CN106908451B - Metal plate defect localization method based on laser Lamb wave frequency-wavenumber analysis - Google Patents
Metal plate defect localization method based on laser Lamb wave frequency-wavenumber analysis Download PDFInfo
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- G01N21/84—Systems specially adapted for particular applications
- G01N21/88—Investigating the presence of flaws or contamination
- G01N21/8851—Scan or image signal processing specially adapted therefor, e.g. for scan signal adjustment, for detecting different kinds of defects, for compensating for structures, markings, edges
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Abstract
The invention discloses the metal plate defect localization methods based on laser Lamb wave frequency-wavenumber analysis, belong to laser-ultrasound field of non destructive testing.Firstly, by laser pumping probe on a metal plate fix position excitation, laser pick-off popped one's head in defect line scanning receive, obtain N group Lamb wave data, obtain the time-space wave field on scan path.Secondly, the method that continuous wavelet transform is respectively adopted to collected N group Lamb wave data extracts the signal under particular centre frequency, guarantee the analyticity of defect location result.It is imaged finally, carrying out space-frequency-wave number using the algorithm of short empty two-dimensional Fourier transform to the wavefield signals after extraction, the positioning of defect is realized according to the feature of wave-number spectrum on scan path.The present invention provides a kind of detection methods of Entirely contactless formula, and the action rule between Lamb wave and defect is being disclosed in wave-number domain.
Description
Technical field
The present invention provides a kind of metal plate defect localization methods based on laser Lamb wave frequency-wavenumber analysis, belong to
Laser-ultrasound technical field of nondestructive testing.
Background technique
Plate profile structure or class plate profile structure have a large amount of application in aircraft industry and petrochemical industry, using lossless inspection
Survey method, which carries out detection research to hardened structure, not only has high scientific research value, but also anticipates with important economy and society
Justice.
Mainly there are X-ray detection method, detection method of eddy, thermal imaging method, ultrasonic method etc. for large panel construction non-destructive testing
Deng.It is actually detected that these methods have been widely used in industry spot, but is inevitable that there are some problems.Such as X-ray
With radiation effect, human body, which is exposed to for a long time under high-intensitive ray, will receive more serious injury；The result of EDDY CURRENT
Be it is vector superposed between each variable, need to could be obtained by various demodulation methods it is required as a result, and the technology be only used for
The detection of conductive material；Thermal imaging method is only used for the detection on plate structure surface, near surface flaw.And laser Lamb wave detects
Technology combines the advantages of Laser Ultrasonic Technique and Lamb wave detection technique, and the defects detection of band large-scale metal plate structure is had more
Apparent advantage.(1) laser ultrasonic detection technology can realize that remote, non-contact detecting can be applied to the toxic particular field of high temperature
It closes；(2) there is laser ultrasonic detection technology high time-space resolution power can acquire all-wave field information；(3) laser pumping Lamb wave wideband,
Multi-modal facilitate realizes multi-modal, multi-frequency data fusion imaging.
Currently, method of the laser Lamb wave defect detecting technique for metal plate structure defects detection is mainly based upon laser
The defect detecting technique of sensor array.Determining for defect is realized in conjunction with different laser sensor array forms and all kinds of imaging algorithms
Position.Due to the characteristics of Lamb wave is multi-modal, frequency dispersion, the defect waves packet under Limited propagation space is caused to be difficult to be identified in time domain
Out.Although enabling defect information preferably to identify in two dimensions of time and frequency using Time-Frequency Analysis Method, only
It can judge that defect whether there is but can not position.By taking elliptic imaging algorithm as an example, this method to and without flaw indication by making
Difference retains flaw echoes and realizes defect location.But direct wave is believed after subtracting each other due to the above reasons, whether there is or not flaw indication
Number and boundary reflection signal be difficult to be completely counterbalanced by, amplitude is even much larger than flaw indication, it is difficult to obtain pure flaw indication.
Therefore, inevitably there is check frequency and artifact in this method.Based on using conventional time domain, frequency domain or time-frequency domain signal
The positioning that laser Lamb wave realizes defect is difficult to be utilized in processing method, and some scholars have difference according to the Lamb wave of different modalities
Spatial characteristic developed the defect inspection method of Lamb wave frequency-wavenumber domain analysis.This method is passed using piezoelectricity at present
Sensor excitation, laser vibration measurer scan in different location line and receive multiple groups Lamb wave signal, real based on frequency-wavenumber analysis method
The positioning of existing defect.The Lamb wave of different center frequency is different to the susceptibility of defect, but piezoelectric excitation can only motivate every time
Single centre frequency can not facilitate and realize multi-frequency defect imaging.Therefore, in order to realize real non-contact detecting, sufficiently
The advantage for playing laser high time, spatial resolution, takes laser pumping, and laser intensively scans the excitation for receiving Lamb wave signal
Reception mode has developed the metal plate defect localization method based on laser-ultrasound and frequency-wavenumber analysis method, is used for industrial and commercial bank, army
Non-contact, quick, the comprehensive non-destructive testing of the widely used band large-scale metal plate structure of industry.
Summary of the invention
To achieve the above object, the invention proposes a kind of metal plates based on laser Lamb wave frequency-wavenumber analysis to lack
Fall into localization method.By specific frequency extraction algorithm, extracts in wideband laser Lamb wave and defect is optimized to the frequency range of weld defect control
The sensitivity of detection and detection efficiency have developed a kind of defect positioning method of Entirely contactless formula.
The detection system for realizing this method includes laser controller, pulse laser, laser pumping probe, digital oscillography
Device, template to be checked, laser pick-off probe, continuous pulse laser, optical splitter, demodulator, computer.Laser-ultrasound driver unit
Including laser controller, pulse laser, laser pumping probe.Laser controller provides energy excitation pulse to pulse laser
Laser generates pulse laser, then pops one's head in and issues through laser pumping.Meanwhile laser controller provides synchronous triggering to oscillograph
Signal realizes the synchronization motivationtheory acquisition of laser ultrasound signal.Laser-ultrasound receive system include continuous pulse laser, optical splitter,
Demodulator, laser pick-off probe.Continuous pulse laser exports continuous laser, and two beams are divided into after optical splitter.It is a branch of to be used as ginseng
It examines light and is directly inputted to demodulator；Another beam is irradiated to surface of aluminum plate through laser pick-off probe, then will reflect through surface of aluminum plate
The laser (signal light) for carrying ultrasonic signal afterwards also enters into demodulator.Use reference light and signal light will in demodulator
Ultrasonic signal is demodulated to be acquired by oscillograph and be shown.Finally, handling collected ultrasound data using MATLAB realizes defect
Positioning.To avoid influence of the air-shock wave generated by Thermoelastic regime to signal is received, excitation and receiving transducer split aluminium sheet
Two sides.
To achieve the above object, the present invention adopts the following technical scheme that:
A kind of metal plate defect localization method based on laser Lamb wave frequency-wavenumber analysis the following steps are included:
Step 1: laser ultrasonic detection system is adjusted to optimum Working；
Hardened structure to be checked is set up first, guarantees plate stable structure.Laser ultrasonic detection system is opened, setting continuous impulse swashs
The output energy of light device and the sampling parameter and channel triggering mode of oscillograph.Between adjustable plate structure and laser pick-off probe
Distance and angle, until system output amplitude is high and stable AC signal.
Step 2: the excitation of Lamb wave signal receives；
Laser pumping probe is motivated in the fixed position of hardened structure, and laser pick-off probe acquires Lamb wave letter with step-length x point by point
Number fx(t), x is the distance between laser pick-off hot spot and laser pumping hot spot.Finally obtain the Lamb wave data square of M row N column
Battle array.M is spatial sampling points, and N is time sampling points.
Step 3: the extraction of wideband laser Lamb wave specific frequency ingredient；
For the laser Lamb wave signal of wideband, set specific frequency signal is extracted using the method for continuous wavelet transform, is convenient for
Defect recognition.For any square-integrable function f (t), i.e. f (t) ∈ L2(R), continuous wavelet transform are as follows:
Wherein t is the time, and R is set of real numbers, L2It (R) is quadractically integrable function collection, ψ (t) is that a wavelet or mother are small
Wave.Generating function ψ (t) can be obtained into wavelet sequence after flexible and translation:
Wherein: a is contraction-expansion factor, and b is shift factor.
Step 4: the construction of two-dimentional Hanning window function；
A two-dimentional Hanning window is constructed in MATLAB, and obtained Lamb matrix signal is spatially intercepted paragraph by paragraph.It should
Dimension is consistent the Lamb wave signal acquired in window and step 2 in time, and dimension spatially is according to the wavelength of Lamb wave
It determines.Two-dimentional window function W (t, x) expression formula is as follows:
Wherein x is spatial position, DxFor the half-breadth of window.
Step 5: to the adding window interception paragraph by paragraph of the time-space wavefield signals u (t, x) after extraction centre frequency, after interception
Every section of time-space wavefield signals do two-dimensional Fourier transform；
The thought for using for reference Short Time Fourier Transform, spatially to the time-space wavefield signals after extraction centre frequency
Interception paragraph by paragraph, enables spatial information to retain, then to each section of progress two-dimensional Fourier transform.To guarantee the accurate of wave number, two dimension
The Spatial Dimension of Hanning window is greater than a wavelength.To prevent the boundary in window from artifact occur, the part of adding window will have weight twice
It is folded.The above process may be expressed as:
Wherein L (s, ω, k) is space-frequency-wave-number spectrum, and s is spatial position index, and ω is angular frequency, and k is wave number；
Step 6: the one-dimensional space-wave number defect location.
Wave-number spectrum in all two-dimensional Fourier transform results that extraction step five obtains under specific frequency, is combined in order
Obtain space-wave-number spectrum.When zero defect in template to be measured, space-wave-number spectrum only exists positive wave-number spectrum；Have when in template to be measured
When defect, there is only positive wave number spectrum, there is also negative wave-number spectrums in space-wave-number spectrum.The end of negative wave-number spectrum and positive wave number are composed
The position that existing energy is die-offed is exactly the position of defect.
Detailed description of the invention
Fig. 1 laser ultrasonic detection system schematic diagram.
Fig. 2 aluminium sheet and defective locations schematic diagram.
Fig. 3 laser Lamb wave time-domain signal.
The excitation of Fig. 4 laser Lamb wave receives schematic diagram.
Fig. 5 laser Lamb wave time-space wavefield signals.
Fig. 6 extracts the time-domain signal near 160kHz frequency.
Fig. 7 extracts the time-space wave field figure near 160kHz frequency.
Fig. 8 two dimension Hanning window.
Time-space wave field adding window near Fig. 9 160kHz frequency intercepts result.
Figure 10 adding window wave field two-dimensional Fourier transform result.
Figure label is described as follows: 1- aluminium sheet, 2- defect, and 3- motivates hot spot, 4- light spot received, 5- flaw echo.
Specific embodiment
Following EXPERIMENTAL EXAMPLE is provided in conjunction with the content of the method for the present invention:
Fig. 1 is laser ultrasonic detection system, is divided into excitation and receives two parts.Laser-ultrasound driver unit includes laser control
Device processed, pulse laser, laser pumping probe.Laser controller provides energy excitation pulse laser to pulse laser and generates
Then pulse laser is popped one's head in through laser pumping and is issued.Meanwhile laser controller provides synchronous triggering signal and realizes and swashs to oscillograph
The synchronization motivationtheory of light ultrasonic signal acquires.It includes continuous pulse laser, optical splitter, demodulator that laser-ultrasound, which receives system, is swashed
Light-receiving probe.Continuous pulse laser exports continuous laser, and two beams are divided into after optical splitter.It is a branch of directly defeated as reference light
Enter to demodulator；Another beam is irradiated to surface of aluminum plate through laser pick-off probe, then will carry after surface of aluminum plate reflects super
The laser (signal light) of acoustical signal also enters into demodulator.Use reference light and signal light by ultrasonic signal solution in demodulator
It recalls to be acquired by oscillograph and show.Finally, handling collected ultrasound data using MATLAB realizes defect location.To avoid
Influence of the air-shock wave generated by Thermoelastic regime to signal is received, so excitation and receiving transducer split aluminium sheet in this example
Two sides.
As shown in Fig. 2, being lacked having a size of the through-hole for processing 20mm × mmm on 1000mm × 1000mmmm × 1mm aluminium sheet
Fall into, defect away from upper surface 490mm, away from lower end surface 490mm, away from left side 300mm, away from right side 698mm.
The present embodiment comprises the following steps:
Step 1: laser ultrasonic detection system is adjusted to optimum Working；
Hardened structure to be checked is set up first, guarantees the stabilization of hardened structure.Laser controller is opened, laser controller is set to arteries and veins
The energy for rushing laser offer is 3.8J.The distance between incentive probe and aluminium sheet and angle are adjusted, distance is in embodiment
100mm, angle 90o, excitation spot diameter is 1mm (focusing).Excitation hot spot is placed on defect or more middle line, away from defect right end
150mm。
It opens laser-ultrasound and receives system, the output energy that continuous pulse laser is arranged is 150mW, by the straight of demodulator
Stream output exports the channel 2 for being respectively connected to oscillograph and channel 4 (other channels can also with) with exchange.It is triggering that channel 2, which is arranged,
Source, direct current triggering, channel impedance are 50 Ω.Mobile laser pick-off probe is motivating light spot received on the left of hot spot at 10.2mm,
Upper and lower position is consistent.It is high to adjust the direct current signal that the distance between laser pick-off probe and aluminium sheet and angle receive channel 2
It is in 500mV, embodiment 1.2V (no more than 3V).Switching channel 4 is trigger source, opens high pressure and calibration on demodulator
Switch adjusts the triggering level in channel 4 until the AC signal amplitude highest and stabilization that channel 4 receives.Laser pick-off at this time
The distance of probe and surface of aluminum plate is about 40mm, and light spot received diameter is about 0.3mm (focusing).Calibration switch is closed, is selected again
The trigger source that channel 2 is oscillograph is selected, synchronization trigger source that laser controller provides and channel 2 are connected.The direct current of demodulator
Output is connected to another oscillograph, the reference voltage as ultrasonic signal amplitude.Oscilloscope sampling rate is set as 50MHz, sampling
Points are 10000, reflect signal into boundary to reduce data volume and prevent from adopting.Single presses actuator buttons, is acquired by oscillograph
The laser Lamb wave signal arrived is as shown in Figure 3.
Step 2: excitation receives Lamb wave signal；
Laser Lamb wave motivates reception mode as shown in Figure 4.Excitation hot spot is placed on defect or more middle line, away from defect right end
The excitation of 150mm fixed position.Excitation, light spot received upper and lower position are consistent, to motivate at the 10.2mm of hot spot left side as starting
Position acquires Lamb wave signal f with step-length 0.2mm point by pointx(t), x is excitation the distance between hot spot and light spot received.This reality
It applies and acquires 1301 groups of data in example altogether, obtain the data matrix of 1301 rows 10000 column, at that time m- space wave field such as Fig. 5 institute
Show.
Step 3: the extraction of wideband laser Lamb wave specific frequency ingredient；
Method in the present embodiment using continuous wavelet transform (CWT) is according to formula (1) to 1301 groups of wideband laser Lamb
The signal that wave signal extraction centre frequency is 160kHz is convenient for defect recognition.CWT is to wherein one group of laser Lamb wave signal to mention
It takes, as a result as shown in fig. 6, can be clearly seen that defect reflection echo compared to time-domain signal before extracting.To laser Lamb wave
Result is as shown in Figure 7 after time-space wave field extracts 160kHz centre frequency using CWT.
Step 4: the construction of two-dimentional Hanning window；
A two-dimentional Hanning window is constructed in MATLAB, and the Chinese is spatially done to obtained Lamb wave matrix data and is rather adjusted
System.In time, the Lamb wave signal acquired in the dimension with step 2 of window is consistent, is 10000.In the present embodiment, laser is connect
Receive probe primary recipient A0The Lamb wave (based on acoplanarity displacement) of mode, the A in 100kHz~400kHz frequency band0On mode wavelength
It is limited to 17mm, to guarantee that wave number is undistorted, the dimension of window function spatially is selected as 80mm.Two-dimentional Hanning window is as shown in Figure 8.
Step 5: to time-space wavefield signals u (t, x) the segmentation adding window interception after extraction centre frequency, after interception
Every section of time-space wavefield signals do two-dimensional Fourier transform；
The Hanning window segmentation for being spatially 80mm with length to the wavefield signals after specific frequency constituents extraction is cut
It takes.It will be intercepted behind the initial position of Hanning window and data matrix alignment, then (guarantee space essence every the mobile Hanning window of 0.2mm
Degree) continue interception until all interception finishes.Time-space wave field after the interception of two-dimentional Hanning window is as shown in Figure 9.To interception
Each segment signal afterwards does two-dimensional Fourier transform according to (4) formula, obtains F-K spectra, as shown in Figure 10.It can be with from figure
The result and theoretic frequency found out-wave number curve kissing merges the energy near 160kHz and is dominant, as a result with expected one
It causes.
Step 6: the one-dimensional space-wave number defect location.
The wave-number spectrum under 160kHz is chosen in the result of the windowed data two-dimensional Fourier transform at every section, these data are pressed
Sequencing when selection discharges from left to right obtains space-wave-number spectrum.When zero defect on scan path, space-wave-number spectrum
Only exist positive wave-number spectrum；When defective on scan path, do not compose and will appear negative there is only positive wave number in space-wave-number spectrum
Wave-number spectrum.The end of negative wave-number spectrum and positive wave number compose the position that the position that existing energy is die-offed is exactly defect.
Claims (1)
1. realizing the detection system packet of this method based on the metal plate defect localization method of laser Lamb wave frequency-wavenumber analysis
Include laser controller, pulse laser, laser pumping probe, digital oscilloscope, template to be checked, laser pick-off probe, continuous arteries and veins
Rush laser, optical splitter, demodulator, computer；Laser-ultrasound driver unit includes laser controller, pulse laser, laser
Incentive probe；Laser controller provides energy excitation pulse laser to pulse laser and generates pulse laser, then through laser
Incentive probe issues；Meanwhile laser controller provides synchronous triggering signal to oscillograph and realizes that synchronizing for laser ultrasound signal is sharp
Encourage acquisition；It includes continuous pulse laser, optical splitter, demodulator, laser pick-off probe that laser-ultrasound, which receives system,；Continuous impulse
Laser exports continuous laser, and two beams are divided into after optical splitter；It is a branch of that reference light is used as to be directly inputted to demodulator；Another Shu Jing
Laser pick-off probe is irradiated to surface of aluminum plate, then also enters into the laser that ultrasonic signal is carried after surface of aluminum plate reflects
In demodulator；Ultrasonic signal is demodulated to be acquired and be shown by oscillograph using reference light and signal light in demodulator；Finally,
Collected ultrasound data, which is handled, using MATLAB realizes defect location；To avoid the air-shock wave pair generated by Thermoelastic regime
The influence of signal is received, excitation and receiving transducer split aluminium sheet two sides；
It is characterized by: method includes the following steps:
Step 1: laser ultrasonic detection system is adjusted to optimum Working；
Hardened structure to be checked is set up first, guarantees plate stable structure；Laser ultrasonic detection system is opened, continuous pulse laser is set
Output energy and oscillograph sampling parameter and channel triggering mode；Adjustable plate structure and laser pick-off probe between away from
From and angle, until system output amplitude is high and stable AC signal；
Step 2: the excitation of Lamb wave signal receives；
Laser pumping probe is motivated in the fixed position of hardened structure, and laser pick-off probe acquires Lamb wave signal f with step-length x point by pointx
(t), x is the distance between laser pick-off hot spot and laser pumping hot spot；Finally obtain the Lamb wave data matrix of M row N column；M
For spatial sampling points, N is time sampling points；
Step 3: the extraction of wideband laser Lamb wave frequency content；
For the laser Lamb wave signal of wideband, frequency signal is extracted using the method for continuous wavelet transform, is convenient for defect recognition；
For any square-integrable function f (t), i.e. f (t) ∈ L2(R), continuous wavelet transform are as follows:
Wherein t is the time, and R is set of real numbers, L2It (R) is quadractically integrable function collection, ψ (t) is a wavelet or morther wavelet；It will
Generating function ψ (t) can obtain wavelet sequence after flexible and translation:
Wherein: a is contraction-expansion factor, and b is shift factor；
Step 4: the construction of two-dimentional Hanning window function；
A two-dimentional Hanning window is constructed in MATLAB, and obtained Lamb matrix signal is spatially intercepted paragraph by paragraph；The window and
Dimension is consistent the Lamb wave signal acquired in step 2 in time, and dimension spatially is determined according to the wavelength of Lamb wave
It is fixed；Two-dimentional window function W (t, x) expression formula is as follows:
Wherein x is spatial position, DxFor the half-breadth of window；
Step 5: to the adding window interception paragraph by paragraph of the time-space wavefield signals u (t, x) after extraction centre frequency, to every after interception
Section time-space wavefield signals do two-dimensional Fourier transform；
The thought for using for reference Short Time Fourier Transform, it is spatially piecewise to the time-space wavefield signals after extraction centre frequency
Interception, enables spatial information to retain, then to each section of progress two-dimensional Fourier transform；To guarantee the accurate of wave number, the two-dimentional Chinese is peaceful
The Spatial Dimension of window is greater than a wavelength；To prevent the boundary in window from artifact occur, the part of adding window will have overlapping twice；On
The process of stating may be expressed as:
Wherein L (s, ω, k) is space-frequency-wave-number spectrum, and s is spatial position index, and ω is angular frequency, and k is wave number；
Step 6: the one-dimensional space-wave number defect location；
Wave-number spectrum in all two-dimensional Fourier transform results that extraction step five obtains under frequency, combination obtains sky in order
M- wave-number spectrum；When zero defect on scan path, space-wave-number spectrum only exists positive wave-number spectrum；When defective on scan path
When, there is only positive wave number spectrum, there is also negative wave-number spectrums in space-wave-number spectrum；The end of negative wave-number spectrum and positive wave number compose existing energy
Measure the position that the position die-offed is exactly defect.
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