Method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration

Abstract

The present invention relates to a method for detecting and identifying pore and fold defects based on the frequency migration of transmitted ultrasound, comprising the following steps: S1. Select the area to be measured of a composite material structure with a thickness greater than 10 mm and the positions of the transmitting phased array and the receiving phased array; S2. Use experimental equipment to collect the ultrasonic transmission signals within the area to be measured and build an experimental system; S3. Use the experimental system built in step S2 to excite the transmitting phased array and save the data collected by the receiving phased array; S4. Perform synchronous wavelet squeezing transformation on the ultrasonic transmission signal data collected in step S3, and normalize the time-frequency spectrum obtained by the processing; S5. According to the time-frequency spectrum normalization diagram obtained in step S4, by judging whether high-frequency signal components and defect types appear, output the corresponding detection results. Compared with the prior art, the present invention utilizes the discovered phenomenon of the frequency migration of ultrasonic transmission signals, and by arranging a transmitting phased array on one side of the composite material to be measured and a receiving phased array on the other side, solves the problem that it is difficult for traditional ultrasonic techniques based on energy loss to detect and identify large-thickness composite material structures with pore and fiber fold mixed defects.

Description

Technical Field

[0001] The present invention relates to the technical field of ultrasonic nondestructive testing, and in particular to a method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration. Background Art

[0002] Composite materials are widely used in the fields of aerospace, military, automotive, etc. due to their advantages of light weight, high strength, good toughness, and corrosion resistance. In recent years, the application parts of composite materials in the fields of aerospace and others have developed from secondary load-bearing structures to primary load-bearing structures. To reduce the number of parts and the weight of the airframe and improve the reliability of products, the main load-bearing composite components in aviation are tending towards large thickness and integration. Since the forming process of large-thickness composite structures involves complex fluid-solid multi-field changes, it is difficult to accurately control the forming parameters. Therefore, defects such as out-of-plane folds, delaminations, and pores will inevitably occur during the manufacturing process of large-thickness composite structures. Among them, out-of-plane folds refer to the bending of fibers in three-dimensional space, resulting in the overall waveform bending of single-layer or multi-layer fibers. The reason for its generation is usually the mismatch of curing deformation of each layer of prepreg during the composite material forming process. On the other hand, pore and delamination defects are often caused by the non-uniform flow of resin during the forming process, and the existence of out-of-plane folds will precisely interfere with the resin flow, thereby inducing defects such as pores. Therefore, defects in large-thickness composite structures often appear in a mixed form. Research shows that the existence of out-of-plane folds, pores, and delamination defects will reduce the compression, tensile, and bending properties of composite materials. If these mixed defects cannot be detected in time, it may lead to major safety accidents.

[0003] At present, nondestructive testing methods are often used to verify the quality of composite materials. Due to its advantages of high sensitivity, strong penetration, and fast detection, ultrasonic testing is widely used in the on-site testing of large composite materials. At present, ultrasonic testing mainly focuses on the detection of single types of defects such as fiber folds. However, in large-thickness composite structures, it is easy to have a situation where multiple defects such as pores, delaminations, and folds are mixed. Both fold and pore defects will cause the distortion and aberration of ultrasonic waves, changing the phase information of ultrasonic waves. Therefore, existing traditional ultrasonic testing methods based on the change of transmission and reflection energy or ultrasonic phase information are difficult to identify and characterize fold and pore mixed defects. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned defects existing in the prior art and provide a method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration, comprising the following steps:

[0007] S1. Select the area to be measured of the composite material structure with a thickness greater than 10 mm and the positions of the transmitting phased array and the receiving phased array.

[0008] S2. Use the experimental equipment to collect the ultrasonic transmission signals in the area to be measured and build an experimental system.

[0009] S3. Use the experimental system built in step S2, excite the transmitting phased array and save the data collected by the receiving phased array.

[0010] S4. Perform synchronous wavelet squeezing transform on the ultrasonic transmission signal data collected in step S3, and normalize the time-frequency spectrum obtained by the processing.

[0011] S5. According to the normalized time-frequency spectrum diagram obtained in step S4, output the corresponding detection results by judging whether high-frequency signal components and defect types appear.

[0012] Further, in step S1, the composite material with a thickness greater than 10 mm is fixed on the workbench with a vise, and the transmitting phased array and the receiving phased array are respectively arranged on both sides of the material according to the possible defect positions in the material.

[0013] Further, in step S2, the experimental equipment includes a phased array probe, a probe fixture and an ultrasonic phased array signal acquisition system. The ultrasonic phased array adopts the direct contact method, and the ultrasonic transmission signals are obtained by full matrix acquisition.

[0014] Further, a coupling agent needs to be added during the process of the ultrasonic phased array adopting the direct contact method.

[0015] Further, in step S2, according to the positions selected in step S1, the transmitting phased array and the receiving phased array are arranged. The signal generator and the transmitting phased array, the power amplifier and the transmitting phased array, and the receiving phased array and the receiving amplifier are respectively connected by wires. Then the computer, the data acquisition card and the receiving amplifier are connected in turn. Finally, the signal generator and the data acquisition card, and the power amplifier and the data acquisition card are connected to ensure the synchronization of the transmitted and received signals and form an experimental system.

[0016] Further, in step S3, use the experimental system built in step S2. The signal generated by the signal generator is amplified by the power generator inside the signal generator, excite the transmitting phased array to generate an incident wave into the specimen to be tested, produce an interaction with the defect in the expected action area and be received by the receiving phased array, and then be amplified by the receiving amplifier and collected by the data acquisition card, and finally input into the computer for storage.

[0017] Furthermore, the step S4 performs synchronous wavelet extrusion transformation on the collected ultrasonic transmission signal: firstly, a continuous wavelet transformation is performed on the time domain ultrasonic signal s(t) to obtain the wavelet coefficient W s (a,b), and then the wavelet coefficients W s (a, b) are converted to the frequency domain to obtain the instantaneous frequency of the signal.

[0018] Furthermore, the wavelet coefficients are:

[0019]

[0020] Among them, a is the scale parameter, b is the wavelet moving distance, s is the time parameter, and ψ′ is the conjugate function of the mother wavelet function;

[0021] Get the wavelet coefficient W s (a, b), let the harmonic signal be x(t) = Acos(ωt), and let the wavelet function ψ be concentrated on the positive frequency axis. When ξ < 0, And perform Fourier transform on s(t) and ψ(t), and transform the wavelet coefficients W s (a,b) converted to frequency domain:

[0022]

[0023] Where ζ is the angular frequency, and are the Fourier transforms of s(t) and ψ(t), respectively.

[0024] Furthermore, the instantaneous frequency of the signal is:

[0025]

[0026] Aggregate distribution at ξ=ω0, W s (a,b) Aggregate distribution at the center, squeezing any center frequency ω l Nearby area The wavelet coefficient W s (a, b) to synchronous extrusion transformation T s (ω l ,b), that is:

[0027]

[0028] Among them, a k is the discrete scale, Δω=ω l -ω l-1 is the frequency transformation value; finally, the processed time-frequency spectrum is normalized.

[0029] Further, based on the time-frequency spectrum normalized diagram obtained in step S4 in step S5, it is determined whether there is a high-frequency signal component higher than the excitation center frequency. Fiber fold defects will cause frequency migration of the transmitted signal, that is, high-frequency signal components appear, while no defects and pore defects will not induce ultrasonic frequency migration characteristics, that is, no high-frequency signal components will appear.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention proposes a method for detecting and identifying pore and fold defects based on transmitted ultrasonic frequency migration. Transmitting and receiving phased arrays are arranged on both sides of the large-thickness composite material to be measured. The ultrasonic phased array uses the direct contact method, and the ultrasonic transmitted signal is collected by the full matrix. The signal is subjected to synchronous wavelet squeezing transformation, and the processed time-frequency spectrum is normalized to obtain a time-frequency spectrum normalized diagram. Whether there is a high-frequency signal component is judged through the time-frequency spectrum normalized diagram, thereby judging whether the type of defect is fiber fold, which solves the problem that traditional ultrasound has difficulty in characterizing pore and fiber fold defects.

[0032] 2. The present invention uses wires to connect the signal generator to the transmitting phased array, the power amplifier to the transmitting phased array, and the receiving phased array to the receiving amplifier respectively, then connects the computer, data acquisition card and receiving amplifier in sequence, and finally connects the signal generator and data acquisition card, and the power amplifier and data acquisition card to form an experimental system, which maximally ensures the synchronization of the transmitted and received signals, and the experimental results are true and effective.

[0033] 3. The present invention proposes a method for detecting and identifying pore and fold defects based on transmitted ultrasonic frequency migration. By designing a probe fixture, the position of the phased array probe can be moved according to the change of the area to be measured of the composite material, which is applicable to different environments and different material applications, and greatly improves the practicability of the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a flowchart of the present invention;

[0035] Figure 2 is a schematic diagram of the propagation path of sound waves in the case of transmitting and receiving phased arrays in the transmission experiment of the present invention;

[0036] Figure 3 is the ultrasonic transmission time-domain signal diagram of the present invention;

[0037] Figure 4(a) is the time-frequency spectrum diagram of the ultrasonic transmission experiment signal of the defect-free specimen;

[0038] Figure 4(b) is the time-frequency spectrum diagram of the ultrasonic transmission experiment signal of the specimen with folds;

[0039] Figure 4(c) is the time-frequency spectrum diagram of the ultrasonic transmission experiment signal of the specimen with pores. Detailed implementation manners

[0040] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives detailed implementation manners and specific operation processes, but the protection scope of the present invention is not limited to the following embodiments.

[0041] As Figure 1 shown, a method for detecting and identifying pore and fold defects based on the frequency migration of transmitted ultrasound includes the following steps:

[0042] S1. Select the area to be measured of the composite material structure with a thickness greater than 10 mm and the positions of the transmitting phased array and the receiving phased array;

[0043] S2. Use the experimental equipment to collect the ultrasonic transmission signals in the area to be measured and build an experimental system;

[0044] S3. Use the experimental system built in step S2 to excite the transmitting phased array and save the data collected by the receiving phased array;

[0045] S4. Perform synchronous wavelet squeezing transform on the ultrasonic transmission signal data collected in step S3, and normalize the processed time-frequency spectrum;

[0046] S5. According to the time-frequency spectrum normalization diagram obtained in step S4, output the corresponding detection results by judging whether high-frequency signal components and defect types appear.

[0047] In step S1, the composite material with a thickness greater than 10 mm is fixed on the workbench with a vise, and the transmitting phased array and the receiving phased array are respectively arranged on both sides of the material according to the possible defect positions in the material.

[0048] In step S2, the experimental equipment includes a phased array probe, a probe fixture, and an ultrasonic phased array signal acquisition system. The ultrasonic phased array uses the direct contact method, and the ultrasonic transmission signals are obtained by full matrix acquisition.

[0049] A coupling agent needs to be added during the process of the ultrasonic phased array using the direct contact method.

[0050] In step S2, the transmitting phased array and the receiving phased array are arranged according to the positions selected in step S1, and the signal generator is connected to the transmitting phased array, the power amplifier is connected to the transmitting phased array, and the receiving phased array is connected to the receiving amplifier respectively by wires. Then, the computer, the data acquisition card, and the receiving amplifier are connected in sequence, and finally the signal generator and the data acquisition card, and the power amplifier and the data acquisition card are connected to ensure the synchronization of the transmitted and received signals, forming an experimental system.

[0051] In step S3, the experimental system built in step S2 is used. The signal generator generates a signal, which is amplified by the power generator inside the signal generator, and then the excitation transmitting phased array generates an incident wave that enters the specimen to be tested, interacts with the defect in the expected action area, and is received by the receiving phased array. After being amplified by the receiving amplifier, it is collected by the data acquisition card and finally input into the computer for storage.

[0052] In step S4, the synchronous wavelet squeezing transform is performed on the collected ultrasonic transmission signal: First, the continuous wavelet transform is performed on the time-domain ultrasonic signal s(t) to obtain the wavelet coefficient W s (a, b), and then the wavelet coefficient W s (a, b) is transformed into the frequency domain, and then the instantaneous frequency of the signal is obtained. The wavelet coefficient is:

[0053]

[0054] where a is the scale parameter, b is the wavelet shift distance, s is the time parameter, and ψ′ is the conjugate function of the mother wavelet function;

[0055] After obtaining the wavelet coefficient W s (a, b), let the harmonic signal be x(t) = Acos(ωt). Assume that the wavelet function ψ is concentrated on the positive frequency axis. When ξ < 0, And perform the Fourier transform on s(t) and ψ(t), and transform the wavelet coefficient W s (a, b) into the frequency domain:

[0056]

[0057] where ξ is the angular frequency, and are the Fourier transforms of s(t) and ψ(t) respectively.

[0058] The instantaneous frequency of the signal is:

[0059]

[0060] is concentrated and distributed at ξ = ω0, and W s (a, b) is concentrated and distributed at Squeeze the wavelet coefficient W l in any interval near the center frequency ω s (a, b) to the synchronous squeezing transform quantity T s (ω l , b), that is:

[0061]

[0062] where a kis the discrete scale, Δω = ω l - ω l-1 is the frequency transformation magnitude; finally, normalize the time-frequency spectrum obtained by the processing.

[0063] In step S5, based on the normalized time-frequency spectrum diagram obtained in step S4, determine whether there is a high-frequency signal component higher than the excitation center frequency. Fiber fold defects will cause frequency migration of the transmitted signal, that is, high-frequency signal components appear, while non-defective and pore defects will not induce ultrasonic frequency migration characteristics, that is, high-frequency signal components will not appear.

[0064] A method for characterizing the mixed defects of pores and folds in thick composite materials based on the frequency migration phenomenon of ultrasonic transmitted signals proposed by the present invention has been verified on carbon fiber resin matrix composite specimens. Taking the non-defective, fiber fold defect-containing, and pore defect-containing composite specimens as examples, the specific implementation manners are described in detail. The layup methods of the non-defective and pore defect-containing specimens are both [0 / 90] 24s , and the layup method of the fiber fold defect-containing specimen is [0 / 90] 30s . For the pore defect-containing specimen, a hole with a radius of 0.5 mm is drilled at the center position. When manufacturing the fiber fold defect-containing specimen, after laying the prepreg, a semi-circular long iron rod with a surface wetted with a release agent is placed at the bottom layer, so as to generate fiber folds during the hot pressing process. After the specimen is formed, it is subjected to cutting treatment to ensure that the surface of the specimen is flat and the dimensions are 12 mm in thickness × 45 mm in width × 100 mm in length.

[0065] In the experiment, build an experimental system according to the Figure 2 shown structural diagram. The computer controls the signal generator to generate a Ricker wave with a center frequency of 5 MHz, which is amplified by the internal power amplifier, and excites the lower phased array (Doppler, 5L64 - 0.6*10) to generate longitudinal waves into the specimen to be tested. After acting on the expected action area and the specimen, it is received by the upper phased array (Doppler, 5L64 - 0.6*10), amplified by the receiving amplifier, received by the data acquisition card and input into the computer, and the signal processing and data analysis are carried out by MATLAB.

[0066] In the experiment, after normalizing the collected signals, perform time-domain analysis, and the results are as Figure 3 shown.

[0067] There are obvious characteristic differences in the transmitted ultrasonic time-domain signals of the three types of specimens: First, the arrival times of the head wave signals are different, because pore and fold defects will change the local material properties and affect the arrival time of the head wave; second, in the fiber fold defect-containing and pore defect-containing specimens, there are large amplitude perturbations after the transmitted head wave, because the ultrasonic waves form multiple reflections between the defect and the upper surface of the specimen and then propagate to the lower surface of the specimen.

[0068] Although Figure 3 the time-domain characteristics of the ultrasonic transmission signals of different defects in [reference] are different, effective quantitative features cannot be extracted for defect identification and characterization. To further study the time-frequency characteristics of the interaction between ultrasound and different defects, the data processing is carried out according to step S4 in the present invention, and the results are shown in Figure 4.

[0069] First of all, strong frequency components appear in all three transmission signals at 4 - 6 MHz, as shown in the highlighted parts of Figure 4(a), Figure 4(b) and Figure 4(c). This is because the central frequency of the incident signal is 5 MHz. Secondly, for the transmitted ultrasonic signal in the specimen with fiber fold defect in Figure 4(b), a large number of signal components with a frequency range of 6 - 7.5 MHz appear in the time range of 5.1 - 5.9 μs, that is, frequency components significantly higher than the central frequency of the incident signal are generated. However, the above high-frequency components do not appear in the same time interval in Figure 4(a) and Figure 4(c). In other words, the fiber fold defect will cause the frequency migration phenomenon of the transmitted signal, while the defect-free and pore defects will not induce the ultrasonic frequency migration characteristics.

[0070] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field of the present invention through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art shall fall within the protection scope determined by the claims.

Claims

1. A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration, characterized in that, It includes the following steps: S1. Select the area to be measured of the carbon fiber resin matrix composite structure with a thickness greater than 10 mm and the positions of the transmitting phased array and the receiving phased array; S2. Use the experimental equipment to collect the ultrasonic transmission signals in the area to be measured and build an experimental system; S3. Use the experimental system built in step S2 to excite the transmitting phased array and save the data collected by the receiving phased array; S4. Perform synchronous wavelet squeezing transform on the ultrasonic transmission signal data collected in step S3, and normalize the obtained time-frequency spectrum; S5. According to the normalized time-frequency spectrum diagram obtained in step S4, output the corresponding detection results by judging whether high-frequency signal components and defect types appear; Step S4 performs synchronous wavelet squeezing transformation on the collected ultrasonic transmission signals: First, perform continuous wavelet transform on the time-domain ultrasonic signal s(t) to obtain wavelet coefficients W s (a, b), and then transform the wavelet coefficients W s (a, b) to the frequency domain to obtain the instantaneous frequency of the signal; In step S5, according to the normalized time-frequency spectrum diagram obtained in step S4, judge whether high-frequency signal components higher than the excitation center frequency appear. Fiber wrinkling defects will cause frequency migration of the transmission signal, that is, high-frequency signal components appear, while no defects and pore defects will not induce ultrasonic frequency migration characteristics, that is, high-frequency signal components will not appear.

2. The pore and fold defect detection and recognition method based on transmission ultrasonic frequency migration according to claim 1, wherein In step S1, the composite material with a thickness greater than 10 mm is fixed on the workbench with a vise, and the transmitting phased array and the receiving phased array are arranged on both sides of the material respectively.

3. A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration according to claim 1, characterized in that, In step S2, the experimental equipment includes a phased array probe, a probe fixture and an ultrasonic phased array signal acquisition system. The ultrasonic phased array adopts the direct contact method, and the ultrasonic transmission signals are obtained by full matrix acquisition.

4. A pore and fold defect detection and identification method based on transmission ultrasonic frequency migration according to claim 3, characterized in that Coupling agent needs to be added during the process of the ultrasonic phased array adopting the direct contact method.

5. A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration according to claim 1, characterized in that In step S2, the transmitting phased array and the receiving phased array are arranged according to the positions selected in step S1. The signal generator and the transmitting phased array, the power amplifier and the transmitting phased array, and the receiving phased array and the receiving amplifier are respectively connected by wires. Then the computer, the data acquisition card and the receiving amplifier are connected in sequence. Finally, the signal generator and the data acquisition card, and the power amplifier and the data acquisition card are connected to ensure the synchronization of the transmitted and received signals and form an experimental system.

6. The pore and fold defect detection and identification method based on transmission ultrasonic frequency migration according to claim 1, characterized in that In step S3, use the experimental system built in step S2. The signal generated by the signal generator is amplified by the power generator inside the signal generator, and the transmitting phased array is excited to generate an incident wave into the specimen to be tested. The interaction with the defect is generated in the expected action area and received by the receiving phased array. After being amplified by the receiving amplifier, it is collected by the data acquisition card and finally input into the computer for storage.

7. A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration according to claim 1, characterized in that, The wavelet coefficient is: where a is the scale parameter, b is the wavelet shift distance, s is the time parameter, and ψ′ is the conjugate function of the mother wavelet function; Obtain the wavelet coefficient W s After (a, b), let the harmonic signal be x(t) = Acos(ωt). Assume that the wavelet function ψ is concentrated on the positive frequency axis. When ξ < 0, And perform Fourier transforms on s(t) and ψ(t), and transform the wavelet coefficient W s (a, b) to the frequency domain: where ξ is the angular frequency, and are the Fourier transforms of s(t) and ψ(t), respectively.

8. A method for detecting and identifying pore and fold defects based on transmission ultrasonic frequency migration according to claim 1, characterized in that, The instantaneous frequency of the signal is: is concentrated and distributed at ξ = ω0, W s (a, b) is in is concentrated and distributed, squeezing the wavelet coefficients W of any interval near the center frequency ω l nearby interval to the synchrosqueezing transform quantity T s (a, b), that is: s (ω l , b), namely: where a k is the discrete scale, Δω = ω l - ω l-1 is the frequency transformation magnitude; finally, normalize the time-frequency spectrum obtained by processing.

Images