Experimentation and Detection Characters of Lamb Wave Phase Array ...

Experimentation and Detection Characters of Lamb Wave Phase Array on a Large Thin Aluminium Plate D. GAO, Z. WU, M. LIU and Z. WANG ABSTRACT Base on the principles of phase array of piezoelectric sensors, experimentation of lamb wave phased array in large thin aluminium plate was processed by an S0 mode Lamb wave resulted from high frequency narrow bandwidth signal. Different locations on large thin-wall structural were effectively focused through adjusting the signal transmission delay phase of elements. The concept of dead zone of phase array is further clarified and several measures to reduce the dead zone detection were given, which was favorable to improve signal-to-noise ratio of Lamb wave detection. Keyword: Structural Health Monitoring; Lamb waves; Phased Array; Phase delay; Dead zone

INTRODUCTION Ultrasonic Lamb wave testing technique is widely used in Structural health monitoring of large thin-wall structure because of its long energy propagation distance, fast detection speed, strong wavefronts plasticity[1]. Although the ultrasonic Lamb wave testing technique have many advantages, its intrinsic characteristics such as low signal-to-noise ratio is still obstacles for them to be used widely. The Introduction of the lamb phase array technology into ultrasonic Lamb wave testing technique will effectively improve signal-to-noise ratio. A lot of work on lamb phase array technology has been done by many scientists. The functional relationship between sensor size, adhesive properties, excitation frequency and the stress wave in structure has been investigated by Victor Giurgiutiu, and the damage scanning and imaging method for lamb phase array has been given[2]. Joseph L. Rose has researched the control of guided wave phase array mode and the application of guided wave phase array to aircraft detection[3]. _____________ Dongyue Gao,Zhanjun Wu,Minjing Liu ,Zhi Wang, State Key Laboratory of Structural Analysis for Industrial Equipment Faculty of Vehicle Engineering and Mechanics, School of Aeronautics and Astronautics Dalian University of Technology, Dalian 116024, P.R. China

In this paper, the effective focusing on the different positions of the large thinwalled structures and identification of unknown wave source location were achieved based on the control of the beam direction and focus position by adjusting the signal transmission delay time of elements. The concept of dead zone of lamb wave phased array is further clarified. The measure method to reduce the dead zone was investigated in detail.

PRINCIPLE OF PIEZOELECTRIC TRANSDUCER EXCITATION LAMB WAVE The elastic wave will produce in the structure and reflect, superimpose between the upper and lower boundaries when the structure was excitated by the external energy perturbation. The longitudinal wave (P) and vertical shear wave (SV) mixed into Lamb waves and the Rayleigh–Lamb dispersion equations is shown as follow: tan pd 4k 2 pq 1  [ 2 ] (1) tan qd (k  q 2 )2 Where the index (+1) and (-1) are symmetric mode antisymmetric mode, respectively. The dispersion curves of different modes were described according to equation (1) and the dispersion curve of Lamb wave in the aluminium plate is shown in Figure.1.

Figure.1. The dispersion curve of Lamb wave in the aluminium plate [4]

According to the dispersion curve, the Lamb waves of different modes with same frequency have different velocities. The different waves are difficult to identify because of the superimposition of the waves of different modes due to the reflection and scattering of waves through damage position. Therefore, the excitation frequency was controlled in order to get a single lamb wave mode when Lamb wave is excited. An S0 mode lamb wave resulted from high frequency narrow bandwidth signal was employed in this work. When the transformation frequency of sensors used in this work was 240 kHz, strain produced from A0 mode was effectively inhibited and single S0 mode lamb was obtained.

ULTRASONIC LAMB WAVE PHASE ARRAY The phase array was composed of 8 sensor elements. Here, the coordinate of ith element was (xi,yi), and the coordinate of focal point and polar coordinator were (xd,yd) and (ρ,θ), respectively. The journey of lamb wave between nth sensors to focal point was d n  ( xd  xn ) 2  ( yd  yn ) 2 . ( xd , y d ); (  d ,  d )





d i 1

d

i



( x1 , y1 ) ( x2 , y2 ) ( x3 , y3 )

( xi 1 , yi1 ) ( xi , yi )

Figure.2. Sketch map of element position of ultrasonic lamb wave phase array

Signal delay time of adjacent sensors was Δti=ti-ti-1=(di-di-1)/c, here c is wave speed. The sensor was excited according to delay time, so as to ensure that the lamb waves simultaneously arrived preconceived focus, (the time was numerically equal to t1), which was the transformation process of phase array technology. According to Huygens principle, if there is scatterer on the focal position, guided wave with high amplitude will be reflect after lamb focusing. Based on Huygens principle, the location of wave source or damage was distinguished. The position of the wave peak after the signal re-delay of each sensor element was superposed was wave source or damage. This is the receive process of phase array technology, specific process is shown in Figure.3.

(a)

(b)

Figure.3. The delay of each element of phase array. (a) Focusing process of phase array (b) The receive process of phase array

In summary, the process of Lamb wave phase array inspection is shown in Figure.4.

Figure.4. The process of Lamb wave phase array inspection.

EXPERIMENT RESEARCH OF LAMB WAVE PHASE ARRAY TECHNOLOGY EXPERIMENTAL SYSTEMS Experimental system used in this work is shown in Figure.5. Size of thin aluminium sheet is1mm×1000mm×1000mm, size of cycle-piezoelectric sensors is d=0.45mm, ф=8mm, and distance of element is 9mm; the adhesives is isotropic conductive adhesive.

Sensors array 传感器阵列

Figure.5. Macrograph of sample and sensor systems.

INVESTIGATION ON FOCUSING OF LAMB PHASE ARRAY As shown in Figure.5, the eight sensors as a simple linear excitation array were placed in the center of a thin aluminium plate and the several sensors located in other positions on the aluminium plate were arranged as the receiving sensors. The location coordinates of the sensors are listed in Table 1. No. 1 2 3 4

TABLE I. THE LOCATION COORDINATES OF THE SENSORS Location (ρ,θ) No. Location (ρ,θ) No. Location (ρ,θ) No. Location (ρ,θ)

(-31.5, 180) (-22.5, 180) (-13.5, 180) (-4.5, 180)

5 6 7 8

(4.5, 0) (13.5, 0) (22.5, 0) (31.5, 0)

9 10 11 12

(200, 90) (300, 90) (200, 60) (250, 60)

13 14 15 16

(400, 60) (200, 0) (400, 0) (408, 50)

In order to verify focusing effect of phase array, the position of the sensors was focused by controlling the Beam. The two waveforms (same phase and delay phase superposition waveforms) were received by the sensor. All array elements were simultaneously excited and the waveform Vi was obtained by superimposing receiving sensor signals. The position of the receiving sensors was respectively focused by adjusting the delay time of each array element and the focusing signals Vi΄ was collected by the receiving sensors. 4

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(b) (c) Figure7. The signals of same phase and phase array (a) Signal at 200mm, 300mm in the direction of 90º (b) Signal at 200mm, 400mm in the direction of 60º (c) Signal at 200mm, 300mm in the direction of 0º

It could be seen from Figure.7 that the focusing effect of the phase array in the direction of 60º and 0º was more obvious than it on the direction of 90º, which revealed that the focusing effect in the direction of 90º is clear.

IDENTIFICATION OF UNKNOWN SIGNAL SOURCE Signals are transmitted from 16th sensor in an unknown location (γ16, θ16), and the location of the wave source was identified by calculating time delay. According to the time delay of the array element and peak position, the direction and distance of the wave source was identified. The received signals of elements in array are shown in Figure.8.

Figure8. The received signals of elements of the unknown location in array.

According to these signals, the polar coordinates of the wave source were calculated to be 409.25, 51 and this position was focused by the method described in 3.3 section. The signals received by 16th sensor are shown in Figure.9. The obtained results were compared with the actual position and the error between the obtained results and the actual position was less than 3%. Based on this method, the positions of the wave source for 9~15th sensors were respectively identified and the results are listed in Table 2. These errors listed in Table 2 indicated that this method of identifying damage location is reasonable.

Figure.9. Focusing signals at 51, 409.25

Table 2 The identified and actual positions of the wave source. No.

Position (γ, θ)

Identified position

Error

No.

Position (γ, θ)

Identified position

Error

9

(200, 90)

(204.5, 85)

2.25%

13

(400, 60)

(400.7, 58)

0.17%

10

(300, 90)

(302.6, 85)

0.87%

14

(200, 0)

(202.7, 0)

1.35%

11

(200, 60)

(205.85, 59)

2.92%

15

(400, 0)

(403.85, 0)

0.97%

12

(250, 60)

(253.1, 58)

1.24%

DEAD ZONE OF ULTRASONIC LAMB PHASE ARRAY No identification of the damage and the greater error of the wave source location were present in the lamb wave phased array focusing technique and recognition algorithms and these zones were called as dead zone. There are two reasons for the presence of the dead zone: first, the feedback signals at near-field of the array were annihilated by electromagnetic coupling signals resulted from between the signal boards. Such zone was called as electromagnetic coupling dead zone. The similar results have been reported elsewhere [5]. Furthermore, the detection accuracy of the lamb phase array at the different positions on the same structure was different, which resulted in the presence of the detection dead zone. The focusing effect was not obvious when the angle between focus direction and array normal was small. It could be obtained from Table 2 that the error between the identified and actual positions of the wave source in the direction of 90° was obvious. The identification experiment

was carried out between 0° -5° and the results revealed that the error between the identified and actual positions of the wave source increased as the angle between focus direction and array normal decreased. The result error was attributed to the smaller phase shift between adjacent sensors because the accuracy of recognition algorithms increased as the phase shift increased. In addition, the focusing effect was not obvious when the displacement of the waveform was not obvious and these zones were called as angle error dead zone. P  2  f  min( n , 0  n  i ) (11) As shown in equation (11), the focusing effect was not obvious when time delay between adjacent sensors was 4.1667e-008 s, namely, the phase shift was 3.6°. In other word, the zone with angle of below 3.6° between the adjacent sensors is angle error dead zone of lamb wave phased array detect technology. Draft angle error dead zone of 1000*1000mm aluminium plate is shown in Figure.10 and Gray Level is phase shift of adjacent sensors.

Figure.10. The sketch map of angle error dead zone.

In order to reduce or eliminate angle error dead zone, the several methods were used, such as the increase in the number of sensors, the use of mobile sensor array elements, change sensor arrays arrangement, and so on. In the present work, the change of the array arrangement was used to reduce the angle error dead zone and the four sensors were added in the normal of original array, the positions the added four sensors are shown in TABLE 3. TABLE.3. THE POSITIONS OF ADDED FOUR SENSORS

No.

Location (γ, θ)

No.

Location

No

Location

No.

Location

17

(31.5, 90)

18

(13.5, 90)

19

(-13.5, 270)

20

(-31.5, 270)

The crisscross array was composed of the added four sensors and the original array, 9th and 10th sensors were re-focused in the direction of 90° and the results are shown in Figure.11.

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Figure.11 Focusing effect of the crisscross array.

CONCLUSIONS In this paper, Lamb wave phased array detect technology was investigated experimentally and theoretically, based on the low signal-to-noise ratio of Lamb wave phased array detect technology. Different locations on large thin-wall structural were effectively focused through adjusting the signal transmission delay phase of elements. The concept of dead zone of phase array is further clarified and several measures to reduce the dead zone detection were given, which was favorable to improve signal-tonoise ratio of Lamb wave detection, such as the increase in the number of sensors, the use of mobile sensor array elements, change sensor arrays arrangement, and so on.

ACKNOWLEDGMENTS This work was supported by China Postdoctoral Science Foundation Funded Project (20100481220) and the Fundamental Research Funds for the Central Universities (3014-852001 and DUT10ZDG05) and the National Natural Science Foundation of China (51002019and 91016024).

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