Method for measuring defect size of equal-thickness foam sandwich component

Abstract

The invention relates to a method for measuring the defect size of an equal-thickness foam sandwich component, and belongs to the technical field of nondestructive testing of composite materials. Comprising the following steps: determining the minimum defect required to be detected according to the thickness, structure and defect detection requirements of a to-be-detected component; preparing reference blocks with defects of different sizes, and measuring the actual sizes of the defects; detecting the defect of the reference block through laser dislocation speckle detection, determining a detection parameter and a vacuum pressure difference, and measuring the defect size of the reference block in a laser dislocation speckle image; establishing a corresponding relation between the actual defect size of the reference block and the size in the laser dislocation speckle image through a mathematical regression method, and obtaining a defect size correction formula; and detecting the to-be-detected component by adopting the detection parameters and the vacuum pressure difference, measuring the size of the defect of the to-be-detected component in the image, and correcting the size of the defect in the image through a defect size correction formula to obtain an actual size. According to the invention, accurate measurement of the defect size of the equal-thickness foam interlayer component can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of nondestructive testing of composite materials, and in particular to a method for measuring defect sizes of foam sandwich components of equal thickness. Background Art

[0002] The foam sandwich structure combines the foam core layer, skin, matrix and other materials together through the glue layer to form a composite material structure with high modulus and light weight, which has been widely used in aerospace and other fields. During the preparation process of the foam sandwich structure components, due to the complex process flow, debonding defects are prone to occur at the bonding interface; and during the service process, due to the influence of mechanical stress, internal stress, impact, external load and environmental temperature alternation, debonding defects may occur at the bonding interface. As a functional structural load-bearing component, the debonding defects at the interface of different materials will cause the mechanical properties, thermal insulation properties and other key technical indicators of the component to decline, and in severe cases, even lead to foam peeling, bulging or cracking, resulting in a decrease in stealth, thermal insulation and mechanical properties, bringing safety hazards. Therefore, before delivery and during service, it is necessary to conduct non-destructive testing on the bonding quality between the skin and the foam, and between the foam and the matrix material, so as to determine the quality of the component. Foam sandwich materials are porous, low-density, have good thermal insulation properties and complex multi-layer structures, making it impossible to effectively detect internal defects of such components using conventional X-ray, ultrasonic, magnetic powder, penetration, infrared and other methods.

[0003] Laser offset speckle is a method of detecting defects by applying a certain vacuum excitation load to the defect, so that the skin surface corresponding to the foam debonding defect produces an off-surface displacement. It has a good detection effect on bonding defects and can be used to detect bonding defects between foam and skin, foam and base material. Since laser offset speckle detection reflects the internal defects of components through the deformation of the outer surface of the component, the corresponding relationship between the area of ​​off-surface displacement (bulge) on the outer surface of the skin and the interface debonding defect is relatively complex, and its size measurement is the difficulty of its detection. The existing relevant detection standards and data documents have not made clear descriptions of the defect measurement method and measurement accuracy. In some documents, the size value of the off-surface displacement area measured on the skin surface in the laser offset speckle image is directly used to replace the actual internal defect size value, and the GB / T34886-2017 composite material speckle detection standard vaguely mentioned the need for correction, but did not mention how to correct the defect size.

[0004] When using vacuum loading to perform laser misalignment speckle detection on foam thickness, the vacuum degree will affect the contour size of the defect in the speckle image. The defect size measured from the laser speckle image at different vacuum degrees is different. In addition, the defect size in the comparison test block used in the detection is fixed, while the defect size in the actual component is random and does not correspond to the defect size in the defect test block. Therefore, how to correctly characterize the actual defect size has become a difficulty in laser misalignment speckle detection. At present, many fields use foam sandwich components with equal thickness structures. When using laser misalignment speckle for bonding quality inspection, the defect size measured in the speckle image is used instead of the measurement result. There is a large difference between the measurement result and the actual defect size. At present, there is no research result on the measurement of defect size of foam sandwich components based on vacuum loading by laser misalignment speckle detection, nor is there any related report on the measurement of actual defect size of sandwich foam components with equal thickness. Summary of the invention

[0005] In view of the above analysis, an embodiment of the present invention aims to provide a method for measuring the defect size of foam sandwich components of equal thickness by laser offset speckle detection, so as to solve the problem that when conventional foam sandwich components of equal thickness are subjected to defect detection by laser offset speckle, the defect size measured in the speckle image is used to replace the measurement result, and the measurement result is greatly different from the actual defect size and has low accuracy.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The present invention provides a method for measuring defect sizes of foam sandwich components of equal thickness, comprising the following steps:

[0008] Step 1: Determine the minimum defect required to be detected for the component to be tested based on its thickness, structure and defect detection requirements;

[0009] Step 2: Prepare comparative test blocks containing debonding defects of different sizes according to the minimum defect required to be detected by the component to be inspected;

[0010] Step 3: Measure the actual size of the debonding defect of the comparison test block;

[0011] Step 4: Detect the debonding defects of the comparison test block by using the laser dislocation speckle detection method to determine the detection parameters and vacuum pressure difference;

[0012] Step 5: collecting a laser misalignment speckle image of the debonding defect of the comparison test block under the detection parameters and vacuum pressure difference in step 4, and measuring the size of the debonding defect of the comparison test block in the laser misalignment speckle image;

[0013] Step 6: Establish the corresponding relationship between the actual size of the debonding defect of the comparison test block and the debonding defect size of the comparison test block in the laser misalignment speckle image by mathematical regression method, and obtain the defect size correction formula;

[0014] Step 7: Using the detection parameters and vacuum pressure difference determined in step 4, the component to be inspected is inspected, a laser misalignment speckle image of the debonding defect of the component to be inspected is collected, and the size of the debonding defect of the component to be inspected in the laser misalignment speckle image is measured;

[0015] Step 8: Correct the size of the debonding defect of the component to be inspected in the laser misalignment speckle image using the defect size correction formula to obtain the actual size of the debonding defect of the component to be inspected.

[0016] Furthermore, in step 2, the comparison test block includes a skin, foam and a matrix material from top to bottom; the brand of the film or adhesive used in the comparison test block is the same as the brand of the film or adhesive used in the component to be tested; the thickness of the film or adhesive in the comparison test block is the same as the thickness of the film or adhesive in the component to be tested; the skin material on the comparison test block is the same as the skin material used in the component to be tested; the skin thickness on the comparison test block is the same as the skin thickness in the component to be tested; the foam material on the comparison test block is the same as the foam material used in the component to be tested, and the foam thickness on the comparison test block is the same as the foam thickness in the component to be tested; the matrix on the comparison test block is the same or similar to the matrix material used in the component to be tested.

[0017] Furthermore, in step 2, the debonding defect size between the foam and the matrix in the comparison test block includes different step sizes, and the step size includes the minimum defect size required to be detected in the component to be tested in the acceptance requirements.

[0018] Furthermore, in step 2, the minimum defect size in the comparison test block should not be larger than the minimum defect required to be detected in the component to be inspected.

[0019] Furthermore, in step 2, the step size of the comparison test block also includes a plurality of sizes that are slightly larger and slightly smaller than the minimum defect size required to be detected in the component to be tested in the acceptance requirements.

[0020] Furthermore, in step 2, the shape of the debonding defect of the comparison test block is rectangular or circular.

[0021] Furthermore, in step 3, the actual size of the debonding defect of the comparison test block is measured by X-ray or CT.

[0022] Furthermore, step 4 includes the following steps:

[0023] The comparison test block is placed in a vacuum box, and the debonding defect of the comparison test block is detected by a laser dislocation speckle detection method. During the detection process, loading is performed step by step, and the pressure difference change in the vacuum box is slowly controlled;

[0024] The vacuum pressure difference value is continuously adjusted until the speckle image of the debonding defect size in the comparison test block that is the same as the minimum defect size required to be detected in the component to be detected is most clearly visible, and the detection parameters and vacuum pressure difference value at this time are determined.

[0025] Furthermore, in step 6, the mathematical regression method is at least two of the four mathematical regression methods of linear, exponential, logarithmic, and polynomial.

[0026] Furthermore, step 6 comprises the following steps:

[0027] Assume that the actual size of the debonding defect of the comparison test block is the strain variable y, and the defect size measured in the laser misalignment speckle image is the variable x;

[0028] The functional relationship between y and x is established by using at least two of the four mathematical regression methods of linear, exponential, logarithmic, and polynomial, and the regression correlation coefficient R^2 values ​​are compared. The regression formula corresponding to the maximum value of the regression correlation coefficient R^2 is used as the defect size correction formula.

[0029] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0030] 1. When the existing foam sandwich components of equal thickness are inspected for defects using laser offset speckle, the defect size measured in the speckle image is used to replace the measurement result. The measurement result is greatly different from the actual defect size and has low accuracy. When the method of the present invention is used for defect detection using laser offset speckle, the measurement is not directly performed through the laser offset speckle system, but the minimum defect to be detected is determined according to the inspection requirements of the foam sandwich component to be tested, and a suitable comparison test block is selected to preset the defect, and the loading conditions are adjusted. The condition in which the speckle image of the debonding defect size in the comparison test block that is the same as the minimum defect size required to be detected in the component to be tested is the clearest is used as the laser offset speckle defect size measurement condition of the component, and then the laser offset speckle detection is performed on the foam sandwich component to be tested under the determined detection parameters and vacuum pressure difference conditions, thereby improving the measurement accuracy.

[0031] 2. The present invention establishes the relationship between the actual size of the debonding defect and the size measured by the speckle image through a mathematical regression method. Firstly, by detecting the preset debonding defect size of the comparison test block, the corresponding relationship between the actual size of the debonding defect under a certain vacuum pressure difference condition and the debonding defect measurement size of the laser dislocation speckle detection image is established. The actual size of the debonding defect of the foam sandwich components of equal thickness can be measured more accurately, the precision and accuracy of the detection results are improved, and the quality of the components can be evaluated more accurately.

[0032] 3. The method of the present invention can realize the dimensional measurement of debonding defects between the skin and the foam, and can also realize the dimensional measurement of debonding defects between the foam and the base material, which can meet the dimensional measurement requirements of debonding defects in foam sandwich structures of equal thickness; it can also be applied to the defect dimensional measurement of other structural components of equal thickness based on vacuum loading laser misalignment speckle detection, and has certain versatility.

[0033] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0035] Figure 1 It is a schematic diagram of the structure of the component to be tested and the comparison test block according to an embodiment of the present invention;

[0036] Figure 2 This is a schematic diagram of the size distribution of debonding defects at the bonding interface between the foam layer and the substrate of a comparative test block of an embodiment of the present invention;

[0037] Figure 3 This is a defect size function relationship diagram established by a linear regression method according to an embodiment of the present invention;

[0038] Figure 4 This is a defect size function relationship diagram established by the exponential regression method according to an embodiment of the present invention;

[0039] Figure 5 This is a defect size function relationship diagram established by logarithmic regression method according to an embodiment of the present invention;

[0040] Figure 6 This is a defect size function relationship diagram established by the polynomial regression method according to an embodiment of the present invention.

[0041] Reference numerals:

[0042] 1-skin; 2-film; 3-foam; 4-substrate. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0044] The present invention provides a method for measuring defect size of a foam sandwich component of equal thickness by laser misalignment speckle detection, comprising the following steps:

[0045] Step 1: Determine the minimum defect required to be detected for the component to be tested based on its thickness, structure and defect detection requirements;

[0046] Step 2: Prepare comparative test blocks containing debonding defects of different sizes according to the minimum defect required to be detected by the component to be inspected;

[0047] Step 3: Measure the actual size of the debonding defect of the comparison test block;

[0048] Step 4: Detect the debonding defects of the comparison test block by using the laser dislocation speckle detection method to determine the detection parameters and vacuum pressure difference;

[0049] Step 5: collecting a laser misalignment speckle image of the debonding defect of the comparison test block under the detection parameters and vacuum pressure difference in step 4, and measuring the size of the debonding defect of the comparison test block in the laser misalignment speckle image;

[0050] Step 6: Establish the corresponding relationship between the actual size of the debonding defect of the comparison test block and the debonding defect size of the comparison test block in the laser misalignment speckle image by mathematical regression method, and obtain the defect size correction formula;

[0051] Step 7: Using the detection parameters and vacuum pressure difference determined in step 4, the component to be inspected is inspected, a laser misalignment speckle image of the debonding defect of the component to be inspected is collected, and the size of the debonding defect of the component to be inspected in the laser misalignment speckle image is measured;

[0052] Step 8: Correct the size of the debonding defect of the component to be inspected in the laser misalignment speckle image using the defect size correction formula to obtain the actual size of the debonding defect of the component to be inspected.

[0053] Specifically, in step 1, the component to be tested is a foam sandwich component of equal thickness, including a skin, foam and a matrix material from top to bottom. The skin is a glass fiber or carbon fiber composite material, the matrix material is a metal material or a composite material, and the foam is a porous foam. The foam and the skin, and the foam and the matrix are bonded with a film or an adhesive. According to the thickness, structure and defect detection requirements of the component to be tested, the minimum defect required to be detected in the component to be tested is first determined.

[0054] Specifically, in step 2, the manufacturing or selection of the comparison test block should adopt the same material, structure and process as the component to be tested. The comparison test block includes skin, foam and base material from top to bottom; the brand of the film or adhesive used in the comparison test block is the same as the brand of the film or adhesive used in the foam sandwich component to be tested; the thickness of the film or adhesive in the comparison test block is the same as the thickness of the film or adhesive in the foam sandwich component to be tested; the skin material on the comparison test block is the same as the skin material used in the foam sandwich component to be tested; the skin on the comparison test block is the same as the skin material used in the foam sandwich component to be tested; The skin thickness is the same as the skin thickness in the foam sandwich component to be tested; the foam material on the comparison test block is the same as the foam material used in the foam sandwich component to be tested, and the foam thickness on the comparison test block is the same as the foam thickness in the foam sandwich component to be tested; considering that in actual products, the foam strength is much smaller than the matrix strength, the foam will deform during the test process, and the matrix generally will not deform. When the matrix does not deform, as long as the matrix strength is sufficient, therefore, the matrix on the comparison test block is the same or similar to the matrix material used in the foam sandwich component.

[0055] It should be noted that the debonding defect size between the foam and the substrate in the comparison test block needs to be designed into different step sizes. The step size of the debonding defect needs to include the minimum defect size required to be detected in the component to be tested in the acceptance requirements, and include multiple sizes that are slightly larger and slightly smaller than the minimum defect size required to be detected. At the same time, the minimum defect size in the comparison test block should not be larger than the minimum defect size required to be detected in the component to be tested. The shape of the debonding defect preset in the comparison test block is determined according to the detection requirements, usually rectangular or circular. For example, in a foam sandwich component, the foam, skin and matrix are all rectangular blocks with dimensions of 200mm×220mm and a foam thickness of 10mm. The minimum defect size required to be detected in the component to be tested in the acceptance requirements is 20mm×20mm. Therefore, when pre-setting debonding defects in the comparison test block, debonding defects with a size of 20mm×20mm must be included. At the same time, in order to characterize the detection capability of this method, it is often necessary to process some sizes slightly larger and smaller than 20mm×20mm. At the same time, the minimum defect size in the comparison test block should not be greater than 20mm×20mm. For example, at least debonding defects of 10mm×10mm, 20mm×20mm, and 30mm×30mm need to be processed in the comparison test block.

[0056] Specifically, in step 3, the actual room temperature sizes of all debonding defects in the comparison test block are measured by X-ray or CT, and are recorded as A1, A2, ..., A m , where A m It is the actual size of the mth debonding defect in the comparison test block.

[0057] Specifically, in step 4, the comparison test block is placed in a vacuum box, and the debonding defect of the comparison test block is detected by the laser dislocation speckle detection method. During the detection process, the loading is gradually carried out, and the pressure difference change in the vacuum box is slowly controlled to ensure that the surface of the debonding defect is deformed within a certain time range and has a relatively obvious deformation process, so that the laser dislocation speckle system can collect enough images during the deformation process of the debonding defect. After that, according to the detection effect of the debonding defect in the comparison test block, a suitable vacuum pressure difference is selected, that is, the value of the vacuum pressure difference is continuously adjusted until the speckle image of the debonding defect size in the comparison test block that is the same as the minimum defect size required to be detected by the component to be detected is the clearest and most visible, and the detection parameters and vacuum pressure difference at this time are recorded for subsequent use in actual component detection, wherein the detection parameters include camera focal length, image acquisition rate and vacuum loading speed.

[0058] Specifically, in step 5, the laser misalignment speckle image of the debonding defect of the comparison test block is collected under the detection parameters and vacuum pressure difference of step 4, and the size of the debonding defect that can be identified by the comparison test block in the laser misalignment speckle image is measured, and the measured sizes are D1, D2, ..., D n , where D n is the size of the nth debonding defect in the comparison test block in the laser misalignment speckle image, and the actual size of all debonding defects in the comparison test block at room temperature (A1, A2, ..., A m), n≤m. It should be noted that the laser misalignment speckle detection of foam sandwich structures realizes defect detection through the off-surface displacement of the skin surface corresponding to the debonding defect, commonly known as bulging. Under different vacuum pressure differences, debonding defects of different sizes gradually cause surface bulging. As long as the minimum defect size required to be detected in the component to be tested in the acceptance requirements is identified (the comparison test block contains defects of this size), all defects larger than this size can be detected, and defects smaller than this size may be detected. However, it is not important whether defects smaller than this size are detected. Defects smaller than this size are not considered in the final product quality. Because the acceptance conditions only require the detection of defects above this size, setting this condition is to ensure that the defects that need to be detected are not missed. For example, when the vacuum pressure difference is 4KPa, a 50*50mm debonding defect will first cause surface bulging. The speckle system can detect the size of the surface bulging area, while the surface of the debonding defect smaller than this size does not change. Then, the vacuum pressure difference is increased to 6KPa. The bulging area of ​​the debonding defect with a size of 50*50mm is further increased, and the surface of the debonding defect with a size of 40*40mm begins to bulge, while no bulge is found on the surface of the defect corresponding to the defect with a size smaller than 40*40mm. At this time, the vacuum pressure difference is increased until it is loaded to 10KPa. The surface of the debonding defect with a size of 10*10mm produces obvious bulging, and the defect image appears clearly in the speckle image. At this time, defects with an actual size larger than 10*10mm can be detected. When the acceptance condition requires the detection of defects larger than 10*10mm, the minimum defect can be detected as a measurement condition to ensure that no defects are missed.

[0059] Specifically, in step 6, a corresponding relationship between the actual size of the debonding defect of the comparison test block and the debonding defect size of the comparison test block in the laser dislocation speckle image is established by a mathematical regression method, and the mathematical regression method is at least two of the four mathematical regression methods of linear, exponential, logarithmic, and polynomial; assuming that the actual size of the debonding defect of the comparison test block is the dependent variable y, and the defect size measured in the laser dislocation speckle image is the variable x, at least two of the four mathematical regression methods of linear, exponential, logarithmic, and polynomial are used to establish a functional relationship between y and x, and the regression correlation coefficient R^2 values ​​are compared, and the regression formula corresponding to the maximum value of the regression correlation coefficient R^2 is used as the defect size correction formula. It should be noted that, with respect to the regression correlation coefficient R^2, R=0 means no correlation, R<0.3 means weak correlation, R=0.3~0.5 means low correlation, R>0.5~0.8 means significant correlation, R>0.8 means high correlation, and R=1 means complete correlation.

[0060] Specifically, in step 7, the inspection parameters and vacuum pressure difference determined in step 4 are used to inspect the component to be inspected, a laser misalignment speckle image of the debonding defect of the component to be inspected is collected, and the sizes of the debonding defect of the component to be inspected in the laser misalignment speckle image are measured, which are x1, x2, x3, ..., x i , where x i is the size of the i-th debonding defect in the laser misalignment speckle image of the component to be inspected;

[0061] Finally, in step 8, the size of the debonding defect of the component to be inspected in the laser misalignment speckle image is corrected according to the defect size correction formula in step 5, and the actual sizes of the debonding defect of the component to be inspected are y1, y2, y3, ..., where y i is the actual size of the i-th debonding defect of the component to be inspected.

[0062] The method of the present invention can realize the size measurement of the debonding defect between the skin and the foam, and can also realize the size measurement of the debonding defect between the foam and the base material, and can meet the debonding defect size measurement requirements of the foam sandwich structure with equal thickness.

[0063] The present invention is also applicable to the defect size measurement of other structural components with equal thickness based on vacuum loading laser misalignment speckle detection, and has certain versatility.

[0064] Example

[0065] Step 1: Determine the minimum defect required to be detected for the component to be tested based on its thickness, structure and defect detection requirements;

[0066] The medium-thickness foam sandwich component in this embodiment includes skin, foam and matrix material from top to bottom; the skin is a glass fiber composite material with a thickness of 1 mm; the matrix material is a metal material; the foam is a porous PMI foam with a thickness of 10 mm; the foam and the skin, and the foam and the matrix are bonded with adhesive film; the foam, skin and matrix are rectangular parallelepipeds with dimensions of 200 mm×220 mm; the minimum defect size required to be detected for this component in the acceptance requirements is 20 mm×20 mm.

[0067] Step 2: Prepare comparison test blocks containing debonding defects of different sizes according to the minimum defect required to be detected by the component to be inspected. The minimum defect size in the comparison test blocks should not be greater than the minimum defect required to be detected by the component to be inspected;

[0068] The comparison test block includes skin, foam and matrix material from top to bottom; the skin is a fiberglass composite material with a thickness of 1mm; the foam is a porous PMI foam with a thickness of 10mm; the skin and foam, and the foam and matrix form a bonding interface through a film, and the thickness of the film between the foam and the upper and lower interfaces is 0.1mm; the matrix material is a 5mm thick aluminum plate, and the skin and foam are bonded to the matrix material in one piece; the foam, skin and matrix are rectangular parallelepipeds with a size of 200mm×220mm;

[0069] On the bonding interface between the foam and the substrate, five debonding defects are formed by controlling the distribution of the adhesive film. The sizes of the debonding defects are 10mm×10mm, 20mm×20mm, 30mm×30mm, 40mm×40mm, and 50mm×50mm.

[0070] Step 3: Measure the actual size of the debonding defect of the comparison test block;

[0071] The comparison block was irradiated with X-rays to obtain an X-ray photographic image. The actual sizes of the five preset defects in the defect comparison block were measured from the X-ray photographic image, and they were 10mm×10mm, 20mm×20mm, 30mm×30mm, 40mm×40mm, and 50mm×50mm, respectively.

[0072] Step 4: Detect the debonding defects of the comparison test block by using the laser dislocation speckle detection method to determine the detection parameters and vacuum pressure difference;

[0073] The component requires a minimum debonding defect of 20mm×20mm to be detected. When selecting loading conditions using the comparison test block, pay attention to the debonding defect of 20mm×20mm. The vacuum loading method is used to perform laser dislocation speckle detection on the comparison test block, and the vacuum pressure difference is adjusted. When the vacuum pressure difference is 6KPa, the corresponding 20mm×20mm defect is the clearest in the speckle image. The loading condition required for speckle detection of the foam sandwich component is set to a vacuum pressure difference of 6KPa, and the detection parameters and vacuum pressure difference at this time are recorded. The detection parameters include camera focal length, image acquisition rate and vacuum loading speed, specifically: camera focal length 500mm, image acquisition rate 25Hz, vacuum loading speed -6KPa / min.

[0074] Step 5: collecting a laser misalignment speckle image of the debonding defect of the comparison test block under the detection parameters and vacuum pressure difference in step 4, and measuring the size of the debonding defect of the comparison test block in the laser misalignment speckle image;

[0075] When the vacuum pressure difference is 6 KPa, the detection image of the laser misalignment speckle of the comparison test block is obtained at this time, and the defect sizes corresponding to the preset debonding defects of 20 mm×20 mm, 30 mm×30 mm, 40 mm×40 mm, and 50 mm×50 mm in the speckle image are measured. The measured sizes corresponding to the four defects with a size not less than 20 mm are 22.5 mm, 35.9 mm, 48.6 mm, and 64.4 mm, respectively.

[0076] Step 6: Establish the corresponding relationship between the actual size of the debonding defect of the comparison test block and the debonding defect size of the comparison test block in the laser misalignment speckle image by mathematical regression method, and obtain the defect size correction formula;

[0077] Assuming that the measured sizes x1, x2, x3, and x4 of the debonding defects of the comparison test block in the speckle image are 22.5, 35.9, 48.6, and 64.4, respectively, and the actual sizes y1, y2, y3, and y4 of the debonding defects of the comparison test block are 20, 30, 40, and 50, respectively, the linear, exponential, logarithmic, and polynomial mathematical regression methods are used to establish the relationship between the actual size y of the debonding defects of the comparison test block and the measured size x of the debonding defects, and the relationship between y and x is shown in Figure 1. Figure 3 , Figure 4 , Figure 5 and Figure 6 ;

[0078] The relationship between the actual size y of the debonding defect of the comparison test block and the measured size x of the debonding defect established by the linear mathematical regression method is y=0.7209x+4.1088; the relationship between the actual size y of the debonding defect of the comparison test block and the measured size x of the debonding defect established by the exponential mathematical regression method is y=13.012e^(0.0218x); the relationship between the actual size y of the debonding defect of the comparison test block and the measured size x of the debonding defect established by the logarithmic mathematical regression method is y=28.462Ln(x)-69.903; the relationship between the actual size y of the debonding defect of the comparison test block and the measured size x of the debonding defect established by the polynomial mathematical regression method is y=-0.0023x^2+0.9175x+0.3659.

[0079] According to the relevant principles of mathematical regression, R^2 is used to evaluate the degree of fit of the regression equation. The value range of R^2 is from 0 to 1. The closer R^2 is to 1, the better the degree of fit of the model, and vice versa. The calculation method of R^2 is as follows:

[0080]

[0081] In the above formula:

[0082] y i To compare the actual size of the debonding defect of the test block;

[0083] The debonding defect size x is calculated by the mathematical regression method according to the measured debonding defect size x of the comparison test block. There will be differences;

[0084] for y i The average value of

[0085] Using different mathematical regression methods to obtain the relationship curve between y and x and the above R^2 calculation formula, we can calculate the correlation coefficient R^2 values ​​of the four mathematical regression methods: linear, exponential, logarithmic, and polynomial. Figures 3 to 6 It can be seen that the R^2 values ​​are 0.9977, 0.9626, 0.9641, and 0.9994, respectively. The polynomial regression method has the largest correlation coefficient R^2 value and is highly correlated. Therefore, the polynomial regression method is used to calculate the actual size of the debonding defect. Therefore, the defect size correction formula for the comparison test block under a vacuum pressure difference of 6KPa is: y = -0.0023x 2 +0.9175x+0.3659. In the formula, x is the measured size in the speckle image, and y is the actual defect size.

[0086] In the comparison test blocks, the size measurement values, regression values ​​and actual values ​​of four typical debonding defects are shown in Table 1. It can be seen from the table that the maximum deviation between the defect measurement value and the actual value reaches 28.80%. After polynomial regression calculation, the maximum deviation of the defect size is only 1.25%.

[0087] Table 1 Comparison of test block debonding defect size results

[0088] Size type / defect number Defect 1 Defect 2 Defect 3 Defect 4 Actual value (mm) 20 30 40 50 Measurement value (mm) 22.5 35.9 48.6 64.4 Deviation between measured value and actual value 2.5 5.9 8.6 14.4 Percentage of deviation between measured value and actual value 12.50％ 19.67％ 21.50％ 28.80％ Regression value (mm) 19.8 30.3 39.5 49.9 Deviation between regression value and actual value -0.2 0.3 -0.5 -0.1 The percentage of deviation between the regression value and the actual value 1.00％ 1.00％ 1.25％ -0.20％

[0089] Step 7: Using the detection parameters and vacuum pressure difference determined in step 4, the component to be inspected is inspected, a laser misalignment speckle image of the debonding defect of the component to be inspected is collected, and the size of the debonding defect of the component to be inspected in the laser misalignment speckle image is measured;

[0090] For the flat foam sandwich component of equal thickness in this embodiment, the same detection parameters as those for the defect comparison test block were adopted. When the vacuum pressure difference was loaded to 6 KPa, the defect size x was measured to be 52 mm through the speckle image.

[0091] Step 8: Correct the size of the debonding defect of the component to be inspected in the laser misalignment speckle image using the defect size correction formula to obtain the actual size of the debonding defect of the component to be inspected.

[0092] By correcting the size of the debonding defect of the component to be inspected in the laser dislocation speckle image using the defect size correction formula, the actual debonding defect size between the foam and the substrate can be obtained: y = -0.0023x2 + 0.9175x + 0.3659 = -0.0023*522 + 0.9175*52 + 0.3659 = 41.9 mm.

[0093] It can be seen from this embodiment that when performing speckle detection on a foam sandwich component, the measurement method of the present invention is adopted, and the defect size correction formula established by the mathematical regression method is used to correct the size of the debonding defect of the component to be inspected in the laser misaligned speckle image, so that a defect size closer to the actual situation can be obtained, thereby improving the accuracy of laser misaligned speckle detection.

[0094] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for measuring defect size of foam sandwich components of equal thickness, characterized in that: The following steps are involved: Step 1: Determine the minimum defect required to be detected for the component to be tested based on its thickness, structure and defect detection requirements; Step 2: Prepare comparative test blocks containing debonding defects of different sizes according to the minimum defect required to be detected by the component to be inspected; Step 3: Measure the actual size of the debonding defect of the comparison test block; Step 4: Detect the debonding defects of the comparison test block by using the laser dislocation speckle detection method to determine the detection parameters and vacuum pressure difference; Step 5: collecting a laser misalignment speckle image of the debonding defect of the comparison test block under the detection parameters and vacuum pressure difference in step 4, and measuring the size of the debonding defect of the comparison test block in the laser misalignment speckle image; Step 6: Establish the corresponding relationship between the actual size of the debonding defect of the comparison test block and the debonding defect size of the comparison test block in the laser misalignment speckle image by mathematical regression method, and obtain the defect size correction formula; Step 7: Using the detection parameters and vacuum pressure difference determined in step 4, the component to be inspected is inspected, a laser misalignment speckle image of the debonding defect of the component to be inspected is collected, and the size of the debonding defect of the component to be inspected in the laser misalignment speckle image is measured; Step 8: Correct the size of the debonding defect of the component to be inspected in the laser misalignment speckle image using the defect size correction formula to obtain the actual size of the debonding defect of the component to be inspected.

2. The measuring method according to claim 1, characterized in that: In the step 2, the comparison test block includes a skin, a foam and a matrix material from top to bottom; the brand of the film or adhesive used in the comparison test block is the same as the brand of the film or adhesive used in the component to be tested; the thickness of the film or adhesive in the comparison test block is the same as the thickness of the film or adhesive in the component to be tested; the skin material on the comparison test block is the same as the skin material used in the component to be tested; the skin thickness on the comparison test block is the same as the skin thickness in the component to be tested; the foam material on the comparison test block is the same as the foam material used in the component to be tested, and the foam thickness on the comparison test block is the same as the foam thickness in the component to be tested; the matrix on the comparison test block is the same or similar to the matrix material used in the component to be tested.

3. The measuring method according to claim 2, characterized in that: In the step 2, the debonding defect size between the foam and the substrate in the comparison test block includes different step sizes, and the step size includes the minimum defect size required to be detected in the component to be tested in the acceptance requirements.

4. The measuring method according to claim 3, characterized in that: In step 2, the minimum defect size in the comparison test block should not be larger than the minimum defect required to be detected in the component to be tested.

5. The measuring method according to claim 3, characterized in that: In the step 2, the step size of the comparison test block further includes a plurality of sizes that are slightly larger and slightly smaller than the minimum defect size required to be detected in the component to be tested in the acceptance requirements.

6. The measuring method according to claim 3, characterized in that: In step 2, the shape of the debonding defect of the comparison test block is rectangular or circular.

7. The measuring method according to claim 1, characterized in that: In step 3, the actual size of the debonding defect of the comparison test block is measured by X-ray or CT.

8. The measuring method according to claim 1, characterized in that: The step 4 comprises the following steps: The comparison test block is placed in a vacuum box, and the debonding defect of the comparison test block is detected by a laser dislocation speckle detection method. During the detection process, loading is performed step by step, and the pressure difference change in the vacuum box is slowly controlled; The vacuum pressure difference value is continuously adjusted until the speckle image of the debonding defect size in the comparison test block that is the same as the minimum defect size required to be detected in the component to be detected is most clearly visible, and the detection parameters and vacuum pressure difference value at this time are determined.

9. The measuring method according to claim 1, characterized in that: In step 6, the mathematical regression method is at least two of the four mathematical regression methods: linear, exponential, logarithmic, and polynomial.

10. The measuring method according to claim 9, characterized in that: The step 6 comprises the following steps: Assume that the actual size of the debonding defect of the comparison test block is the strain variable y, and the defect size measured in the laser misalignment speckle image is the variable x; The functional relationship between y and x is established by using at least two of the four mathematical regression methods of linear, exponential, logarithmic, and polynomial, and the regression correlation coefficient R^2 values ​​are compared. The regression formula corresponding to the maximum value of the regression correlation coefficient R^2 is used as the defect size correction formula.

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