A wood crack depth detection method based on acoustic emission multi-parameter coupling

Abstract

The present invention discloses a wood crack depth detection method based on acoustic emission multi-parameter coupling. Based on the high sensitivity of acoustic emission parameter peak value, frequency center of gravity, and energy to cracks, a weighted average method is used to normalize the three parameters. A comprehensive index value for wood crack depth detection based on acoustic emission multi-parameter coupling is proposed, and the coefficient of difference of the comprehensive index value is used as a detection index. This method has sufficient theoretical basis. A judgment threshold for the crack depth detection index is proposed. The threshold of the comprehensive index coefficient of difference is determined by the trimmed mean, making it possible to intuitively and conveniently judge the crack depth of the specimen.

Description

Technical Field

[0001] The present invention relates to the technical field of structural health monitoring, and in particular to a wood crack depth detection method based on acoustic emission multi-parameter coupling. Background Art

[0002] Wood crack depth is a key indicator for assessing wood damage. Acoustic emission technology, highly sensitive to the appearance and growth of cracks, can be used to detect the state of acoustic emission sources and assess internal characteristics such as material damage and defects. Consequently, it has been gradually applied to damage detection of wood and wood-based materials.

[0003] At present, the research on using acoustic emission technology to detect wood damage is mainly qualitative, lacking quantitative detection of crack damage, and the acoustic emission parameters analyzed are relatively single, with few comprehensive multi-parameter analyses of structural damage.

[0004] Currently, the most commonly used nondestructive testing methods for detecting internal wood damage include stress wave testing, ultrasonic testing, and micro-drill resistance testing. However, these methods are primarily used to determine the location and size of internal defects in wood, while relatively few are used to detect crack depth, which is also a key indicator for evaluating wood damage.

[0005] Research using acoustic emission technology to detect wood cracks has primarily focused on qualitative analysis, lacking quantitative detection of crack depth. Furthermore, the parameters used in wood damage analysis are relatively simple, considering only the relationship between individual parameters such as energy, amplitude, and ring count and crack development. Extraction of a single acoustic emission parameter is susceptible to factors such as ambient noise, making it difficult to quantitatively determine the extent of wood damage based solely on a single acoustic emission parameter. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for detecting wood crack depth based on acoustic emission multi-parameter coupling to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A wood crack depth detection method based on acoustic emission multi-parameter coupling, comprising:

[0009] 1. Extracting acoustic emission signal parameters

[0010] Through the lead breaking test, the acoustic emission signal received by the sensor is collected, and the acoustic emission parameter peak, frequency center and energy in the signal are extracted.

[0011] 2. Standardized processing and testing indicators

[0012] (1) The obtained peak value, frequency center of gravity, and energy form the original judgment matrix R = (r ij) m×3 ,Right now:

[0013]

[0014] Among them, r ij is the jth parameter of the i-th lead break point, where i∈[1,m], m is the number of lead break points, and j=1, 2, and 3 refer to the peak value, frequency center of gravity, and energy, respectively.

[0015] (2) Normalize the original judgment matrix (1), that is:

[0016]

[0017] The obtained normalized matrix K is:

[0018]

[0019] Among them, k ij is the normalized value of the jth parameter of the i-th lead break point after normalization processing according to formula (2).

[0020] 3. Determine the weight of each indicator

[0021] Based on the normalized matrix formula (3), the information entropy and difference coefficient of each indicator are calculated first, and then the weight of each indicator is determined. The specific calculation steps are as follows:

[0022] (1) Calculate the information entropy of the jth indicator

[0023]

[0024] in, According to n, we can know e j The value of is in the interval [0,1].

[0025] (2) Calculate the difference coefficient of the jth indicator

[0026] g j =1-e j (j=1,2,3) (5)

[0027] (3) Calculate the weight of the jth indicator

[0028]

[0029] 4. Determine the coefficient of difference of comprehensive index values

[0030] Substitute the above standardized index values ​​and weight coefficients into formula (7), namely:

[0031]

[0032] Then, based on the difference in comprehensive index values ​​between those that passed through the cracks and those that did not, the comprehensive index value difference coefficient was proposed as the judgment criterion. The calculation formula for the difference coefficient is:

[0033]

[0034] Among them, U i is the comprehensive index value of the i-th lead break point; U i+1 is the comprehensive index value of the i+1th lead break point; i is the difference coefficient of the comprehensive index value between the i-th lead-breaking point and the i+1-th lead-breaking point.

[0035] 5. Determine the depth of the crack

[0036] (1) Determine the crack depth detection threshold

[0037] In the crack-free area of ​​the test piece to be tested, the lead breaking test is carried out using the same sensor arrangement as the crack depth detection. The acoustic emission parameters (peak value, frequency center of gravity, energy) in the acoustic emission signal are extracted, and the peak value, frequency center of gravity, and energy are normalized by formulas (1) to (7). The comprehensive index value of each lead breaking point is calculated, and then the comprehensive index value difference coefficient λ between each two consecutive lead breaking points is obtained by formula (8). i . Remove λ according to formula (9) i The maximum and minimum values ​​of λ are averaged, and the obtained λ is L is the threshold for judging the crack depth.

[0038]

[0039] Among them, λ L is the comprehensive index value difference coefficient threshold; i,max is the maximum value of the comprehensive index value difference coefficient; i,min is the minimum value of the comprehensive index difference coefficient; m is the number of lead break points.

[0040] (2) Determine the crack depth

[0041] When λ i Greater than the threshold λ L When λ i Less than the threshold λ L When , it means that both lead-breaking points pass through the crack, and it is necessary to continue to judge the next lead-breaking point until the crack depth is determined.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] Unlike most studies that use acoustic emission technology to qualitatively detect wood cracks, the method proposed in this patent can quantitatively detect the crack depth; it overcomes the limitations of single acoustic emission parameter evaluation and proposes a scientific detection method for crack depth detection by multi-parameter coupling; it has high accuracy and can effectively predict the degree of wood damage, avoiding the loss of life and property that may be caused by wood damage; it provides new ideas for wood crack depth detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0045] Figure 1 This is the detection method process of the present invention;

[0046] Figure 2 A top view of the arrangement of the sensor and the lead break point of the present invention;

[0047] Figure 3 A side view of the arrangement of the sensor and the lead break point of the present invention;

[0048] Figure 4 This is a front view of the arrangement of the sensor and the lead breaking point of the present invention;

[0049] Figure 5 Parameters of the Chinese fir specimen of the present invention;

[0050] Figure 6 These are the test parameters for Chinese fir with different crack depths according to the present invention;

[0051] Figure 7 This is the coefficient of difference diagram of the comprehensive index value of MC17 of the present invention;

[0052] Figure 8 This is a coefficient of difference diagram of the MC20 comprehensive index value of the present invention.

[0053] Figure 9 This is a graph of the difference coefficients of the MC23 comprehensive index values ​​of the present invention.

[0054] Figure 10 It is the comprehensive index value and its difference coefficient of the crack-free Chinese fir of the present invention. DETAILED DESCRIPTION

[0055] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0056] Example:

[0057] In order to test the effectiveness and accuracy of the method proposed in this patent, the detection of three pieces of wood with actual cutting cracks is used as an example for analysis and explanation.

[0058] (1) Experimental Overview

[0059] The test material is Fujian fir, with an average moisture content of about 12% and a density of 0.315g / cm3. The dimensions of all specimens are 500mm (length) × 50mm (width) × 50mm (height). Simulated cracks were cut in the specimens, with a crack length (l) of 300mm, a width (w) of 3mm, and a depth (d) of 17mm, 20mm, and 23mm, respectively. The details of the cut cracks are as follows: Figure 2-4 For the convenience of description, wood with crack depths of 17mm, 20mm, and 23mm are defined as MC17, MC20, and MC23, respectively. Figure 5 .

[0060] (2) Acoustic emission signal acquisition process

[0061] Considering that there is no acoustic emission source in wood crack detection, a 0.5mm HB lead core breaking method is used to simulate the acoustic emission source. The sensor is placed on the north side of the specimen, 60mm away from the east side, and the lead is broken on the south side of the specimen. Figure 2 In order to test the influence of different lead-breaking distances on the signal received by the sensor under the same crack depth, the lead-breaking point is moved, that is, the distance between the lead-breaking point and the east side of the specimen is recorded as N, and the distance to the corresponding position of the sensor on the south side of the specimen is recorded as the sound source distance G. When N is 120mm and 140mm, the sound source distance G is 60mm and 80mm, as shown in the figure. Figure 6 The lead break points are evenly distributed in the longitudinal direction and are recorded as lead break point positions 1 to 9. When the lead breaks at positions 1 to 3, the corresponding sensor position is a, 4 to 6 correspond to sensor b, and 7 to 9 correspond to sensor c. "·" represents the lead break position, such as Figure 4 In order to reduce the uncertainty caused by artificial acoustic emission sources, 5 effective lead cuttings were performed at each lead cutting point.

[0062] (3) Results and analysis

[0063] This test first extracts the peak value, frequency center of gravity, and energy of the acoustic emission parameters, and uses the comprehensive index value calculation method to calculate the comprehensive index value difference coefficient λ of different specimens at different sound source distances. The results are as follows: Figure 7-9 As shown. The dotted line is the threshold value of the comprehensive index value difference coefficient, and its calculation method is based on the threshold setting method mentioned above. Considering that the test is the same batch of Chinese fir, the threshold calculation method is: randomly select a Chinese fir (MC20 is selected in this test), and conduct a lead breaking test at its crack-free part (west side of the specimen) using the same arrangement as the crack depth detection method. Collect the acoustic emission signals of 9 lead-breaking points, calculate the comprehensive index value difference coefficient between each lead-breaking point, and calculate the average value of the distance λ between the two sound sources by formula (9). The results are shown as follows: Figure 10 As shown; In order to make the λ for judging the crack depth more objective and reliable, Figure 10 The average value of the difference coefficient of the comprehensive index value of the two sound source distances is summed up and the average value is set as the threshold value. Then the threshold value λ L =0.346.

[0064] Depend on Figure 7 It can be seen that the difference coefficient of the comprehensive index value of MC17 at the two sound source distances will mutate between the lead break point depth of 15 and 20 mm, and both are greater than 0.9, which is much greater than the threshold value of 0.346 set in this paper. In addition, except for a few abnormal lead break points, the λ between the other lead break points is less than the threshold value. Therefore, it can be judged that the crack depth is 15 mm < d ≤ 20 mm. Similarly, according to Figure 8 It can be seen that the variation pattern of the comprehensive index value difference coefficient λ of MC20 is the same as that of MC17. When the distance between the two sound sources is between 15 and 20 mm, the λ is much larger than the threshold value, so the crack depth is judged to be 15 mm < d ≤ 20 mm. Figure 9 It can be seen that the coefficient of difference of the comprehensive index value of MC23 is greater than the threshold value of 0.346 when the lead break point depth is between 20 and 25 mm, and the crack depth is judged to be 20 mm<d≤25 mm.

[0065] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0066] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for detecting wood crack depth based on acoustic emission multi-parameter coupling, characterized by: The following steps are involved: S1. Extract at least two acoustic emission signal parameter indicators; S2. Use the weighted average method to normalize all parameter indicators to obtain the standardized value of each parameter indicator; S3. Calculate the weight of each acoustic emission signal parameter index, and obtain a comprehensive index value based on the parameter index weight and the standardized value of the parameter index; S4. Based on the difference in comprehensive index values ​​between those that pass through the crack and those that do not, a comprehensive index value difference coefficient is proposed as a judgment criterion; S5. Determine a judgment threshold of the crack depth detection index by using the comprehensive index value difference coefficient, and judge the crack depth according to the threshold; In S1, the lead breaking test is performed to collect the acoustic emission signal received by the sensor, and the peak value, frequency center of gravity and energy of the acoustic emission parameters in the signal are extracted; The peak value, frequency center of gravity, and energy obtained in S2 form the original judgment matrix R =( r ij ) m×3 ,Right now: Among them, r ij is the jth parameter of the i-th lead break point, where i∈[1,m], m is the number of lead break points, and j=1,2,3 refers to the peak value, frequency center of gravity, and energy respectively; Normalize the original judgment matrix (1), that is: The obtained normalized matrix K is: Among them, k ij is the normalized value of the jth parameter of the i-th lead break point after normalization processing by formula (2); In S3, the information entropy and difference coefficient of each indicator are calculated based on the normalized matrix K (3), and then the weight of each indicator is determined. The specific calculation steps are as follows: Calculate the information entropy of the j-th indicator: Where n= , i=1, 2, 3, …, m; Calculate the coefficient of variation of the j-th indicator: Calculate the weight of the j-th indicator: ； In S4, the standardized index values ​​and weight coefficients of each index are substituted into formula (7), namely: ； The calculation formula of the coefficient of difference in S4 is: Among them, U i is the comprehensive index value of the i-th lead break point; U i+1 is the comprehensive index value of the i+1th lead break point; i is the difference coefficient of the comprehensive index value between the i-th lead-breaking point and the i+1-th lead-breaking point; In S5: First determine the crack depth detection threshold: In the crack-free area of ​​the test piece to be tested, the lead breaking test is carried out using the same sensor arrangement as the crack depth detection. The comprehensive index value of each lead breaking point is calculated, and then the comprehensive index value difference coefficient λ between each two consecutive lead breaking points is obtained by formula (8): i ; According to formula (9), λ is obtained L is the threshold for judging the crack depth; Among them, λ L is the comprehensive index value difference coefficient threshold; i,max is the maximum value of the comprehensive index value difference coefficient; i,min is the minimum value of the comprehensive index difference coefficient; m is the number of lead break points; Next, determine the crack depth: When λ i Greater than the threshold λ L When λ i Less than the threshold λ L When , it means that both lead-breaking points pass through the crack, and it is necessary to continue to judge the next lead-breaking point until the crack depth is determined.

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