Full-focus imaging optimization method applied to multi-layer composite material

Abstract

The invention relates to the technical field of nondestructive testing, in particular to a full-focus imaging optimization method applied to a multi-layer composite material. According to the method, full-focus imaging of the multi-layer composite material is optimized through a systematic process, FMC data of a defect-free area are collected, elastic constant combinations of all layers are enumerated, a sound velocity curve is calculated, a full-focus image is generated based on equivalent sound velocity, and finally the optimal elastic constant is screened according to the average amplitude of the bottom surface. The large-angle sound velocity information is covered through the elastic constant combination, the problem that angle coverage is limited is solved, and higher imaging precision is achieved for a large-thickness composite material; parameters are automatically screened by using image amplitudes, so that low efficiency and errors caused by manual judgment of attenuation signal peak values are avoided, and meanwhile, the upper bound of the range of detectable propagation angles is improved; and the sound velocity is calculated by combining heterogeneous layering characteristics, so that the defect identification precision is greatly improved compared with that of a traditional homogeneous model.

Description

Technical Field

[0001] The present invention relates to the technical field of nondestructive testing, and in particular to a full-focus imaging optimization method applied to multi-ply composite materials. Background Art

[0002] When identifying defects in composite materials, the backwall echo method is usually used to calibrate the sound velocity first, and then the subsequent defect identification steps are based on the calibrated sound velocity. Although the backwall echo method is widely used, it also has the following disadvantages: 1. Limited angular coverage: Due to the physical length of the ultrasonic array, the propagation angle range of the effective signal can be collected is small. However, defect identification requires sound velocity curve information with a larger propagation angle.

[0003] 2. Peak extraction is difficult: For ultrasonic waves with larger propagation angles, the propagation distance is longer, resulting in significant attenuation of the bottom echo signal and weak peak value. Human intervention is often required for judgment, which is inefficient and unreliable.

[0004] 3. Poor model applicability: The bottom surface echo method is based on the assumption of homogeneous materials. The sound velocity curve obtained by bottom surface calibration is directly applied to all imaging areas, failing to consider the sound velocity changes of different plies (heterogeneous areas) of the composite material.

[0005] Due to the combined effects of the above shortcomings, the defect recognition accuracy of multi-ply composite materials is severely restricted, and an effective solution is urgently needed. Summary of the Invention

[0006] To avoid and overcome the technical problems existing in the prior art, the present invention provides a method for optimizing full-focus imaging of multi-ply composite materials. This method effectively improves the accuracy of defect identification in multi-ply composite materials by adaptively correcting the composite material's sound velocity curve at different depths.

[0007] To achieve the above object, the present invention provides the following technical solutions: A full-focus imaging optimization method for multi-ply composite materials includes the following optimization steps: S1. Obtain FMC data of a multi-ply composite material with no defect areas; S2. determining all possible values ​​of the elastic constants of each ply, and combining all possible values ​​of the elastic constants of different plies related to longitudinal waves with one another to obtain all possible combinations of relevant elastic constants of the multi-ply composite material; S3, calculating the original sound velocity curve of the ultrasonic wave under each set of elastic constant combinations; S4. Based on the ratio of distance to time, obtain an equivalent sound speed curve corresponding to each original sound speed curve; S5. For each set of elastic constant combinations, a corresponding fully focused image is generated according to the corresponding equivalent sound velocity curves and the FMC data in step S1; S6. Obtain the average amplitude of the bottom area of ​​each fully focused image, and select the elastic constant combination corresponding to the maximum average amplitude as the optimal elastic constant; S7 . Obtain FMC data of the multi-ply composite material with the defective area, and execute steps S3 - S5 using the FMC data and the optimal elastic constants to generate a fully focused image with the defective area.

[0008] As a further solution of the present invention: the calculation process of the original sound velocity curve is as follows: S31. Selecting one ply in the multi-ply composite material as a base ply, and establishing a three-dimensional reference coordinate system on the base ply, with the X-axis of the reference coordinate system parallel to the fiber direction of the ply; executing steps S32-S39 for each set of elastic constant combinations; S32. Under the current elastic constant combination, the physical relationship between the phase velocity of the ultrasonic wave in the foundation ply and the elastic constant is established by the Christoffel equation, and the following stiffness matrix composed of the various elastic constants is constructed; S33. Calculate the phase velocity of the ultrasonic wave in the foundation ply based on the stiffness matrix; S34. Calculate the group velocity of the ultrasonic wave in the base ply based on the phase velocity; S35, traversing the propagation angle range, calculating the group velocity corresponding to each propagation angle; and using the propagation angle as the horizontal coordinate and the group velocity as the vertical coordinate, smoothly connecting each group velocity in sequence to obtain the original sound velocity curve of the foundation ply; S36, constructing a coordinate rotation matrix of the rotating ply that rotates relative to the base ply; S37. Calculate the rotation stiffness matrix of the current rotating ply based on the coordinate rotation matrix; S38, substituting the rotational stiffness matrix of the current rotational ply into step S33, and executing steps S33-S35 to obtain the original sound velocity curve of the current rotational ply; S39. Execute step S38 for each ply in the multi-ply composite material to obtain an original sound velocity curve of each rotating ply in the multi-ply composite material.

[0009] As a further solution of the present invention: the stiffness matrix is ​​expressed as follows: (1); in, represents the stiffness matrix of the foundation ply; Indicates the tensile stiffness along the positive direction of the X axis; represents the coupling stiffness in the XY plane; represents the coupling stiffness in the XZ plane; Indicates the tensile stiffness along the positive direction of the Y axis; represents the coupling stiffness in the YZ plane; Indicates the tensile stiffness along the positive direction of the Z axis; represents the shear stiffness around the X-axis; represents the shear stiffness around the Y axis; represents the shear stiffness around the Z axis; under transversely isotropic conditions, , , , .

[0010] As a further solution of the present invention: Phase velocity The calculation formula is as follows: (2); (3); (4); (5); (6); Where, and Both represent transition parameters; Indicates the material density of the base ply; Indicates the angle between the propagation direction of the ultrasonic phase velocity in the base layer and the positive direction of the Z axis The sine value of Indicates the angle between the ultrasonic propagation direction in the base layer and the positive direction of the Z axis The cosine value of . and By correlating the phase velocity angle with the speed of sound and solving the square root expression using transition parameters, the phase velocity is ensured to conform to the physical meaning of the wave equation. The phase velocity angle (0°-90°) is directly incorporated into the phase velocity calculation, resolving the large angle extrapolation errors found in traditional methods. The derivation of the formula is based on the eigenvalue solution of the Christoffel equation, avoiding nonphysical solutions (such as imaginary velocity). This ensures that the phase velocity calculation is strictly consistent with the phase propagation characteristics of ultrasound in anisotropic materials, laying a precise foundation for group velocity calculations.

[0011] As a further solution of the present invention: group velocity The calculation formula is as follows: (7); (8); (9); ; Where, Represents the group velocity component of the ultrasonic wave along the positive direction of the X axis; represents the group velocity component of the ultrasonic wave along the positive direction of the Z axis, Represents the ultrasonic propagation angle. Decompose the group velocity into X-axis components and the Z-axis component The actual propagation velocity is synthesized by square root. The direction of the group velocity is consistent with the propagation direction of ultrasonic energy and matches the stress distribution between composite layers. It can more accurately describe the difference in the propagation of acoustic energy in the fiber direction and the perpendicular direction, providing a more realistic velocity parameter for sound velocity curve fitting.

[0012] As a further solution of the present invention: coordinate rotation matrix It is expressed as follows: (10); in, Indicates the rotation angle of the current rotating layer relative to the reference coordinate system around the Z axis. Indicates the rotation angle of the ply around the Z axis. 、 The equivalent term directly corresponds to the shear effect after coordinate system rotation. Coordinate transformations are supported for arbitrary ply angles from 0° to 180° (e.g., ±45°, 90°), covering common composite layup structures. The matrix format supports GPU parallel computing, reducing rotational stiffness matrix calculation time when processing multi-layer composite materials. The geometric transformation process is intuitive and explainable, making it easier for engineers to understand the impact of ply angle on sound velocity calculations and improving model debugging efficiency.

[0013] As a further solution of the present invention: the calculation formula of the rotational stiffness matrix is ​​as follows: (11); Where, represents the rotational stiffness matrix, express The transposed matrix of . Using The stiffness matrix of the base ply is converted to the stiffness matrix of the rotated ply, where the transposed matrix Energy conservation is ensured before and after the transformation (the matrix determinant remains unchanged). Anisotropic properties are mapped angularly through linear algebraic operations, ensuring that the stiffness matrix of the rotated ply strictly conforms to the fiber orientation changes. Physical parameter distortion caused by coordinate transformation (such as the stiffness matrix non-positive definiteness) is avoided, ensuring the physical rationality of subsequent sound velocity calculations. The matrix transformation process can be automated through programming, adapting to the batch calculation requirements of complex multi-ply structures.

[0014] As a further solution of the present invention: the process of obtaining the equivalent sound speed curve is as follows: S41. In the reference coordinate system, calculate the current propagation angle of the ultrasonic wave from the ultrasonic probe array element point Spread to the focal point The propagation time : (12); Where, Indicates the number of plies where the focal point is located in a multi-ply composite material; Indicates that ultrasound Propagation distance in a layer stack, Indicates that ultrasound Group velocity in a ply layup; S42. Calculate the equivalent speed based on the ratio of distance to time based on the propagation time; (13); Where, Indicates focus The equivalent speed of ultrasonic wave propagation in the ply; Indicates focus Ultrasonic probe array element point The Euclidean distance between S43. Traverse the propagation angle range, calculate the equivalent velocity corresponding to each layer at each propagation angle, and use the propagation angle as the horizontal coordinate and the equivalent velocity as the vertical coordinate to smoothly connect each equivalent velocity in sequence to obtain the equivalent sound velocity curve of each layer.

[0015] The equivalent velocity is defined as the ratio of Euclidean distance to total propagation time, simplifying multi-ply polyline propagation to a straight-line distance calculation. The equivalent sound velocity curve is smoothly fitted using the "angle-velocity" equation, allowing direct insertion into the fully focused imaging formula, eliminating the time-consuming point-by-point sound velocity correction required by traditional methods. By accumulating the propagation time of each ply and comprehensively considering the impact of material heterogeneity on sound velocity, the equivalent sound velocity is closer to the average propagation velocity of ultrasound along the actual path, improving imaging accuracy.

[0016] As a further solution of the present invention, the process of generating the all-focus image in step S5 is as follows: S51, select one array element from the ultrasonic transmitting probe as the transmitting array element, and the rest of the array elements are used as receiving array elements; calculate the ultrasonic wave emitted from the transmitting array element and propagated to the focal point The total propagation time after it is reflected into the ultrasonic transmitting probe and received by each receiving array element; (14); Where, Indicates that the ultrasound is The array elements are emitted and spread to the focal point After that, it is reflected into the ultrasonic transmitting probe and The total propagation time when each receiving element receives; represents the coordinates of the th array element as the transmitting array element in the XZ plane of the reference coordinate system; Indicates the first receiving element The coordinates of each array element in the XZ plane of the reference coordinate system; S52: For each focal point, traverse all transmit-receive array element pairs , get each FMC data ;From FMC data Extract The signal amplitude in the , and accumulate to get the focus point The composite signal amplitude at ; (15); S53: Obtain the synthetic signal amplitudes at all focus points, and combine them to form a fully focused image.

[0017] The total propagation time of all transmitting-receiving array element pairs is calculated, and the signal amplitudes of the corresponding FMC data are accumulated. The coherence of ultrasonic waves is used to enhance the defect reflection signals at the imaging point while suppressing random noise in non-defect areas. The spatial relationship between the focal point and all array elements is considered to form a two-dimensional focusing effect, which significantly improves the recognition rate of internal defects in composite materials. The full-matrix acquisition and accumulation algorithm is compatible with existing ultrasonic array hardware, and imaging quality can be upgraded without additional hardware costs.

[0018] As a further solution of the present invention: the process of obtaining the optimal elastic constant is as follows: S61, for each fully focused image, select an area of ​​the bottom surface with a thickness H upward as the bottom surface area; S62: Calculate the average synthetic signal amplitude within the bottom area of ​​each fully focused image. ; ; Where, and are the X-axis coordinates of the leftmost element and the rightmost element of the phased array probe respectively; K At depth H, and The number of pixels contained in between.

[0019] S63, obtaining the maximum average synthetic signal amplitude The full-focus image is obtained, and the elastic constant combination of the full-focus image is taken as the optimal elastic constant.

[0020] An area of ​​a set thickness upward from the bottom surface is selected to calculate the average composite signal amplitude, and the elastic constant combination corresponding to the maximum value is taken as the optimal solution. The bottom surface echo amplitude is strongly correlated with the accuracy of the sound velocity calibration, and using the amplitude as an indicator can directly reflect the degree of material parameter matching. Using the bottom surface amplitude of the fully focused image as an evaluation indicator solves the instability of extracting signal peak points in traditional methods, while also expanding the range of detectable propagation angles to include more material acoustic properties. Setting the ROI area avoids interference from surface defects or boundary effects, improving screening reliability. No material parameters need to be preset, and data-driven automatic optimization is used to adapt to multi-ply composite material inspection scenarios. The screening process can be automated through programming, reducing manual intervention costs and improving inspection efficiency.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention optimizes full-focus imaging of multi-ply composite materials through a systematic process: first, FMC data is collected from defect-free areas, the elastic constant combinations of each ply are enumerated and the sound velocity curve is calculated. A full-focus image is then generated based on the equivalent sound velocity, and finally, the optimal elastic constant is selected using the bottom surface average amplitude. Its core advantage lies in overcoming the limitations of traditional bottom surface echo methods. It not only covers large-angle sound velocity information through elastic constant combinations, addressing the issue of limited angular coverage, but also automatically selects parameters using image amplitude, avoiding the inefficiency and errors of manual determination of attenuated signal peaks. Furthermore, the sound velocity calculation, combined with the characteristics of heterogeneous plies, significantly improves defect identification accuracy compared to traditional homogeneous models. The highly automated process is also well-suited for composite material inspection scenarios.

[0022] 2. The anisotropic properties of each composite material layer are incorporated into the sound velocity calculation. Through the recursive relationship between phase velocity and group velocity, the sound velocity curve is made consistent with the actual propagation law of ultrasonic waves. The layered modeling strategy solves the influence of the rotation angle of multiple layers on the sound velocity, providing sound velocity parameters that match the material structure for subsequent imaging, effectively reducing the proportion of imaging blind spots.

[0023] 3. Decoupling complex material parameters into standardized matrix elements facilitates experimental measurement and simulation input. The independent elastic constants are simplified through the transverse isotropy assumption, reducing computational complexity and adapting to rapid engineering modeling. The matrix form is compatible with finite element software (such as ABAQUS) and can be directly connected to existing simulation platforms to improve model implementation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Flowchart of the present invention.

[0025] Figure 2 Schematic diagram of the reference coordinate system in the present invention.

[0026] Figure 3 Schematic diagram of the ply direction in the present invention.

[0027] Figure 4 for Figure 3 Equivalent sound velocity curves corresponding to plies with different rotation angles.

[0028] Figure 5 This is a fully focused image containing defects in the present invention. DETAILED DESCRIPTION

[0029] 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 only 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 making creative efforts are within the scope of protection of the present invention.

[0030] See also Figures 1 to 4 In the embodiment of the present invention, the ultrasonic measurement equipment for detection includes: a 64-element ultrasonic probe with an element center distance of 0.60mm and a nominal center frequency of 5MHz, a full-matrix acquisition phased array controller, a computer with a phased array detection platform operating environment, and a composite material acoustic test block with built-in defects. The reference coordinate system established is as follows: Figure 2 As shown. The density value of carbon fiber multi-ply board 1566m 3 / kg, thickness is 10mm, The layers are laid in a way of 120 layers in total. Figure 3 As shown, Figure 3 The arrows indicate the depth direction. A 6mm diameter delamination defect was simulated in the plate using a polytetrafluoroethylene film, buried 8.5mm from the surface. Ultrasonic phased array testing was used to test composite material delamination defects to verify the reliability of the present invention.

[0031] 1. Obtain FMC data for defect-free areas When performing FMC (Full Matrix Capture) data acquisition on a defect-free area, the primary focus is on the data acquisition process and data integrity, rather than defect identification or characterization. FMC data acquisition is a technique used in ultrasonic imaging that does not require prior knowledge (such as shape or sound velocity) of the part being inspected (e.g., metal parts or composite materials). The following are detailed steps and precautions for performing FMC data acquisition on a defect-free area: 1. Preparation Equipment preparation: Ensure that the phased array ultrasound instrument and related software are operating normally and the probe is connected correctly.

[0032] Calibration: Perform necessary calibration on the instrument to ensure the accuracy of data collection.

[0033] Set parameters: According to the material characteristics of the inspected component and the inspection requirements, set appropriate inspection parameters, such as frequency, gain, and focus depth.

[0034] 2. FMC data collection steps 2.1. Excitation and reception Single element excitation: First, select an element in the array as the emission source to excite ultrasonic waves.

[0035] Full-element reception: When the transmitter excites ultrasonic waves, all other elements in the array act as receivers to receive ultrasonic echo signals from the inspected area.

[0036] Repeating process: Repeat the above process, each time selecting a new array element as the transmitting source, until all array elements have been excited in turn. In this way, each array element will act as a transmitting source to send ultrasonic waves, and all other array elements will receive the echo signals of these ultrasonic waves.

[0037] 2.2 Data Recording Recording A-waves: During each transmission and reception process, the A-wave (i.e., time-amplitude waveform) data received by each array element is recorded. This data contains information about the propagation and reflection of the ultrasonic wave in the test area.

[0038] Data storage: The data transmitted and received by all array elements are stored in the instrument or computer for subsequent TFM (Total Focusing Method) image reconstruction.

[0039] 3. Notes Ensure integrity: During the data acquisition process, ensure that each array element is excited in turn and all received data are recorded completely.

[0040] Avoid interference: During the data collection process, external interference (such as electromagnetic interference, mechanical vibration, etc.) should be avoided as much as possible to ensure the accuracy of the data.

[0041] Quality Control: Regularly maintain and inspect the instrument to ensure its stability and reliability during data acquisition.

[0042] 4. Subsequent processing TFM image reconstruction: After FMC data acquisition is complete, the acquired data can be processed using the TFM algorithm to generate a high-resolution ultrasound image. The TFM algorithm superimposes all received ultrasound signals to enhance the signal strength of the target imaging area, thereby improving image clarity and resolution.

[0043] In summary, the primary focus of FMC data acquisition in a defect-free area is data integrity and accuracy. Proper setup and rigorous operational procedures ensure that the acquired data meets the requirements for subsequent TFM image reconstruction.

[0044] 2. Determine the elastic constant combination From formulas (3) to (4), we can see that there are four elastic constants related to quasi-longitudinal waves in anisotropic materials, namely 、 、 、 ;in, The value of can be determined in advance based on the pulse echo mode to simplify the entire optimization process. When propagating at 0°, according to formulas (1) to (4), , , the phase velocity of the quasi-longitudinal wave and group velocity Degenerates into The group velocity can be obtained by the pulse echo signal, and the flight time of the self-transmitting and self-receiving array element to the bottom surface can be obtained. , combined with the thickness of the carbon fiber composite material H , calculated The expression is: (16); According to formula (16), The final value is 12.6GPa.

[0045] For the remaining parameters related to longitudinal wave imaging , this embodiment will The range is set to , the sampling interval is 5 GPa, with a total of 21 sampling points. The range is , the sampling interval is 2GPa, with a total of 6 sampling points. The range is , the sampling interval is 2 GPa, with a total of 10 sampling points.

[0046] 3. Calculate the original sound speed curve Substitute these elastic constants into formulas (1) to (5) in turn to calculate Sound velocity curve under the base ply at 0°.

[0047] Combined with formulas (6) to (7), the stiffness matrix of the plies at 90° and ±45° is calculated. C , and calculate the original sound velocity curves under the 90° and ±45° plies.

[0048] 4. Calculate the equivalent sound speed curve According to formula (8), the equivalent sound velocity curve is calculated. The calculation results are as follows: Figure 4 shown. Figure 4 The equivalent sound velocity curve calculated according to formula (8) is shown. Figure 4 The "bottom" in the figure indicates that the sound velocity measurement is based on the reflected signal from the material's bottom surface. The overall curve exhibits a certain fluctuation trend. Analyzing its changing characteristics can provide a basis for subsequent defect identification or material performance evaluation.

[0049] 5. Fully Focused Image According to formula (16), full-focus imaging is performed. The selected imaging area is the cross section below the array, with a size of 37.8 mm × 12 mm. The length of the grid points in the X-axis direction is 0.1 times the spacing between adjacent array elements, and the width in the Z-axis direction is 0.5 times the thickness of a single layer. The number of grids in the imaging range is 631 × 313.

[0050] 6. Obtaining the Optimal Elastic Constants Since the thickness of the material is known to be 10 mm, the average of the echo amplitude at 10 mm from the bottom of the full-focus imaging is used as the evaluation index. According to step 2, there are 21×2×10=420 output points. The sampling point corresponding to the maximum evaluation index is found. The results show that the elastic constant vector related to the longitudinal wave , which is the optimal elastic constant.

[0051] 7. Defect Identification Store the above optimal elastic constants The corresponding equivalent sound velocity curve is used to perform full-focus imaging on a multi-ply composite material plate with defects to identify the defects. The imaging results are shown in Figure 2. Figure 5 shown. Figure 5This image shows the results of fully focused imaging based on an optimized elastic constant equivalent velocity curve, used to identify defects in multi-ply composite panels. The horizontal axis (-15 to 15) represents spatial position, the left vertical axis (0 to 10) reflects the depth of the composite material, and the right vertical axis (0 to 1) represents signal amplitude or energy intensity. Figure 5 The red and yellow areas in the middle correspond to defect locations, primarily concentrated near zero on the horizontal axis, coinciding with the curve's peak, indicating significant energy reflection or anomalies. Total Focus Imaging technology achieves high-precision defect detection by synthesizing the acoustic wave path and optimizing the equivalent velocity of sound curve. The red and yellow gradient area intuitively illustrates the distribution and severity of defects.

[0052] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A full focus imaging optimization method applied to multi-ply composite materials, characterized in that: The optimization steps include: S1. Obtain FMC data of a multi-ply composite material with no defect areas; S2. determining all possible values ​​of the elastic constants of each ply, and combining all possible values ​​of the elastic constants of different plies related to longitudinal waves with one another to obtain all possible combinations of relevant elastic constants of the multi-ply composite material; S3, calculating the original sound velocity curve of the ultrasonic wave under each set of elastic constant combinations; S4. Based on the ratio of distance to time, obtain an equivalent sound speed curve corresponding to each original sound speed curve; S5. For each set of elastic constant combinations, a corresponding fully focused image is generated according to the corresponding equivalent sound velocity curves and the FMC data in step S1; S6. Obtain the average amplitude of the bottom area of ​​each fully focused image, and select the elastic constant combination corresponding to the maximum average amplitude as the optimal elastic constant; S7 . Obtain FMC data of the multi-ply composite material with the defective area, and execute steps S3 - S5 using the FMC data and the optimal elastic constants to generate a fully focused image with the defective area.

2. The total focus imaging optimization method for multi-ply composite materials according to claim 1, characterized in that: The calculation process of the original sound speed curve is as follows: S31. Selecting one ply in the multi-ply composite material as a base ply, and establishing a three-dimensional reference coordinate system on the base ply, with the X-axis of the reference coordinate system parallel to the fiber direction of the ply; executing steps S32-S39 for each set of elastic constant combinations; S32. Under the current elastic constant combination, the physical relationship between the phase velocity of the ultrasonic wave and the elastic constant in the foundation ply is established through the Christoffel equation, and a stiffness matrix composed of the various elastic constants is constructed; S33. Calculate the phase velocity of the ultrasonic wave in the foundation ply based on the stiffness matrix; S34. Calculate the group velocity of the ultrasonic wave in the base ply based on the phase velocity; S35, traversing the propagation angle range, calculating the group velocity corresponding to each propagation angle; and using the propagation angle as the horizontal coordinate and the group velocity as the vertical coordinate, smoothly connecting each group velocity in sequence to obtain the original sound velocity curve of the foundation ply; S36, constructing a coordinate rotation matrix of the rotating ply that rotates relative to the base ply; S37. Calculate the rotation stiffness matrix of the current rotating ply based on the coordinate rotation matrix; S38, substituting the rotational stiffness matrix of the current rotational ply into step S33, and executing steps S33-S35 to obtain the original sound velocity curve of the current rotational ply; S39. Execute step S38 for each ply in the multi-ply composite material to obtain an original sound velocity curve of each rotating ply in the multi-ply composite material.

3. The total focus imaging optimization method for multi-ply composite materials according to claim 2, characterized in that: The stiffness matrix is ​​expressed as follows: ； in, represents the stiffness matrix of the foundation ply; Indicates the tensile stiffness along the positive direction of the X axis; represents the coupling stiffness in the XY plane; represents the coupling stiffness in the XZ plane; Indicates the tensile stiffness along the positive direction of the Y axis; represents the coupling stiffness in the YZ plane; Indicates the tensile stiffness along the positive direction of the Z axis; represents the shear stiffness around the X-axis; represents the shear stiffness around the Y axis; represents the shear stiffness around the Z axis; under transversely isotropic conditions, , , , .

4. The total focus imaging optimization method for multi-ply composite materials according to claim 3, characterized in that: Phase velocity The calculation formula is as follows: ； ； ； ； ； Where, and Both represent transition parameters; Indicates the material density of the base ply; Indicates the angle between the propagation direction of the ultrasonic phase velocity in the base layer and the positive direction of the Z axis The sine value of Indicates the angle between the ultrasonic propagation direction in the base layer and the positive direction of the Z axis The cosine of .

5. The total focus imaging optimization method for multi-ply composite materials according to claim 4, characterized in that: Group velocity The calculation formula is as follows: ； ； ； ； Where, Represents the group velocity component of the ultrasonic wave along the positive direction of the X axis; represents the group velocity component of the ultrasonic wave along the positive direction of the Z axis, Indicates the ultrasonic wave propagation angle.

6. The total focus imaging optimization method for multi-ply composite materials according to claim 5, characterized in that: Coordinate rotation matrix It is expressed as follows: ； in, Indicates the rotation angle of the current rotated ply relative to the reference coordinate system around the Z axis.

7. The total focus imaging optimization method for multi-ply composite materials according to claim 6, characterized in that: The calculation formula of the rotational stiffness matrix is ​​as follows: ； Where, represents the rotational stiffness matrix, express The transposed matrix of .

8. The total focus imaging optimization method for multi-ply composite materials according to claim 7, characterized in that: The process of obtaining the equivalent sound speed curve is as follows: S41. In the reference coordinate system, calculate the current propagation angle of the ultrasonic wave from the ultrasonic probe array element point Spread to the focal point The propagation time : ； Where, Indicates the number of plies where the focal point is located in a multi-ply composite material; Indicates that ultrasound Propagation distance in a layer stack, Indicates that ultrasound Group velocity in a ply layup; S42. Calculate the equivalent speed based on the ratio of distance to time based on the propagation time; ； Where, Indicates focus The equivalent speed of ultrasonic wave propagation in the ply; Indicates focus Ultrasonic probe array element point The Euclidean distance between S43. Traverse the propagation angle range, calculate the equivalent velocity corresponding to each layer at each propagation angle, and use the propagation angle as the horizontal coordinate and the equivalent velocity as the vertical coordinate to smoothly connect each equivalent velocity in sequence to obtain the equivalent sound velocity curve of each layer.

9. The total focus imaging optimization method for multi-ply composite materials according to claim 8, characterized in that: The process of generating the all-focus image in step S5 is as follows: S51, select one array element from the ultrasonic transmitting probe as the transmitting array element, and the rest of the array elements are used as receiving array elements; calculate the ultrasonic wave emitted from the transmitting array element and propagated to the focal point The total propagation time after it is reflected into the ultrasonic transmitting probe and received by each receiving array element; ； Where, Indicates that the ultrasound is The array elements are emitted and spread to the focal point After that, it is reflected into the ultrasonic transmitting probe and The total propagation time when each receiving element receives; Indicates the first transmitting element The coordinates of each array element in the XZ plane of the reference coordinate system; Indicates the first receiving element The coordinates of each array element in the XZ plane of the reference coordinate system; S52: For each focal point, traverse all transmit-receive array element pairs , get each FMC data ; From FMC data Extract The signal amplitude in the , and accumulate to get the focus point The composite signal amplitude at ; ； S53: Obtain the synthetic signal amplitudes at all focus points, and combine them to form a fully focused image.

10. The total focus imaging optimization method for multi-ply composite materials according to claim 9, characterized in that: The process of obtaining the optimal elastic constants is as follows: S61, for each fully focused image, select an area of ​​the bottom surface with a thickness H upward as the bottom surface area; S62: Calculate the average synthetic signal amplitude within the bottom area of ​​each fully focused image. : ； Where, and are the X-axis coordinates of the leftmost element and the rightmost element of the phased array probe respectively; K At depth H, and The number of pixels contained in between; S63, obtaining the maximum average synthetic signal amplitude The full-focus image is obtained, and the elastic constant combination of the full-focus image is taken as the optimal elastic constant.

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