CN113340996B - Online detection method for laser impact strengthening defect based on acoustic emission attenuation energy - Google Patents
Online detection method for laser impact strengthening defect based on acoustic emission attenuation energy Download PDFInfo
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Abstract
The invention discloses a laser shock strengthening on-line detection method based on attenuation energy of acoustic emission signals. Firstly, the acoustic emission signals collected by the invention come from the detected material, so that the internal structure of the material can be better represented; and secondly, the laser impact strengthening acoustic emission signals acquired in real time are strong pulse signals and have strong attenuation characteristics, so that the information of the acoustic emission signal attenuation section is richer than other sections, attenuation energy is defined as oscillation energy of the signal attenuation section, the characteristics of the attenuation oscillation energy are taken as the basis, defect information can be detected more clearly, and the method has higher distinction degree. The method provided by the invention has the advantages of simple calculation, clear characteristic meaning, good real-time detectability and engineering applicability, and provides an effective implementation way for realizing the online defect detection in the laser shock peening process.
Description
Technical Field
The invention belongs to the field of laser shock peening, and particularly relates to a laser shock peening defect online detection method based on acoustic emission attenuation energy.
Background
Laser shock peening (Laser Shocking Peening, LSP) is a new technique for strengthening metallic materials using transient shock waves excited by high-energy high-pressure lasers. Under the irradiation of strong laser, the grains on the surface of the material are subjected to the phenomena of distortion, shearing slip, dislocation twinning and the like, so that a residual stress layer with a certain size and depth is formed on the surface, the strength, hardness, wear resistance and fatigue strength of the metal material are greatly improved, and the service life of the material can be effectively prolonged. Compared with the traditional surface strengthening technology, the method has the advantages of obvious strengthening effect, large influence layer depth, strong process controllability and the like, and is widely applied to the fields of aviation, ships and the like.
The strengthening effect of the metal material is closely related to the surface quality after laser shock strengthening, and the better the processed surface quality is, the more obvious the strengthening effect of the material is. The quality of laser processing depends on the state of the metal material itself, in addition to the process factors of the LSP process. When the metal material to be processed has defects, if no post-treatment process is adopted, the laser shock strengthening effect cannot reach the expectations, and the problem of poor quality in actual production can be caused. For this phenomenon, it is first necessary to detect defects in the plate to be processed, and then perform impact reinforcement after performing subsequent processing on the defective plate. This undoubtedly increases the production links, reduces the production efficiency, and increases the labor cost. Therefore, if the material defects can be detected simultaneously in the laser shock peening process, the production efficiency can be greatly improved.
Aiming at how to improve the laser shock strengthening effect, chinese patent CN109136529A proposes a laser shock strengthening method, and the surface roughness Ra of the impact area of a workpiece to be processed is reduced to below 3 mu m by carrying out smoothing treatment on the impact area, so that the strengthening effect after laser shock strengthening is more remarkable. The conventional LSP patent disclosed or authorized at present processes normal plate, improves the strengthening effect of the material by improving the process parameters, has certain ideal and does not meet the actual production and processing conditions.
Aiming at material defect detection, most of the current methods adopt ultrasonic waves for defect detection and positioning, chinese patent CN107782787A proposes an ultrasonic defect detection method, an ultrasonic probe is utilized to excite an ultrasonic signal, another ultrasonic probe is utilized to receive an ultrasonic echo signal, and ultrasonic echo parameters obtained after modulating, FIR filtering and EM algorithm optimization are analyzed to realize the defect detection of the material. The ultrasonic defect detection patents of the materials disclosed or authorized at present are all used for detecting defects by utilizing an ultrasonic probe, are mainly limited in that the real-time detection cannot be realized, the ultrasonic probe with a built-in driver is required, and meanwhile, the excitation signal mode and parameters are required to be set, so that the ultrasonic defect detection method is not simple and convenient.
Therefore, the laser shock peening and the material defect detection are both capable of solving only one problem in the respective fields, and no patent is available to solve the problem of defect on-line detection in the laser shock peening processing involving both fields.
Disclosure of Invention
The invention combines the laser shock peening process and the defect detection process, thereby realizing the purpose of online defect detection in the laser shock peening process. Based on the acoustic emission signals in the laser shock strengthening process acquired by the acoustic emission sensor in real time, carrying out envelope processing after carrying out frequency band filtering on the signals in which spectral peaks are positioned, and dividing the signals into three time periods; the method is stable and reliable, has simple characteristic characterization, is easy to explain and has strong engineering applicability.
In order to achieve the above purpose, the invention is realized by adopting the following technical scheme:
the laser impact strengthening defect on-line detection method based on acoustic emission attenuation energy comprises the following steps:
firstly, installing an acoustic emission sensor on the front surface of a plate to be impacted, and sequentially connecting the acoustic emission sensor, a preamplifier, an A/D data acquisition card and an industrial personal computer with each other; performing laser impact strengthening processing on the flat plate, and collecting acoustic emission signals in the impact strengthening process in real time;
step two, in order to increase the data processing speed, under the condition of meeting the Nyquist sampling theorem, carrying out downsampling processing on the acoustic emission signal to obtain an undistorted downsampled signal;
step three, further carrying out Fourier transform on the downsampled signal, determining the frequency band of each spectral peak by a spectrogram of the channel signal, and carrying out filtering processing on the signal to obtain a filtered signal;
step four, carrying out envelope processing on the filtered signal to obtain an envelope curve of the signal, dividing the signal into three parts of an ascending section, a stable section and an attenuation section on a time domain waveform according to peak points on the envelope curve, and taking the signal attenuation section as a research object;
step five, defining damping energy as damping oscillation energy for further characterizing the fluctuation intensity of the signal damping section; and calculating the attenuation oscillation energy of the plate acoustic emission signal, and using the attenuation oscillation energy to characterize defect information characteristics so as to realize the online detection of the laser shock peening defect.
In the first step, the real-time acoustic emission signal in the laser shock strengthening processing process adopts an RS-2A resonant narrowband acoustic emission sensor, the central frequency is 150KHz, the frequency response range is 50-400KHz, the signal to noise ratio of the signal can be improved by a preamplifier with the amplification gain of 20dB, an A/D data acquisition card is used for acquiring the signal, an industrial personal computer displays the waveform of the acoustic emission signal in real time and stores the signal, and the acoustic emission signal is acquired in real time through a signal acquisition device in the laser shock strengthening process.
In the second step, the signal is first fourier transformed, and after the maximum frequency component of the signal is determined, the signal is downsampled under the condition that the Nyquist sampling theorem is satisfied, so as to improve the signal processing efficiency.
In the third step, the down-sampling signal in the second step is subjected to Fourier transform, and then the signal is subjected to band-pass filtering according to the frequency band of each spectral peak on the spectrogram.
In the fourth step, the maximum and minimum values are adopted for the time domain waveform of the filtered signal to obtain the signal envelope, the position and the amplitude of the maximum and minimum values of the signal are determined first, and then the signal envelope is obtained by adopting cubic spline interpolation.
In the fourth step, the filtering signal is divided into three sections in the time domain according to the peak point on the envelope line, namely a rising section, a stable section and an attenuation section, and the laser impact signal belongs to a pulse signal and has strong attenuation characteristic, so that the effective information in the attenuation section of the signal is more abundant than that in the other two sections.
In the fifth step, defining the damping energy as damping oscillation energy, and representing the fluctuation intensity of the signal in a specific time interval; the physical meaning is the average kinetic energy value of mass points of unit mass in a time interval at a specific position of a plate, and the calculation formula is as follows:
wherein: n (N) 1 And N 2 Respectively a starting point and an ending point of the selected time period, T is a signal sampling interval time, N 1 、N 2 E (0, 1, 2. N), n is the total sampling point number, and f (iT) is the value of the signal at iT.
In the fifth step, the oscillation energy of the signal attenuation section is calculated and is used as a characteristic to characterize defect information, so that the defect on-line detection of laser shock reinforcement is realized.
Compared with the prior art, the invention has at least the following beneficial technical effects:
(1) The acoustic emission detection technology adopted by the invention has the advantage that ultrasonic waves or other nondestructive detection cannot be replaced in defect detection, and the acoustic emission detection technology has the advantages that the signal received by the acoustic emission sensor comes from the detected material, so that the internal structure of the material can be better represented; therefore, the acoustic emission signal is more sensitive to structural defects in the material, and the acoustic emission signal changes more when the structure of the material itself changes.
(2) The laser shock enhanced acoustic emission signal acquired in real time belongs to a strong pulse signal and has strong attenuation characteristics, so that in the acquired acoustic emission signal, the attenuation section information of the signal is richer relative to other time sections, and defect information is richer, so that the attenuation oscillation energy is defined as the oscillation energy of the signal attenuation section, the characteristics of the attenuation oscillation energy are used as the characterization parameters of defect detection, the defect information can be detected more clearly, and the characteristic distinction degree is high.
The method provided by the invention has the advantages of simple calculation, clear characteristic meaning, good real-time detectability and engineering applicability, and provides an effective implementation way for realizing the online defect detection in the laser shock peening process.
Drawings
FIG. 1 is a technical flow chart of the present invention;
FIG. 2 is a schematic diagram of a method for online detection of laser shock peening defects in an embodiment of the present invention;
FIG. 3 is a diagram of the shape and dimensions of a hollow blank panel and a defective panel according to an embodiment of the present invention; wherein a is a front view and b is a side view;
FIG. 4 is a plot of frequency domain magnitudes before and after downsampling of acoustic emission signals for a hollow white panel and a defective panel in accordance with an embodiment of the present invention; a and b are blank panel original signals and downsampled signal spectrograms respectively, and c and d are defect panel original signals and downsampled signal spectrograms respectively;
FIG. 5 is a time domain waveform of a filtered signal of a hollow white panel and a defective panel according to an embodiment of the present invention; wherein a is a blank plate, b is a defect plate;
FIG. 6 is a time domain envelope and time domain segmentation of a filtered signal according to an embodiment of the present invention; wherein a is a blank plate, b is a defect plate;
FIG. 7 is a graph showing the comparison of the damping oscillation energy of a hollow white panel and a defective panel according to an embodiment of the present invention.
Reference numerals illustrate:
the system comprises a 1-industrial personal computer, a 2-A/D data acquisition card, a 3-preamplifier, a 4-acoustic emission sensor, a 5-water constraint layer, a 6-black tape absorption layer, a 7-flat plate, an 8-facula projection system, a 9-laser generator, a 10-laser controller, an 11-laser impact area and 12-prefabricated defects.
Detailed Description
In order to make the problems solved by the present invention more clear, the present invention will be further explained with reference to the drawings and examples.
In the invention, an acoustic emission technology is selected AS a defect detection technology, acoustic emission is an excellent nondestructive detection technology, and has been sufficiently developed in the past decades, and an acoustic emission sensor 4 selected by the invention is an AS-B2 type sensor. The whole acoustic emission detection system consists of an acoustic emission sensor 4, a preamplifier 3, an A/D data acquisition card 2 and an industrial personal computer 1, so that acoustic emission signals can be normally acquired, stored and analyzed, and the acoustic emission signals are received by the acoustic emission signals sent by the material itself, unlike other nondestructive detection, so that the acoustic emission initial sampling rate is higher and generally must not be lower than 5MHz.
The invention provides a laser shock enhancement on-line detection method based on acoustic emission signal damping oscillation energy, wherein a technical flow chart is shown in figure 1, and mainly comprises the following steps:
the method comprises the steps of firstly, correctly arranging the acoustic emission sensor on a plate to be impacted, ensuring the close fit between the sensor and the plate by using an industrial couplant, ensuring that the sensor does not move in the laser impact process by using a clamp, and ensuring that acoustic emission signals in laser impact strengthening can be normally acquired in real time. FIG. 2 is a schematic diagram of an on-line detection method of laser shock peening defects.
And secondly, after the acoustic emission signals are acquired in real time, carrying out spectrum analysis on the signals, determining the maximum frequency component of the acoustic emission signals, carrying out downsampling on the signals according to the Nyquist sampling theorem, and compressing the data length of the signals by a plurality of times on the premise of ensuring that the signals are not distorted, so that the data processing speed is improved.
And thirdly, performing Fourier transformation on the downsampled signal, determining the frequency band of each spectral peak through a spectrogram of the signal, mainly determining the frequency band of the main and auxiliary spectral peaks on the signal, and performing filtering processing on the downsampled signal according to the frequency band to obtain a filtered signal time domain diagram of each frequency band.
Fourthly, carrying out maximum and minimum value envelope processing on the filtered signal to obtain a time domain envelope of the filtered signal, dividing the signal into three parts of an ascending section, a stable section and an attenuation section from the time domain according to peak points and trends on the envelope, obtaining a starting time point and a termination time point of each part, and taking the signal attenuation section as a study object; .
And fifthly, defining an oscillation energy characteristic parameter to represent the fluctuation intensity of the signal in a specific time interval. The physical meaning is the average kinetic energy value of mass points of unit mass in a time interval at a specific position of a plate, and the calculation formula is as follows:
wherein: n (N) 1 And N 2 Respectively, a start point and an end point of the selected time period, T is a signal sampling interval time,
f s for the sampling rate of the signal, N 1 、N 2 E (0, 1, 2. N), n is the total sampling point number, and f (iT) is the value of the signal at iT.
As the acoustic emission of laser shock reinforcement belongs to strong pulse signals and has strong attenuation characteristics, the effective information in the attenuation section of the signals is more abundant than that of the other two sections. Therefore, the signal attenuation oscillation energy is calculated and is used as a characteristic to characterize defect information, so that the defect on-line detection of laser shock reinforcement is realized.
Examples:
the on-line detection schematic diagram of the laser shock peening defect in the embodiment is shown in fig. 2, and an acoustic emission defect detection system is composed of an industrial personal computer 1, an A/D data acquisition card 2, a preamplifier 3 and an acoustic emission sensor 4; a laser controller 10, a laser generator 9 and a facula projection system 8 form a laser shock strengthening system; the metal plate 7, the black tape absorbing layer 6 and the water constraint 5 form a laser shock strengthening processing technology; the three parts form the laser shock peening defect on-line detection system together.
In this example, the validity of the proposed method was experimentally verified by prefabricating defects on a flat plate for simulating defective plates in the process. The blank plate and the defect plate are in the shape and size, the arrangement of acoustic emission sensors and the position of a laser impact area are shown in figure 3, the length of the metal plate is 300mm, the width of the metal plate is 50mm, the thickness of the metal plate is 4mm, the acoustic emission sensors are 60mm away from the center of the laser impact strengthening area, and the prefabricated defect is located in the middle of the blank plate and the defect plate.
In this example, the sensors are correctly arranged as required, and the sensors are tightly attached by using an industrial couplant and a clamp, so that the sensors are ensured not to shake in laser processing. Then, before laser shock peening, lead breaking experiments are carried out on the plate, so that the acoustic emission detection system is ensured to normally collect signals, finally, laser shock peening processing is carried out on the blank plate and the defect plate, and acoustic emission signals are collected in real time, wherein the laser shock peening process parameters selected in the example are as follows: laser energy is 3J, the diameter of a light spot is 3mm, and the blank plate and the defect plate are respectively subjected to single-point continuous impact for 5 times.
In this example, according to the first step of the present invention, after the sensor is arranged on the plate to be impacted as required, the laser impact sound emission signal is synchronously collected by the sound emission detection system. According to the second step of the invention, after determining the maximum frequency component of the acoustic emission signal, the blank panel and the defect panel signal are subjected to downsampling, wherein the initial sampling rate of the blank panel is 5MHz, the initial sampling rate of the defect panel is 3MHz, the blank panel is reduced by 5 times, the defect panel is reduced by 3 times, the sampling rates after downsampling are 1MHz, and the frequency domain amplitude diagram of the acoustic emission signal before and after downsampling is shown in figure 4. According to the third step of the invention, the down-sampled signal is subjected to spectrum analysis, the frequency band of each spectral peak on the spectrum is determined, the signal is subjected to filtering processing, and the frequency band of the secondary spectral peak on the spectrum is selected: 31250Hz-62500Hz, and filtering to obtain a filtered signal time domain diagram shown in figure 5. According to the fourth step of the invention, the maximum and minimum value envelope processing is performed on the filtered signal, the time domain envelope of the filtered signal is shown in fig. 6, the signal is divided into three parts of a rising section, a stable section and a decay section from the time domain according to the peak point of the envelope, and the starting time point and the ending time point of each part are determined as shown in table 1. According to the fifth step of the present invention, the ringing energy is calculated to characterize the defect information, and a graph of the ringing energy characteristics of the blank panel and the defect panel is shown in fig. 7.
TABLE 1 starting and ending time points for various portions of the signals
As can be seen from the results of the above experiments and examples, the laser shock peening defect online detection method of the present invention can combine the laser shock peening process with the defect detection process, and realize online detection of material defects by utilizing the acoustic emission phenomenon of the material during laser shock peening. The laser impact strengthening acoustic emission signal belongs to a strong pulse signal and has the characteristic of strong attenuation, and the information contained in a signal attenuation section is richer than the rest part, so the invention provides a characteristic parameter of the attenuation oscillation energy, which is defined as the fluctuation intensity of the signal attenuation section and is used for representing defect information. The method provided by the invention is simple, has definite characteristic meaning, strong characterization capability, good instantaneity, engineering applicability and the like, and provides an effective technical means for realizing the on-line detection of the laser shock peening defect.
Claims (8)
1. The laser impact strengthening defect on-line detection method based on acoustic emission attenuation energy is characterized by comprising the following steps of:
firstly, installing an acoustic emission sensor on the front surface of a plate to be impacted, and sequentially connecting the acoustic emission sensor, a preamplifier, an A/D data acquisition card and an industrial personal computer with each other; performing laser impact strengthening processing on the flat plate, and collecting acoustic emission signals in the impact strengthening process in real time;
step two, in order to increase the data processing speed, under the condition of meeting the Nyquist sampling theorem, carrying out downsampling processing on the acoustic emission signal to obtain an undistorted downsampled signal;
step three, further carrying out Fourier transform on the downsampled signal, determining the frequency band of each spectral peak by a spectrogram of the channel signal, and carrying out filtering processing on the signal to obtain a filtered signal;
step four, carrying out envelope processing on the filtered signal to obtain an envelope curve of the signal, dividing the signal into three parts of an ascending section, a stable section and an attenuation section on a time domain waveform according to peak points on the envelope curve, and taking the signal attenuation section as a research object;
step five, defining damping energy as damping oscillation energy for further characterizing the fluctuation intensity of the signal damping section; and calculating the attenuation oscillation energy of the plate acoustic emission signal, and using the attenuation oscillation energy to characterize defect information characteristics so as to realize the online detection of the laser shock peening defect.
2. The method for detecting the laser shock peening defect on line based on the acoustic emission attenuation energy according to claim 1, wherein in the first step, a real-time acoustic emission signal in the laser shock peening processing process adopts an RS-2A type resonant narrowband acoustic emission sensor, the center frequency is 150KHz, the frequency response range is 50-400KHz, the signal to noise ratio of the signal can be improved by a preamplifier with the amplification gain of 20dB, the A/D data acquisition card is used for acquiring the signal, the industrial personal computer displays the waveform of the acoustic emission signal in real time and stores the signal, and the acoustic emission signal is acquired in real time by the signal acquisition device in the laser shock peening process.
3. The method for on-line detection of laser shock peening defect based on acoustic emission attenuation energy according to claim 1, wherein in the second step, fourier transform is performed on the signal first, and after a maximum frequency component of the signal is determined, downsampling processing is performed on the signal under a condition that Nyquist sampling theorem is satisfied, so as to improve signal processing efficiency.
4. The method for detecting the laser shock peening defect on line based on the acoustic emission attenuation energy according to claim 1, wherein in the third step, after fourier transforming the downsampled signal in the second step, band-pass filtering is performed on the signal according to the frequency band of each spectral peak on the spectrogram.
5. The method for online detection of laser shock peening defects based on acoustic emission attenuation energy according to claim 1, wherein in the fourth step, a signal envelope is obtained by using a maximum and a minimum value for a time domain waveform of a filtered signal, a position and an amplitude of the maximum and the minimum value of the signal are determined, and then the signal envelope is obtained by using cubic spline interpolation.
6. The method for on-line detection of laser shock peening defect based on acoustic emission attenuation energy according to claim 1, wherein in step four, the filtered signal is divided into three sections, namely, a rising section, a stationary section and an attenuation section, from the time domain according to the peak point on the envelope line, and since the laser shock signal belongs to a pulse signal, the effective information in the attenuation section of the signal is more abundant than the other two sections.
7. The method for online detection of laser shock peening defects based on acoustic emission attenuation energy according to claim 1, wherein in step five, the attenuation energy is defined as attenuation oscillation energy, and the intensity of signal fluctuation in a specific time interval is characterized; the physical meaning is the average kinetic energy value of mass points of unit mass in a time interval at a specific position of a plate, and the calculation formula is as follows:
wherein: n (N) 1 And N 2 Respectively a starting point and an ending point of the selected time period, T is the signal samplingInterval time N 1 、N 2 E (0, 1, 2. N), n is the total sampling point number, and f (iT) is the value of the signal at iT.
8. The method for online detection of laser shock peening defects based on acoustic emission attenuation energy according to claim 1, wherein in step five, oscillation energy of a signal attenuation section is calculated and is used as a characteristic to characterize defect information, thereby realizing online detection of defects of laser shock peening.
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