CN114441638B - A flaw detection method for corrugated plate - Google Patents

Abstract

The invention belongs to the field of early damage detection of plates, and particularly relates to a flaw detection method for corrugated plates, which comprises the following steps: s1: obtaining parameters of corrugated plates; s2: the fundamental wave parameters are determined, and the fundamental wave frequency of the corresponding corrugated plate is selected according to the parameters of the corrugated plate; s3: detecting damage, namely transmitting fundamental waves at signal excitation points of the corrugated plates, generating second harmonic waves in the corrugated plates by the fundamental waves, and receiving second harmonic signals at signal receiving points of the corrugated plates; s4: and (5) damage analysis. According to the scheme, the influence of the corrugated plate boundary on the ultrasonic wave phase is utilized, the limit of the phase-rate matching condition is overcome through reasonable frequency selection, so that the second harmonic energy is kept continuously growing in the whole, and further, the nonlinear ultrasonic nondestructive detection of the early damage of the corrugated plate structure is realized.

Description

A flaw detection method for corrugated plate

Technical Field

The invention belongs to the field of early damage detection of plate materials, and in particular relates to a flaw detection method for corrugated plates.

Background Art

During the service process, plate and shell structures inevitably experience gradual degradation of material properties, reduced load-bearing capacity, and damage due to internal factors (alternating energy transfer inside the plate) and external factors (external loads, temperature changes, corrosion). When the damage accumulates to a certain extent, it rapidly develops and leads to catastrophic accidents. Therefore, timely prediction and location of material performance changes and damage are crucial to ensure the normal operation of equipment and the safety of people's lives and property.

Compared with other traditional detection methods, ultrasonic testing has the advantages of safety, no radiation, no pollution, no additional damage, and short detection time. Ultrasonic testing is divided into linear ultrasonic and nonlinear ultrasonic. Nonlinear ultrasonic is better than linear ultrasonic. It can detect microstructural changes with geometric dimensions smaller than the ultrasonic wavelength and predict material performance degradation earlier. Among them, Lamb wave has a wide propagation range, fast detection, and excellent plate performance. It is a widely used structure. Nonlinear ultrasonic testing is widely used in early damage detection of plates.

However, because the dispersion and mode conversion of Lamb waves are relatively complex, and nonlinear Lamb waves also require certain conditions to generate accumulative nonlinear signals, the research progress of nonlinear Lamb waves is relatively slow compared to linear Lamb waves. It was not until the late 20th century and the early 21st century that Deng Mingxi obtained an analytical solution to the second harmonic problem for nonlinear Lamb waves without obvious cracks. In 2003, with the help of the guided wave mode expansion method developed by Auld, Deng Mingxi, Lima and Hamilton obtained a consistent second harmonic solution, which profoundly revealed the generation law of second harmonics.

At present, the nonlinear Lamb wave detection technology for thin plates with uniform thickness has gradually matured, and the theoretical system is relatively complete. People point out that there are two main conditions for the generation of cumulative second harmonics: one is that non-zero energy flow flows from the fundamental wave to the second harmonic; the other is that the phase velocities of the incident fundamental wave and the second harmonic are the same. For the non-zero energy flow condition, in general, as long as the material is nonlinear, this condition is automatically satisfied. For the second phase velocity matching condition, the variation law of the nonlinear coefficient of the nonlinear Lamb wave is distinguished from the variation law of the nonlinear coefficient of the nonlinear body wave. Once the phase velocity matching condition cannot be met, the second harmonic will increase with the increase of the fundamental wave propagation distance, and will undergo periodic oscillation of the sine and cosine functions, and will no longer increase linearly with the increase of the fundamental wave propagation distance like the variation law of the second harmonic of the body wave. Lamb waves have dispersive properties. Except for a few limited frequency pairs, the phase velocities of the second harmonic and the fundamental wave are usually not equal. Then, during the propagation of the Lamb wave fundamental wave, the newly generated second harmonic will interfere with the previously generated second harmonic, which will lead to the phenomenon of periodic variation of the second harmonic on the wave propagation path.

Although the second harmonic detection technology has many advantages, there are also many difficulties that need to be overcome. Lamb waves themselves have the complexity of dispersion and multi-mode; the accumulation of second harmonics requires the two conditions discussed above to be met: non-zero energy flow flows from the fundamental wave to the second harmonic, and the phase velocities of the fundamental wave and the second harmonic are equal. In practical applications, if it is convenient for signal processing, the group velocities of the fundamental wave and the second harmonic are also required to be equal; under normal circumstances, the energy of the second harmonic is weak and easily interfered by noise. Generally speaking, the secondary nonlinearity of the material is a weaker nonlinearity. After the phase velocity mismatch limits the sustainable accumulation of the second harmonic, the second harmonic signal is usually weak and easily submerged by noise. In order to overcome the phase velocity mismatch, researchers have roughly proposed two solutions. The first is to find a frequency mode pair so that the phase velocities of the screened fundamental wave and the second harmonic are exactly the same, so as to achieve the purpose of linear growth of the second harmonic intensity as the fundamental wave propagates. People use the S1-S2 mode pair (S1 is the fundamental wave of the first-order symmetric mode, and S2 is the second-order symmetric mode second harmonic) to measure material nonlinearity, such as tensile plastic damage and fatigue damage, temperature fatigue damage, and creep damage. In addition, other mode pairs, such as A2-S4, S2-S4, etc., have also been studied. However, this method also has many problems that need to be overcome. For example, these frequency mode pairs are dispersed and limited in number, and phase velocity matching cannot be achieved in a wide bandwidth; in practical applications, it is difficult for the center frequency of the excitation signal to strictly match the expected selected frequency, and the received signal is usually complex and difficult to process. The second method is to use the low dispersion of the low-frequency S0 mode wave to approximately meet the phase velocity matching condition, so that the second harmonic energy is approximately linearly accumulated within a large distance range of the fundamental wave propagation. The low-frequency S0 mode wave approximately meets the phase velocity matching condition in a wider frequency range, breaking through the limitation of a single frequency pair and having good robustness; a single low-frequency S0 mode signal is easier to excite, the dispersion is relatively weak, and the signal processing is also simpler. In recent years, the low-frequency S0 mode has received widespread attention from researchers. Some researchers have used Lamb wave S0 modal waves to detect microcracks, pointing out that low-frequency S0 modal waves can make up for the shortcomings of modal frequency pairs and have conducted systematic simulation studies on low-frequency S0 modal waves.

In order to overcome the restriction of phase velocity matching conditions on the second harmonic and realize the long-distance detection of the second harmonic, various schemes such as modal pairs and low-frequency S0 modal waves have been proposed. Even if the shortcomings of the relevant schemes are ignored, the relevant technologies are mainly aimed at plates with uniform thickness, and are powerless to detect early damage of thin plates with complex structures, such as corrugated plates. It is necessary to propose new and effective detection methods for early damage of corrugated plate structures.

The document with application number CN201810186104.4 discloses a mixed frequency Lamb wave detection method, which belongs to the field of nondestructive testing. There is a strong engineering demand for quantitative identification and location of early damage of materials using nonlinear ultrasonic detection technology. Coaxial co-directional mixing technology can avoid the interference of the same-frequency harmonic noise signal generated by the measurement system, and the inside of the sample can be scanned without moving the ultrasonic probe. The above scheme uses Lamb coaxial co-directional mixing technology to detect and locate early damage in thin plates. Lamb waves of S0 mode and A0 mode are excited successively at the same position, and the time interval is determined by the scanning position; when the frequencies of the two Lamb waves meet specific conditions, the early damage in the thin plate will excite a mixed wave. The mixed wave is an A0 mode Lamb wave, the frequency is the difference between the two fundamental frequencies, and the propagation direction is opposite to the two fundamental waves. At the same time, the degree of damage in the early damage area of the thin plate, the location of the damage position and the damage range can be evaluated based on the mixed wave information. This method provides a feasible way to detect and locate early damage in thin plates. However, hybrid wave detection technology also has many disadvantages. For example, it is limited by resonance conditions, requires the fundamental wave to meet a specific frequency ratio and mixing angle, and often requires scanning of the test piece, which is not convenient for actual engineering applications. Currently, hybrid wave technology is limited to laboratory research and has no engineering applications.

Traditional nonlinear Lamb wave detection technologies such as second harmonic and hybrid waves are all used for testing thin plates with uniform thickness. In actual material flaw detection, in addition to thin plates with uniform thickness, there are also corrugated plate structures, such as corrugated plates with an upper boundary that is a sinusoidal function curve (x ₃ (U) = h-Asin(2π/Λ)x ₁ , where A is the amplitude of the sine function and Λ is the spatial period length of the surface morphology profile of the corrugated plate), and a lower boundary that is symmetrical with the upper boundary about the neutral plane of the thin plate (x ₃ (L) = Asin(2π/Λ)x ₁ -h, where A is the amplitude of the sine function and Λ is the spatial period length of the surface morphology profile of the corrugated plate). At present, there is no accurate flaw detection method for this type of corrugated thin plates.

Summary of the invention

This solution provides a more accurate flaw detection method for corrugated plates.

In order to achieve the above object, the present invention provides a flaw detection method for corrugated plates, comprising:

S1: Acquisition of corrugated plate parameters, including the spatial period wavelength Λ (Λ = L _n ) of the corrugated plate boundary change and the thickness h;

S2: Determine the fundamental wave parameters. According to the parameters of the corrugated plate, select the fundamental wave frequency of the corresponding corrugated plate. The spatial period wavelength of the change of the corrugated plate boundary is the same as the change period distance of the second harmonic excited by the corresponding frequency in the corresponding flat plate of the same thickness.

S3: Damage detection, transmitting the fundamental wave at the signal excitation point of the corrugated plate, the fundamental wave generates the second harmonic in the corrugated plate, and the second harmonic signal is received at the signal receiving point of the corrugated plate;

S4: Damage analysis: judging the damage degree of the corrugated plate according to the slope of the fitted straight line of the received second harmonic signal.

The principle and beneficial effects of this scheme: From a physically intuitive qualitative point of view, as long as there is secondary nonlinearity in the material, the fundamental wave will continuously generate secondary harmonics during the propagation process. Due to the dispersion properties of Lamb waves, except for a small number of limited frequency pairs, the phase velocities of the second harmonic and the fundamental wave are usually not equal. During the propagation of the Lamb wave fundamental wave, the newly generated second harmonic will interfere with the previously generated second harmonic, making it impossible for the energy of the second harmonic to continue to accumulate. By utilizing the influence of the corrugated plate boundary on the ultrasonic phase and through reasonable frequency selection, the second harmonic energy can generally maintain a continuous growth, thereby realizing the nonlinear ultrasonic nondestructive testing of early damage to the corrugated plate structure, which provides a strong guarantee for more accurate damage detection and judgment for nonlinear ultrasonic early damage nondestructive testing of corrugated plates.

The present invention proposes a second harmonic detection technology for early damage of corrugated plate structures, which can overcome the problem that the second harmonics of traditional detection technologies cannot be continuously accumulated due to the limitation of phase velocity matching conditions, achieve the goal of sustainable growth of the second harmonics, and can smoothly detect early damage of corrugated plates such as thin plates with uneven thickness, thereby achieving the purpose of more accurate damage detection.

Furthermore, the spatial periodic wavelength of the corrugated plate boundary change is Λ, satisfying Λ=L _n , where L _n is the periodic distance of the cumulative change of the second harmonic in a thin plate having the same thickness as the average thickness of the corrugated plate, and L _n is specifically:

为基波的相速度，其中

为二次谐波的相速度，其中l_n为二次谐波可累积的最大空间距离。等式左边包含的未知数为L_n，右边包含的未知数为ω，

和

由ω决定(具体的决定关系为著名的兰姆波频散方程)。即由波纹板周期长度Λ可以确定二次谐波累积变化周期距离L_n，进而通过计算可以确定基波频率ω，反之亦然。in

is the phase velocity of the fundamental wave, where

is the phase velocity of the second harmonic, where l _n is the maximum spatial distance that the second harmonic can accumulate. The unknown number on the left side of the equation is L _n , and the unknown number on the right side is ω,

and

Determined by ω (the specific determining relationship is the famous Lamb wave dispersion equation). That is, the cumulative change period distance _Ln of the second harmonic can be determined by the period length Λ of the corrugated plate, and then the fundamental frequency ω can be determined by calculation, and vice versa.

Furthermore, the fundamental wave signal is: x(t)=0.5Psin(2πft)(1-cos(2πft/N)), wherein f is the center frequency, N is the pulse frequency, and P is the pulse amplitude.

Furthermore, in S3, an ultrasonic probe is used to receive the second harmonic signal; and the spacing between the ultrasonic probes is an integer multiple of l _n .

Furthermore, the signal excitation points and signal receiving points of the corrugated plate are distributed on the surface of the corrugated plate, and the damage of the corrugated plate is judged by using the received second harmonic signal.

Furthermore, there are multiple signal receiving points, and the signal receiving points are evenly arranged on the corrugated plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the theoretical model structure of the corrugated plate according to Example 1 of the present invention.

2 is a schematic diagram of the structure of a simulation model of a corrugated plate according to Example 1 of the present invention (the spatial period wavelength of the corrugated plate is 41 mm, the average thickness is 2 mm, and the total length of the model is 900 mm).

FIG3 is a symmetric modal phase velocity dispersion diagram of a 2 mm thick aluminum plate in Example 1 of the present invention (thin plate with uniform thickness).

4 is a graph showing theoretical results of second harmonic energy varying with propagation distance in Example 1 of the present invention without using structural quasi-phase matching technology (thin plate with uniform thickness, fundamental frequency of 500 kHz).

5 is a diagram showing simulation results of second harmonic energy varying with propagation distance in Example 1 of the present invention without using structural quasi-phase matching technology (thin plate with uniform thickness, fundamental frequency of 500 kHz).

6 is a graph showing theoretical results of second harmonic energy varying with propagation distance using the structural quasi-phase matching technique in FIG. 2 of Example 1 of the present invention (corrugated plate with an average thickness of 2 mm and a fundamental frequency of 500 kHz).

7 is a simulation result diagram of the variation of the second harmonic energy with the propagation distance using the structural quasi-phase matching technology in FIG. 2 of Example 1 of the present invention (corrugated plate with an average thickness of 2 mm and a fundamental frequency of 500 kHz).

8 is a theoretical solution of the variation of the second harmonic energy with the propagation distance using the structural quasi-phase matching technology in Example 1 of FIG. 2 of the present invention (corrugated plate with an average thickness of 2 mm, fundamental frequency of 500 kHz, Λ=41 mm, |ΔΦ(Λ,x _Λ )|≠0).

9 is a simulation result of the variation of the second harmonic energy with the propagation distance using the structural quasi-phase matching technology in FIG. 2 of Example 1 of the present invention (corrugated plate with an average thickness of 2 mm, a fundamental frequency of 500 kHz, Λ=38 mm, |ΔΦ(Λ,x _Λ )|≠0 but less than the corresponding value in FIG. 8 ).

FIG10 is a graph showing theoretical results of second harmonic energy variation with propagation distance using the structural quasi-limited matching technology in FIG2 of Example 1 of the present invention (the ideal case where |ΔΦ(Λ,x _Λ )| is zero).

FIG. 11 is a schematic diagram of the structure of Embodiment 2 of the present invention.

DETAILED DESCRIPTION

The following is a further detailed description through specific implementation methods:

The reference numerals in the drawings of the specification include: housing 1 , flexible substrate 2 , acoustic lens 3 , CMUT sheet 4 , piezoelectric ceramic sheet 5 , electrode 6 , and pad layer 7 .

Embodiment 1:

Introduction to the principle of structural quasi-phase matching technology:

The present invention is based on the basic principle first proposed by the inventor in this patent, which the inventor calls: structural quasi-phase matching technology principle, referred to as SQPM. Before introducing the embodiments, the structural quasi-phase matching technology is first introduced.

According to the literature "X.Sun, X.Ding, F.Li, S.Zhou, Y.Liu, N.Hu., Z.Su, Y.Zhao, J.Zhang, M.Deng, Interaction of Lamb Wave Modes with Weak Material Nonlinearity: Generation of Symmetric Zero-Frequency Mode, Sensors, 18 (2018) 2451", the solution of the second harmonic is:

方向传播，

和

分别为从薄板表面和内部流入第m阶传播模态的能量流。当

和f_n≠0时，非线性解的幅值随基波传播距离增加线性增加。在非线性兰姆波领域内，条件

被称为相速度匹配条件；条件f_n≠0被称为非零能量流条件。相速度匹配条件不满足，二次谐波幅值随基波传播距离的增加周期性正余弦函数振荡；非零能量流条件不满足，二次谐波无法激发。P _mn (when m = n) is not zero, indicating that the mth order propagation mode is along

Direction propagation,

and

are the energy flows from the surface and the interior of the thin plate into the mth order propagation mode, respectively.

When and f _n ≠ 0, the amplitude of the nonlinear solution increases linearly with the propagation distance of the fundamental wave. In the nonlinear Lamb wave domain, the condition

It is called the phase velocity matching condition; the condition f _n ≠ 0 is called the non-zero energy flow condition. If the phase velocity matching condition is not met, the amplitude of the second harmonic will oscillate periodically like a sine and cosine function as the propagation distance of the fundamental wave increases; if the non-zero energy flow condition is not met, the second harmonic cannot be excited.

二次谐波可累积的最大空间距离为：The spatial period distance of the second harmonic energy change is

The maximum spatial distance over which the second harmonic can accumulate is:

和

分别为基波和二次谐波的相速度。in,

and

are the phase velocities of the fundamental wave and the second harmonic, respectively.

Formula (1) can also be written as:

As shown in FIG1 , the periodic structure of the corrugated plate is made of isotropic uniform material. The upper surface of the corrugated plate satisfies the function x ₃ (U) = h-Asin((2π/Λ)x ₁ ), and the lower surface satisfies the function x ₃ (L) = Asin((2π/Λ)x ₁ )-h, where 2h is the average thickness of the corrugated plate, A is the amplitude of the sine function, Λ is the spatial period length of the surface morphology of the corrugated plate, and L _n is the spatial period length of the periodic variation of the second harmonic of the Lamb wave in a uniform plate with a thickness of 2h. In the model, for the structural quasi-phase matching technology, Λ = L _n is set, A < h, and in addition, numbers close to L _n can be considered as the value of Λ.

Assume that the fundamental wave energy does not decay. Taking the S0 mode Lamb wave as an example, the energy flow _fn is related to the thickness of the corrugated plate. Assuming that the energy flow in the plate with a thickness of 2h is _f0 , the energy flow in the corrugated plate is:

f _n (x ₁ )＝f ₀ ·2h/h(x ₁ )＝f ₀ ·2h/(2h-Asin((2π/Λ)x ₁ ). (4)

Where h(x ₁ ) is the thickness function of the corrugated board. Substituting equation (3) into equation (2) yields:

Wherein, _xΛ ＝[ _x1 /Λ]·Λ∈[0,Λ] represents the relative position coordinate of the excitation position in one period unit of the corrugated plate (in this example, _xΛ ＝0). Definition:

而对空间周期为Λ＝L_n的波纹板，Δk(x₁,x_Λ)为Λ和x_Λ的函数：For a uniform thin plate with a thickness of 2h, within the wave propagation distance Λ＝ _Ln , Δk(x ₁ ,x _Λ )＝Δk remains unchanged. It is easy to know that at this time:

For a corrugated plate with a spatial period of Λ = L _n , Δk(x ₁ ,x _Λ ) is a function of Λ and x _Λ :

Formula (6) means that when the fundamental wave propagates in the corrugated plate for a distance of L _n , the amplitude of the second harmonic will not be zero as it is in a plate of uniform thickness. Due to the complex dispersion relation of Lamb waves, it is difficult to obtain an explicit expression for ΔΦ(Λ,x _Λ ) in a corrugated plate.

As shown in Figure 1, the corrugated plate can be regarded as a periodic unit with the same spatial length of Λ = L _n , so ΔΦ(Λ = L _n , x _Λ = 0) = ΔΦ. The second harmonic generated by the second unit will interfere with the second harmonic generated by the first unit. Therefore, formula (5) can be written as:

Among them, g ₁ (Λ, x _Λ ) and g ₂ (Λ, x _Λ ) are two adjustable constants related to Λ and x _Λ . Due to the complex dispersion characteristics of Lamb waves, as usual, their specific explicit expressions are not given. Therefore, the second harmonic energy can be written as:

If Λ≈L _n , ΔΦ(Λ,x _Λ ) is used instead of ΔΦ, and formula (1) can be written in a more general form:

From formula (9) and formula (10), we can know that the length L' _n of the second harmonic energy space variation in the corrugated plate is:

And the maximum cumulative distance l _n ' of the second harmonic in the corrugated plate is:

Considering the relationship |ΔΦ(Λ,x _Λ )|＜|Δk| in this example, the maximum cumulative distance of the second harmonic in the corrugated plate is greater than the corresponding value in the plate with uniform thickness of 2h. The term related to the coefficient g ₁ (Λ,x _Λ ) causes the second harmonic energy of the corrugated plate to oscillate during the growth process. The smaller |ΔΦ(Λ,x _Λ )|, the more maximum energy the second harmonic obtains. If |ΔΦ(Λ,x _Λ )|＝0, the second harmonic energy can grow infinitely, and equation (10) can be rewritten as:

Equation (13) means that the second harmonic energy grows in the form of a quadratic function with the wave propagation distance, and the growth process is accompanied by oscillation.

The above is the theoretical principle of structural quasi-phase matching technology. According to the spatial period length Λ of the corrugated plate, the corresponding fundamental frequency can be determined so that |ΔΦ(Λ,x _Λ )| is small enough to allow the second harmonic to obtain a larger cumulative distance. The ideal situation is to make |ΔΦ(Λ,x _Λ )| zero so that the second harmonic energy increases in the form of a quadratic function with the wave propagation distance.

Figures 7 to 10 show that as |ΔΦ(Λ,x _Λ )| decreases to zero, the distance over which the second harmonic can be accumulated increases, and the theoretical and simulation results are in good agreement.

2 is a schematic diagram of the simulation model of the embodiment, the model length is 900mm, including 22 repeating periodic units, the average thickness of the model is 2mm, the amplitude A of the morphology function of the boundary contour is set to 0.7mm, from _x1 = 0mm to _x1 = 680mm, a signal extraction point is set every 1mm, and the total number of signal extraction points is 681. Among them, the fundamental excitation frequency is 500kHz, the phase velocities of the fundamental wave and the second harmonic are 5314.8m/s and 4709.9m/s respectively, and Λ = _Ln = 41mm.

FIG4 is a diagram showing theoretical results of an embodiment of the present invention without using structural quasi-phase matching technology (thin plate with uniform thickness, fundamental frequency of 500 kHz). It can be seen from the figure that the energy of the second harmonic varies periodically, the maximum cumulative distance of the second harmonic is about 21 mm, and the maximum periodic cumulative distance is about 41 mm.

FIG5 is a simulation result diagram of the change of the second harmonic energy with the propagation distance in FIG2 of the embodiment of the present invention without using the structural quasi-phase matching technology (thin plate with uniform thickness and fundamental frequency of 500kHz). FIG4 and FIG5 respectively show the theoretical solution and simulation results of the corresponding model without using the structural quasi-phase matching technology. Obviously, the simulation results are consistent with the theoretical solution. Although the curve of the simulation result is not as smooth as the curve of the theoretical result, it may be caused by the fact that the divided grid cannot be infinitely small, but it does not affect the trend of the change of the relevant results.

Figures 6 and 7 are the theoretical solution and simulation results of the corresponding model using the structural quasi-phase matching technology (corrugated plate with an average thickness of 2 mm, a fundamental frequency of 500 kHz, Λ=41 mm, |ΔΦ(Λ,x _Λ )|≠0). The simulation results are in good agreement with the simulation results, and the maximum cumulative distance of the second harmonic is increased from 22.5 mm to 175 mm through the structural quasi-phase matching technology.

Continue to adjust the parameters and reduce the value of |ΔΦ(Λ,x _Λ )|, and the distance over which the second harmonic energy can be accumulated will continue to increase. Within the range of ±18mm, the periodic unit length Λ of the model is modified with a spacing of 1mm. After multiple finite element simulations, it is found that 38mm is the optimal periodic unit length for the model (i.e., the Λ value that maximizes the second harmonic accumulation distance is 38mm when searching at intervals of 1mm in the range Λ∈[23,59]mm. Searching at intervals of 0.1mm or 0.01mm should be able to obtain a more ideal Λ value, but the amount of simulation calculations will be greatly increased). Figures 8 and 9 show the theoretical solution and simulation results of the corresponding model respectively (corrugated plate with an average thickness of 2 mm, fundamental frequency of 500kHz, Λ=38mm, |ΔΦ(Λ, _xΛ )|≠0). The simulation solution is consistent with the theoretical results. Compared with the previous model with a period unit length Λ of 41 mm, the cumulative distance of the second harmonic energy is increased from 175 mm to 503 mm. At the same time, more energy flows from the fundamental wave to the second harmonic. Through the structural quasi-phase matching technology, the second harmonic obtains a longer cumulative distance and more energy. Figure 10 shows the relationship between the second harmonic energy and wave propagation when |ΔΦ(Λ, _xΛ )| is zero. It is easy to see that in this ideal case, the second harmonic energy can grow rapidly in the form of a quadratic function as the fundamental wave propagation distance increases.

Guided by the principle of structural quasi-phase matching technology, the limitation of the phase velocity matching principle of the traditional second harmonic detection technology can be overcome, and the early damage non-destructive detection of the corrugated plate structure can be implemented. As shown in Figures 7 to 9, by drawing a straight line at the peak, trough or any position of the wave in the figures, the intersection of the straight line and the curve is marked at equal intervals. When the intersection begins to decrease, the maximum linear cumulative distance of the second harmonic is reached at this time. The ultrasonic probes are arranged on the corrugated plate at an integer multiple of l _n , and the measured data is exactly the intersection of such a straight line and the curve. The specific implementation process is: determine the periodic variation spatial distance L _n (Λ＝L _n ) of the second harmonic according to the spatial period wavelength Λ of the corrugated plate to be tested, and determine ω according to L _n and formula (2), that is, realize the frequency selection of the corrugated plate to be tested. Then arrange the probes, and the spacing between the probes is an integer multiple of l _n to ensure that the energy of the detected second harmonic signal grows approximately linearly with the propagation of the fundamental wave (such as the points represented by circles in Figures 7 to 9 are distributed on a growing straight line). Finally, according to the traditional method of nonlinear ultrasonic testing, the degree of early damage of the test material is judged according to the slope of the straight line fitted by the test signal points, thus realizing the early damage detection of corrugated plates, which suddenly breaks the bottleneck of traditional detection methods.

This solution provides a flaw detection method for corrugated plates, including:

S1: Acquisition of corrugated plate parameters, including the spatial period wavelength Λ and thickness h of the corrugated plate boundary change; the structural diagram of the corrugated plate is specifically shown in Figure 1, and Figure 2 is a structural diagram of the simulation model when the wavelength of the corrugated plate in this embodiment is 41 mm.

S2: Determine the fundamental wave parameters. According to the parameters of the corrugated plate, select the fundamental wave frequency of the corresponding corrugated plate. The spatial period wavelength of the change of the corrugated plate boundary is the same as the change period distance of the second harmonic excited by the corresponding frequency in the corresponding flat plate of the same thickness. The fundamental wave signal is: x(t) = 0.5Psin(2πft)(1-cos(2πft/N)), where f is the center frequency, N is the pulse frequency, and P is the pulse amplitude.

S3: Damage detection, the fundamental wave is emitted at the signal excitation point of the corrugated plate, the fundamental wave generates the second harmonic in the corrugated plate, and the second harmonic signal is received at the signal receiving point of the corrugated plate. The signal excitation point and the signal receiving point of the corrugated plate are distributed on the surface of the corrugated plate. It is convenient to receive the second harmonic signal to judge the damage of the corrugated plate. There are multiple signal receiving points, and the signal receiving points are evenly arranged on the corrugated plate. In this step, an ultrasonic probe is used to receive the second harmonic signal; the spacing between the ultrasonic probes is an integer multiple of l _n .

S4: Damage analysis: based on the received second harmonic signal, calculate and make a data point graph of the square of the second harmonic subvalue (or according to the traditional method: the ratio of the second harmonic subvalue to the square of the fundamental subvalue) versus the fundamental wave propagation distance, make a straight line fitting curve of these data points, and judge the degree of damage of the corrugated plate according to the slope of the fitting line (consistent with the traditional method in this field, the fitting line with a lower slope measured at the factory is used as the standard reference for no damage, and the larger the slope, the greater the degree of damage).

The specific implementation process is as follows: determine the periodic variation spatial distance _Ln (Λ＝ _Ln ) of the second harmonic according to the spatial period wavelength Λ of the corrugated plate to be tested, and determine the fundamental frequency according to _Ln and formula (2), that is, to achieve the frequency selection of the corrugated plate to be tested. Then arrange the probes, and the spacing between the probes is an integer multiple of _ln , so as to ensure that the energy of the detected second harmonic signal grows approximately linearly with the propagation of the fundamental wave (such as the points represented by circles in Figures 7 to 9 are distributed on a growing straight line). Finally, according to the traditional practice of nonlinear ultrasonic testing, the degree of early damage of the test material is judged according to the slope of the straight line fitted by the test signal points, that is, the early damage detection of the corrugated plate is achieved, which suddenly changes the bottleneck of the traditional detection method.

For example, the fundamental frequency determination process of the corresponding simulation case in this article: the wavelength of the corrugated plate is Λ=41mm, the average thickness is 2mm, and for the 2mm thick plate, the period of the second harmonic change is determined to be 41mm. According to formula (2), the fundamental frequency can be selected to be 500kHz, and the spacing between the probes is about 20.5mm. The measured signal point may be the point indicated by the circle in Figure 7. It is recommended that the layout of the detection probes remain unchanged once determined to ensure that the change in the slope of the measured straight line is mainly caused by the change in the degree of damage.

FIG3 is a symmetrical modal phase velocity dispersion diagram of a 2 mm thick aluminum plate according to an embodiment of the present invention. This diagram is obtained by numerically solving the famous Lamb wave dispersion equation. According to the angular velocity ω, the fundamental wave phase velocity and the second harmonic phase velocity generated by the fundamental wave can be obtained from this diagram. Generally, by clicking in software such as MATLAB or Origin, the corresponding coordinate values can be read and obtained, and the values of the frequency and the corresponding velocity can be obtained. The purpose of showing FIG3 is that the dispersion curve can assist in frequency selection. By clicking the dispersion curve of the example in FIG3, the relevant parameter values in formula (2) can be obtained, the frequency of the fundamental wave excited on the corrugated plate can be determined, and the fundamental wave of the frequency can be input to implement early damage detection of the corrugated plate.

From a physical and intuitive qualitative point of view, as long as there is secondary nonlinearity in the material, the fundamental wave will continuously generate secondary harmonics during the propagation process. Due to the dispersion properties of Lamb waves, except for a small number of limited frequency pairs, the phase velocities of the second harmonic and the fundamental wave are usually not equal. During the propagation of the Lamb wave fundamental wave, the newly generated second harmonic will interfere with the previously generated second harmonic, making it impossible for the second harmonic energy to accumulate continuously. This scheme utilizes the influence of the corrugated plate boundary on the ultrasonic phase and through reasonable frequency selection, so that the second harmonic energy can maintain a continuous growth overall, thereby realizing nonlinear ultrasonic nondestructive testing of early damage to the corrugated plate structure. This detection method can overcome the problem of the inability to continuously accumulate the second harmonic due to the limitation of phase velocity matching conditions in traditional detection technology, achieve the goal of sustainable growth of the second harmonic, and thus smoothly implement early damage detection of corrugated plates such as thin plates with uneven thickness.

Embodiment 2:

The difference between this embodiment and the first embodiment is that the ultrasonic probe (mainly referring to the receiving point) in this embodiment includes: a housing 1, a CMUT sheet 4, an acoustic lens 3 and a controller; the CMUT sheet 4 includes a plurality of CMUT elements; the acoustic lens 3 is located on the surface of the CMUT sheet 4; a cushion layer 7 is provided on the bottom surface of the CMUT sheet 4, the cushion layer 7 is bonded to the CMUT sheet 4, and the cushion layer 7 and the acoustic lens 3 are respectively located on the top and bottom of the CMUT sheet 4; the CMUT sheet 4 is connected to the flexible substrate 2 by a metal wire; the flexible substrate 2 is electrically connected to the controller (the flexible substrate 2 and the controller are also connected). There is an external power lamp and other equipment); a piezoelectric ceramic piece 5 is arranged inside the shell 1, and an electrode 6 with opposite polarities is arranged on the piezoelectric ceramic piece 5, and the electrode 6 is connected to a power source; the acoustic lens 3 is located between the CMUT piece 4 and the piezoelectric ceramic piece 5; the piezoelectric ceramic piece 5 is located at the end of the shell 1 close to the object to be detected; the power supply is electrically connected to the controller; the ultrasonic wave generated by the piezoelectric ceramic piece 5 has the same wavelength and period as the fundamental wave; the ultrasonic wave generated by the piezoelectric ceramic piece 5 is opposite to the direction of the fundamental wave; the percentage of the amplitude of the ultrasonic wave generated by the piezoelectric ceramic piece 5 to the amplitude of the fundamental wave signal obtained by the ultrasonic probe at the signal receiving point is 98%.

The ultrasonic probe is provided with an acoustic lens 3, a converter for converting ultrasonic waves into electrical signals, and a backing layer 7 for absorbing ultrasonic waves radiated on the back side of the converter. In the transducer using the CMUT element, the CMUT element is a structure in which a concave portion is formed in an insulating layer formed on a semiconductor substrate, the opening of the concave portion is blocked by a film body to form a vacuum (or gas-sealed) gap, and a pair of electrodes 6 are arranged opposite to each other on the surface of the film body and the back side of the insulating layer across the vacuum gap. This transducer uses the film body to receive reflected echoes from the inside of the detected body, and converts the displacement of the film body into an electrical signal as a change in electrostatic capacitance between a pair of electrodes 6. The CMUT converter has a low conversion efficiency of ultrasonic waves. When performing conversion, the reflected echoes from the detected body are not converted into electrical signals but pass through the semiconductor substrate and reach the interface of the backing layer 7 and are reflected. As a result, the reflected echo repeatedly reflects at the interface between the detected object and the cushion layer 7, which will cause the fundamental wave to form multiple reflection excitation at the ultrasonic probe at the signal receiving point in this embodiment. The fundamental wave will generate a second harmonic signal in the material. The energy intensity of the second harmonic is quite different from that of the fundamental wave. The wave energy of the fundamental wave (equivalent to the square of the amplitude) is about 1000 times that of the second harmonic signal (equivalent to the square of the amplitude). Due to the difference in energy intensity between the fundamental wave and the second harmonic, the second harmonic signal is relatively weak. When the ultrasonic wave receives the signal at the signal receiving point, the reflection of the fundamental wave will interfere with the second harmonic signal, which will cause inaccurate received signals and affect the flaw detection of the material.

By setting a piezoelectric ceramic sheet 5 in the ultrasonic probe, the controller controls the power supply to apply current to the electrode 6, so as to form a sinusoidal alternating current on the piezoelectric ceramic sheet 5, so that the piezoelectric ceramic sheet 5 can have a sinusoidal length expansion and contraction, forming a vibrating ultrasonic wave to be emitted outward, and the ultrasonic wave generated at the piezoelectric ceramic sheet 5 is opposite to the direction of the fundamental wave, so that before the ultrasonic probe at the signal receiving point receives the signal, the ultrasonic wave generated at the piezoelectric ceramic sheet 5 offsets the fundamental wave, so that the fundamental wave signal can be weakened, thereby reducing the multiple reflection excitation of the fundamental wave at the ultrasonic probe, reducing the interference of the fundamental wave on the required second harmonic signal, so that the received second harmonic signal is more accurate. After the controller receives the ultrasonic probe signal at the signal receiving point, the controller controls the change of the sinusoidal alternating current of the piezoelectric ceramic sheet 5, thereby controlling the ultrasonic amplitude generated by the piezoelectric ceramic sheet 5, so as to better offset the fundamental wave, make the received second harmonic signal more accurate, and make the flaw detection of the corrugated plate more accurate.

The traditional method of canceling the influence of the fundamental wave at the signal receiving point is generally to emit a fundamental wave with an opposite phase at the signal excitation point to eliminate the fundamental wave. This embodiment can still adopt this traditional fundamental wave elimination method at the signal excitation point, and eliminate the fundamental wave signal at the signal receiving point to reduce the interference of the fundamental wave signal on the second harmonic signal. The sinusoidal alternating current of the piezoelectric ceramic piece 5 can also be adjusted according to the signal received by the ultrasonic probe at the signal receiving point, so as to control the ultrasonic amplitude generated by the piezoelectric ceramic piece 5, so as to better cancel the fundamental wave signal at the signal receiving point, make the received second harmonic signal more accurate, and thus make the flaw detection of the corrugated plate more precise (the direction of the wave is also considered, but at the receiving point, the amplitude of the wave is mainly measured, which is a scalar and the influence of the direction can be ignored).

The above is only an embodiment of the present invention, and the common knowledge such as the known specific structure and characteristics in the scheme is not described in detail here. It should be pointed out that for those skilled in the art, several deformations and improvements can be made without departing from the structure of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims (4)

1. A flaw detection method for corrugated plate, comprising: S1: Acquisition of corrugated plate parameters, including the spatial period wavelength Λ and thickness h of the corrugated plate boundary change; S2: Determine the fundamental wave parameters, determine the periodic variation spatial distance _Ln of the second harmonic according to the spatial period wavelength Λ of the corrugated plate, determine the fundamental wave frequency ω according to _Ln , and realize the frequency selection of the corrugated plate; the spatial period wavelength Λ of the corrugated plate boundary variation is the same as the periodic variation spatial distance _Ln of the second harmonic excited by the corresponding frequency in the corresponding flat plate of the same thickness, Λ= _Ln ; The spatial distance of the periodic variation of the second harmonic is:

in,

and

are the phase velocities of the fundamental wave and the second harmonic, respectively; S3: Damage detection, using ultrasonic probes to receive second harmonic signals, arrange the probes, and the spacing between the probes is an integer multiple of the maximum spatial distance _ln that the second harmonic can accumulate, so as to ensure that the energy of the detected second harmonic signal grows approximately linearly with the propagation of the fundamental wave, emit the fundamental wave at the signal excitation point of the corrugated plate, the fundamental wave generates the second harmonic in the corrugated plate, and receive the second harmonic signal at the signal receiving point of the corrugated plate; S4: Damage analysis: judging the damage degree of the corrugated plate according to the slope of the fitted straight line of the received second harmonic signal. 2. A flaw detection method for corrugated plate according to claim 1, characterized in that the fundamental wave signal is: x(t) = 0.5Psin(2πft)(1-cos(2πft/N)), where f is the center frequency, N is the pulse frequency, P is the pulse amplitude, and t is time. 3. A flaw detection method for corrugated plate according to claim 1, characterized in that the signal excitation points and signal receiving points of the corrugated plate are distributed on the surface of the corrugated plate. 4. A flaw detection method for corrugated plate according to claim 1, characterized in that: there are multiple signal receiving points, and the signal receiving points are evenly arranged on the corrugated plate.

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