A method and system for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement

Abstract

The present invention relates to a method and system for ultrasonic imaging of the out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement. The method uses 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. First, the resonance frequency of the single-layer prepreg is determined, and an ultrasonic probe is used to perform water immersion ultrasonic testing on the composite laminate. Then, the reflected signals at each position on the scanning path are collected and band-pass filtered and Hilbert transformed respectively to locate the front wall and the back wall of the composite laminate. The instantaneous phase at the front wall is used to determine the position of the interlayer resin. Finally, the interlayer resin positions corresponding to each filtered reflection signal are presented in a graph to achieve ultrasonic imaging of the out-of-plane fiber orientation of the composite material. Compared with the prior art, the present invention has the advantages of increasing the effective imaging thickness, improving imaging accuracy, and reliability.

Description

Technical Field

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

[0002] Carbon fiber reinforced composites (CFRPs) are increasingly being used in industry due to their lightweight, high specific strength, high specific modulus, and corrosion resistance. This trend places higher demands on their design and manufacturing compliance, as well as their reliability during service.

[0003] In aerospace engineering, due to the need for precise matching to complex load environments, thick carbon fiber reinforced composite laminate components are widely used in critical structural parts. In the domestic large aircraft manufacturing sector, engine blade components can reach a local thickness of up to 50mm. The processing of such thick composite laminates is extremely challenging. The large number of plies and the complex layup structure can easily lead to a series of processing difficulties, including but not limited to unreasonable layup design, air entrapment during layup, and improper execution of processing steps. These difficulties directly affect the effective penetration and uniform distribution of resin between layers, resulting in significant deviations from the design expectations of the molded composite laminate components. Specifically, these defects manifest as non-volumetric defects such as out-of-plane fiber wrinkles, as well as typical volumetric defects such as pores and cracks. The presence of these defects not only weakens the compliance between composite design and manufacturing, but also poses potential risks during the service life of the structure, and may even lead to premature failure. Therefore, achieving out-of-plane fiber orientation imaging of thick composite laminates is particularly important for ensuring structural safety and the safety of people's lives and property.

[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 resonant frequency of a single-layer prepreg to perform water-immersion ultrasonic scanning on the sample to obtain a reflected signal, and then use a bandpass filter with a bandwidth of 100% of the center frequency to filter the reflected signal. Finally, the instantaneous phase of the filtered signal is used to locate the position of the interlayer resin, thereby achieving out-of-plane fiber orientation imaging of the component. In the aerospace field, considering that the thickness of a single-layer carbon fiber prepreg in China is generally between 0.1mm and 0.2mm, and its corresponding resonant frequency is generally between 14MHz and 7MHz, the existing technology has two limitations: first, the high-frequency signal is severely attenuated and has difficulty penetrating thick composite laminates, limiting the imaging thickness; second, the high-frequency signal has a small wavelength and is more sensitive to volume defects such as pores, resulting in a deviation in the instantaneous phase, causing imaging artifacts and limited imaging accuracy.

[0005] For example, the invention patent with publication number CN113686959B discloses an imaging method and apparatus based on ultrasonic Lamb wave defect detection. This method focuses Lamb wave echo signals of a first mode and a second mode to determine a focused echo signal. Based on the focused echo signal, a first distance is determined, which represents the actual distance between the sensor location corresponding to the defect echo signal and the defect location. 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. While this method can obtain high-resolution detection images, its detection thickness is limited, making it difficult to detect thick composite laminates. It also fails to overcome the disadvantage of being sensitive to volumetric defects such as pores, which causes instantaneous phase deviations and leads to 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 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 the excitation signal to perform out-of-plane fiber orientation ultrasonic imaging of a thick composite material. The method comprises the following steps:

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

[0010] S2. Band-pass filter each reflected signal to obtain a corresponding filtered reflected signal;

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

[0012] S4. Determine the interlayer resin position corresponding to each filtered reflection signal using the instantaneous phase at each front wall, and present 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.

[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 setting the filter bandwidth 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, Hilbert transform is performed on the filtered reflected signal 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 from 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:

[0021]

[0022] Where 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: first calculate the instantaneous phase from the complex signal, and then determine the instantaneous phase at the front wall of the composite laminate according to the positions of the front wall and the rear wall. The specific formula for calculating the instantaneous phase from a complex signal is:

[0024]

[0025] Where, 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 front wall is used to locate the instantaneous phase at the interlayer resin. The specific formula is:

[0027]

[0028] Where, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase at 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 resonant frequency enhancement is provided. The system operates using the above-mentioned ultrasonic imaging method for out-of-plane fiber orientation of composite materials. The system includes a low-frequency water immersion ultrasound module, a signal processing module, and a fiber orientation imaging module.

[0030] The low-frequency water immersion ultrasonic module is used to determine the resonant frequency of a single layer of prepreg. An ultrasonic probe with a center frequency of half the resonant frequency of the single layer of prepreg is used to perform water immersion ultrasonic testing on the composite laminate, collecting the reflected signals at various positions along the scanning path.

[0031] The signal processing module uses a Butterworth filter to bandpass filter the reflected signal to obtain a filtered signal. The filtered signal is then Hilbert transformed to locate the front and rear walls 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 single image, realizing ultrasonic imaging of the out-of-plane fiber orientation 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 resonant 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 thick composite materials. 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 thick composite laminates.

[0035] 2. The ultrasonic imaging method for out-of-plane fiber orientation of composite materials in the present invention first determines the resonant frequency of a single-layer prepreg, uses an ultrasonic probe with a center frequency of half the resonant frequency of the single-layer prepreg to perform water immersion ultrasonic testing on the composite laminate, and collects the reflected signals at each position on the scanning path; then, each reflected signal is bandpass filtered to obtain the corresponding filtered reflected signal; the filtered reflected signals are Hilbert transformed to respectively locate the front wall and the back wall of the composite laminate, and the instantaneous phase at the front wall corresponding to the filtered reflected signal is obtained; 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 single image to realize ultrasonic imaging of out-of-plane fiber orientation of composite materials. This method is less affected by small volume defects such as pores and cracks in thick composite laminate components, can effectively reduce the interference caused by small volume defects on the instantaneous phase, improve imaging accuracy, and further improve the reliability and robustness of imaging of out-of-plane fiber orientation.

[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 order of the filter 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 out-of-plane fiber orientation ultrasonic imaging of a composite material using a half-layer resonant frequency according to the present invention;

[0039] Figure 2 is a flow chart of a method for ultrasonic imaging of out-of-plane fiber orientation in 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 1 is the 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 in the embodiment at an excitation frequency of 5 MHz and a bandpass filter with a bandwidth of 25% of the center frequency;

[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 the laminate component obtained by bandpass filtering with a 100% center frequency bandwidth at an excitation frequency of 10 MHz 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] Figure 9 This is a diagram showing the out-of-plane fiber orientation imaging results of a 96-layer composite laminate model under a 5 MHz excitation frequency and a bandpass filter with a bandwidth of 25% of the center frequency in the embodiment;

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

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

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

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

[0051] Carbon fiber reinforced composites (CFRPs) are increasingly being used in industry due to their lightweight, high specific strength, high specific modulus, and corrosion resistance. This trend places higher demands on their design and manufacturing compliance, as well as their reliability during service.

[0052] In aerospace engineering, due to the need for precise matching to complex load environments, thick carbon fiber reinforced composite laminate components are widely used in critical structural parts. In the domestic large aircraft manufacturing sector, engine blade components can reach a local thickness of up to 50mm, making the processing of such thick composite laminates extremely challenging. The large number of plies and the complex layup structure can easily lead to a series of processing difficulties, including but not limited to unreasonable layup design, air entrapment during layup, and improper execution of processing steps. These difficulties directly affect the effective penetration and uniform distribution of resin between layers, resulting in significant deviations from design expectations in the molded composite laminate components. These defects manifest as non-volumetric defects such as out-of-plane fiber wrinkles, as well as typical volumetric defects such as pores and cracks. The presence of these defects not only weakens the design-manufacturing compliance of composite materials but also poses potential risks during the service life of the structure, even leading to premature failure. Therefore, achieving out-of-plane fiber orientation imaging in thick composite laminates is particularly important for ensuring structural safety and the safety of people's lives and property.

[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 resonant frequency of a single-layer prepreg to perform water-immersion ultrasonic scanning on the sample to obtain a reflected signal, and then use a bandpass filter with a bandwidth of 100% of the center frequency to filter the reflected signal. Finally, the instantaneous phase of the filtered signal is used to locate the position of the interlayer resin, thereby achieving out-of-plane fiber orientation imaging of the component. In the aerospace field, considering that the thickness of a single-layer carbon fiber prepreg in China is generally between 0.1mm and 0.2mm, and its corresponding resonant frequency is generally between 14MHz and 7MHz, the existing technology has two limitations: first, the high-frequency signal is severely attenuated and has difficulty penetrating thick composite laminates, limiting the imaging thickness; second, the high-frequency signal has a small wavelength and is more sensitive to volume defects such as pores, resulting in a deviation in the instantaneous phase, causing 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 imaging accuracy, reduce the interference of inevitable small 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 this method is as follows: Figure 1 As shown, the method includes the following steps:

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

[0058] S2. Band-pass filter each reflected signal to obtain a corresponding filtered reflected signal;

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

[0060] S4. Determine the interlayer resin position corresponding to each filtered reflection signal using the instantaneous phase at each front wall, and present 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.

[0061] In this embodiment, the process of applying 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. The specific process is as follows:

[0062] Step S1 specifically uses an immersion ultrasonic probe to determine the resonant 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 resonant frequency of the single-layer prepreg, and using an ultrasonic probe with a center frequency near half the resonant 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, we built the acquisition system and established connections between the LTPA system, laptop, and ABB robotic arm. The parameters for 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 order 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, and reflection signals at various positions on the scanning path are collected.

[0070] Each reflected signal along the scanning path is then selected and Hilbert transformed to obtain its instantaneous amplitude. The maximum and minimum peak values ​​of the instantaneous amplitude are used to locate the front and back walls of the specimen.

[0071] In this embodiment, based on the thickness of the sample and the time the ultrasound travels 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 a single-layer prepreg is 11.7 MHz. Based on this, the resonant frequency of a half-layer prepreg is determined to be 5.85 MHz.

[0072] Finally, an ultrasonic probe with a center frequency near half the resonant frequency of a single-layer prepreg is used to perform water immersion ultrasonic testing on the laminate to collect the reflected signals of the laminate components.

[0073] In this embodiment, an immersion ultrasonic probe with a center frequency of 5 MHz was used to transmit the excitation signal. Because the immersion ultrasonic probe used has a broadband excitation frequency, and the difference between 5 MHz and 5.85 MHz is only 0.85 MHz, the signal emitted by the immersion probe contains a significant signal component at half the resonant frequency of a single layer of prepreg. Therefore, using a probe with a center frequency of 5 MHz as the excitation signal is both effective and reasonable.

[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 reflection 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, in this scheme, the order of the filter is weighed and a second-order Butterworth filter is used to improve the imaging quality and obtain the filtered reflection signal.

[0076] Step S3 is to perform Hilbert transform on the filtered signal, calculate the instantaneous amplitude and instantaneous phase of the filtered signal, locate the front wall and the back wall of the laminate, and determine 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 to calculate its instantaneous amplitude and instantaneous phase. The positions of the front and rear walls of the laminate component are determined based on 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 for the instantaneous amplitude is:

[0082]

[0083] Where A inst (t) is the instantaneous amplitude.

[0084] In this solution, 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] Where, 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 back 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 at an excitation frequency of 5 MHz. The dotted line indicates the position of the front wall, and the dashed line indicates the position of the back wall.

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

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

[0090]

[0091] Where, 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. 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 graph to realize out-of-plane fiber orientation imaging of the component. In this embodiment, the out-of-plane fiber orientation imaging diagram of the laminate component with a bandpass filter of 25% of the center frequency bandwidth at a 5MHz excitation frequency is shown as follows: Figure 5 shown.

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

[0094]

[0095]

[0096] Where, 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] In this embodiment, the out-of-plane fiber orientation imaging diagram of the laminate component with a 5 MHz excitation frequency and a bandpass filter with a bandwidth of 25% of the center frequency 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 based on 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 based on 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 of prepreg is used to perform water immersion ultrasonic testing on the same component, and the imaging results are as follows: Figure 7 shown.

[0099] The error index E of this scheme and the existing method i Comparison of Figure 8As shown in the figure, 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 the interlayer resin of 64 layers, while this solution can effectively locate the interlayer resin of 82 layers, and the positioning depth is increased by 18 layers, about 2.25mm.

[0101] Error index E of existing methods i The fluctuation decreases before the 22nd floor, and increases rapidly after the 22nd floor due to the rapid attenuation characteristics of high-frequency signals, and E i The maximum occurs at the 64th layer, 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 is relatively stable. 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 to increase the effective imaging thickness of thick composite laminates while ensuring the positioning accuracy of the resin layer, thereby achieving 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 this method is as follows: Figure 1 As shown, the method includes the following steps:

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

[0106] S2. Band-pass filter each reflected signal to obtain a corresponding filtered reflected signal;

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

[0108] S4. Determine the interlayer resin position corresponding to each filtered reflection signal using the instantaneous phase at each front wall, and present 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.

[0109] In this embodiment, the process of applying 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. 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 of prepreg and to collect reflection signals from a thick composite laminate.

[0111] First, establish the ply order as [0 / 90] 24s A 2D finite element model of a composite laminate component, simulating an immersion ultrasonic testing environment. In this 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. 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, the resonant frequency of the single-layer prepreg is determined to be 9.9 MHz. Therefore, the resonant frequency of the half-single-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 actual engineering, 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] The center frequency of the model excitation signal is 5 MHz, and the reflection signal of the laminate model is calculated.

[0115] In this embodiment, in order to make the reflected signal smoother and have a higher resolution, a 5-fold interpolation process is performed on the collected signal.

[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, with its passband set to 25% of the center frequency of the excitation signal (4.375MHz-5.675MHz) and its filter order set to 2 to obtain a filtered reflection signal.

[0118] Step S3 is to perform Hilbert transform on the filtered signal, calculate the instantaneous amplitude and instantaneous phase of the filtered signal, locate the front wall and the back wall of the laminate, and determine 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 to calculate its instantaneous amplitude and instantaneous phase. The positions of the front and rear walls of the laminate component are determined based on 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 for the instantaneous amplitude is:

[0124]

[0125] Where A inst (t) is the instantaneous amplitude.

[0126] In this solution, 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] Where, 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 back 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 at an excitation frequency of 5 MHz. The dotted line indicates the position of the front wall, and the dashed line indicates the position of the back wall.

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

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

[0132]

[0133] Where, 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. 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 graph 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 of 25% center frequency bandwidth at an excitation frequency of 4.95 MHz is shown as follows: Figure 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: Figure 10 shown.

[0136] In this embodiment, the error index E is used i Quantitatively evaluate the imaging accuracy and compare the error of this solution with that of existing methods. The error of imaging accuracy E i The comparison results are shown in Figure 11 It can be seen that compared with the existing method, this solution 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 layering order is [0 / 90] 4s Pores were set in a two-dimensional finite element model of a laminated plate component. The specific method was to insert circles with radii of 5 μm and 2.5 μm, respectively, into the resin layer between the sixth and seventh fiber layers of the two-dimensional model, and assign the circles 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 of 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 error comparison between this solution and the existing method is shown in Figure 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, the TOF of the resins between the layers affected by the pores in the existing method is dif Compared with the existing methods, the instantaneous phase obtained by this scheme is less affected 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. It 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 description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and such modifications or substitutions are intended to be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement, characterized in that: The method uses half the resonant 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 method includes the following steps: S1. Determine the resonant frequency of a single-layer prepreg, perform water immersion ultrasonic testing on the composite laminate using an ultrasonic probe with a center frequency half the resonant frequency of the single-layer prepreg, and collect reflected signals at various positions along the scanning path; S2. Band-pass filter each reflected signal to obtain a corresponding filtered reflected signal; S3. performing Hilbert transform on the filtered reflection signal to locate the front wall and the rear wall of the composite laminate, and obtaining the instantaneous phase of the filtered reflection signal at the front wall; S4. Determine the interlayer resin position corresponding to each filtered reflection signal using the instantaneous phase at each front wall, and present 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.

2. The method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement according to claim 1, 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 for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement according to claim 1, 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, wherein the method comprises: The specific step of performing band-pass filtering on the reflected signal in S2 is: using a Butterworth filter to perform band-pass filtering on the collected reflected signal, and setting the filter bandwidth to 25% of the center frequency of the excitation signal.

5. The method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonance frequency enhancement according to claim 4, characterized in that: The Butterworth filter is a second-order Butterworth filter.

6. The method of claim 1, wherein the method comprises: 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: Where, is a real signal, that is, the signal actually measured. is a complex signal, Is an imaginary unit.

7. The method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement according to claim 5, 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 from 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: Where, is the instantaneous amplitude.

8. The method for ultrasonic imaging of out-of-plane fiber orientation of composite materials using half-layer resonant frequency enhancement according to claim 6, characterized in that: The specific process of obtaining the instantaneous phase at the front wall in S3 is as follows: first, the instantaneous phase is calculated from the complex signal, and then the instantaneous phase at the front wall of the composite 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: Where, is the instantaneous phase.

9. The method of claim 1, wherein the method comprises: The S4 uses the instantaneous phase at the front wall to determine the position of the interlayer resin. Specifically, the instantaneous phase at the front wall is used to locate the instantaneous phase at the interlayer resin. The specific formula is: Where, is the instantaneous phase at the front wall of the laminate, is the instantaneous phase at 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 resonant frequency of a single-layer prepreg, and an ultrasonic probe with a center frequency of half the resonant frequency of the single-layer prepreg is used to perform water immersion ultrasonic testing on the composite laminate, collecting the reflected signals at each position along the scanning path; The signal processing module performs bandpass filtering on the reflected signal using a Butterworth filter 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.

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