A method for identifying internal cracks in cylindrical shell components based on modal vibration shapes

Abstract

A method for identifying internal cracks in cylindrical shell components based on modal vibration modes belongs to the technical field of structural nondestructive testing, and is characterized by: obtaining the radial displacement modal vibration modes of the cylindrical shell components; performing fractal transformation on each circumferential line of a certain order of radial displacement modal vibration modes; identifying and locating cracks on the fractal shell surface of the radial displacement modal vibration modes after fractal transformation, thereby realizing damage detection of cylindrical shell components. By improving the existing damage identification method for cylindrical shell components based on modal vibration modes, damage features are directly extracted from the modal vibration modes in the service state, solving the bottleneck problem of the need for healthy vibration mode comparison in the existing vibration nondestructive testing technology; this identification method can pre-arrange sensors on the object to be tested and directly transmit the sensor data wirelessly; at the same time, the entire structure can be directly detected at one time without the need for regional detection, which can significantly improve the detection efficiency and is suitable for popularization and application.

Description

Technical Field

[0001] The invention belongs to the technical field of structural nondestructive testing, and in particular relates to a method for identifying internal cracks of a cylindrical shell component based on modal vibration shapes. Background Art

[0002] Cylindrical shell components are key basic components in aviation power equipment and new energy equipment, such as aircraft engine and gas turbine brakes, hydrogen storage tanks, wind turbine towers, etc. Under long-term service, common cracks such as chemical corrosion, material aging, stress corrosion cracking, fatigue, and temperature difference cracking will cause major hidden dangers in the design bearing capacity of cylindrical shell components. Regular structural health monitoring and maintenance are essential to ensure the safety and reliability of cylindrical shell components.

[0003] Traditional non-destructive testing methods such as acoustic emission, magnetic field, eddy current, thermal imaging and X-ray are local methods that can only detect a limited area at a time. Multiple measurements must be made to inspect the structure or a specific area. At the same time, traditional non-destructive testing methods require close proximity to the structure to be tested and continuous testing to detect in-service structures, which has low detection efficiency.

[0004] The vibration method nondestructive testing technology can globally identify structural damage and is sensitive to early damage, so it can detect potential problems in cylindrical shell components early and help prevent crack damage from expanding. Among them, the modal vibration shape contains local information and can accurately locate and quantify damage, so the modal vibration shape is the most commonly used modal parameter in the vibration method.

[0005] The bottleneck of the current damage identification method for cylindrical shell components based on modal vibration shapes is that it is difficult to observe the damage characteristics directly from the modal vibration shape surface. The usual practice is to perform difference comparison between the undamaged modal vibration shape and the damaged modal vibration shape. However, for cylindrical shell components that have been in service for a long time, it is extremely difficult to obtain undamaged modal vibration shapes with the same boundary conditions, aging degree and many other parameters. These obstacles seriously hinder the widespread and effectiveness of the application of vibration nondestructive testing technology. Summary of the invention

[0006] The present invention aims to solve the above problems and provides a method for identifying internal cracks of cylindrical shell components based on modal vibration shapes.

[0007] In a first aspect, the present invention provides a method for identifying internal cracks in a cylindrical shell component based on modal vibration shapes, comprising the following steps:

[0008] 1) Obtaining radial displacement modal vibration shapes of cylindrical shell components; the radial displacement modal vibration shapes include multi-order radial displacement modal vibration shapes;

[0009] 2) Perform fractal transformation on each circumferential line of a certain order radial displacement mode shape;

[0010] 3) According to the fractal shell surface of the radial displacement mode shape after fractal transformation, cracks are identified and located to realize damage detection of cylindrical shell components.

[0011] Furthermore, in the method for identifying internal cracks in a cylindrical shell component based on modal vibration modes described in the present invention, the fractal transformation of each circumferential line of a certain order radial displacement modal vibration mode described in step 2) includes: defining a fractal dimension value that describes the degree of irregularity of the circumferential line; for a certain circumferential line, specifying a fan-shaped sliding window size and a moving step length, the fan-shaped sliding window starts from a measuring point on the circumferential line and returns to the measuring point, and during the movement, the fractal dimension values ​​of the circumferential lines of the radial displacement modal vibration modes in the fan-shaped sliding window area are calculated respectively.

[0012] The size of the fan-shaped sliding window determines the feasibility of crack detection. A window that is too large will result in a small effect of the crack on the fractal dimension. At the same time, a window that is too small will result in the crack being divided into different sliding windows, which will affect the incompleteness of crack size detection. The window size is generally selected when the fractal dimension of the circumferential line in the fan-shaped window reaches the maximum value.

[0013] The step size of the fan-shaped sliding window determines the smoothness of the shell surface of the fractal dimension of the radial displacement modal vibration mode. The step size should be as small as possible, generally as long as it can meet the real-time requirements of crack detection.

[0014] Furthermore, in the method for identifying internal cracks in cylindrical shell components based on modal vibration modes of the present invention, the step 3) of identifying and locating cracks on the fractal shell surface according to the radial displacement modal vibration modes after fractal transformation comprises:

[0015] The spatial position of the fractal dimension value calculated according to the fan-shaped sliding window area is represented by the coordinate value of the middle measuring point in the fan-shaped area;

[0016] Traverse and scan each circumferential line in the radial displacement modal vibration mode, and determine the fractal shell surface corresponding to the radial displacement modal vibration mode according to the fractal dimension value and position of each fan-shaped sliding window;

[0017] If there are local cracks in the cylindrical shell component, the fractal shell surface after fractal transformation forms a local singular peak at the end of the local crack. The crack is located and quantified according to the singular peak, so as to realize the internal crack detection of the cylindrical shell component.

[0018] Furthermore, in the method for identifying internal cracks in cylindrical shell components based on modal vibration shapes of the present invention, the method for obtaining the fractal dimension comprises the following steps:

[0019] The cylindrical shell component adopts two sets of coordinate systems: cylindrical coordinate system and rectangular coordinate system. The spatial position of each measuring point in the cylindrical coordinate system is expressed by coordinates ( θ,ρ,z) is represented by the coordinates ( x , y , z )express;

[0020] The circumferential line of the radial displacement mode shape in a fan-shaped sliding window area is composed of a number of preset measuring points. The circumferential line measuring points in the fan-shaped sliding window area are interpolated by using the point-to-point dichotomy method through the interpolation function;

[0021] The fractal dimension uses the radial displacement change rate of each measuring point as a description index, and the change rate is expressed as follows:

[0022] τ =| r - R | / R

[0023] in τ i is the radial displacement change rate of the measuring point, R is the radius of the cylindrical shell component, r is the radial displacement of the measuring point, , ( x 0 ,y 0 ,z 0 ) is the coordinate of the center of each circumferential line;

[0024] The sum of the radial displacement change rates of the fan-shaped sliding window area under different binary division times is:

[0025]

[0026] in θ =2(m-1)π / (n(k-1)) represents the central angle scale of the fan-shaped sliding window area, n is the total number of measuring points on a complete circle of the circumferential line, k It is the sum of the number of measuring points on the circumferential line and the number of bisection points in the fan-shaped sliding window area. m is the number of initial measurement points contained in the fan-shaped sliding window area;

[0027] The fractal dimension of the circumferential line in the fan-shaped sliding window area is calculated by log(T θ ) and log( θ ) is obtained by taking the slope of the least squares linear fit line between:

[0028] T θ~ θ -D

[0029] in DIt is the fractal dimension value of the circumferential line in the typical fan-shaped sliding window area.

[0030] Furthermore, the method for identifying internal cracks in cylindrical shell components based on modal vibration modes of the present invention further includes performing multi-resolution filtering on the axial line of the radial displacement modal vibration mode when traversing and scanning each circumferential line in the radial displacement modal vibration mode; the advantage of multi-resolution filtering is that it retains the high-frequency fluctuations caused by the cracks, while also weakening the fractal dimension peak caused by the trend term in the high-order radial displacement vibration mode. The multi-resolution filtering includes the following steps:

[0031] a) The same θ The z value of the measuring point is used as the horizontal coordinate, and the radial displacement r The value is used as the vertical coordinate to form a one-dimensional signal;

[0032] b) performing multi-layer wavelet decomposition of the one-dimensional signal using the Symlet wavelet function to obtain a number of sub-band signals, some of which represent the low-frequency trend of the signal, setting the coefficients of the sub-band signals with low-frequency trends to 0, and then reconstructing the signal by inverse discrete wavelet transform;

[0033] c) Arrange the reconstructed measuring points according to their original spatial positions to obtain the de-trended radial displacement mode shape.

[0034] Furthermore, in the method for identifying internal cracks in cylindrical shell components based on modal vibration shapes of the present invention, step 1) of obtaining the radial displacement modal vibration shapes of the cylindrical shell components comprises the following steps:

[0035] Firstly, the measuring points are arranged, and the circumferential lines are evenly arranged along the axial direction on the outer surface of the cylindrical shell component, and the measuring points are evenly arranged on each circumferential line;

[0036] Use a mobile force hammer to traverse all measuring points and fix an accelerometer at any measuring point for measurement;

[0037] A pulse excitation is applied to the measuring points on the surface of the cylindrical shell component using a force hammer. At the same time, an accelerometer is used to collect the radial hammer vibration response of the fixed measuring points of the cylindrical shell component. The frequency response function matrix is ​​obtained according to the Fourier transform of the excitation and corresponding response signals. The radial displacement mode shape of the cylindrical shell component is obtained using the imaginary peak value and spatial position information of the frequency response function of each measuring point.

[0038] Furthermore, in the method for identifying internal cracks in a cylindrical shell component based on modal vibration modes described in the present invention, the hammer head of the force hammer adopts a hard plastic hammer head so as to obtain the vibration response of the cylindrical shell component in a wider frequency range; and the hammering direction of the force hammer is along the radial direction of the cylindrical shell component.

[0039] In a second aspect, the present invention provides a cylindrical shell component internal crack identification system based on modal vibration shape, including a vibration shape acquisition module, a fractal transformation module and a crack identification module;

[0040] The vibration mode acquisition module is used to acquire the radial displacement mode vibration mode of the cylindrical shell component; the radial displacement mode vibration mode includes multi-order radial displacement mode vibration modes;

[0041] The fractal transformation module is used to perform fractal transformation on each circumferential line of a certain order radial displacement modal vibration shape;

[0042] The crack identification module is used to identify and locate cracks according to the fractal shell surface of the radial displacement mode vibration type after fractal transformation, so as to realize damage detection of the cylindrical shell component.

[0043] In a third aspect, the present invention provides a device for identifying internal cracks in cylindrical shell components based on modal vibration shapes, comprising a memory and a processor; the memory is used to store a computer program; the processor is used to implement the method for identifying internal cracks in cylindrical shell components based on modal vibration shapes as described in the first aspect above when executing the computer program.

[0044] In a fourth aspect, the present invention provides a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for identifying internal cracks in a cylindrical shell component based on modal vibration shapes as described in the first aspect above is implemented.

[0045] The vibration mode-based internal crack identification method for cylindrical shell components described in the present invention improves the existing damage identification method for cylindrical shell components based on modal vibration modes, directly extracts damage characteristics of the modal vibration modes under service status, and solves the bottleneck problem of the need for healthy vibration mode comparison in the existing vibration non-destructive testing technology; the identification method can pre-arrange sensors on the object to be tested and directly transmit the sensor data wirelessly; at the same time, the entire structure can be directly detected at one time without the need for regional detection, which can significantly improve the detection efficiency and is suitable for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a schematic diagram of the arrangement of measuring points according to an embodiment of the present invention;

[0047] Figure 2 A schematic diagram of the radius change rate of a measuring point on the inner circumference of a fan-shaped sliding window according to an embodiment of the present invention;

[0048] Figure 3 A model diagram of a cylindrical shell component with internal cracks in abaqus according to an embodiment of the present invention;

[0049] Figure 4 This is a schematic diagram of a cylindrical shell with cracks according to an embodiment of the present invention;

[0050] Figure 5is the 40th order radial displacement mode vibration shape diagram of the cylindrical shell component according to the embodiment of the present invention;

[0051] Figure 6 The 40th-order radial displacement mode vibration shape diagram of the cylindrical shell component after multi-resolution filtering according to the embodiment of the present invention;

[0052] Figure 7 It is a schematic diagram of the fractal dimension shell surface corresponding to the 40th-order radial displacement mode shape of the cylindrical shell component described in an embodiment of the present invention. DETAILED DESCRIPTION

[0053] The vibration mode-based internal crack identification method for cylindrical shell components of the present invention is described in detail below through the accompanying drawings and embodiments.

[0054] Embodiment 1

[0055] This embodiment discloses a method for identifying internal cracks in a cylindrical shell component based on modal vibration shapes, comprising the following steps:

[0056] 1) Obtaining radial displacement modal vibration shapes of cylindrical shell components; the radial displacement modal vibration shapes include multi-order radial displacement modal vibration shapes;

[0057] 2) Perform fractal transformation on each circumferential line of a certain order radial displacement mode shape;

[0058] 3) According to the fractal shell surface of the radial displacement mode shape after fractal transformation, cracks are identified and located to realize damage detection of cylindrical shell components.

[0059] In this embodiment, the step 1) of obtaining the radial displacement modal vibration type of the cylindrical shell component includes the following steps: firstly, arranging the measuring points, evenly arranging the circumferential lines along the axial direction on the outer surface of the cylindrical shell component, and evenly arranging the measuring points on each circumferential line; Figure 1 As shown, the dots are the arranged measuring points;

[0060] A mobile force hammer is used to traverse all measuring points and an accelerometer is fixed at any measuring point for measurement; the hammer head of the force hammer is a hard plastic hammer head, so as to obtain the vibration response of the cylindrical shell component in a wider frequency range; the hammering direction of the force hammer is along the radial direction of the cylindrical shell component. In specific applications, the position of the accelerometer can be changed as needed, and then the force hammer is used to traverse all measuring points to ensure that the mode of the cylindrical shell component is not missed.

[0061] Secondly, a force hammer is used to apply pulse excitation to the measuring points on the surface of the cylindrical shell component. At the same time, an accelerometer is used to collect the radial hammer vibration response of the fixed measuring points of the cylindrical shell component. The frequency response function matrix is ​​obtained according to the Fourier transform of the excitation and corresponding response signals. The radial displacement modal vibration shape of the cylindrical shell component is obtained using the imaginary peak value and spatial position information of the frequency response function of each measuring point.

[0062] In the embodiment of the present disclosure, step 2) of performing fractal transformation on each circumferential line of a certain order radial displacement modal vibration type includes: first defining a fractal dimension value describing the degree of irregularity of the circumferential line; for a certain circumferential line, specifying a fan-shaped sliding window size and a moving step length, the fan-shaped sliding window starts from a certain measuring point on the circumferential line and returns to the measuring point, and calculating the fractal dimension values ​​of the circumferential lines of the radial displacement modal vibration types in the fan-shaped sliding window area during the movement.

[0063] The window size is generally selected to be the size of the window size corresponding to when the fractal dimension of the circumferential line in the fan-shaped window reaches the maximum value. The step length of the fan-shaped sliding window is generally sufficient as long as it can meet the real-time requirements of crack detection.

[0064] In the embodiment of the present disclosure, the method for obtaining the fractal dimension comprises the following steps:

[0065] The cylindrical shell component adopts two sets of coordinate systems: cylindrical coordinate system and rectangular coordinate system. The spatial position of each measuring point in the cylindrical coordinate system is expressed by coordinates ( θ,ρ,z ) is represented by the coordinates ( x , y , z )express;

[0066] The circumferential line of the radial displacement mode shape in a fan-shaped sliding window area is composed of several preset measuring points, such as Figure 2 As shown in the figure. The shaded area is the fan-shaped sliding window area. The circumferential line on this area is composed of 5 measuring points. The deformed circumferential line is as follows: Figure 2 As shown in the partial enlarged figure.

[0067] The interpolation function uses the point-to-point dichotomy to interpolate the circumferential line measurement points in the fan-shaped sliding window area; in this embodiment, the cubic spline interpolation function is used to interpolate the circumferential line measurement points in the fan-shaped sliding window area into denser measurement points. Assuming that there are 5 initial measurement points, the point-to-point dichotomy is used, and the total number of dichotomies is 3 times, and the final number of measurement points becomes 5+22+23+24=33 points. Generally speaking, in order to ensure the accuracy of the fractal dimension value, at least 3 times or more of dichotomy is required.

[0068] The fractal dimension uses the radial displacement change rate of each measuring point as a description index, and the change rate is expressed as follows:

[0069] τ =| r - R | / R

[0070] in τ i is the radial displacement change rate of the measuring point,R is the radius of the cylindrical shell component, r is the radial displacement of the measuring point, , ( x 0 ,y 0 ,z 0 ) is the coordinate of the center of each circumferential line;

[0071] The sum of the radial displacement change rates of the fan-shaped sliding window area under different binary division times is:

[0072]

[0073] in θ =2(m-1)π / (n(k-1)) represents the central angle scale of the fan-shaped sliding window area, n is the total number of measuring points on a complete circle of the circumferential line, k It is the sum of the number of measuring points on the circumferential line and the number of bisection points in the fan-shaped sliding window area. m is the number of initial measurement points contained in the fan-shaped sliding window area;

[0074] The fractal dimension of the circumferential line in the fan-shaped sliding window area is calculated by log(T θ ) and log( θ ) is obtained by taking the slope of the least squares linear fit line between:

[0075] T θ~ θ -D

[0076] in D It is the fractal dimension value of the circumferential line in the typical fan-shaped sliding window area.

[0077] At the same time, in the embodiment of the present disclosure, the traversal scanning of each circumferential line in the radial displacement modal vibration mode also includes multi-resolution filtering of the axial line of the radial displacement modal vibration mode; the advantage of multi-resolution filtering is that it retains the high-frequency fluctuations caused by the cracks, while also weakening the fractal dimension peak caused by the trend term in the high-order radial displacement vibration mode.

[0078] In the embodiment of the present disclosure, the step 3) of identifying and locating cracks according to the fractal shell surface of the radial displacement mode shape after fractal transformation includes:

[0079] The spatial position of the fractal dimension value calculated according to the fan-shaped sliding window area is represented by the coordinate value of the middle measuring point in the fan-shaped area;

[0080] Traverse and scan each circumferential line in the radial displacement modal vibration mode, and determine the fractal shell surface corresponding to the radial displacement modal vibration mode according to the fractal dimension value and position of each fan-shaped sliding window;

[0081] If there are local cracks in the cylindrical shell component, the fractal shell surface after fractal transformation forms a local singular peak at the end of the local crack. The crack is located and quantified according to the singular peak to realize the internal crack detection of the cylindrical shell component. For more intuitiveness, a fractal dimension threshold can be set, and the fractal dimension below the threshold is set to 0.

[0082] In this embodiment, a cylindrical shell with internal cracks is constructed by using the commercial finite element software Abaqus. Figure 3 As shown. The cylindrical shell material is Q345 steel, density ρ =7850 kg / m 3 , elastic modulus E =206 Gpa , Poisson's ratio v =0.33. The length of the cylindrical shell is 700mm, the outer diameter is 260mm, and the wall thickness is 8mm. The cylindrical shell crack is cut by rotary cutting, with a rotation angle of 15° around the center axis, a cutting depth of 5mm, and a cutting width of 1mm. The detailed size of the cylindrical shell with cracks is shown in Figure 4 As shown;

[0083] In this embodiment, the numerical model is composed of solid units, and the overall grid spacing is 3.5 mm; the grid points represent the actual measurement points. The boundary conditions of the cylindrical shell components are completely free; the radial displacement mode vibration shape of the cracked cylindrical shell is calculated using the frequency analysis in the linear perturbation module.

[0084] In this embodiment, the size of the fan-shaped sliding window is 5 measuring points, and the step length is 1 measuring point; the fractal analysis of the radial displacement modal vibration type is implemented by programming with MATLAB software.

[0085] The following is a detailed description of this method using the 40th order radial displacement mode vibration mode of a cylindrical shell component as a specially selected vibration mode. Of course, other high-order radial displacement mode vibration modes can also be selected for crack detection. The selection of a certain order mode vibration mode is generally a vibration mode that is symmetrical about the central axis.

[0086] A: The 40th radial displacement mode of the cylindrical shell component is as follows Figure 5 shown.

[0087] B: Multi-resolution filtering is performed on the 40th order radial displacement mode shape of the cylindrical shell component. The Symlet wavelet basis function is used, the number of decomposition layers is 6, the coefficients of the 2nd layer subband signal with low frequency trend are set to 0, and then the radial displacement mode shape is reconstructed by inverse discrete wavelet transform. The reconstructed mode shape is as follows: Figure 6 shown.

[0088] C: Traverse each circumferential line of the reconstructed radial displacement modal vibration mode through the given fan-shaped sliding window size and sliding step size. At each position of the fan-shaped sliding window, calculate the fractal dimension of the circumferential line of the radial displacement modal vibration mode in the area surrounded by the fan-shaped sliding window, and obtain the fractal dimension shell of the circumferential line, such as Figure 7 shown.

[0089] D: By Figure 7 It can be seen that there are two pairs of circular alternating areas on both sides of the gray circular area at the bottom of the fractal dimension shell surface, corresponding to the two end points of the crack, so that the position, length and other information of the crack can be located. This proves the effectiveness of the present invention in detecting cracks inside cylindrical shells.

[0090] Embodiment 2

[0091] This embodiment discloses a cylindrical shell component internal crack identification system based on modal vibration shape, including a vibration shape acquisition module, a fractal transformation module and a crack identification module;

[0092] The vibration mode acquisition module is used to acquire the radial displacement mode vibration mode of the cylindrical shell component; the radial displacement mode vibration mode includes multi-order radial displacement mode vibration modes;

[0093] The fractal transformation module is used to perform fractal transformation on each circumferential line of a certain order radial displacement modal vibration shape;

[0094] The crack identification module is used to identify and locate cracks according to the fractal shell surface of the radial displacement mode vibration type after fractal transformation, so as to realize damage detection of the cylindrical shell component.

[0095] The process of crack identification by the cylindrical shell component internal crack identification system based on modal vibration shape described in this embodiment is the same as the cylindrical shell component internal crack identification method based on modal vibration shape described in the aforementioned embodiment 1, and will not be repeated here.

[0096] Embodiment 3

[0097] The present embodiment discloses a device for identifying internal cracks in cylindrical shell components based on modal vibration shapes, comprising a memory and a processor; the memory is used to store a computer program; the processor is used to implement the method for identifying internal cracks in cylindrical shell components based on modal vibration shapes as described in the aforementioned embodiment 1 when executing the computer program. The specific steps of the identification method are the same as those in the aforementioned embodiment 1 and will not be repeated here.

[0098] Embodiment 4

[0099] This embodiment discloses a computer-readable storage medium having a computer program stored thereon. When the computer program is executed by a processor, the method for identifying internal cracks in a cylindrical shell component based on modal vibration shapes as described in the first embodiment above is implemented. The specific steps of the identification method are the same as those in the first embodiment above and will not be repeated here.

[0100] The computer described in the embodiment of the present application may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium. The computer-readable storage medium may be any available medium that a computer can read or a data storage device such as a server or a data center that includes one or more available media integrations. The available medium may be a magnetic medium, (e.g., a floppy disk, a hard disk, a magnetic tape), an optical medium (e.g., a digital versatile disc (DVD)) or a semiconductor medium (e.g., a solid-state drive (SSD)), etc. The software formed by the computer storage code may be located in a random access memory, a flash memory, a read-only memory, a programmable read-only memory or an electrically erasable programmable memory, a register, and other mature storage media in the art.

[0101] Each functional module in each embodiment of the present application can be integrated into a processing unit or module, or each module can exist physically separately, or two or more modules can be integrated into one unit or module. In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When software is used for implementation, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function described in the embodiment of the present application is generated in whole or in part.

[0102] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for identifying internal cracks in cylindrical shell components based on modal vibration shapes, characterized in that The following steps are involved: 1) Obtain the radial displacement modal vibration mode of the cylindrical shell component; the radial displacement modal vibration mode includes multi-order radial displacement modal vibration modes; the cylindrical shell component adopts two sets of coordinate systems: cylindrical coordinate system and rectangular coordinate system, and the spatial position of each measuring point in the cylindrical coordinate system is expressed by coordinates ( θ,ρ,z ) is represented by the coordinates ( x , y , z )express; 2) Perform fractal transformation on each circumferential line of a certain order radial displacement modal vibration type; define the fractal dimension value that describes the degree of irregularity of the circumferential line; for a certain circumferential line, specify the fan-shaped sliding window size and moving step length, and the fan-shaped sliding window starts from a certain measuring point on the circumferential line and returns to the measuring point. During the movement, the fractal dimension values ​​of the circumferential lines of the radial displacement modal vibration types in the fan-shaped sliding window area are calculated respectively; 3) Identify and locate cracks based on the fractal shell surface of the radial displacement mode after fractal transformation; the spatial position of the fractal dimension value calculated from the fan-shaped sliding window area is represented by the coordinate value of the middle measuring point in the fan-shaped area; Traverse and scan each circumferential line in the radial displacement modal vibration mode, and determine the fractal shell surface corresponding to the radial displacement modal vibration mode according to the fractal dimension value and position of each fan-shaped sliding window; If there are local cracks in the cylindrical shell component, the fractal shell surface after fractal transformation forms a local singular peak at the end of the local crack. According to the singular peak, the crack is located and quantified, so as to realize the internal crack detection of the cylindrical shell component and the damage detection of the cylindrical shell component. The traversal scanning of each circumferential line in the radial displacement modal vibration mode also includes performing multi-resolution filtering on the axial line of the radial displacement modal vibration mode; the multi-resolution filtering includes the following steps: a) The same θ The z value of the measuring point is used as the horizontal coordinate, and the radial displacement r The value is used as the vertical coordinate to form a one-dimensional signal; b) performing multi-layer wavelet decomposition of the one-dimensional signal using the Symlet wavelet function to obtain a number of sub-band signals, some of which represent the low-frequency trend of the signal, setting the coefficients of the sub-band signals with low-frequency trends to 0, and then reconstructing the signal by inverse discrete wavelet transform; c) Arrange the reconstructed measuring points according to their original spatial positions to obtain the de-trended radial displacement mode shape.

2. The method for identifying internal cracks in cylindrical shell components based on modal vibration shapes according to claim 1, characterized in that: The method for obtaining the fractal dimension comprises the following steps: The circumferential line of the radial displacement mode shape in a sliding window area is composed of a number of preset measuring points. The circumferential line measuring points in the fan-shaped sliding window area are interpolated by using the point-to-point dichotomy method through the interpolation function; The fractal dimension uses the radial displacement change rate of each measuring point as a description index, and the change rate is expressed as follows: τ =| r - R | / R in τ is the radial displacement change rate of the measuring point, R is the radius of the cylindrical shell component, r is the radial displacement of the measuring point, , ( x 0 ,y 0 ,z 0 ) is the coordinate of the center of each circumferential line; The sum of the radial displacement change rates of the fan-shaped sliding window area under different binary division times is: ,in θ =2(m-1)π / (n(k-1)) represents the central angle scale of the fan-shaped sliding window area, n is the total number of measuring points on a complete circle of the circumferential line, k It is the sum of the number of measuring points on the circumferential line and the number of bisection points in the fan-shaped sliding window area. m is the number of initial measurement points contained in the fan-shaped sliding window area; The fractal dimension of the circumferential line in the fan-shaped sliding window area is calculated by log(T θ ) and log( θ ) is obtained by taking the slope of the least squares linear fit line between: T θ~ θ -D in D It is the fractal dimension value of the circumferential line in the typical fan-shaped sliding window area.

3. The method for identifying internal cracks in cylindrical shell components based on modal vibration shapes according to claim 1 or 2, characterized in that: Step 1) The step of obtaining the radial displacement modal vibration shape of the cylindrical shell component comprises the following steps: Firstly, the measuring points are arranged, and the circumferential lines are evenly arranged along the axial direction on the outer surface of the cylindrical shell component, and the measuring points are evenly arranged on each circumferential line; Use a mobile force hammer to traverse all measuring points and fix an accelerometer at any measuring point for measurement; A pulse excitation is applied to the measuring points on the surface of the cylindrical shell component using a force hammer. At the same time, an accelerometer is used to collect the radial hammer vibration response of the fixed measuring points of the cylindrical shell component. The frequency response function matrix is ​​obtained according to the Fourier transform of the excitation and corresponding response signals. The radial displacement mode shape of the cylindrical shell component is obtained using the imaginary peak value and spatial position information of the frequency response function of each measuring point.

4. The method for identifying internal cracks in cylindrical shell components based on modal vibration shapes according to claim 3 is characterized in that: The hammer head of the power hammer is a hard plastic hammer head; the hammering direction of the power hammer is along the radial direction of the cylindrical shell component.

5. A cylindrical shell component internal crack identification system based on modal vibration shape, characterized in that: It includes vibration mode acquisition module, fractal transformation module and crack identification module; The vibration mode acquisition module is used to obtain the radial displacement mode vibration mode of the cylindrical shell component; the radial displacement mode vibration mode includes multi-order radial displacement mode vibration modes; the cylindrical shell component adopts two sets of coordinate systems: a cylindrical coordinate system and a rectangular coordinate system, and the spatial position of each measuring point in the cylindrical coordinate system is represented by coordinates ( θ,ρ,z ) is represented by the coordinates ( x , y , z )express; The fractal transformation module is used to perform fractal transformation on each circumferential line of a certain order radial displacement modal vibration type; define a fractal dimension value describing the degree of irregularity of the circumferential line; for a certain circumferential line, specify the size of the fan-shaped sliding window and the moving step length, the fan-shaped sliding window starts from a certain measuring point on the circumferential line and returns to the measuring point, and calculates the fractal dimension values ​​of the circumferential lines of the radial displacement modal vibration type in the fan-shaped sliding window area during the movement; The crack identification module is used to identify and locate cracks according to the fractal shell surface of the radial displacement mode vibration mode after fractal transformation; the spatial position of the fractal dimension value calculated according to the fan-shaped sliding window area is represented by the coordinate value of the middle measuring point in the fan-shaped area; Traverse and scan each circumferential line in the radial displacement modal vibration mode, and determine the fractal shell surface corresponding to the radial displacement modal vibration mode according to the fractal dimension value and position of each fan-shaped sliding window; If there are local cracks in the cylindrical shell component, the fractal shell surface after fractal transformation forms a local singular peak at the end of the local crack. According to the singular peak, the crack is located and quantified, so as to realize the internal crack detection of the cylindrical shell component and the damage detection of the cylindrical shell component. The traversal scanning of each circumferential line in the radial displacement modal vibration mode also includes performing multi-resolution filtering on the axial line of the radial displacement modal vibration mode; the multi-resolution filtering includes the following steps: a) The same θ The z value of the measuring point is used as the horizontal coordinate, and the radial displacement r The value is used as the vertical coordinate to form a one-dimensional signal; b) performing multi-layer wavelet decomposition of the one-dimensional signal using the Symlet wavelet function to obtain a number of sub-band signals, some of which represent the low-frequency trend of the signal, setting the coefficients of the sub-band signals with low-frequency trends to 0, and then reconstructing the signal by inverse discrete wavelet transform; c) Arrange the reconstructed measuring points according to their original spatial positions to obtain the de-trended radial displacement mode shape.

6. A device for identifying internal cracks in cylindrical shell components based on modal vibration shapes, characterized in that: It comprises a memory and a processor; the memory is used to store a computer program; the processor is used to implement the method for identifying internal cracks in a cylindrical shell component based on a modal vibration shape as described in any one of claims 1 to 4 when executing the computer program.

7. A computer-readable storage medium, characterized in that: The storage medium stores a computer program, and when the computer program is executed by a processor, the method for identifying internal cracks in a cylindrical shell component based on modal vibration shapes as described in any one of claims 1 to 4 is implemented.

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