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Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique
Accepted Manuscript Title: Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique Author: Wei Zeng Haitao Wang GuiYun Tian Wen Wang PII: DOI: Reference:
S0030-4026(15)01256-5 http://dx.doi.org/doi:10.1016/j.ijleo.2015.09.175 IJLEO 56380
To appear in: Received date: Accepted date:
12-11-2014 25-9-2015
Please cite this article as: W. Zeng, H. Wang, G.Y. Tian, W. Wang, Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.09.175 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript
Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique
Wei Zeng 1 Haitao Wang 1 GuiYun Tian 1, 2 Wen Wang 1 1 College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016，China； 2 School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan, 611731, China；
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Abstract: The main objective of this paper was to use the p-waves to quantitatively evaluate the surface cracks by a laser ultrasonic imaging technique. The p-waves were generated by a pulsed laser illuminated on the specimen with a surface damage and received by a piezo transducer. According the relationship between the propagation distances with the p-waves velocity, we analyzed the time (1.5µs) of the maximum forward amplitude of the once P-waves arrived at the receiver when the pulse laser illuminated at different excitation points, and analyzed the vibration of the p-waves at different distance from the surface damage at 1.5µs. At last, C-scan image and estimated size and location as determined by p-waves amplitude at 1.5µs were used to detect the sizes of the damage. The experimental results showed that using longitudinal acoustic waves by a laser ultrasonic imaging technique is an effectively way for the investigation of surface damage. Keywords: pulsed laser; surface damage; the velocity; longitudinal acoustic waves; laser ultrasonic imaging technique;
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1. Introduction In order to detect different kinds of manufacturing damages and in-service defects, a large number of non-destructive testing (NDT) methods are proposed to avoid catastrophic failure or costly repairs. Surface cracks are the common type of defects in the production process, and the conventional methods for detection of surface cracks are X-rays, electromagnetic detection [1], eddy current testing [2], ultrasonic testing [3] and so on. Laser ultrasonic technology [4-7], as a widely used technique, has a good performance. Usually, surface acoustic wave detection method (including the pulsed echo [8] and pitch-catch method [9]) is sensitive to the surface and sub-surface damages and is an ideal way. Laser ultrasonic imaging technology has proved to be an efficient way to detect cracks, which has many advantages over conventional ultrasound technology, such as being noncontact, easy to focus on, realizing fast scanning imaging and so on. Some scholars [10-11] adopt laser ultrasonic wave propagation imaging technology to visualize ultrasonic wave propagation, which can effectively detect the position of surface defects. Some other scholars [12-15] proposed methods for enhancing the visibility of the ultrasonic wave propagation images, which is to achieve the purpose of quantitative defect detection. Jia [16] et al proposed the scanning heating laser source technique to analyze the frequency characteristics of ultrasonic waves. The results show that the frequency domain analysis can detect surface cracks. These methods only take surface acoustic wave to detect cracks; however, a method of detecting surface defects is put forward by using the longitudinal acoustic waves. According to the amplitude of the p-waves change at the propagation of surface defects at time of the maximum forward amplitude of the once P-waves at the position of damages arrived at the receiver, and analyze the B-scans of the time-domain data (X-axis or Y-axis) as the laser beam is scanned over the specimen with the damage or no defects. The P-waves propagation image at damage area demonstrates that the information of the surface damage can be efficiently evaluated. In this study, we proposed a laser ultrasonic imaging method to detect surface damage in time domain for longitudinal acoustic waves by a laser ultrasonic technique. A laser ultrasonic system is developed, and typical B-scan time imaging of surface damage is realized. The P-waves propagation image at 1.5µs demonstrates that the sizes, location and shape of the surface damage can be efficiently estimated. The outline of this research is as follows: Section 2 briefly describes the laser ultrasonic technique and the sketch for imaging method. The experimental results and discussion are shown in Section 3. Finally, section 4 presents concluding remarks.
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2. Experimental System and Methods
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Fig.1 illustrates the diagram of the experimental setup. A Q-switched Nd:YAG laser (DAWA-200) with a wavelength of 1064 nm, a maximum pulse duration of 8 ns and a repetition rate of 100 Hz was used to excite ultrasonic waves. The diameter of the incident laser beam is less than 0.1mm. The longitudinal waves were detected by an ultrasound receiver (the resonance frequency of 2.5MHz). The shape of the receiver was a circular, 2cm in diameter. The laser source was scanned by using the galvanometer. In order to analyze the signal efficiently, we used an amplifier to improve the signals amplitude and the signals were digitized with a sampling frequency of 25MHz. In the excitation process, the laser energy with 15 mW output power is below the injury threshold of the specimen. GPIB
Trigger
Trigger
Nd:YAG laser
Galvanometer
Amplifiler
us
Damage Spicemen
ip t
Computer
cr
A/D
Transducer
an
Fig. 1. Experimental laser ultrasonic inspection setup
Table 1 the parameters used in the experimental system
The austenitic stainless steel
12 mm
Condition (Length×Width×Depth) A surface hole flaw size of( 2×2× 2 mm3)
M
Thick
Scanning length 10× 10 mm2
The pitch of the scanning length 0.2 mm
d
Material
The austenitic stainless steel plate with a surface damage size of 2 mm ×2 mm × 2 mm was used by the laser ultrasonic inspection system, are shown in Fig.1. The thickness of the austenitic stainless steel plate is 12 mm. The pitch of the scanning was 0.2 mm and the scan length was 10 mm, as shown in Table.1. In the received signal process, a 1MHz-4MHz bandpass filter was used to enhance the signal-to-noise ratio (SNR). The excitation points are at one side of the surface crack, the transducer was at the other side of the surface crack, are shown in Fig,2.
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X Laser beam
Damage
Z
Y
Specimen Transducer
Fig. 2. Schematic diagram of detection the surface cracks
Fig.3 The sketch for imaging method
The laser ultrasonic inspection system collects 2601 waveforms by the transducer after completion of a scan. The collected signals pile up into a data cube in different directions (X, Y, t), as shown in Fig.3. According to the reciprocal theorem, we could select the longitude wave propagation images at the time of P-waves reached at the receiver generated by laser excited at the damage regions to detect the surface damage.
3. Experimental results and discussion Fig.4 illustrates the typical waveform detected by the receiver. The once p-waves and the
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secondary p-waves were shown in Fig.4. The experiment results illustrate that the amplitude of the longitudinal acoustic waves at different distances away from the surface crack. When the received signals at the excitation point (Y=2 mm) are almost coincided with that of the laser excitation point(Y=8 mm). However, when the laser excitation point (Y=6 mm) is at the position of surface crack, the amplitude of the received p-waves are greater than that of the laser excitation point(Y=2 mm or Y=8 mm) and the arrival time of the P-waves is significantly ahead due to the propagation distance decrease. So we could conclude that the maximum forward amplitude of the P-waves produced at the position of the cracks is greater than that of the P-waves produced at other places. Fig.3 clearly shows the maximum forward amplitude of the P-waves at the excitation point (Y=6 mm) arrived to the receiver at 1.5µs, in order to analysis of the changes with the amplitude of the once p-waves along the scanning area (X-axis or Y-axis), we would select the amplitude changes along the X-axis or Y-axis at 1.5µs.
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Fig.4 the P-wave signals at the excitation points(Y=2 mm, Y=6 mm and Y=8 mm)
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Figure 5 Laser ultrasonic evaluation image of 1-D in the austenitic stainless steel with (a) no damage (b) a damage
Laser ultrasonic evaluation image of 1-D in the austenitic stainless steel with no damage and damage are presented in Fig.5 (a)-(b), as can be seen that the once p-waves and secondary p-waves can be clearly measured by this technique. When the laser scanned over the specimen with no damage, the P-wave signals and the echo signals reached the transducer at 2µs and 6µs respectively, are depicted in Fig.5 (a). As can be seen from the Fig. 5(a), the interval time between the two longitudinal waves was about 4.2µs. The distances between the two longitudinal waves is twice the thickness of the sample, according to the relationship between distance and velocity of the ultrasonic wave propagation, we could use this method to calculate velocity of the p-waves and the depth of the damage. 24 2H 24 0.4 2.28mm V 5714m / s Depth Vt 4.2 4.2 4.2 The measured velocity and the depth of the damage are consistent with the actual value. When the laser beam scanned over the specimen over with a damage, the once P-waves at the damge region arrived (between the Y position of 5 mm and 7 mm) at the receiver at 1.5µs, are shown in Fig.5(b). Due to the propagation distance decrease, the arrival time of P-wave signals produced at damage region is significantly earlier than that of P-wave signals produced at intact region. In order to analyze the changes of P-waves at the scanning area at 1.5µs, we selected the distribution of normalized amplitude at 1.5µs by parallel to the X axis at different Y value and Y axis at
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different X value to analyze in detail.
Fig.6 Distribution of normalized amplitude at 1.5µs (a) by scanning parallel to the X axis at Y=5.2 mm, Y=5.5 mm Y=5.7 mm and Y=6 mm (b) by scanning parallel to the X axis at Y=6.1 mm, Y=6.4 mm, Y=6.7 mm and Y=6.9 mm
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We performed the distribution of normalized amplitude at 1.5µs by scanning parallel to the X axis at Y=5.2 mm, Y=5.5 mm, Y=5.7 mm and Y=6 mm, as shown in Fig.6 (a). When the laser excited at the Y position of 5.2 mm, 5.5 mm, 5.7 mm and 8 mm, the p-waves amplitude at intact regions kept at the negative 0.005 V and the p-waves amplitude at the damage region(between the X position of 4.3 mm and 6.3 mm) has a remarkable increase. The experiment results clearly show the presence of a defect and estimates its length of about 2 mm at X-axis. Fig.6 (a) also illustrated us that the p-waves amplitude is the largest at Y position of 5.3 mm and increases with Y increases (between the Y position of 5.2 mm and 6.0 mm). The distribution of normalized amplitude by scanning parallel to the X axis at Y=6.1 mm, Y =6.4 mm, Y =6.7 mm and Y =6.9 mm, as illustrated in Fig.6 (b). According to the above analysis, the p-waves amplitude detected between the X position of 4.3 mm and 6.3 mm has a sudden increase, which is provided in Fig.6 (b). So the width of the damage can be estimated as nearly 2 mm based on the results of scanning parallel to X axis.
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Fig. 7 Distribution of normalized amplitude at 1.5µs (a) by scanning parallel to the Y axis at X=4.5 mm, X=4.7 mm X=5 mm and X=5.2 mm (b) by scanning parallel to the Y axis at X=5.3 mm, X=5.6 mm, X=5.9mm and X=6.2 mm The distribution of normalized amplitude at 1.5µs by scanning parallel to the Y axis at
X=4.5 mm, X=4.7 mm, X=5 mm and X=5.2 mm, as depicted in Fig.7 (a). When the laser scanned in parallel to Y axis at the X position of 5.2 mm, 5.0 mm, 4.7 mm and 4.5 mm. A remarkable decrease can be seen between the Y position of 5 mm and 7 mm, which clearly demonstrates the presence of a defect and estimates its length of about 2mm. The distribution of normalized amplitude by scanning parallel to the Y axis at X=5.3 mm, X=5.5 mm, X=5.7 mm and X=6.2 mm, are shown in Fig.7 (b). According to the detected results at the X position of 5.3 mm, 5.5 mm, 5.7 mm and 6.2 mm as a reference, the amplitude detected between the Y position of 5 mm and 7 mm has a sudden decrease, which is provided in Fig.7 (b). We could detect the damage and estimate the length of the damage as approximately 2 mm.
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Figure 8 C-scan image depicting the surface-damage and estimated size and location as determined by the p-waves amplitude (a) at 1.5µs (a) at 1.55µs (c) at 1.6µs (d) at 1.7µs
4. Conclusion
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According to the above analysis, in order to evaluation of the damages, we proposed the p-waves amplitude at 1.5µs to improve the visualization of the damage. Fig.8 (a) presents the C-scan image and estimated size and location as determined by p-waves amplitude at 1.5µs when the laser scanned over the specimen. A hole diameter of 2 mm can be clearly detected from the Fig.8 (a). The estimated sizes, shape and location agree with the actual the sizes, location and shape of the surface defects. In order to understand the P-waves signal propagation process, Fig.8(b)-(d) depicted the amplitude of the longitudinal wave signal gradually decreased, which is correspond with the change trend of the p-waves in Fig.4. It is important that the proposed method, using laser ultrasonic imaging technology, is simple, reliable and quantitative.
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In this research, a laser ultrasonic imaging technology for detecting surface damage based on the P-waves propagation is developed. According to the relationship between distance and velocity of ultrasonic wave propagation, we select the time of the P-waves produced by laser excited at the position of surface damage arrived at the receiver, and analysis of the changes with the amplitude of the once p-waves along the scanning area (X-axis or Y-axis) at 1.5µs was undertaken. The experimental results demonstrated that the surface damage can cause a significant increase of the p-waves propagation image at1.5µs. The shape, size, and distribution of surface defects can be intuitive and efficiently detected, which demonstrated that the proposed method could be an effective tool. Therefore, the laser ultrasonic imaging method could be applied to industrial non-destructive testing. The next study will be made on the effects of the inner defects or different depths of the damages.
Acknowledgments
This work in this paper was supported by the FP7 project " Health Monitoring of Offshore Wind Farm (HEMOW)" (FP7-PEOPLE-2010-IRSES), supported by the Fundamental Research Funds for the Central Universities and Graduate Education Innovation Project of Jiangsu Province (KYLX_0252) and Chinese Ministry of Science and International Cooperation Project (“2011DFR71080”).
References [1] Z. J. Chen, J. Q. Xuan, P. Wang, et al. Simulation on High Speed Rail Magnetic Flux Leakage Inspection. In 2011 IEEE International Instrumentation and Measurement Technology Conference. New York, 760-764(2011). [2] J. W. Wilson, M. Kaba, G. Y. Tian, et al. Feature extraction and integration for the quantification of PMFL data. Nondestructive Testing and Evaluation. 25(2), 101-109 (2010) [3]J.Li,L.Dong, C.Ni, Z.Shen, and H.Zhang, Application of ultrasonic surface waves in the
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detection of microcracks using the scanning heating laser source technique. Chinese optics letters 10(11),111403 (2012). [4] Zeng Wei,Yang Xiangming,Wang Haitao,et al. Laser ultrasonic technology and its applications. Nondestructive Testing, 35(12):49-52(2013)(in chinese) [5] A. Moura, A. M. Lomonosov, and P. Hess, Depth evaluation of surface-breaking cracks using laser-generated transmitted rayleigh waves, J. Appl. Phys. 103, 084911 (2008). [6] S. Dixon, S. E. Burrows, B. Dutton, and Y. Fan, Detection of cracks in metal sheets using pulsed laser generated ultrasound and EMAT detection, Ultrasonics 51, 7–16 (2011).. [7] Y. Sohn and S. Krishnaswamy, Interaction of a scanning laser-generated ultrasonic line source with a surface-breakingflaw, J. Acoust. Soc. Am. 115, 172–181 (2004) [8] J. A. Cooper, R. A. Crosbie, R. J. Dewhurst, et al. Surface acoustic wave interactions with cracks and slots: a noncontacting study using lasers. IEEE Trans.Ultrason. Ferroelectr. Freq. Contr. (33), 462–470 (1986). [9] I.-Y. Yang, K.-H. Im, U. Heo, D. K. Hsu, J.-W. Park, H.-J. Kim, and S.-J. Song, Ultrasonic approach of Rayleigh pitch-catch contact ultrasound waves on CFRP laminated composites, J. Mater. Sci. Technol. 24, 407–409 (2008).. [10] Takatsubo J, Wang B, Tsuda H, et al. Generation laser scanning method for the visualization of ultrasounds propagating on a 3-D object with an arbitrary shape. J Solid Mech Mater Eng, 1 (12) ,1405–1411(2007). [11] Zeng Wei, Wang Haitao, Tian Guiyun,et al.. Application of laser ultrasonic imaging technique for detection defect based on Hilbert transform. Chinese Journal of Laser, 41 (5):0508004 1-7(2014)(in chinese) [12] Zeng Wei,Wang Haitao, Tian Guiyun, et al. Research on laser ultrasonic defects signal detection technology based on energy analysis. Chinese Journal of Scientific Instrument, 35 (3):650-655(2014) (in chinese) [13] Ruzzene M. Frequency–wavenumber domain filtering for improved damage visualization. Smart Materials and Structures 16(6):2116–2129(2007) [14]J.E. Michaels, T.E. Michaels, Guided wave signal processing and image fusion for in situ damage localization in plates. Wave Motion 44 (6):482–492(2007) [15]Zeng Wei, Wang Haitao, Tian Guiyun, etal. Research on laser ultrasound imaging in the detection of austenitic stainless steel weld. Chinese Journal of Lasers, 41(7): 07030041-7 (2014)(in chinese) [16]Li Jia, Hongchao Zhang, Chenyin Ni, and Zhonghua Shen, Analysis of laser generated ultrasonic wave frequency characteristics induced by a partially closed surface-breaking crack. Applied optics 52 (18):4179–4185(2013)
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S0030-4026(15)01256-5 http://dx.doi.org/doi:10.1016/j.ijleo.2015.09.175 IJLEO 56380
To appear in: Received date: Accepted date:
12-11-2014 25-9-2015
Please cite this article as: W. Zeng, H. Wang, G.Y. Tian, W. Wang, Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.09.175 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript
Detection of surface defects for longitudinal acoustic waves by a laser ultrasonic imaging technique
Wei Zeng 1 Haitao Wang 1 GuiYun Tian 1, 2 Wen Wang 1 1 College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016，China； 2 School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan, 611731, China；
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Abstract: The main objective of this paper was to use the p-waves to quantitatively evaluate the surface cracks by a laser ultrasonic imaging technique. The p-waves were generated by a pulsed laser illuminated on the specimen with a surface damage and received by a piezo transducer. According the relationship between the propagation distances with the p-waves velocity, we analyzed the time (1.5µs) of the maximum forward amplitude of the once P-waves arrived at the receiver when the pulse laser illuminated at different excitation points, and analyzed the vibration of the p-waves at different distance from the surface damage at 1.5µs. At last, C-scan image and estimated size and location as determined by p-waves amplitude at 1.5µs were used to detect the sizes of the damage. The experimental results showed that using longitudinal acoustic waves by a laser ultrasonic imaging technique is an effectively way for the investigation of surface damage. Keywords: pulsed laser; surface damage; the velocity; longitudinal acoustic waves; laser ultrasonic imaging technique;
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1. Introduction In order to detect different kinds of manufacturing damages and in-service defects, a large number of non-destructive testing (NDT) methods are proposed to avoid catastrophic failure or costly repairs. Surface cracks are the common type of defects in the production process, and the conventional methods for detection of surface cracks are X-rays, electromagnetic detection [1], eddy current testing [2], ultrasonic testing [3] and so on. Laser ultrasonic technology [4-7], as a widely used technique, has a good performance. Usually, surface acoustic wave detection method (including the pulsed echo [8] and pitch-catch method [9]) is sensitive to the surface and sub-surface damages and is an ideal way. Laser ultrasonic imaging technology has proved to be an efficient way to detect cracks, which has many advantages over conventional ultrasound technology, such as being noncontact, easy to focus on, realizing fast scanning imaging and so on. Some scholars [10-11] adopt laser ultrasonic wave propagation imaging technology to visualize ultrasonic wave propagation, which can effectively detect the position of surface defects. Some other scholars [12-15] proposed methods for enhancing the visibility of the ultrasonic wave propagation images, which is to achieve the purpose of quantitative defect detection. Jia [16] et al proposed the scanning heating laser source technique to analyze the frequency characteristics of ultrasonic waves. The results show that the frequency domain analysis can detect surface cracks. These methods only take surface acoustic wave to detect cracks; however, a method of detecting surface defects is put forward by using the longitudinal acoustic waves. According to the amplitude of the p-waves change at the propagation of surface defects at time of the maximum forward amplitude of the once P-waves at the position of damages arrived at the receiver, and analyze the B-scans of the time-domain data (X-axis or Y-axis) as the laser beam is scanned over the specimen with the damage or no defects. The P-waves propagation image at damage area demonstrates that the information of the surface damage can be efficiently evaluated. In this study, we proposed a laser ultrasonic imaging method to detect surface damage in time domain for longitudinal acoustic waves by a laser ultrasonic technique. A laser ultrasonic system is developed, and typical B-scan time imaging of surface damage is realized. The P-waves propagation image at 1.5µs demonstrates that the sizes, location and shape of the surface damage can be efficiently estimated. The outline of this research is as follows: Section 2 briefly describes the laser ultrasonic technique and the sketch for imaging method. The experimental results and discussion are shown in Section 3. Finally, section 4 presents concluding remarks.
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2. Experimental System and Methods
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Fig.1 illustrates the diagram of the experimental setup. A Q-switched Nd:YAG laser (DAWA-200) with a wavelength of 1064 nm, a maximum pulse duration of 8 ns and a repetition rate of 100 Hz was used to excite ultrasonic waves. The diameter of the incident laser beam is less than 0.1mm. The longitudinal waves were detected by an ultrasound receiver (the resonance frequency of 2.5MHz). The shape of the receiver was a circular, 2cm in diameter. The laser source was scanned by using the galvanometer. In order to analyze the signal efficiently, we used an amplifier to improve the signals amplitude and the signals were digitized with a sampling frequency of 25MHz. In the excitation process, the laser energy with 15 mW output power is below the injury threshold of the specimen. GPIB
Trigger
Trigger
Nd:YAG laser
Galvanometer
Amplifiler
us
Damage Spicemen
ip t
Computer
cr
A/D
Transducer
an
Fig. 1. Experimental laser ultrasonic inspection setup
Table 1 the parameters used in the experimental system
The austenitic stainless steel
12 mm
Condition (Length×Width×Depth) A surface hole flaw size of( 2×2× 2 mm3)
M
Thick
Scanning length 10× 10 mm2
The pitch of the scanning length 0.2 mm
d
Material
The austenitic stainless steel plate with a surface damage size of 2 mm ×2 mm × 2 mm was used by the laser ultrasonic inspection system, are shown in Fig.1. The thickness of the austenitic stainless steel plate is 12 mm. The pitch of the scanning was 0.2 mm and the scan length was 10 mm, as shown in Table.1. In the received signal process, a 1MHz-4MHz bandpass filter was used to enhance the signal-to-noise ratio (SNR). The excitation points are at one side of the surface crack, the transducer was at the other side of the surface crack, are shown in Fig,2.
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X Laser beam
Damage
Z
Y
Specimen Transducer
Fig. 2. Schematic diagram of detection the surface cracks
Fig.3 The sketch for imaging method
The laser ultrasonic inspection system collects 2601 waveforms by the transducer after completion of a scan. The collected signals pile up into a data cube in different directions (X, Y, t), as shown in Fig.3. According to the reciprocal theorem, we could select the longitude wave propagation images at the time of P-waves reached at the receiver generated by laser excited at the damage regions to detect the surface damage.
3. Experimental results and discussion Fig.4 illustrates the typical waveform detected by the receiver. The once p-waves and the
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secondary p-waves were shown in Fig.4. The experiment results illustrate that the amplitude of the longitudinal acoustic waves at different distances away from the surface crack. When the received signals at the excitation point (Y=2 mm) are almost coincided with that of the laser excitation point(Y=8 mm). However, when the laser excitation point (Y=6 mm) is at the position of surface crack, the amplitude of the received p-waves are greater than that of the laser excitation point(Y=2 mm or Y=8 mm) and the arrival time of the P-waves is significantly ahead due to the propagation distance decrease. So we could conclude that the maximum forward amplitude of the P-waves produced at the position of the cracks is greater than that of the P-waves produced at other places. Fig.3 clearly shows the maximum forward amplitude of the P-waves at the excitation point (Y=6 mm) arrived to the receiver at 1.5µs, in order to analysis of the changes with the amplitude of the once p-waves along the scanning area (X-axis or Y-axis), we would select the amplitude changes along the X-axis or Y-axis at 1.5µs.
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Fig.4 the P-wave signals at the excitation points(Y=2 mm, Y=6 mm and Y=8 mm)
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Figure 5 Laser ultrasonic evaluation image of 1-D in the austenitic stainless steel with (a) no damage (b) a damage
Laser ultrasonic evaluation image of 1-D in the austenitic stainless steel with no damage and damage are presented in Fig.5 (a)-(b), as can be seen that the once p-waves and secondary p-waves can be clearly measured by this technique. When the laser scanned over the specimen with no damage, the P-wave signals and the echo signals reached the transducer at 2µs and 6µs respectively, are depicted in Fig.5 (a). As can be seen from the Fig. 5(a), the interval time between the two longitudinal waves was about 4.2µs. The distances between the two longitudinal waves is twice the thickness of the sample, according to the relationship between distance and velocity of the ultrasonic wave propagation, we could use this method to calculate velocity of the p-waves and the depth of the damage. 24 2H 24 0.4 2.28mm V 5714m / s Depth Vt 4.2 4.2 4.2 The measured velocity and the depth of the damage are consistent with the actual value. When the laser beam scanned over the specimen over with a damage, the once P-waves at the damge region arrived (between the Y position of 5 mm and 7 mm) at the receiver at 1.5µs, are shown in Fig.5(b). Due to the propagation distance decrease, the arrival time of P-wave signals produced at damage region is significantly earlier than that of P-wave signals produced at intact region. In order to analyze the changes of P-waves at the scanning area at 1.5µs, we selected the distribution of normalized amplitude at 1.5µs by parallel to the X axis at different Y value and Y axis at
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different X value to analyze in detail.
Fig.6 Distribution of normalized amplitude at 1.5µs (a) by scanning parallel to the X axis at Y=5.2 mm, Y=5.5 mm Y=5.7 mm and Y=6 mm (b) by scanning parallel to the X axis at Y=6.1 mm, Y=6.4 mm, Y=6.7 mm and Y=6.9 mm
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We performed the distribution of normalized amplitude at 1.5µs by scanning parallel to the X axis at Y=5.2 mm, Y=5.5 mm, Y=5.7 mm and Y=6 mm, as shown in Fig.6 (a). When the laser excited at the Y position of 5.2 mm, 5.5 mm, 5.7 mm and 8 mm, the p-waves amplitude at intact regions kept at the negative 0.005 V and the p-waves amplitude at the damage region(between the X position of 4.3 mm and 6.3 mm) has a remarkable increase. The experiment results clearly show the presence of a defect and estimates its length of about 2 mm at X-axis. Fig.6 (a) also illustrated us that the p-waves amplitude is the largest at Y position of 5.3 mm and increases with Y increases (between the Y position of 5.2 mm and 6.0 mm). The distribution of normalized amplitude by scanning parallel to the X axis at Y=6.1 mm, Y =6.4 mm, Y =6.7 mm and Y =6.9 mm, as illustrated in Fig.6 (b). According to the above analysis, the p-waves amplitude detected between the X position of 4.3 mm and 6.3 mm has a sudden increase, which is provided in Fig.6 (b). So the width of the damage can be estimated as nearly 2 mm based on the results of scanning parallel to X axis.
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Fig. 7 Distribution of normalized amplitude at 1.5µs (a) by scanning parallel to the Y axis at X=4.5 mm, X=4.7 mm X=5 mm and X=5.2 mm (b) by scanning parallel to the Y axis at X=5.3 mm, X=5.6 mm, X=5.9mm and X=6.2 mm The distribution of normalized amplitude at 1.5µs by scanning parallel to the Y axis at
X=4.5 mm, X=4.7 mm, X=5 mm and X=5.2 mm, as depicted in Fig.7 (a). When the laser scanned in parallel to Y axis at the X position of 5.2 mm, 5.0 mm, 4.7 mm and 4.5 mm. A remarkable decrease can be seen between the Y position of 5 mm and 7 mm, which clearly demonstrates the presence of a defect and estimates its length of about 2mm. The distribution of normalized amplitude by scanning parallel to the Y axis at X=5.3 mm, X=5.5 mm, X=5.7 mm and X=6.2 mm, are shown in Fig.7 (b). According to the detected results at the X position of 5.3 mm, 5.5 mm, 5.7 mm and 6.2 mm as a reference, the amplitude detected between the Y position of 5 mm and 7 mm has a sudden decrease, which is provided in Fig.7 (b). We could detect the damage and estimate the length of the damage as approximately 2 mm.
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Figure 8 C-scan image depicting the surface-damage and estimated size and location as determined by the p-waves amplitude (a) at 1.5µs (a) at 1.55µs (c) at 1.6µs (d) at 1.7µs
4. Conclusion
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According to the above analysis, in order to evaluation of the damages, we proposed the p-waves amplitude at 1.5µs to improve the visualization of the damage. Fig.8 (a) presents the C-scan image and estimated size and location as determined by p-waves amplitude at 1.5µs when the laser scanned over the specimen. A hole diameter of 2 mm can be clearly detected from the Fig.8 (a). The estimated sizes, shape and location agree with the actual the sizes, location and shape of the surface defects. In order to understand the P-waves signal propagation process, Fig.8(b)-(d) depicted the amplitude of the longitudinal wave signal gradually decreased, which is correspond with the change trend of the p-waves in Fig.4. It is important that the proposed method, using laser ultrasonic imaging technology, is simple, reliable and quantitative.
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In this research, a laser ultrasonic imaging technology for detecting surface damage based on the P-waves propagation is developed. According to the relationship between distance and velocity of ultrasonic wave propagation, we select the time of the P-waves produced by laser excited at the position of surface damage arrived at the receiver, and analysis of the changes with the amplitude of the once p-waves along the scanning area (X-axis or Y-axis) at 1.5µs was undertaken. The experimental results demonstrated that the surface damage can cause a significant increase of the p-waves propagation image at1.5µs. The shape, size, and distribution of surface defects can be intuitive and efficiently detected, which demonstrated that the proposed method could be an effective tool. Therefore, the laser ultrasonic imaging method could be applied to industrial non-destructive testing. The next study will be made on the effects of the inner defects or different depths of the damages.
Acknowledgments
This work in this paper was supported by the FP7 project " Health Monitoring of Offshore Wind Farm (HEMOW)" (FP7-PEOPLE-2010-IRSES), supported by the Fundamental Research Funds for the Central Universities and Graduate Education Innovation Project of Jiangsu Province (KYLX_0252) and Chinese Ministry of Science and International Cooperation Project (“2011DFR71080”).
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