Method and system for enhancing out-of-plane fiber trend ultrasonic imaging of composite material by using half-layer resonance frequency

Abstract

The invention relates to a method and a system for enhancing out-of-plane fiber trend ultrasonic imaging of a composite material by utilizing half-layer resonance frequency. According to the method, half of the resonant frequency of a single-layer prepreg is adopted as the center frequency of an excitation signal, the out-of-plane fiber trend ultrasonic imaging of the large-thickness composite material is completed, firstly, the resonant frequency of the single-layer prepreg is determined, and an ultrasonic probe is adopted for carrying out water immersion ultrasonic detection on a composite material laminated plate; then reflected signals at all positions on the scanning path are collected, band-pass filtering and Hilbert transformation are carried out respectively so as to position the front wall and the rear wall of the composite material laminated plate, the position of interlayer resin is determined by using the instantaneous phase at the front wall, and finally the position of the interlayer resin corresponding to all the filtered reflected signals is presented in a graph. And the out-of-plane fiber trend ultrasonic imaging of the composite material is realized. Compared with the prior art, the method has the advantages that the effective imaging thickness is improved, and the imaging accuracy and reliability are improved.

Description

Technical Field

[0001] The invention relates to the field of nondestructive testing of composite materials, and in particular to a method and a system for ultrasonic imaging of out-of-plane fiber orientation of composite materials by using half-layer resonance frequency enhancement. Background Art

[0002] Carbon fiber reinforced composite materials are increasingly used in the industry due to their excellent properties such as light weight, high specific strength, high specific modulus and corrosion resistance. This trend places higher demands on the compliance of their design and manufacturing as well as their reliability during service.

[0003] In the field of aerospace engineering, in view of the need for precise matching of complex load environments, thick carbon fiber reinforced composite laminate components are widely used in key structural parts. In the field of domestic large aircraft manufacturing, the local thickness of engine blade components can reach 50mm. The processing process of such thick composite laminates is extremely challenging. Due to the large number of layers and the complex layer structure, it is easy to cause a series of processing problems, including but not limited to unreasonable layer design, air entrainment during the layering process, and improper execution of the processing procedures. These problems directly affect the effective penetration and uniform distribution of the resin between the layers, which in turn causes the formed composite laminate components to show significant deviations from the design expectations, specifically manifested in non-volumetric defects in the form of out-of-plane fiber wrinkles, as well as typical volumetric defects such as pores and cracks. The existence of these defects not only weakens the compliance between composite material design and manufacturing, but also constitutes a potential hidden danger during the service life of the structure, and even causes premature failure of the structure. Therefore, it is particularly important to achieve out-of-plane fiber orientation imaging of thick composite laminates to ensure the safety of the structure and the safety of life and property of the people.

[0004] One of the existing methods for imaging the out-of-plane fiber orientation of composite laminates is to use a probe with a center frequency near the resonance frequency of a single-layer prepreg to perform water immersion ultrasonic scanning on the sample to obtain a reflection signal, and use a bandpass filter with a bandwidth of 100% of the center frequency to filter the reflection signal, and finally use the instantaneous phase of the filtered signal to locate the position of the interlayer resin, thereby achieving out-of-plane fiber orientation imaging of the component. In the field of aerospace, considering that the thickness of a single-layer carbon fiber prepreg in China is generally between 0.1mm and 0.2mm, and its corresponding resonance frequency is generally between 14MHz and 7MHz, the limitations of the existing technology are two: first, the high-frequency signal is severely attenuated and difficult to penetrate a thick composite laminate, so the imaging thickness is limited; second, the wavelength of the high-frequency signal is small, and it is more sensitive to volume defects such as pores, resulting in deviations in the instantaneous phase, resulting in imaging artifacts and limited imaging accuracy.

[0005] For example, the invention patent with the publication number CN113686959B discloses an imaging method and device based on ultrasonic Lamb wave defect detection. This method performs focusing processing on the Lamb wave echo signal of the first mode and the Lamb wave echo signal of the second mode to determine the focused echo signal; based on the focused echo signal, a first distance is determined, and the first distance represents the actual distance between the position of the sensor corresponding to the defect echo signal and the position of the defect; based on the first distances corresponding to N defect echo signals, a defect detection image of the plate-like structure to be detected is determined. Although this method can obtain high-resolution detection images, its detection thickness is limited, and it is difficult to detect thick composite laminates. At the same time, it also fails to overcome the shortcomings of being sensitive to volume defects such as pores, causing deviations in instantaneous phases, and resulting in imaging artifacts. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a method and system for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] According to one aspect of the present invention, a method for enhancing the out-of-plane fiber orientation ultrasonic imaging of a composite material using a half-layer resonant frequency is provided. The method uses half the resonant frequency of a single-layer prepreg as the center frequency of an excitation signal to complete the out-of-plane fiber orientation ultrasonic imaging of a thick composite material. The method comprises the following steps:

[0009] S1. Determine the resonance frequency of a single-layer prepreg, use an ultrasonic probe with a center frequency of half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the composite laminate, and collect reflection signals at various positions on the scanning path;

[0010] S2, performing bandpass filtering on each reflected signal to obtain a corresponding filtered reflected signal;

[0011] S3, performing Hilbert transform on the filtered reflection signal respectively to locate the front wall and the rear wall of the composite laminate, and obtaining the instantaneous phase at the front wall corresponding to the filtered reflection signal;

[0012] S4. The interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall, and the interlayer resin position corresponding to each filtered reflection signal is presented in a graph to realize ultrasonic imaging of the fiber orientation out of the composite material surface.

[0013] As a preferred technical solution, the specific steps for determining the resonant frequency of a single layer of prepreg in S1 are: using finite element software or experiments to find the frequency of maximum interlayer reflection of the laminate, and determining the resonant frequency of the single layer of prepreg used in the composite laminate.

[0014] As a preferred technical solution, when collecting the reflected signals at various positions on the scanning path in S1, the reflected signals are interpolated 5 times.

[0015] As a preferred technical solution, the specific step of bandpass filtering the reflected signal in S2 is: using a Butterworth filter to bandpass filter the collected reflected signal, and the filter bandwidth is set to 25% of the center frequency of the excitation signal.

[0016] As a preferred technical solution, the Butterworth filter is a second-order Butterworth filter.

[0017] As a preferred technical solution, in S3, the filtered reflected signal is subjected to Hilbert transform to obtain a complex signal of the reflected signal. The specific formula of the complex signal of the reflected signal is:

[0018] s a (t) = Re (t) + iIm (t)

[0019] Where Re(t) is a real signal, that is, the signal actually measured, Im(t) is a complex signal, and i is an imaginary unit.

[0020] As a preferred technical solution, the positions of the front wall and the rear wall of the composite laminate in S3 are determined by the instantaneous amplitude calculated by the complex signal. The first peak of the instantaneous amplitude corresponds to the position of the front wall, and the tail peak of the instantaneous amplitude corresponds to the position of the rear wall. The specific formula of the instantaneous amplitude is:

[0021]

[0022] In the formula, A inst (t) is the instantaneous amplitude.

[0023] As a preferred technical solution, the specific process of obtaining the instantaneous phase at the front wall in S3 is: firstly calculate the instantaneous phase from the complex signal, and then determine the instantaneous phase at the front wall of the laminate according to the positions of the front wall and the rear wall of the composite laminate. The specific formula for calculating the instantaneous phase from a complex signal is:

[0024]

[0025] In the formula, is the instantaneous phase.

[0026] As a preferred technical solution, S4 uses the instantaneous phase at the front wall to determine the position of the interlayer resin. Specifically, the instantaneous phase at the interlayer resin is located using the instantaneous phase at the front wall. The specific formula is:

[0027]

[0028] In the formula, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase of the interlayer resin.

[0029] According to another aspect of the present invention, a system for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement is provided. The system is operated by the above-mentioned ultrasonic imaging method of out-of-plane fiber orientation of composite materials. The system includes a low-frequency water immersion ultrasonic module, a signal processing module and a fiber orientation imaging module.

[0030] The low-frequency water immersion ultrasonic module is used to determine the resonance frequency of a single-layer prepreg. An ultrasonic probe with a center frequency of half the resonance frequency of a single-layer prepreg is used to perform water immersion ultrasonic testing on the composite laminate, and the reflected signals at each position on the scanning path are collected.

[0031] The signal processing module uses a Butterworth filter to perform bandpass filtering on the reflected signal to obtain a filtered signal; and performs a Hilbert transform on the filtered signal to locate the front wall and the rear wall of the laminate and obtain the instantaneous phase at the front wall;

[0032] The fiber orientation imaging module uses the instantaneous phase at the front wall to determine the interlayer resin position corresponding to the filtered reflection signal, and presents the interlayer resin position corresponding to each filtered reflection signal in a graph to achieve out-of-plane fiber orientation ultrasonic imaging of the composite material.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention adopts half the resonance frequency of a single-layer prepreg as the center frequency of the excitation signal to complete ultrasonic imaging of the out-of-plane fiber orientation of a thick composite material. The characteristics of low-frequency ultrasound, such as slow attenuation, high depth resolution, and long wavelength, can be utilized to ensure the positioning accuracy of the resin layer while increasing the effective imaging thickness of a thick composite laminate.

[0035] 2. The method for ultrasonic imaging of fiber orientation out of the composite material in the present invention first determines the resonance frequency of a single-layer prepreg, uses an ultrasonic probe with a center frequency of half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic detection on the composite laminate, and collects the reflected signals at each position on the scanning path; then bandpass filters each reflected signal to obtain the corresponding filtered reflected signal; Hilbert transforms the filtered reflected signal to correspond to the front wall and the rear wall of the composite laminate, and obtains the instantaneous phase at the front wall corresponding to the filtered reflected signal; finally, the instantaneous phase at each front wall is used to determine the interlayer resin position corresponding to each filtered reflected signal, and the interlayer resin position corresponding to each filtered reflected signal is presented in a picture, so as to realize ultrasonic imaging of fiber orientation out of the composite material. This method is less affected by small volume defects such as pores and cracks in thick composite laminate components, can effectively reduce the interference of small volume defects on the instantaneous phase, improve imaging accuracy, and further improve the reliability and robustness of imaging of fiber orientation out of the plane.

[0036] 3. The present invention adopts a second-order Butterworth filter to perform bandpass filtering on the collected reflection signal, and the filter bandwidth is set to 25% of the center frequency of the excitation signal. If the filter order is too small, the filtering strength will be insufficient, and if it is too large, it will cause signal disorder. Therefore, the present invention weighs the filter order and uses a second-order Butterworth filter to improve the imaging quality.

[0037] 4. When collecting the reflected signals at various positions on the scanning path in the present invention, the reflected signals are interpolated 5 times to make the reflected signals smoother and have higher resolution, thereby improving the accuracy of imaging of the out-of-plane fiber orientation. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 A schematic diagram of the steps of a method for enhancing the out-of-plane fiber orientation ultrasonic imaging of a composite material using a half-layer resonance frequency in the present invention;

[0039] Figure 2 is a flow chart of a method for ultrasonic imaging of out-of-plane fiber orientation of a composite material in an embodiment;

[0040] Figure 3 Schematic diagram of the positions of the front wall and the rear wall of the laminate member in the embodiment;

[0041] Figure 4 It is an instantaneous phase diagram of the 150th reflected signal after filtering on the planned path at an excitation frequency of 5 MHz in the embodiment;

[0042] Figure 5 This is an out-of-plane fiber orientation imaging diagram of the laminate component at an excitation frequency of 5 MHz and a bandpass filter with a center frequency bandwidth of 25% in the embodiment;

[0043] Figure 6 Schematic diagram of the resin positioning error between layers in the embodiment;

[0044] Figure 7 This is an out-of-plane fiber orientation imaging diagram of a laminate component at an excitation frequency of 10 MHz and a bandpass filter with 100% center frequency bandwidth in the embodiment;

[0045] Figure 8 is the error E between the imaging accuracy of the present method and the existing method in the embodiment i Comparison chart;

[0046] Fig. 9 This is a diagram showing the out-of-plane fiber orientation imaging result of a 96-layer composite laminate model with a bandpass filter having a bandwidth of 25% of the center frequency at an excitation frequency of 5 MHz in the embodiment;

[0047] Fig.10 It is an out-of-plane fiber orientation imaging diagram of a 96-layer composite laminate model with a bandpass filter at an excitation frequency of 10 MHz and a 100% center frequency bandwidth in the embodiment;

[0048] Fig.11 is the error E between the imaging accuracy of the present method and the existing method in the embodiment i Comparison chart;

[0049] Fig.12 The error TOF of the method in the embodiment and the existing method affected by the pores dif Comparison chart. DETAILED DESCRIPTION

[0050] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0051] Carbon fiber reinforced composite materials are increasingly used in the industry due to their excellent properties such as light weight, high specific strength, high specific modulus and corrosion resistance. This trend places higher demands on the compliance of their design and manufacturing as well as their reliability during service.

[0052] In the field of aerospace engineering, in view of the need for precise matching of complex load environments, thick carbon fiber reinforced composite laminate components are widely used in key structural parts. In the field of domestic large aircraft manufacturing, the local thickness of engine blade components can reach 50mm. The processing process of such thick composite laminates is extremely challenging. Due to the large number of layers and the complex layer structure, it is easy to cause a series of processing problems, including but not limited to unreasonable layer design, air entrainment during the layering process, and improper execution of the processing procedures. These problems directly affect the effective penetration and uniform distribution of the resin between the layers, which in turn causes the formed composite laminate components to show significant deviations from the design expectations, specifically manifested in non-volumetric defects in the form of out-of-plane fiber wrinkles, as well as typical volumetric defects such as pores and cracks. The existence of these defects not only weakens the compliance between composite design and manufacturing, but also poses a potential hidden danger during the service life of the structure, and even causes premature failure of the structure. Therefore, it is particularly important to achieve out-of-plane fiber orientation imaging of thick composite laminates to ensure the safety of the structure and the safety of life and property of the people.

[0053] One of the existing methods for imaging the out-of-plane fiber orientation of composite laminates is to use a probe with a center frequency near the resonance frequency of a single-layer prepreg to perform water immersion ultrasonic scanning on the sample to obtain a reflection signal, and use a bandpass filter with a bandwidth of 100% of the center frequency to filter the reflection signal, and finally use the instantaneous phase of the filtered signal to locate the position of the interlayer resin, thereby achieving out-of-plane fiber orientation imaging of the component. In the field of aerospace, considering that the thickness of a single-layer carbon fiber prepreg in China is generally between 0.1mm and 0.2mm, and its corresponding resonance frequency is generally between 14MHz and 7MHz, the limitations of the existing technology are two: first, the high-frequency signal is severely attenuated and difficult to penetrate a thick composite laminate, so the imaging thickness is limited; second, the wavelength of the high-frequency signal is small, and it is more sensitive to volume defects such as pores, resulting in deviations in the instantaneous phase, resulting in imaging artifacts and limited imaging accuracy.

[0054] In summary, there is an urgent need to develop a detection technology for the manufacturing conformity of thick composite materials, which can improve the effective imaging thickness while ensuring the imaging accuracy, reduce the interference of inevitable tiny volume defects on the imaging, and construct an imaging method for the out-of-plane fiber orientation of thick carbon fiber reinforced composite materials.

[0055] Example 1

[0056] In this embodiment, a method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement is applied. The flow chart of the method is as follows: Figure 1 As shown, the method comprises the following steps:

[0057] S1. Determine the resonance frequency of a single-layer prepreg, use an ultrasonic probe with a center frequency of half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the composite laminate, and collect reflection signals at various positions on the scanning path;

[0058] S2, performing bandpass filtering on each reflected signal to obtain a corresponding filtered reflected signal;

[0059] S3, performing Hilbert transform on the filtered reflection signal respectively to locate the front wall and the rear wall of the composite laminate, and obtaining the instantaneous phase at the front wall corresponding to the filtered reflection signal;

[0060] S4. The interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall, and the interlayer resin position corresponding to each filtered reflection signal is presented in a graph to realize ultrasonic imaging of the fiber orientation out of the composite material surface.

[0061] In this embodiment, the process of using this method to perform ultrasonic imaging of the out-of-plane fiber orientation of a composite material is as follows: Figure 2 As shown, the interlayer reflection signal is the reflection signal at each position on the scanning path, and the specific process is as follows:

[0062] Step S1 specifically uses a water immersion ultrasonic probe to determine the resonance frequency of a single-layer prepreg and collect reflection signals of thick composite laminates, including: building a collection system, determining the actual thickness of the sample, collecting reflection signals, calculating the resonance frequency of the single-layer prepreg, and using an ultrasonic probe with a center frequency near half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the laminate, and collecting reflection signals at various positions on the scanning path;.

[0063] First, the acquisition system is built to establish the connection between the LTPA system, laptop computer and ABB robot arm. The parameter settings of the phased array system are shown in Table 1.

[0064] In this embodiment, in order to make the reflection signal smoother and have a higher resolution, a 5-fold interpolation process is performed on the collected reflection signal.

[0065] Table 1 Phased array system parameter settings

[0066]

[0067] Then determine the actual thickness of the sample and measure the actual thickness of the sample using a vernier caliper.

[0068] In this embodiment, the thickness of the single-layer prepreg used is 0.125 mm, and the ply sequence is [0 / 90] 24s The actual thickness of the sample was measured to be 11.7 mm.

[0069] In this embodiment, an immersion ultrasonic probe with a center frequency of 10 MHz is used to perform immersion ultrasonic testing to collect reflection signals at various positions on the scanning path.

[0070] Then select each reflected signal on the scanning path in turn, perform Hilbert transform on the signal, and obtain its instantaneous amplitude. Use the maximum and minimum peak values ​​of the instantaneous amplitude to locate the front wall and rear wall of the sample.

[0071] In this embodiment, based on the sample thickness and the time of ultrasound in the sample, the speed of ultrasound propagation in the sample is calculated to be 2925 m / s. Where v is the speed of ultrasound propagation in the material, d is the corresponding thickness, and the calculated resonant frequency of the single-layer prepreg is 11.7 MHz. Based on this, the resonant frequency of the half-single-layer prepreg is determined to be 5.85 MHz.

[0072] Finally, the laminate is subjected to water immersion ultrasonic testing using an ultrasonic probe whose center frequency is near half the resonance frequency of the single-layer prepreg to collect the reflected signals of the laminate components.

[0073] In this embodiment, an immersion ultrasonic probe with a center frequency of 5 MHz is used to transmit the excitation signal. Since the immersion ultrasonic probe used is a broadband excitation, and the difference between 5 MHz and 5.85 MHz is only 0.85 MHz, there are more signal components of half the resonance frequency of the single-layer prepreg in the signal transmitted by the immersion probe, so it is effective and reasonable to select a probe with a center frequency of 5 MHz as the excitation.

[0074] Step S2 specifically involves performing narrowband filtering processing on the collected reflection signal with a bandwidth of 25% of the center frequency of the excitation signal to eliminate interference from other frequency components in the signal.

[0075] In this embodiment, a bandpass Butterworth filter is used to filter the collected reflected signal, and its passband is set to 25% of the center frequency of the excitation signal (4.375MHz~5.675MHz), and its filter order is set to 2. If the filter order is too small, the filtering strength will be insufficient, and if it is too large, it will cause signal disorder. Therefore, the order of the filter is weighed in this scheme, and a second-order Butterworth filter is used to improve the imaging quality and obtain the filtered reflected signal.

[0076] Step S3 specifically performs Hilbert transform on the filtered signal, calculates the instantaneous amplitude and instantaneous phase of the filtered signal, locates the front wall and the rear wall of the laminate, and determines the instantaneous phase at the front wall.

[0077] First, the Hilbert transform is performed on the filtered signal, that is, the Hilbert transform is performed on the filtered signal obtained by S2, and its instantaneous amplitude and instantaneous phase are calculated. And the positions of the front wall and the rear wall of the laminate component are determined according to the time values ​​corresponding to the maximum and minimum peaks of the instantaneous amplitude.

[0078] Among them, after performing Hilbert transform on the filtered signal, the complex signal of the reflected signal is obtained. The specific formula of the complex signal of the reflected signal is:

[0079] s a (t) = Re (t) + iIm (t)

[0080] Where Re(t) is a real signal, that is, the signal actually measured, Im(t) is a complex signal, and i is an imaginary unit.

[0081] In this scheme, the positions of the front wall and the rear wall of the composite laminate are determined by the instantaneous amplitude calculated by the complex signal, that is, the first peak of the instantaneous amplitude corresponds to the position of the front wall, and the tail peak of the instantaneous amplitude corresponds to the position of the rear wall. The positions of the front wall and the rear wall of the laminate component are shown as follows Figure 3 As shown, the specific formula of instantaneous amplitude is:

[0082]

[0083] In the formula, A inst (t) is the instantaneous amplitude.

[0084] In this scheme, the instantaneous phase at the front wall of the laminate is determined The specific formula for calculating the instantaneous phase from a complex signal is:

[0085]

[0086] In the formula, is the instantaneous phase.

[0087] Then determine the instantaneous phase of the front wall position, and according to the determined positions of the front wall and the rear wall, obtain the instantaneous phase of the signal at the front wall Figure 4 This is the instantaneous phase diagram of the 150th reflected signal after filtering on the planned path under the excitation frequency of 5 MHz, where the dotted line indicates the position of the front wall and the dashed line indicates the position of the rear wall.

[0088] Step S4 specifically determines the position of the interlayer resin, realizes the imaging of the fiber orientation outside the component surface, and quantitatively evaluates the imaging accuracy.

[0089] First, locate the interlayer resin. Use the following formula to locate the position of the interlayer resin:

[0090]

[0091] In the formula, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase at the interlayer resin. The corresponding time here is the time when it reaches the i-th layer of resin.

[0092] Then, out-of-plane fiber orientation imaging is realized, and the interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall. The interlayer resin position corresponding to each filtered reflection signal is presented in a figure to realize out-of-plane fiber orientation imaging of the component. In this embodiment, the out-of-plane fiber orientation imaging of the laminate component with a bandpass filter at a 5 MHz excitation frequency and a 25% center frequency bandwidth is shown in FIG. Figure 5 shown.

[0093] In this embodiment, in order to quantitatively evaluate the imaging accuracy and measure the imaging error, an error index E is defined: i , which is defined as follows:

[0094]

[0095]

[0096] In the formula, is the flight time of the reflected signal of the i-th layer of prepreg calculated from the component geometric characteristics, is the flight time of the reflected signal of the i-th layer of prepreg obtained according to the instantaneous phase positioning, is the flight time of ultrasound passing through a single layer of prepreg; t prepreg is the thickness of a single layer of prepreg, which is 0.125 mm in this embodiment; v is the velocity of the ultrasonic signal in the composite material, which is 2925 m / s in this embodiment.

[0097] The out-of-plane fiber orientation imaging diagram of the laminate component in this embodiment at a 5 MHz excitation frequency and a bandpass filter with a 25% center frequency bandwidth is shown in FIG. Figure 6 As shown, the hollow rectangle is the flight time of the reflection signal of the i-th layer of prepreg calculated from the geometric characteristics of the component, the solid circle is the flight time of the reflection signal of the i-th layer of prepreg obtained according to the instantaneous phase positioning, and the straight line is the instantaneous phase.

[0098] In this embodiment, an ultrasonic probe with a center frequency near the resonant frequency of a single-layer prepreg is used to perform water immersion ultrasonic testing on the same component, and the imaging result of a bandpass filter with a bandwidth of 100% of the center frequency is as follows: Figure 7 shown.

[0099] The error index E of this scheme and the existing method i Comparison of Figure 8As shown. It can be seen that this scheme has a higher effective imaging depth and accuracy for out-of-plane fiber orientation imaging of a 96-layer thick carbon fiber reinforced composite laminate:

[0100] The existing method can only effectively locate 64 layers of interlayer resin, while the present solution can effectively locate 82 layers of interlayer resin, and the positioning depth is increased by 18 layers, about 2.25 mm.

[0101] Error index E of existing methods i The fluctuation decreases before the 22nd floor, and rapidly increases after the 22nd floor due to the rapid attenuation characteristics of high-frequency signals, and E i The maximum occurs at layer 64, with a maximum value of 44.60%; the error index E of the method proposed in this scheme is i It fluctuates and rises before the 28th floor, and fluctuates around 13% after the 28th floor, which has high stability. i The maximum occurs on the 28th floor, with a maximum value of 15.45%.

[0102] In summary, this scheme utilizes the characteristics of slow attenuation, high depth resolution and long wavelength of low-frequency ultrasound. While ensuring the positioning accuracy of the resin layer, it increases the effective imaging thickness of thick composite laminates and has higher imaging accuracy and reliability.

[0103] Example 2

[0104] In this embodiment, a method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement is applied. The flow chart of the method is as follows: Figure 1 As shown, the method comprises the following steps:

[0105] S1. Determine the resonance frequency of a single-layer prepreg, use an ultrasonic probe with a center frequency of half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the composite laminate, and collect reflection signals at various positions on the scanning path;

[0106] S2, performing bandpass filtering on each reflected signal to obtain a corresponding filtered reflected signal;

[0107] S3, performing Hilbert transform on the filtered reflection signal respectively to locate the front wall and the rear wall of the composite laminate, and obtaining the instantaneous phase at the front wall corresponding to the filtered reflection signal;

[0108] S4. The interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall, and the interlayer resin position corresponding to each filtered reflection signal is presented in a graph to realize ultrasonic imaging of the fiber orientation out of the composite material surface.

[0109] In this embodiment, the process of using this method to perform ultrasonic imaging of the out-of-plane fiber orientation of a composite material is as follows: Figure 2 As shown, the interlayer reflection signal is the reflection signal at each position on the scanning path, and the specific process is as follows:

[0110] Step S1 specifically involves using an immersion ultrasonic probe to determine the resonance frequency of a single-layer prepreg and to collect reflection signals from a thick composite laminate.

[0111] First, establish the ply order as [0 / 90] 24s A two-dimensional finite element model of a composite laminate component, simulating an immersion ultrasonic testing environment. In the model, the thickness of a single fiber layer is 0.125 mm, and the thickness of a single resin layer is 0.01 mm.

[0112] Then the center frequency of the ultrasonic signal is changed, and the resonance frequency of the single-layer prepreg is determined to be 9.9 MHz according to the maximum value of the average amplitude of all echoes in the first reflection signal between the front wall and the back wall corresponding to each frequency. Therefore, the resonance frequency of the half-layer prepreg is 4.95 MHz.

[0113] In this embodiment, considering that it is difficult to find ultrasonic probes with center frequencies of 9.9 MHz and 4.95 MHz in engineering practice, and the bandwidths of 10 MHz and 5 MHz excitation signals are wide enough to include signal components of 9.9 MHz and 4.95 MHz, 10 MHz and 5 MHz are used as the center frequencies of the excitation signals, respectively, to excite the analysis model and collect reflected signals at various positions on the scanning path.

[0114] Taking 5 MHz as the center frequency of the model excitation signal, the reflection signal of the model of the laminate is calculated.

[0115] In this embodiment, in order to make the reflected signal smoother and have a higher resolution, the collected signal is interpolated 5 times.

[0116] Step S2 specifically involves performing narrowband filtering processing on the collected reflection signal with a bandwidth of 25% of the center frequency of the excitation signal to eliminate interference from other frequency components in the signal.

[0117] In this embodiment, a bandpass Butterworth filter is used to filter the collected reflection signal, the passband is set to 25% of the center frequency of the excitation signal (4.375MHz-5.675MHz), and the filter order is set to 2 to obtain a filtered reflection signal.

[0118] Step S3 specifically performs Hilbert transform on the filtered signal, calculates the instantaneous amplitude and instantaneous phase of the filtered signal, locates the front wall and the rear wall of the laminate, and determines the instantaneous phase at the front wall.

[0119] First, the Hilbert transform is performed on the filtered signal, that is, the Hilbert transform is performed on the filtered signal obtained by S2, and its instantaneous amplitude and instantaneous phase are calculated. And the positions of the front wall and the rear wall of the laminate component are determined according to the time values ​​corresponding to the maximum and minimum peaks of the instantaneous amplitude.

[0120] Among them, after performing Hilbert transform on the filtered signal, the complex signal of the reflected signal is obtained. The specific formula of the complex signal of the reflected signal is:

[0121] s a (t) = Re (t) + iIm (t)

[0122] Where Re(t) is a real signal, that is, the signal actually measured, Im(t) is a complex signal, and i is an imaginary unit.

[0123] In this scheme, the positions of the front wall and the rear wall of the composite laminate are determined by the instantaneous amplitude calculated by the complex signal, that is, the first peak of the instantaneous amplitude corresponds to the position of the front wall, and the tail peak of the instantaneous amplitude corresponds to the position of the rear wall. The positions of the front wall and the rear wall of the laminate component are shown as follows Figure 3 As shown, the specific formula of instantaneous amplitude is:

[0124]

[0125] In the formula, A inst (t) is the instantaneous amplitude.

[0126] In this scheme, the instantaneous phase at the front wall of the laminate is determined The specific formula for calculating the instantaneous phase from a complex signal is:

[0127]

[0128] In the formula, is the instantaneous phase.

[0129] Then determine the instantaneous phase of the front wall position, and according to the determined positions of the front wall and the rear wall, obtain the instantaneous phase of the signal at the front wall Figure 4 This is the instantaneous phase diagram of the 150th reflected signal after filtering on the planned path under the excitation frequency of 5 MHz, where the dotted line indicates the position of the front wall and the dashed line indicates the position of the rear wall.

[0130] Step S4 specifically determines the position of the interlayer resin, realizes the imaging of the fiber orientation outside the component surface, and quantitatively evaluates the imaging accuracy.

[0131] First, locate the interlayer resin. Use the following formula to locate the position of the interlayer resin:

[0132]

[0133] In the formula, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase at the interlayer resin. The corresponding time here is the time when it reaches the i-th layer of resin.

[0134] Then, out-of-plane fiber orientation imaging is realized, and the interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall. The interlayer resin position corresponding to each filtered reflection signal is presented in a figure to realize out-of-plane fiber orientation imaging of the component. In this embodiment, the out-of-plane fiber orientation imaging of the 32-layer composite laminate model with a bandpass filter at an excitation frequency of 4.95 MHz and a bandwidth of 25% of the center frequency is shown in FIG. Fig. 9 shown.

[0135] In this embodiment, the established model was also studied using the existing method. The imaging results are as follows: at an excitation frequency of 9.9 MHz, with a bandpass filter of 100% of the center frequency bandwidth, the out-of-plane fiber orientation imaging of the 32-layer composite laminate model is as follows: Fig.10 shown.

[0136] In this embodiment, the error index E is used i Quantitatively evaluate the imaging accuracy and compare the error of this scheme with that of existing methods. The error of imaging accuracy E i The comparison results are shown in Fig.11 It can be seen that compared with the existing method, this scheme can effectively locate more resin layers, and the error index E i This is consistent with the conclusion in Example 1.

[0137] In this example, in order to verify that the proposed method is less sensitive to pores than the original method, the ply order is [0 / 90] 4s The pores are set in the two-dimensional finite element model of the laminated plate component. The specific method is to insert circles with radii of 5um and 2.5um respectively in the resin layer between the sixth and seventh fiber layers of the two-dimensional model, and assign their materials to air to simulate the tiny pores between the layers.

[0138] In order to quantitatively evaluate the degree to which this scheme and the existing methods are affected by pores of different sizes, the time-of-flight error is defined as:

[0139] TOF dif =TOF por -TOF ori

[0140] In the formula, TOF por TOF is the flight time at the maximum value of the instantaneous phase fluctuation caused by the pores in a certain layer of resin. ori It is the arrival time of the instantaneous phase when there are no pores in the resin layer.

[0141] In this embodiment, the flight time error comparison between this scheme and the existing method is shown in Fig.12 For pores with a radius of 5 μm, the TOF of each layer of resin affected by the pores in the existing method is dif It is about 1.5 times that of this solution. For pores with a radius of 2.5um, compared with this solution, the TOF of each interlayer resin affected by the pores in the existing method is dif It is about 1 times that of this scheme. Compared with the existing methods, the instantaneous phase obtained by this scheme is less disturbed by the pores and has higher reliability in actual detection.

[0142] In summary, the imaging method proposed in this scheme is less affected by tiny volume defects such as pores and cracks in thick composite laminate components, and can effectively reduce the interference of tiny volume defects on the instantaneous phase, improve imaging accuracy, and further improve the reliability of imaging of out-of-plane fiber orientation.

[0143] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement, characterized in that: The method uses half the resonant frequency of a single-layer prepreg as the center frequency of an excitation signal to complete ultrasonic imaging of the out-of-plane fiber orientation of a thick composite material. The method includes the following steps: S1. Determine the resonance frequency of a single-layer prepreg, use an ultrasonic probe with a center frequency of half the resonance frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the composite laminate, and collect reflection signals at various positions on the scanning path; S2, performing bandpass filtering on each reflected signal to obtain a corresponding filtered reflected signal; S3, performing Hilbert transform on the filtered reflection signal respectively to locate the front wall and the rear wall of the composite laminate, and obtaining the instantaneous phase at the front wall corresponding to the filtered reflection signal; S4. The interlayer resin position corresponding to each filtered reflection signal is determined by using the instantaneous phase at each front wall, and the interlayer resin position corresponding to each filtered reflection signal is presented in a graph to realize the out-of-plane fiber orientation ultrasonic imaging of the composite material.

2. The method of claim 1 for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement, characterized in that: The specific steps of determining the resonant frequency of a single layer of prepreg in S1 are: using finite element software or experiments to find the frequency with the maximum interlayer reflection of the laminate, and determining the resonant frequency of the single layer of prepreg used in the composite laminate.

3. The method of claim 1 for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement, characterized in that: When collecting the reflected signals at various positions on the scanning path in S1, the reflected signals are interpolated 5 times.

4. The method of claim 1 for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement, characterized in that: The specific step of performing bandpass filtering on the reflection signal in S2 is: using a Butterworth filter to perform bandpass filtering on the collected reflection signal, and the filtering bandwidth is set to 25% of the center frequency of the excitation signal.

5. The method of using half-layer resonance frequency to enhance the out-of-plane fiber orientation ultrasonic imaging of composite materials according to claim 3, characterized in that: The Butterworth filter is a second-order Butterworth filter.

6. The method of claim 1 for utilizing half-layer resonance frequency to enhance out-of-plane fiber orientation ultrasonic imaging of composite materials, characterized in that: In S3, the filtered reflected signal is subjected to Hilbert transform to obtain a complex signal of the reflected signal. The specific formula of the complex signal of the reflected signal is: s a (t)=Re(t)+iIm(t) Where Re(t) is a real signal, that is, the signal actually measured, Im(t) is a complex signal, and i is an imaginary unit.

7. The method of claim 5 for using half-layer resonance frequency enhanced composite material out-of-plane fiber orientation ultrasonic imaging, characterized in that: The positions of the front wall and the rear wall of the composite laminate in S3 are determined by the instantaneous amplitude calculated by the complex signal. The first peak of the instantaneous amplitude corresponds to the position of the front wall, and the last peak of the instantaneous amplitude corresponds to the position of the rear wall. The specific formula of the instantaneous amplitude is: In the formula, A inst (t) is the instantaneous amplitude.

8. The method of claim 6 for utilizing half-layer resonance frequency to enhance out-of-plane fiber orientation ultrasonic imaging of composite materials, characterized in that: The specific process of obtaining the instantaneous phase at the front wall in S3 is: firstly, the instantaneous phase is calculated from the complex signal, and then the instantaneous phase at the front wall of the laminate is determined according to the positions of the front wall and the rear wall of the composite laminate. The specific formula for calculating the instantaneous phase from a complex signal is: In the formula, is the instantaneous phase.

9. The method of claim 1 for enhancing the out-of-plane fiber orientation ultrasonic imaging of composite materials using half-layer resonance frequency, characterized in that: The S4 uses the instantaneous phase at the front wall to determine the position of the interlayer resin. Specifically, the instantaneous phase at the interlayer resin is located using the instantaneous phase at the front wall. The specific formula is: In the formula, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase of the interlayer resin.

10. A system for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement, characterized in that: The system is operated by the composite material out-of-plane fiber orientation ultrasonic imaging method as described in any one of claims 1 to 9, and the system includes a low-frequency water immersion ultrasonic module, a signal processing module and a fiber orientation imaging module; The low-frequency water immersion ultrasonic module is used to determine the resonance frequency of a single-layer prepreg, and to perform water immersion ultrasonic testing on the composite laminate using an ultrasonic probe whose center frequency is half the resonance frequency of the single-layer prepreg, and to collect reflection signals at various positions on the scanning path; The signal processing module uses a Butterworth filter to perform bandpass filtering on the reflected signal to obtain a filtered signal; and performs a Hilbert transform on the filtered signal to locate the front wall and the rear wall of the laminate, and obtain the instantaneous phase at the front wall; The fiber orientation imaging module uses the instantaneous phase at the front wall to determine the interlayer resin position corresponding to the filtered reflection signal, and presents the interlayer resin position corresponding to each filtered reflection signal in a graph to achieve out-of-plane fiber orientation ultrasonic imaging of the composite material.

Images