CN111735774A - Method for quantifying size of crack defect based on time-distance curve of laser ultrasonic surface wave - Google Patents
Method for quantifying size of crack defect based on time-distance curve of laser ultrasonic surface wave Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- 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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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
- G01N2021/1706—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- 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
- G01N2021/8854—Grading and classifying of flaws
- G01N2021/8874—Taking dimensions of defect into account
Abstract
The invention discloses a method for quantifying the size of a crack defect based on a laser ultrasonic surface wave time distance curve, which comprises the steps of operating a displacement table to enable excitation laser to carry out one-dimensional scanning at a distance which does not pass through a crack and is parallel to the detection direction of a surface wave probe; the excited surface wave is scattered at four corners of the crack; fitting four time distance curves according to the time change of the four scattered waves, and carrying distance information from an excitation point to the edge of the crack to calculate the size information of the crack; extracting by adopting a mode of reference wave and cross-correlation function; predicting the position of the next point by fitting the known points to reduce the search range and increase the accuracy; then, the waveform of the next point is windowed to be used as a new reference waveform, and finally, the time-distance curve of each waveform is obtained through iteration. And the time distance curve is fitted through a polynomial, so that the resolution is increased and the filtering effect is achieved. The method utilizes waveform information generated by laser ultrasound to carry out quantitative characterization on the length and the width of the crack defect, has low equipment requirement and high efficiency, and can better quantify the size of the crack.
Description
Technical Field
The invention belongs to the technical field of material defect detection, and particularly relates to a method for quantifying the size of a crack defect based on a laser ultrasonic surface wave time-distance curve.
Background
Cracks are a common defect in materials that can expand under external loads, ultimately causing serious damage. Nondestructive testing commonly used for crack detection includes radiation inspection, ultrasonic inspection, magnetic particle inspection, eddy current inspection, penetrant inspection, thermodynamic methods, chemical analysis, and the like. The laser ultrasonic detection has the advantages of flexible sound source, non-contact and the like.
At present, the crack detection based on laser ultrasound can be divided into one-dimensional detection and two-dimensional detection, wherein the former generally detects one dimension of a defect, such as the measurement of parameters such as the position, the depth and the like of the defect, and the measurement of the length and the width of the crack is rarely involved; the two-dimensional detection needs to perform complicated scanning to describe cracks, often pays attention to the representation of images in two dimensions and ignores rich information in one dimension, and at the moment, the efficiency and the precision of the detection mostly depend on a scanning mechanism, so the requirement on equipment is high. Reflected waves are generally used as the detection target in terms of wave types, and scattered waves at the upper and lower edges of a crack, which also carry abundant crack information, are often ignored.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a method for quantifying the size of the crack defect based on the laser ultrasonic surface wave time-distance curve, which fully utilizes rich information of a laser ultrasonic generated waveform to quantify and characterize the length and the width of the crack defect, has low equipment requirement and high efficiency, and can better quantify the size of the crack.
The invention adopts the following technical scheme:
a method for quantifying crack defect size, comprising the steps of:
s1, installing a surface wave probe on one side of the workpiece to be detected, which is close to the crack, moving a displacement table, scanning N groups of data by using excitation laser along the parallel direction of the surface wave probe detection at the position far away from the crack, and marking four corner points of the crack respectively according to the sequence from near to far away from the surface wave probe and from near to far away from the scanning path;
s2, preprocessing the N groups of data collected in the step S1;
s3, performing visualization processing on the data preprocessed in the step S2 by taking the scanning point serial number n as a vertical axis, time t as a horizontal axis and signal amplitude as a z axis;
s4, extracting the peak time of the direct wave of each point of the visual signal in the step S3, taking the position of the point as a vertical axis and the peak time as a horizontal axis to obtain a slope k, if the ratio of k to the known material surface wave speed is less than a set threshold value, the error is larger, the scanning path is adjusted to carry out the step S1 to carry out re-measurement, and if the ratio is greater than the set threshold value, the step S5 is continued;
s5, marking four scattered waves corresponding to four corner points of the crack in the sequence of the sequence numbers n of the scanning points from small to large and the sequence of the time t from small to large, and presetting a sequence for storing time distance information of each scanning point obtained in the following steps;
s6, extracting first distinguishable points of each scattered wave in the visual map, windowing in a time domain, intercepting scattered wave parts as respective first reference waves, and extracting scattered wave time distance information of each first point, wherein the reference waves are used for extracting the position of the scattered wave of the next scanning point in the step S7;
s7, in the signal of the scanning point N, N is 2,3, … … N, the scattered wave position of the scanning point N is obtained through a cross correlation function with the reference wave of the scanning point N-1, then a new reference wave is extracted by taking the scattered wave of the scanning point N as the center and windowing is performed, and the information of the scattered wave is obtained iteratively until all points are extracted;
and S8, fitting time distance curves of four scattered waves at the upper edge and the lower edge by a polynomial function according to the time distance information of all the scattered waves obtained in the step S6 and the step S7, extracting the size and the position (y1, t1), (y2, t2), (y3, t3), (y4, t4) of the time minimum value of the four scattered waves L1, L2, L3 and L4), wherein the limit is the transverse position of four points of the crack, and the time is the scattered wave propagation distance of the crack, so that the crack defect size quantification is completed.
Specifically, in step S1, the excitation laser scans N points at an initial position y0 at a scanning displacement interval dy, and records a set of signals z (N, t) at dt intervals from a start time Tb to an end time Te at each point, where N is a scanning point number, N is 1,2, … … N, t is a time value t Tb, Tb + dt, Tb +2dt, … …, Te, and four corner points of a marked crack are P1, P2, P3, and P4.
Specifically, in step S2, the N sets of data acquired in step S1 are low-pass filtered respectively; and then carrying out wavelet decomposition on the N groups of data.
Specifically, in step S4, if the slope k and the known material surface wave velocity crThe ratio of the first to the second is less than a set threshold
And adjusting the scanning path for re-measurement.
Specifically, in step S5, four scattered waves are labeled as L1, L2, L3, and L4, each of the scattered waves is stored in the sequence li (n) from the time value of the extracted point, i is the scattered wave number, and i is 1,2, 3, 4, and n is the extracted point number.
Specifically, in step S6, the index and time when the amplitude is maximum or minimum are recorded as Li (n0) t0, and when i is 1,4, the maximum value is obtained, and when i is 2,3, the minimum value is obtained.
Specifically, in step S7, a cross-correlation function is performed between the reference wave at the scanning point n-1 and the waveform at the scanning point n
Taking the maximum value of the function in the left and right ranges Rn, and taking the peak value of the waveform in the range Mn as the position of the scattered wave waveform of the scanning point n, and recording as Li (n) t; then, taking the scattered wave of the scanning point n as a center, windowing to extract a new reference wave, and fitting the n-1 th point and the n-1 th point to predict the position of the waveform of the n +1 th point; step S7 is repeated until all the point extractions are completed.
Specifically, in step S8, the average crack length l is:
the crack widths w are averaged as:
wherein p is a compensation coefficient.
Compared with the prior art, the invention has at least the following beneficial effects:
the method for quantifying the size of the crack defect can calculate the size of the crack through the information of the scattered wave by only one-dimensional scanning, and is small in program calculation amount and simple to operate.
Further, in step S1, laser ultrasound is used as an excitation source, and data is acquired and reasonably stored in a one-dimensional scanning manner.
Furthermore, most of high-frequency noise is removed through low-pass filtering in step S2, so that the signal-to-noise ratio is improved; and removing the low-frequency trend term by removing the approximate coefficient through wavelet decomposition, and reducing the interference.
Further, step S4 is to compare the slope calculated from the incident wave with the known speed of the surface wave of the material to detect the tilt of the scanning path, thereby reducing the error caused by the experimental operation.
Further, step S5 is used to pre-establish a stored sequence of scattered waves and match the corner points of the crack.
Further, in step S6, in order to extract the scattered wave, a first distinguishable point is identified in the visualization as a first reference wave, and the next iteration is performed based on this.
Furthermore, scattered waves can be accurately identified in signals with low signal-to-noise ratio by a method of adding a cross-correlation function to reference waves; the fitting prediction of the calculated points can reduce the search range of the cross-correlation function and improve the iteration efficiency.
Furthermore, the time distance curve of the scattered waves is fitted through a polynomial, so that the effects of filtering and improving the resolution ratio can be achieved, and the obtained size is more accurate.
In conclusion, the method utilizes waveform information generated by laser ultrasound to carry out quantitative characterization on the length and the width of the crack defect, has low equipment requirement and high efficiency, and can better quantify the size of the crack.
The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.
Drawings
FIG. 1 is a schematic view of a measurement step;
FIG. 2 is a wavefield propagation simulation;
FIG. 3 is a schematic diagram of a simulation waveform stack;
FIG. 4 is a graph showing simulation results of 10mm long and 2mm wide cracks;
FIG. 5 is a graph showing the simulation result of a crack 5mm long and 0.5mm wide;
FIG. 6 is a graph of experimental data extraction for a crack 5mm long and 0.5mm wide;
FIG. 7 is a schematic diagram of crack size calculation.
Wherein: 1. a workpiece; 2. a surface wave probe; 3. and (4) cracking.
Detailed Description
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
Various structural schematics according to the disclosed embodiments of the invention are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and possibly omitted for clarity of presentation. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are merely exemplary, and deviations may occur in practice due to manufacturing tolerances or technical limitations, and a person skilled in the art may additionally design regions/layers having different shapes, sizes, relative positions, according to actual needs.
The invention provides a method for quantifying the size of a crack defect based on a laser ultrasonic surface wave time distance curve, which comprises the steps of firstly installing a surface wave probe, operating a displacement table to enable excitation laser to carry out one-dimensional scanning at a distance which does not pass through a crack and is parallel to the detection direction of the surface wave probe; the excited surface wave interacts with the crack and bypasses the crack, and scattering occurs at four corners of the upper edge and the lower edge; the probe receives the time change of the four scattered waves and fits four time distance curves, and the four time distance curves carry the distance information from the excitation point to the edge of the crack; the position of the probe is unchanged, and the receiving time is shortest when the scanning point to the edge is vertical to the scanning path, so that the size information of the crack is calculated according to the limit position of the time-distance curve; in order to extract a large amount of waveform peak time, a mode of adding a cross-correlation function to reference waves is adopted for extraction; the reference wave is obtained by windowing a known waveform time domain, and the position of the next point is predicted by fitting a known point to reduce the search range and increase the accuracy; then, the waveform of the next point is windowed to be used as a new reference waveform, and finally, the time-distance curve of each waveform is obtained through iteration. And the time distance curve is fitted through a polynomial, so that the resolution is increased and the filtering effect is achieved.
Referring to fig. 1, the method for quantifying the size of a crack defect based on a time-distance curve of a laser ultrasonic surface wave according to the present invention includes the following steps:
s1, a surface wave probe is mounted on the crack side, a displacement stage is moved, N points are scanned at a position far from the crack in a direction parallel to the detection of the surface wave probe at an initial position y0 by scanning a displacement interval dy, a set of signals z (N, t) from a start time Tb to an end time Te at each point is recorded at a time interval dt, N is a scanning point number (N is 1,2, … … N), and t is a time value (t is Tb, Tb + dt, Tb +2dt, … …, Te). Marking four corner points of the crack as P1, P2, P3 and P4 in the sequence from near to far away from the surface wave probe and from near to far away from the scanning path;
s2, preprocessing data;
respectively performing low-pass filtering on the N groups of data acquired in the step S1 to remove high-frequency clutter; performing wavelet decomposition on the data, selecting a sym6 wavelet base, decomposing 7 layers, and removing an approximation coefficient to remove a low-frequency trend item;
s3, visualizing the signal by taking the scanning point serial number n as a vertical axis, time t as a horizontal axis and signal amplitude as a z axis;
s4, extracting the peak time of the direct wave of each point of the visual signal, and obtaining a slope k by taking the position of the point as a vertical axis and the peak time as a horizontal axis;
if the ratio of the slope k to the surface wave velocity of the known material is less than the set threshold
If the tilt error is larger, adjusting the scanning path to perform step S1, re-measuring, and if the scan error is larger than the set threshold, performing step S5;
s5, marking the four scattered waves as L1, L2, L3 and L4 in the sequence of the scanning point sequence number n from small to large and the time t from small to large, and respectively corresponding to corner points P1, P2, P3 and P4; storing each scattered wave in a sequence Li (n) from the time value of the extracted point, wherein i is the serial number of the scattered wave, i is 1,2, 3, 4, and n is the serial number of the extracted point;
s6, extracting first points of the scattering waves distinguishable in the visualization map, windowing and intercepting the scattering waveform portion in the time domain as respective first reference waves, where the reference waves are used in step S7 to extract the position of the scattering wave at the next scanning point, and taking the serial number and time when the amplitude is maximum or minimum as Li (n0) t0, i is 1, and when i is 4, taking the maximum value, i is 2, and when i is 3, taking the minimum value;
s7, cross-correlation function is carried out by the reference wave of the scanning point N-1(N is 2,3, … … N) and the wave form of the scanning point N
The position of the waveform is represented as li (n) t by taking the maximum value of the function in the left-right range Rn and taking the peak of the waveform as the center of the maximum value in the range Mn. And then, taking the scattered wave of the scanning point n as a center, windowing to extract a new reference wave, and fitting the n-1 th point and the n +1 th point to predict the position of the waveform of the n +1 th point. Repeating the step S7 until all the points are extracted, and adjusting the matching range parameters Rn and Mn to improve the extraction effect;
and S8, removing aliasing parts of the incident wave and the scattered wave and points with overlarge errors, converting the sequence into Li (y) 0+ dy (n-1), and fitting time distance curves of the four scattered waves at the upper edge and the lower edge by a polynomial function to increase the resolution and play a role in filtering. The size and position of the time minimum (y1, t1), (y2, t2), (y3, t3), (y4, t4) of the four scattered waves L1, L2, L3, L4 are extracted, the limit of the minimum is the lateral position of four points of the crack, and the time represents the scattered wave propagation distance of the crack, as shown in fig. 7. The crack length l is then averaged as:
the crack widths w are averaged as:
wherein p is a compensation coefficient, the value of which is related to the wavelength and diffraction width of the surface wave.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, a surface wave probe 2 is mounted on a workpiece 1, and a displacement table or a scanning galvanometer is used to scan laser light in a direction parallel to the detection of the surface wave probe 2 at a position away from a crack and record a signal. The four corner points of the crack are designated as P1, P2, P3 and P4 in the order of distance from the surface wave probe 2 to the scanning path.
The effect of interchanging excitation and detection is equivalent due to the acoustic reversibility, and therefore can be considered as a probe exciting surface wave, received by the scanning point. And because the surface wave probe has directivity, only the ultrasonic wave parallel to the probe detection direction can be received, the whole can be regarded as a linear ultrasonic wave perpendicular to the probe detection direction passing through the crack, and the interacted sound wave is received by each scanning point. As shown in fig. 2, it can be seen from the simulated wave field propagation that the surface wave and the scattered wave generated by the crack after the action have different polarities, and the scattered wave having the same polarity as the ultrasonic wave propagation direction has a positive polarity and the scattered wave having a negative polarity has a negative polarity. The scattered waves L1, L2, L3 and L4 correspond to edge points P1, P2, P3 and P4, respectively.
And visualizing the signal by taking the position of the scanning point as a vertical axis, the time as a horizontal axis and the signal amplitude as a z-axis. Fig. 3 is a schematic diagram showing a stack of simulation waveforms, fig. 4 and 5 are visualization results of simulation data, and fig. 6 is a visualization result of experimental data. The direct wave and four scattered waves can be extracted according to the position, time and polarity.
In order to extract the information of scattered waves from a plurality of points, the actual signal scattered waves are weak and have more noise, a cross-correlation function is adopted to extract the waveforms of the scattered waves. And firstly, respectively manually windowing and intercepting a visible first reference wave. And then in the signal of the next scanning point, the position of the second wave is obtained through a cross-correlation function, and a window is added to be used as a new reference wave. This iteration yields information on scattered waves of as long as possible, as shown in fig. 6. The center of the circle is the position of the peak or trough of the scattered wave.
The point of the time minimum of the four scattered waves L1, L2, L3, L4 is extracted, the physical meaning of this pole being the shortest distance from the probe to the crack and then to the scan path. The lateral position of this scanning point is thus the lateral position of the corner of the crack, the time of which represents the propagation distance of the scattered wave from the corner. From the path and geometry of the ultrasonic wave propagation, as shown in FIG. 7, the transverse distance of P1 corresponds to y1, P2 corresponds to y2, and the sound path c of P1rt1P2 Sound Path crt2The length is calculated by the pythagorean theorem, where the crack is rectangular, and can be calculated twice, so the average of the two times is taken.
When the size is larger than the wavelength, the above formula has good linearity; when the size is smaller than the wavelength, the monotonous relation is still formed but the linearity is poor, and the needed coefficient p needs to be compensated.
In summary, compared with other nondestructive tests, the method for quantifying the size of the crack defect based on the laser ultrasonic surface wave time-distance curve has the advantages of non-contact, flexible sound source and the like, and is widely applied to the industrial fields of aviation, railway transportation, welding and the like, but complex operation and expensive equipment are often required for crack detection. The method measures the size of the crack by using the time-distance curve of the scattered wave generated after the laser ultrasonic surface wave acts on the crack, combines the flexible characteristic of laser ultrasonic, better uses the time domain characteristic of the scattered wave, simplifies the experimental operation and reduces the requirement on equipment.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (8)
1. The method for quantifying the size of the crack defect based on the time-distance curve of the laser ultrasonic surface wave is characterized by comprising the following steps of:
s1, installing a surface wave probe on one side of the workpiece to be detected, which is close to the crack, moving a displacement table, scanning N groups of data by using excitation laser along the parallel direction of the surface wave probe detection at the position far away from the crack, and marking four corner points of the crack respectively according to the sequence from near to far away from the surface wave probe and from near to far away from the scanning path;
s2, preprocessing the N groups of data collected in the step S1;
s3, performing visualization processing on the data preprocessed in the step S2 by taking the scanning point serial number n as a vertical axis, time t as a horizontal axis and signal amplitude as a z axis;
s4, extracting the peak time of the direct wave of each point of the visual signal in the step S3, taking the position of the point as a vertical axis and the peak time as a horizontal axis to obtain a slope k, if the ratio of k to the known material surface wave speed is less than a set threshold value, the error is larger, the scanning path is adjusted to carry out the step S1 to carry out re-measurement, and if the ratio is greater than the set threshold value, the step S5 is continued;
s5, marking four scattered waves corresponding to four corner points of the crack in the sequence of the sequence numbers n of the scanning points from small to large and the sequence of the time t from small to large, and presetting a sequence for storing time distance information of each scanning point obtained in the following steps;
s6, extracting first distinguishable points of each scattered wave in the visual map, windowing in a time domain, intercepting scattered wave parts as respective first reference waves, and extracting scattered wave time distance information of each first point, wherein the reference waves are used for extracting the position of the scattered wave of the next scanning point in the step S7;
s7, in the signal of the scanning point N, N is 2,3, … … N, the scattered wave position of the scanning point N is obtained through a cross correlation function with the reference wave of the scanning point N-1, then a new reference wave is extracted by taking the scattered wave of the scanning point N as the center and windowing is performed, and the information of the scattered wave is obtained iteratively until all points are extracted;
and S8, fitting time distance curves of four scattered waves at the upper edge and the lower edge by a polynomial function according to the time distance information of all the scattered waves obtained in the step S6 and the step S7, extracting the size and the position (y1, t1), (y2, t2), (y3, t3), (y4, t4) of the time minimum value of the four scattered waves L1, L2, L3 and L4), wherein the limit is the transverse position of four points of the crack, and the time is the scattered wave propagation distance of the crack, so that the crack defect size quantification is completed.
2. The method of claim 1, wherein in step S1, the excitation laser scans N points at an initial position y0 scan displacement intervals dy, and records a set of signals z (N, t) at dt intervals from a start time Tb to an end time Te at each point, N being the number of the scanning points, N being 1,2, … … N, t being the time value t Tb, Tb + dt, Tb +2dt, … …, Te, marking four corner points of the crack as P1, P2, P3, P4.
3. The method according to claim 1, wherein in step S2, the N groups of data collected in step S1 are low-pass filtered respectively; and then carrying out wavelet decomposition on the N groups of data.
4. The method of claim 1, wherein in step S4, if the slope k is equal to the known material surface wave velocity crThe ratio of the first to the second is less than a set threshold
And adjusting the scanning path for re-measurement.
5. The method according to claim 1, wherein in step S5, four scattered waves are labeled as L1, L2, L3 and L4, each of the scattered waves is stored from the time value of the extracted point to the sequence li (n), i is the scattered wave number, and i is 1,2, 3, 4, n is the extracted point number.
6. The method according to claim 1, wherein in step S6, the sequence number and time when the amplitude is maximum or minimum are represented as Li (n0) ═ t0, and when i is 1,4, the sequence number and time are maximum, and when i is 2, and when i is 3, the sequence number and time are minimum.
7. The method of claim 1, wherein in step S7, the cross-correlation function is performed with the reference wave at scanning point n-1 and the waveform at scanning point n
Taking the maximum value of the function in the left and right ranges Rn, and taking the peak value of the waveform in the range Mn as the position of the scattered wave waveform of the scanning point n, and recording as Li (n) t; then, taking the scattered wave of the scanning point n as a center, windowing to extract a new reference wave, and fitting the n-1 th point and the n-1 th point to predict the position of the waveform of the n +1 th point; step S7 is repeated until all the point extractions are completed.
8. The method of claim 1, wherein in step S8, the crack lengths l are averaged as:
the crack widths w are averaged as:
wherein p is a compensation coefficient.
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| CN114295731A (en) * | 2021-12-28 | 2022-04-08 | 杭州电子科技大学 | Method for measuring depth of subsurface defect based on laser excitation longitudinal wave |
| CN115406384A (en) * | 2022-09-05 | 2022-11-29 | 天津大学 | Method for measuring opening width of crack on machined surface based on laser surface acoustic wave |
| CN116593400A (en) * | 2023-07-17 | 2023-08-15 | 国家电投集团江西电力有限公司 | Method and system for detecting black spot damage of solar panel |
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