hole corroded area 1r .. ide view of a rivet hole Figure 7. An anomaly and a decrease displayed in phase and amplitude curves respectively when Lamb waves scattered by an area with intergranular corrosion. Spatial separations between rivet holes (open circles) vary as shown. Tests have also been done to examine how the plates acoustically coupled in our mechanically fastened two-layer structures. With our method for specimen preparation, the best coupled areas are these surrounding the rivets. As expected, amplitude and phase of waves in these tightened locations showed significant difference from corresponding areas in a single plate. The middle areas between rivet rows have least coupling, and experimental results imply that waves tend to stay in the top layer. Thickness reduction in the subsurface of top layer can thus be obtained once the effect caused by rivets is removed. On the other hand, disbond caused by the weakening of mechanical strength of rivets can also be detected because of the resultant change of coupling condition between plates. There were three rivets on the path of waves in our measurements as it was mentioned earlier. In the experiments, both amplitude and phase have responded proportionally to the number of rivets being loosened. It has not been understood at this moment that how one wave mode converts to another or how particle displacements [5] vary when waves are made to scan across a riveted layered structure where acoustic coupling in not uniform. However, variations of amplitude and phase were very different when plates were fastened again after a trace of water was introduced into the interface. The very thin water layer may mediate particle displacements continuing from the top to the bottom layer. However, it may not transfer out-of-plate displacement and in-plate displacement with same effectiveness. These intriguing properties may find their applications in the future. In summary, amplitude and phase of wave show variation patterns when Lamb waves were made to scan on riveted aluminum plates. It was observed that the parameters which determine these variation patterns are wavelength, separation of rivet rows, diameter ratio of rivet hole to transducer, and individual thickness and total thickness of the plate. By comparing the variation patterns, structural flaws, such as dis bond, loosened fasteners, top layer corrosion and cracks, which effectively change one or more of the parameters as to incoming waves, can be detected. However, more work need to be done in order to determine resolutions for detecting these flaws. Thickness thinning in the top layer of a mechanically fastened structure can be quantified when variation pattern (obtained either through modelling or data averaging) caused by rivet rows is removed from test results of phase shift. 1575 ACKNOWLEDGEMENTS The authors would like to thank Richard Churray for his help for specimen preparation. The work of KJS is supported by NASA-Langley Research Center under Airframe Structural Integrity Program. REFERENCES 1. K. J. Sun and P. H. Johnston, IEEE 1992 Ultrasonics Symposium Proceedings, 763 (1992) 2. K. J. Sun and P. H. Johnston, in Review of Progress in Quantitative Nondestructive Evaluation, vol.XIII, eds. D. O. Thompson and D. E. Chimenti, 1507 (Plenum Press, New York, 1994) 3. K. J. Sun, D. Kishoni, and P. H. Johnston, IEEE 1993 Ultrasonics Symposium Proceedings 733 (1994) 4. 1. S. Heyman and E. 1. Chern, Journal of Testing and Evaluation, vol. 10, No.5, 202 (1982) 5. D. C. Worlton, J. Appl. Phys., 32,967 (1961) 6. T. R. Meeker, and A. H. Meitzler, in Physical Acoustics, vol. I, part A, 111, edited by R. N. Thurston (Academic Press, Inc. 1964) 1576 

Effect of Rivet Rows on Propagation of Lambn Waves in Mechanically Fastened Two-Layer Aluminum Plates

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2562&amp;context=qnde

Effect of Rivet Rows on Propagation of Lambn Waves in Mechanically Fastened Two-Layer Aluminum Plates

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