CN114720560B - Air coupling Lamb wave ultrasonic detection method for layered defects of carbon fiber composite material plate based on modal decomposition imaging algorithm - Google Patents
Air coupling Lamb wave ultrasonic detection method for layered defects of carbon fiber composite material plate based on modal decomposition imaging algorithm Download PDFInfo
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- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/041—Analysing solids on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
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Abstract
The invention discloses an air coupling Lamb wave ultrasonic detection method for layered defects of a carbon fiber composite plate based on a modal decomposition imaging algorithm. Connecting and debugging equipment; processing the detected signals through modal decomposition; performing feature extraction and analysis on instantaneous energy of the detection signal and the defect-free reference signal which are processed through modal decomposition; the cross correlation coefficient of the instantaneous energy of the detection signal and the non-defective reference signal in the scanning direction forming the same angle with the fiber direction is used as a damage index to realize the characterization of the layering defect of the carbon fiber composite board; and obtaining a two-dimensional image from the A scanning data of the rotary scanning by the rotary scanning defect probability imaging method for the damage index to realize qualitative analysis and quantitative characterization of the carbon fiber composite plate. The invention realizes accurate characterization of the layering defect and solves the problem of secondary pollution of the coupling material to the piece to be detected in the existing detection process.
Description
Technical Field
The invention belongs to the field of ultrasonic detection; in particular to a method for detecting layering defects of a carbon fiber composite plate by air coupling Lamb wave ultrasonic based on a modal decomposition imaging algorithm.
Background
The composite material has the advantages of light weight, high strength, high temperature resistance, high fatigue resistance and the like, and is widely applied to the fields of aerospace and the like. The carbon fiber composite material is an important novel material, and plays an important role in the civil and military fields due to the excellent performance and low cost of the carbon fiber composite material. Various defects and damages inevitably occur during the production and use of the carbon fiber composite material. The main types are wrinkling, delamination and debonding, and potential safety hazards exist. Therefore, the method has important significance and value for detecting and monitoring the structural health of the composite material. A large number of researches and applications show that the ultrasonic detection method is the most practical, most effective and widely applied nondestructive detection technology for the composite material. Lamb waves can completely cover the thickness direction of the whole material on a propagation path, and particularly have the advantages of small attenuation, long propagation distance, high detection sensitivity and the like, so the Lamb waves are widely used for detecting board structures such as carbon fiber composite materials. Generally, a large amount of couplant is needed for detection by exciting Lamb waves in a carbon fiber composite material plate by using a contact sensor, and the performance and detection result of the carbon fiber composite material are influenced to a certain extent. Although laser ultrasound does not require a coupling agent, the carbon fiber composite material is easily damaged when the energy is too high. Due to the advantages of no coupling agent, no contact, no secondary pollution and the like, the air-coupled ultrasound has unique advantages in the detection of the carbon fiber composite board. At present, the most widely used air-coupled ultrasonic C-scan detection system can only set probes on two sides of a material to perform point-by-point detection, but in the in-situ detection of a composite material, such as the detection of an airplane skin material, the probes can only be set on the same side of the material. Therefore, the detection method of the Lamb waves excited by the air-coupled ultrasound can meet the actual requirements and quickly realize the quality detection of the carbon fiber composite material plate.
The characterization of defects by Lamb wave signals has been a research hotspot of Lamb wave nondestructive testing. Lamb wave signals are mostly non-stationary and nonlinear signals due to their chromatic dispersion and multimode characteristics. Researches and analysis show that the method for analyzing the nonlinear non-stationary time sequence of the Lamb wave signal generally meets the requirements of completeness, orthogonality, locality and adaptability, otherwise, the accuracy of characteristic extraction and analysis of the Lamb wave signal is influenced, and the accuracy of subsequent defect display is directly influenced. As a spectrum analysis method suitable for analyzing a non-stationary process and representing a non-linear structure, hilbert-yellow transform (HHT) based on an Empirical Mode Decomposition (EMD) method has been developed. HHT has unique advantages in the aspect of feature extraction of Lamb wave data in air coupling carbon fiber composite plate detection. In order to visually reflect the damage characteristics of a material or structure, more and more researches have been conducted to display a damage recognition result using a two-dimensional or three-dimensional image and to apply an image characterization method to Lamb wave nondestructive testing.
Disclosure of Invention
The invention provides an air coupling Lamb wave ultrasonic detection method for a layering defect of a carbon fiber composite material plate based on a modal decomposition imaging algorithm, which is used for solving the problem of secondary pollution to a to-be-detected piece caused by a coupling material in the existing detection process while realizing accurate characterization of the layering defect.
The invention is realized by the following technical scheme:
a carbon fiber composite material plate layering defect air coupling Lamb wave ultrasonic detection method based on a modal decomposition imaging algorithm comprises the following steps:
step 1: connecting equipment and debugging;
and 2, step: processing the signals detected based on the step 1 through modal decomposition;
and 3, step 3: performing feature extraction and analysis on the instantaneous energy of the detection signal and the defect-free reference signal processed by the modal decomposition in the step 2;
and 4, step 4: based on the step 3, the cross correlation coefficient of the instantaneous energy of the detection signal and the non-defective reference signal in the scanning direction forming the same angle with the fiber direction is used as a damage index to realize the characterization of the layering defect of the carbon fiber composite board;
and 5: and (4) obtaining a two-dimensional image from the A scanning data of the rotary scanning by a rotary scanning defect probability imaging method based on the damage index of the step (4) to realize qualitative analysis and quantitative characterization of the carbon fiber composite plate.
In the detection method, the equipment in the step 1 specifically comprises an excitation air coupling transducer 1, a receiving air coupling transducer 2, a carbon fiber composite material plate 3, a rotating platform 4, a two-dimensional motion platform 5, a signal generator, a voltage amplifier, an upper computer, an oscilloscope and a preamplifier;
the linear guide rail 5-1 of the two-dimensional motion platform 5 is respectively connected with an excitation air coupling transducer 1 and a receiving air coupling transducer 2 through a fixed connector 1-1, the excitation air coupling transducer 1 and the receiving air coupling transducer 2 are respectively arranged at two sides of a carbon fiber composite material plate 3, the carbon fiber composite material plate 3 is arranged on a rotary platform 4,
the excitation air coupling energy converter 1 is sequentially connected with a voltage amplifier and a signal generator, and the receiving air coupling energy converter 2 is sequentially connected with an upper computer, an oscilloscope and a preamplifier.
In the detection method, the modal decomposition processing specifically includes the following steps:
step 2.1: extracting the characteristics of an original signal F (t) through wavelet transformation, and performing threshold filtering reconstruction to obtain a signal F (t);
step 2.2: based on the signal F (t) in the step 1, according to a cubic spline function, fitting a local maximum point and a local minimum point to respectively obtain an upper envelope line F and a lower envelope line F max (t) and F min (t)；
Step 2.3: upper envelope F based on step 2.2 max (t) and the lower envelope F min (t) calculating the average value thereof
m(t)＝[F max (t)+F min (t)]/2；
Step 2.4: obtaining h (t) = F (t) -m (t) by subtraction based on the average value of step 2.3; judging whether h (t) meets two conditions of the intrinsic mode function, if not, taking h (t) as an original signal, repeating the steps 2.2-2.4 until the conditions are met, and if so, performing the step 2.5;
step 2.5: extracting the intrinsic mode function IMF, i.e. I 1 (t) = h (t); while the residual term r (t) = F (t) -I 1 (t) is considered a new signal; judging whether r (t) meets the conditions of a monotone sequence or a constant value sequence, if not, repeating the step 2.3-2.5 until all intrinsic mode functions IMF are extracted to obtain I 2 (t)、I 3 (t) and other IMF components, if the IMF components meet the requirement, the intrinsic mode functions IMF are all extracted, and the step 2.6 is carried out;
step 2.6: the instantaneous energy IE (t) is obtained by a hilbert transform.
According to the detection method, empirical Mode Decomposition (EMD) of modal decomposition is used for determining the instantaneous equilibrium position of an original signal by using the average value of upper and lower envelopes of a time sequence and further extracting an Intrinsic Mode Function (IMF); since the IMF component is a data sequence, it needs to satisfy two conditions at the same time: the number of the maximum points of the signal andthe number of zeros is equal or different by 1; the local mean of the upper envelope defined by the maximum and the lower envelope defined by the minimum is zero; after obtaining each IMF component I j (t) after, processing the IMF component I by Hilbert transform j (t)：
Wherein P is the Cauchy principal value;
the Hilbert spectrum H (omega, t) of the signal is obtained by representing the corresponding relation between the instantaneous amplitude and the instantaneous frequency of each IMF component in the same time-frequency space, and can fully reflect the distribution of signal energy in a time-frequency domain; the instantaneous energy IE (t) is obtained to reflect the energy change of the signal at different times:
in the detection method, the step 4 of characterizing the delamination defect of the carbon fiber composite material plate is specifically that, assuming that N directions are selected for detection at equal intervals in a 360 ° direction, the defect probability estimation P (x, y) at the position of the detection area (x, y) is written as:
wherein P is i (x, y) is a probability estimate of the defect distribution of the air-coupled transducer pair in the ith rotational scan direction, A i Determining the signal difference coefficient of an air coupling transducer pair in the ith rotating scanning direction, determining the attenuation rate of Lamb wave energy on two sides of a scanning path by alpha, and calculating the probability of defect distribution by R, namely 1/2 of the distribution width of Lamb wave energy on two sides of the scanning path;
if the imaging point of the defect distribution probability is in the energy range of the Lamb wave scanning path, calculating the defect distribution probability according to the distance from the energy central line; in contrast, the probability of defect distribution is 0;
when the central lines of the two air coupling transducers are parallel to the fiber direction of the carbon fiber composite material plate, the central lines are designated as 0 degree; at this time, the center lines of the two air-coupled transducers are designated as a first rotational scan path; and taking the middle point of the central lines of the two air coupling transducers as a rotation center, and taking the central lines of the two air coupling transducers which rotate for i-1 times at a fixed rotation angle as an ith rotation scanning path.
The detection method comprises the specific steps that the excitation frequency and the inclination angle of the air coupling transducer are determined, lamb waves have symmetrical modes, antisymmetric modes and frequency dispersion characteristics, and a multi-order symmetrical mode (S) can be excited under the same excitation frequency 0 ，S 1 ，…， S i ) With anti-symmetric mode (A) 0 ，A 1 ，…，A i ) (ii) a In order to excite the air coupling transducer to a relatively pure mode in the piece to be detected, the excitation frequency of the transmitting air coupling transducer is less than a certain upper limit value f according to the dispersion curve of the guided wave and the thickness of the piece to be detected 0 (ii) a Then, determining the excitation frequency f according to the actual performance of the air coupling transducer; according to research and analysis, the in-plane displacement of the symmetric mode is large, and the out-of-plane displacement of the anti-symmetric mode is large, so that the anti-symmetric mode is adopted for air coupling ultrasonic detection; when the frequency-thickness product is determined, the antisymmetric mode A 0 The group velocity of (a) can also be known, and then the tilt angle θ of the air-coupled transducer is determined according to the first critical refraction angle of snell's law in combination with the propagation velocity in air.
The detection method is characterized in that the rotational scanning defect probability imaging method specifically comprises the steps of taking the difference value between the instantaneous energy curve of a detection signal and a corresponding non-defective signal as a damage index, and taking the rotational scanning defect probability calculation as an imaging method; the damage indices are expressed by their cross-correlation coefficients:
where A and ρ are the damage index and cross-correlation coefficient, respectively, of the instantaneous energy of the detected signal and its corresponding instantaneous energy of the defect-free signal, X is the instantaneous energy of the defect-free reference signal, Y is the instantaneous energy of the detected signal, C XY Is the covariance of X and Y, σ X And σ Y Standard deviations for X and Y, respectively.
In the detection method, the qualitative analysis and the quantitative characterization in the step 5 are specifically that an air coupling transducer is placed on one side of a composite material plate sample according to the previously determined inclination angle theta, and the distance between an exciting air coupling transducer and a receiving air coupling transducer is set to be L; using ipsilateral and pitch capture methods, and using A 0 Carrying out layering defect detection on the composite material plate in a modal mode;
the signal generator is used for generating a sine pulse series excitation signal with the center frequency of f, hanning window modulation and the pulse number of 5, which is required by the air coupling transducer;
the voltage amplifier is used for carrying out voltage increase on the excitation signal generated by the signal generator so as to ensure that the air coupling transducer excites enough sound energy;
the two-dimensional motion platform is used for adjusting the horizontal distance between the transmitting air coupling transducer and the receiving air coupling transducer and the height distance between the transmitting air coupling transducer and the carbon fiber composite material plate;
the rotating platform is used for realizing 360-degree rotating scanning of the carbon fiber composite material plate;
the preamplifier is used for amplifying and receiving an echo signal of the air coupling transducer;
the oscilloscope is used for displaying Lamb wave signals and storing data;
the upper computer is used for displaying the imaging qualitative analysis and quantitative characterization results.
A computer-readable storage medium, in which a computer program is stored which, when being executed by a processor, carries out the method steps of the above.
The invention has the beneficial effects that:
according to the invention, air coupling ultrasonic Lamb wave detection is adopted, and air is used as a transmission medium to replace a coupling agent in the traditional ultrasonic nondestructive detection in the detection process, so that the problem of secondary pollution of a coupling material to a piece to be detected can be fundamentally avoided, the advantages of complete non-contact, non-invasion and non-damage are achieved in the detection process, the service life of an air coupling ultrasonic transducer can be greatly prolonged, the on-line rapid detection of the air coupling ultrasonic Lamb wave detection is realized, and the method is suitable for the defect ultrasonic detection of a composite material plate which can not be subjected to contact detection by using the coupling agent.
The air coupling Lamb wave ultrasonic detection method based on the modal decomposition imaging algorithm can effectively realize the layered defect detection of the carbon fiber composite material plate.
The invention adopts 360-degree rotary scanning to acquire the omnibearing information of the detection area.
Compared with the traditional time domain analysis method based on amplitude difference, the invention provides a modal decomposition imaging algorithm to analyze nonlinear non-stationary leakage Lamb wave signals, more accurate characteristic extraction and analysis are carried out on the Lamb wave signals by obtaining instantaneous energy, and more accurate characterization of the layering defects of the carbon fiber composite plate is realized by taking the cross correlation coefficient in the same direction as a damage index.
The method is suitable for realizing defect in-situ test and accurate characterization of large-area rapid scanning of the aerospace composite material plate after the approximate central position of the defect is obtained through rapid scanning.
Drawings
FIG. 1 is a flow chart of the modal decomposition process of the present invention.
FIG. 2 is a schematic diagram of a rotational scanning defect probability imaging method of the present invention.
FIG. 3 is a graph showing Lamb wave dispersion characteristics according to the present invention.
Fig. 4 is a schematic structural diagram of the present invention.
FIG. 5 is a graph of the angle versus cross-correlation coefficient of the present invention.
FIG. 6 is an imaging result of the carbon fiber composite material plate containing the artificial circular delamination defect of the invention, wherein FIG. 6- (a) is an imaging result of a defect probability imaging algorithm, and FIG. 6- (b) is an imaging result of a modal decomposition imaging algorithm.
Detailed Description
The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. 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.
A carbon fiber composite material plate layering defect air coupling Lamb wave ultrasonic detection method based on a modal decomposition imaging algorithm comprises the following steps:
step 1: connecting equipment and debugging;
step 2: processing the signals detected based on the step 1 through modal decomposition;
and step 3: performing feature extraction and analysis on the instantaneous energy of the detection signal and the defect-free reference signal processed by the modal decomposition in the step 2;
and 4, step 4: based on the step 3, the cross correlation coefficient of the instantaneous energy of the detection signal and the non-defective reference signal in the scanning direction forming the same angle with the fiber direction is used as a damage index to realize the characterization of the layering defect of the carbon fiber composite board;
and 5: and (5) obtaining a two-dimensional image from the A scanning data of the rotary scanning by a rotary scanning defect probability imaging method based on the damage index of the step (4) to realize qualitative analysis and quantitative characterization of the carbon fiber composite plate.
In the detection method, the equipment in the step 1 specifically comprises an excitation air coupling transducer 1, a receiving air coupling transducer 2, a carbon fiber composite material plate 3, a rotating platform 4, a two-dimensional motion platform 5, a signal generator, a voltage amplifier, an upper computer, an oscilloscope and a preamplifier;
the linear guide rail 5-1 of the two-dimensional motion platform 5 is respectively connected with an excitation air coupling transducer 1 and a receiving air coupling transducer 2 through a fixed connector 1-1, the excitation air coupling transducer 1 and the receiving air coupling transducer 2 are respectively arranged at two sides of a carbon fiber composite material plate 3, the carbon fiber composite material plate 3 is arranged on a rotary platform 4,
the excitation air coupling transducer 1 is sequentially connected with a voltage amplifier and a signal generator, and the receiving air coupling transducer 2 is sequentially connected with an upper computer, an oscilloscope and a preamplifier.
In the detection method, the modal decomposition processing specifically includes the following steps:
step 2.1: extracting the characteristics of an original signal F (t) through wavelet transformation, and carrying out threshold filtering reconstruction to obtain a signal F (t);
step 2.2: based on the signal F (t) in the step 1, according to a cubic spline function, fitting a local maximum point and a local minimum point to respectively obtain an upper envelope line F and a lower envelope line F max (t) and F min (t)；
Step 2.3: upper envelope F based on step 2.2 max (t) and the lower envelope F min (t) calculating the average value m (t) = [ F) max (t)+F min (t)]/2；
Step 2.4: obtaining h (t) = F (t) -m (t) by subtraction based on the average value of step 2.3; judging whether h (t) meets two conditions of the intrinsic mode function, if not, taking h (t) as an original signal, repeating the steps 2.2-2.4 until the conditions are met, and if so, performing the step 2.5;
step 2.5: extracting the intrinsic mode function IMF, i.e. I 1 (t) = h (t); while the residual term r (t) = F (t) -I 1 (t) is regarded asA new signal; judging whether r (t) meets the conditions of a monotone sequence or a constant value sequence, if not, repeating the step 2.3-2.5 until all intrinsic mode functions IMF are extracted to obtain I 2 (t)、I 3 (t) and other IMF components, if the IMF components meet the requirement, the intrinsic mode functions IMF are all extracted, and the step 2.6 is carried out;
step 2.6: the instantaneous energy IE (t) is obtained by the hilbert transform.
The detection method has the advantages that the modal decomposition processing process can be divided into three contents, namely wavelet threshold denoising, empirical Mode Decomposition (EMD) and Hilbert transformation; the basic idea of wavelet threshold denoising is to select a proper threshold after wavelet transformation is performed on a signal, so as to achieve the purpose of denoising; the Empirical Mode Decomposition (EMD) of the modal decomposition is to determine the instantaneous equilibrium position of an original signal by using the average value of upper and lower envelopes of a time sequence and further extract an Intrinsic Mode Function (IMF); since the IMF component is a data sequence, it needs to satisfy two conditions at the same time: the number of the maximum points of the signals is equal to or different from the number of the zeros by 1; the local mean of the upper envelope defined by the maximum and the lower envelope defined by the minimum is zero; after obtaining each IMF component I j (t) after, processing the IMF component I by Hilbert transform j (t)：
Wherein P is the Cauchy principal value;
the Hilbert spectrum H (omega, t) of the signal is obtained by representing the corresponding relation between the instantaneous amplitude and the instantaneous frequency of each IMF component in the same time-frequency space, and can fully reflect the distribution of signal energy in a time-frequency domain; the instantaneous energy IE (t) is obtained to reflect the energy change of the signal at different times:
in the detection method, the characterization of the delamination defect of the carbon fiber composite plate in the step 4 is specifically that, because the air coupling transducer has a certain size, the generated Lamb wave has a certain diffusion angle and directivity, the energy distribution on the propagation path between the excitation sensor and the receiving sensor has a certain width, and energy superposition exists between each path of the rotary scanning; in a certain range, the geometrical attenuation characteristic of sound wave propagation is combined, the sound wave at each position on a scanning path is regarded as an independent sound source, and the defect probability of all grid units in the detection area relative to different scanning paths is quantized; if a certain range is exceeded, the energy will be ignored as 0; the defect probability of the grid cells in the detection area is the sum of the defect probabilities of the grid cells relative to all the rotating scanning paths; the method for imaging the defect probability by rotating scanning is specifically described with reference to fig. 2;
assuming that N directions are selected for detection at equal intervals in the 360 ° direction, the defect probability estimate P (x, y) at the location of the detection area (x, y) is written as:
wherein P is i (x, y) is a probability estimate of the defect distribution for the air-coupled transducer pair in the ith rotational scan direction, A i The signal difference coefficient of an air coupling transducer pair in the ith rotating scanning direction, alpha determines the attenuation rate of Lamb wave energy on two sides of a scanning path, and R is a calculation threshold value of defect distribution probability, namely 1/2 of the distribution width of the Lamb wave energy on the two sides of the scanning path;
if the imaging point of the defect distribution probability is in the energy range of the Lamb wave scanning path, calculating the defect distribution probability according to the distance from the energy central line; in contrast, the probability of defect distribution is 0;
when the central lines of the two air coupling transducers are parallel to the fiber direction of the carbon fiber composite material plate, the central lines are designated as 0 degree; at this time, the center lines of the two air-coupled transducers are designated as a first rotational scan path; and taking the middle point of the central lines of the two air coupling transducers as a rotation center, and taking the central lines of the two air coupling transducers which rotate for i-1 times at a fixed rotation angle as an ith rotation scanning path.
The detection method, the determination of the excitation frequency and the inclination angle of the air coupling transducer, is specifically described in conjunction with fig. 3; lamb waves have symmetrical and antisymmetric modes and frequency dispersion characteristics, and can excite multi-order symmetrical modes (S) under the same excitation frequency 0 ，S 1 ，…，S i ) With anti-symmetric mode (A) 0 ，A 1 ，…，A i ) (ii) a In order to excite the air coupling transducer to a relatively pure mode in the piece to be detected, the excitation frequency of the transmitting air coupling transducer is less than a certain upper limit value f according to the dispersion curve of the guided wave and the thickness of the piece to be detected 0 (ii) a Then, determining the excitation frequency f according to the actual performance of the air coupling transducer; according to research and analysis, the in-plane displacement of the symmetric mode is large, and the out-of-plane displacement of the anti-symmetric mode is large, so that the anti-symmetric mode is adopted for air coupling ultrasonic detection; when the frequency-thickness product (frequency x the thickness of the object) is determined, the antisymmetric mode A 0 The group velocity of (c) is also known, and then the tilt angle θ of the air coupled transducer is determined according to the first critical refraction angle of snell's law in combination with the propagation velocity in air.
The detection method is characterized in that the rotational scanning defect probability imaging method specifically comprises the steps of taking the difference between an instantaneous energy curve of a detection signal and a corresponding non-defective signal as a damage index, and taking the rotational scanning defect probability calculation as an imaging method; the damage indices are represented by their cross correlation coefficients:
wherein A and ρ are damage index and cross-correlation coefficient of instantaneous energy of the detected signal and instantaneous energy of its corresponding defect-free signal, respectively, X is instantaneous energy of the defect-free reference signal, Y is instantaneous energy of the detected signal, C is instantaneous energy of the detected signal, and XY is the covariance, σ, of X and Y X And σ Y Standard deviations for X and Y, respectively.
In the detection method, the qualitative analysis and the quantitative characterization in the step 5 are specifically described in detail by combining the figure 4 to specifically illustrate the qualitative analysis and the quantitative characterization of the delamination defect of the carbon fiber composite plate. Placing an air coupling transducer at one side of the composite plate sample according to the determined inclination angle theta, and setting the distance between an excitation air coupling transducer and a receiving air coupling transducer to be L; using ipsilateral and elevation trapping methods, and using A 0 Carrying out layering defect detection on the composite material plate in a modal mode;
the signal generator is used for generating a sine pulse series excitation signal with the center frequency f, hanning window modulation and the pulse number of 5 required by the air coupling transducer.
The voltage amplifier is used to increase the voltage of the excitation signal generated by the signal generator to ensure that the air-coupled transducer excites sufficient acoustic energy.
The two-dimensional motion platform is used for adjusting the horizontal distance between the transmitting air coupling transducer and the receiving air coupling transducer and the height distance between the transmitting air coupling transducer and the carbon fiber composite material plate.
The rotary platform is used for realizing 360-degree rotary scanning of the carbon fiber composite material plate.
Because Lamb wave signals are greatly attenuated in air, the preamplifier is used for amplifying and receiving echo signals of the air coupling transducer;
the oscilloscope is used for displaying Lamb wave signals and storing data;
the upper computer is used for displaying the imaging qualitative analysis and quantitative characterization results.
Selecting N directions at equal intervals in the 360-degree direction for detecting the defect-free carbon fiber composite board and the defect-free carbon fiber composite board, and respectively obtaining N A scanning data; the N A-scanning data are subjected to modal decomposition processing to obtain instantaneous energy of N non-defective reference signals and instantaneous energy of N detection signals respectively, and N corresponding cross-correlation coefficients are obtained and used as damage indexes to assign values to the grid of the region to be detected in combination with a rotational scanning defect probability imaging algorithm, so that imaging and qualitative analysis are realized; quantitative characterization of defects is achieved through a 6dB method, and half of the maximum value of an experimental result is selected as a threshold value. If the pixel value is greater than the threshold, the pixel is considered part of the defect, otherwise it is discarded. By analogy, the defect size can be obtained to realize the quantitative characterization of the defect.
A computer-readable storage medium, in which a computer program is stored which, when being executed by a processor, carries out the method steps of the above.
The test sample used in the present invention was a T300/QY8911 carbon fiber composite panel, the parameters of which are shown in the following Table. The carbon fiber composite board comprises 40 layers in sequence of [0/45/90/-45]2s, wherein the thickness of each layer is 0.125mm, and the thickness of the whole board is 5mm. In the process of laying the prepreg on the carbon fiber composite material plate, a polyethylene film with the thickness of 0.05mm is pre-laid between the 20 th layer and the 21 st layer so as to simulate the lamination defect. The carbon fiber composite board to be tested is 300mm long and 300mm wide. Samples included two categories: defect free and circular delamination defects with a diameter of 30 mm.
Carbon fiber composite board material parameters
The center frequency of the air-coupled transducer was determined from the dispersion curve to be f =200kHz, and the tilt angle θ =15 °. The signal generator generates a 200kHz sine pulse series excitation signal modulated by a Hanning window and having a pulse number of 5, and the sine pulse series excitation signal is amplified to a peak-to-peak value of 400V by a voltage amplifier. The horizontal distance between the two air-coupled transducers was set at 200mm. The rotating platform is rotated from 0 ° to 360 ° in 5 ° increments and Lamb wave signals are collected at each angle. And taking Lamb wave signals received by the carbon fiber composite plates with each angle passing through the defect-free angle as reference signals to obtain the cross-correlation coefficient of Lamb waves passing through the carbon fiber composite plates with the artificial delamination defects at the corresponding angles. As can be seen from the relationship graph between the angle and the cross-correlation coefficient respectively obtained by the time-domain amplitude and the instantaneous energy, the relationship between the angle and the cross-correlation coefficient obtained by the instantaneous energy has smaller fluctuation and is more consistent with the actual defect characteristics qualitatively.
And normalizing all the defect probability evaluation results P by taking the maximum defect probability evaluation value Pmax in the experimental results as a reference. And respectively adopting the existing defect probability imaging algorithm and the modal decomposition imaging algorithm provided by the invention to carry out defect imaging qualitative analysis on the carbon fiber composite plate and quantitatively represent the defects according to a 6dB method. Experimental results show that the qualitative characterization result of the modal decomposition imaging algorithm using the instantaneous energy provided by the invention is more in line with the actual shape of the defect than the existing defect probability imaging algorithm using the time domain amplitude. For a diameter of 30mm, the area size is 225 π mm 2 The size error of the defect probability imaging algorithm is 53.6mm 2 The size error of the modal decomposition imaging algorithm provided by the invention is 19.4mm 2 The characterization is more accurate.
Two steps are needed for realizing the in-situ test and the large-area rapid scanning of the aerospace carbon fiber composite plate. The first step is to rapidly scan a large area to obtain the approximate center position of the defect. And secondly, placing the detection device in the defect center, and realizing accurate characterization of the layered defects by combining an imaging method. The modal decomposition imaging algorithm provided by the invention is used for realizing the second step of in-situ testing and large-area rapid scanning of the aviation composite material plate. The traditional detection mostly adopts a step scanning mode in X and Y directions. The invention adopts 360-degree rotary scanning to acquire the omnibearing information of the detection area. Compared with the traditional time domain analysis method based on amplitude difference, the invention provides a modal decomposition imaging algorithm to analyze nonlinear non-stationary leakage Lamb wave signals, obtain instantaneous energy to more accurately extract and analyze characteristics of the Lamb wave signals, and realize more accurate characterization of the layering defects of the carbon fiber composite material plate by taking the cross correlation coefficient in the same direction as a damage index. And then, obtaining a two-dimensional image from the A-scan data of the rotational scanning by a modal decomposition imaging algorithm of a rotational scanning defect probability imaging method to realize qualitative analysis and quantitative characterization of the carbon fiber composite plate.
Claims (8)
1. A carbon fiber composite material plate layering defect air coupling Lamb wave ultrasonic detection method based on a modal decomposition imaging algorithm is characterized by comprising the following steps:
step 1: connecting equipment and debugging;
step 2: processing the signals detected based on the step 1 through modal decomposition;
and step 3: performing feature extraction and analysis on the instantaneous energy of the detection signal and the defect-free reference signal processed by the modal decomposition in the step 2;
and 4, step 4: based on the step 3, the cross correlation coefficient of the instantaneous energy of the detection signal and the non-defective reference signal in the scanning direction forming the same angle with the fiber direction is used as a damage index to realize the characterization of the layering defect of the carbon fiber composite board;
and 5: obtaining a two-dimensional image from the A scanning data of the rotary scanning by a rotary scanning defect probability imaging method based on the damage index of the step 4 to realize qualitative analysis and quantitative characterization of the carbon fiber composite plate;
the modal decomposition processing specifically comprises the following steps:
step 2.1: extracting the characteristics of an original signal F (t) through wavelet transformation, and carrying out threshold filtering reconstruction to obtain a signal F (t);
step 2.2: based on the signal F (t) in the step 1, according to a cubic spline function, fitting a local maximum point and a local minimum point to respectively obtain an upper envelope line F and a lower envelope line F max (t) and F min (t)；
Step 2.3: upper envelope F based on step 2.2 max (t) and the lower envelope F min (t) calculating the average value m (t) = [ F ] thereof max (t)+F min (t)]/2；
Step 2.4: obtaining h (t) = F (t) -m (t) by subtraction based on the average value of step 2.3; judging whether h (t) meets two conditions of the intrinsic mode function, if not, taking h (t) as an original signal, repeating the steps 2.2-2.4 until the conditions are met, and if so, performing the step 2.5;
step 2.5: extracting the intrinsic mode function IMF, i.e. I 1 (t) = h (t); while the residual term r (t) = F (t) -I 1 (t) is considered a new signal; judging whether r (t) meets the conditions of a monotone sequence or a constant value sequence, if not, repeating the step 2.3-2.5 until all intrinsic mode functions IMF are extracted to obtain I 2 (t)、I 3 (t) and other IMF components, if the IMF components meet the requirement, the intrinsic mode functions IMF are all extracted, and the step 2.6 is carried out;
step 2.6: the instantaneous energy IE (t) is obtained by a hilbert transform.
2. The detection method according to claim 1, wherein the device of step 1 specifically comprises an excitation air coupling transducer (1), a reception air coupling transducer (2), a carbon fiber composite material plate (3), a rotating platform (4), a two-dimensional motion platform (5), a signal generator, a voltage amplifier, an upper computer, an oscilloscope and a preamplifier;
the linear guide rail (5-1) of the two-dimensional motion platform (5) is respectively connected with the excitation air coupling transducer (1) and the receiving air coupling transducer (2) through the fixed connectors (1-1), the excitation air coupling transducer (1) and the receiving air coupling transducer (2) are respectively arranged on two sides of the carbon fiber composite material plate (3), the carbon fiber composite material plate (3) is arranged on the rotary platform (4),
the excitation air coupling transducer (1) is sequentially connected with the voltage amplifier and the signal generator, and the receiving air coupling transducer (2) is sequentially connected with the upper computer, the oscilloscope and the preamplifier.
3. According toThe detection method as claimed in claim 1, wherein the empirical mode decomposition EMD of the modal decomposition is to determine the instantaneous equilibrium position of the original signal by using the average value of the upper and lower envelopes of the time series, and further extract the intrinsic mode function IMF; since the IMF component is a data sequence, it needs to satisfy two conditions at the same time: the number of the maximum points of the signals is equal to or different from the number of the zeros by 1; the local mean of the upper envelope defined by the maximum and the lower envelope defined by the minimum is zero; after obtaining each IMF component I j (t) after, processing the IMF component I by Hilbert transform j (t)：
Wherein P is the Cauchy principal value;
the Hilbert spectrum H (omega, t) of the signal is obtained by representing the corresponding relation between the instantaneous amplitude and the instantaneous frequency of each IMF component in the same time-frequency space, and can fully reflect the distribution of signal energy in a time-frequency domain; the instantaneous energy IE (t) is obtained to reflect the energy change of the signal at different times:
4. the detection method according to claim 1, wherein the characterization of the delamination defect of the carbon fiber composite sheet in step 4 is carried out by writing the probability of defect estimate P (x, y) at the location of the detection area (x, y) assuming that N directions are selected at equal intervals in the 360 ° direction:
wherein P is i (x, y) is a probability estimate of the defect distribution for the air-coupled transducer pair in the ith rotational scan direction，A i The signal difference coefficient of an air coupling transducer pair in the ith rotating scanning direction, alpha determines the attenuation rate of Lamb wave energy on two sides of a scanning path, and R is a calculation threshold value of defect distribution probability, namely 1/2 of the distribution width of the Lamb wave energy on the two sides of the scanning path;
if the imaging point of the defect distribution probability is in the energy range of the Lamb wave scanning path, calculating the defect distribution probability according to the distance from the energy central line; in contrast, the probability of defect distribution is 0;
when the central lines of the two air coupling transducers are parallel to the fiber direction of the carbon fiber composite material plate, the central lines are designated as 0 degree; at this time, the center lines of the two air-coupled transducers are designated as a first rotational scan path; and taking the midpoint of the central lines of the two air coupling transducers as a rotation center, and taking the central lines of the two air coupling transducers which rotate for i-1 times by a fixed rotation angle as an ith rotation scanning path.
5. The method as claimed in claim 4, wherein the exciting frequency and the inclination angle of the air-coupled transducer are determined such that Lamb waves have symmetric and anti-symmetric modes and dispersion characteristics, and multiple orders of the symmetric modes (S) can be excited at the same exciting frequency 0 ，S 1 ，…，S i ) With anti-symmetric mode (A) 0 ，A 1 ，…，A i ) (ii) a In order to excite the pure mode in the piece to be detected by the air coupling transducer, the excitation frequency of the transmitting air coupling transducer is less than a certain upper limit value f according to the dispersion curve of the guided wave and the thickness of the piece to be detected 0 (ii) a Then, determining the excitation frequency f according to the actual performance of the air coupling transducer; according toResearch and analysis show that the in-plane displacement of the symmetric mode is large, and the out-of-plane displacement of the anti-symmetric mode is large, so that the anti-symmetric mode is adopted for air coupling ultrasonic detection; when the frequency-thickness product is determined, the antisymmetric mode A 0 The group velocity of (a) can also be known, and then the tilt angle θ of the air-coupled transducer is determined according to the first critical refraction angle of snell's law in combination with the propagation velocity in air.
6. The detection method according to claim 1, wherein the rotational scanning defect probability imaging method is specifically a rotational scanning defect probability imaging method which uses a difference between an instantaneous energy curve of a detection signal and a corresponding non-defective signal as a damage index, and uses a rotational scanning defect probability calculation as an imaging method; the damage indices are represented by their cross correlation coefficients:
wherein A and ρ are damage index and cross-correlation coefficient of instantaneous energy of the detected signal and instantaneous energy of its corresponding defect-free signal, respectively, X is instantaneous energy of the defect-free reference signal, Y is instantaneous energy of the detected signal, C is instantaneous energy of the detected signal, and XY is the covariance of X and Y, σ X And σ Y Standard deviations for X and Y, respectively.
7. The detection method according to claim 6, wherein the qualitative analysis and quantitative characterization of step 5 are implemented by placing an air coupling transducer on one side of the composite plate sample according to the previously determined inclination angle θ, and setting the distance between the exciting air coupling transducer and the receiving air coupling transducer to be L; using ipsilateral and elevation trapping methods, and using A 0 Carrying out layering defect detection on the composite material plate in a modal mode;
the signal generator is used for generating a sine pulse series excitation signal with the center frequency of f, hanning window modulation and the pulse number of 5, which is required by the air coupling transducer;
the voltage amplifier is used for carrying out voltage increase on the excitation signal generated by the signal generator so as to ensure that the air coupling transducer excites enough sound energy;
the two-dimensional motion platform is used for adjusting the horizontal distance between the transmitting air coupling transducer and the receiving air coupling transducer and the height distance between the transmitting air coupling transducer and the carbon fiber composite material plate;
the rotating platform is used for realizing 360-degree rotating scanning of the carbon fiber composite material plate;
the preamplifier is used for amplifying and receiving an echo signal of the air coupling transducer;
the oscilloscope is used for displaying Lamb wave signals and storing data;
the upper computer is used for displaying the imaging qualitative analysis and quantitative characterization results.
8. A computer-readable storage medium, characterized in that a computer program is stored in the computer-readable storage medium, which computer program, when being executed by a processor, carries out the method steps of any one of the claims 1-7.
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