CN112147225B - Nonlinear wave detection method for underwater gate - Google Patents
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Abstract
The invention discloses a nonlinear wave detection method of an underwater gate, which comprises the steps of firstly determining the relation between the group velocity of lamb waves and the product of the frequency and plate thickness through wave velocity calculation, then carrying out coupling optimization, reducing the energy of the lamb waves leaked into water, increasing the energy of the lamb waves in a gate plate, adopting 7 transducer excitation waves to reach a focusing point simultaneously, generating maximum scattered waves at the focusing point, improving the signal to noise ratio, obtaining detection signals, extracting integral frequency multiplication components of the excitation waves in a frequency spectrum, adding the frequency multiplication components, and sequentially converting the focusing point to form a damage image of the whole gate as the damage degree of the focusing point; according to the invention, the linear wave scattered by the structure is removed, and only the nonlinear wave scattered by damage is used for detecting the gate, so that the lamb wave energy can be used for detecting the metal plate with a complex structure, and the detection can be carried out without affecting the water blocking of the gate; the overall damage visualization of the gate can be realized, and the damage degree of each point can be quantitatively represented.
Description
Technical Field
The invention belongs to the technical field of nondestructive testing, and particularly relates to a nonlinear wave detection method of an underwater gate.
Background
The hydraulic gate is a thin-wall metal structure and is easily damaged by aging, freeze thawing, corrosion and cavitation. Under the condition of not affecting water retaining, nondestructive testing is carried out on the hydraulic gate at regular intervals, which is the key for guaranteeing engineering safety and realizing scientific maintenance, and therefore, an integral, rapid and nondestructive testing method of the underwater gate needs to be developed.
The existing hydraulic gate detection mainly comprises coating and corrosion detection, and gate safety is evaluated by calculating the erosion thickness of the gate and stress. The existing detection method can only detect point by point, cannot evaluate the overall safety of the gate, and has hidden danger. Particularly for deep water gates of high dam vaults and large-span gates with spans of nearly hundred meters, the risk of structural failure is higher, and an integral detection method is more needed.
The whole detection method mature at present comprises A scanning, B scanning and C scanning. A, exciting and receiving ultrasonic waves at one point of the plate by the scanning needle, wherein the abscissa is time, and the ordinate is ultrasonic signals, so that defects in the plate thickness direction can be detected; the B sweep is directed at a vertical section of the plate along the thickness direction, the probe moves along a line, the abscissa is the displacement of the probe, and the ordinate is the thickness of the plate; the C-sweep is directed to a horizontal section of the plate at a fixed depth, as shown in fig. 13, where the probe is moved across the surface of the plate to image the signal at a specific depth. It can be seen that the scanning process described above requires moving the probe, which is time consuming and laborious.
For metal plates, one of the more common overall detection methods is guided wave detection. When the guided wave encounters damage, scattering phenomena occur. By collecting and analyzing scattered waves, information such as whether damage occurs, the position of the damage, the size of the damage and the like can be reversely deduced. The advantage of guided wave detection is: the position of the probe does not need to be moved during detection, and large-area integral detection can be realized through propagation, scattering and returning of guided waves in the plate. Meanwhile, when water exists on one side of the metal plate, the guided wave method still has good detection capability.
However, the gate is not a smooth metal plate, and the surface of the gate is often provided with structures such as stiffening and riveting, and the structures can cause scattering of guided waves, so that the structures scatter to submerge damage scattering, and damage cannot be identified. Indoor experiments show that only damage far from the structure can be detected by using only damaged linear scattered waves (when damaged linear scattered waves are not submerged by the structure scattered waves), and a detection blind area exists, and damage in reality is likely to occur near the structure (such as fatigue cracks at holes). Therefore, although lamb wave energy is better for detecting smooth metal plates, it is difficult to apply to gate detection having a complicated configuration. Meanwhile, common damages of the gate, such as corrosion, fatigue, bolt loosening and the like, are difficult to detect through linear scattered waves.
In essence, the lesion scattering removes linear components, including nonlinear components as well. The frequency of the linear scattering of the damage is the same as the frequency of the structural scattering wave, while the frequency of the nonlinear scattering of the damage is an integer multiple of the structural scattering wave. Linear and nonlinear scattering can be separated by time-frequency analysis. Because nonlinear scattering can only come from damage, damage can be detected, positioned and quantified only by considering nonlinear scattering waves, interference of constructing linear scattering waves can be avoided, and lamb wave can be applied to detection of structural complex components, so that the problem of integral nondestructive detection of an underwater gate is solved.
Disclosure of Invention
In order to solve the problems, the invention discloses a nonlinear wave detection method of an underwater gate, which can realize the integral damage visualization of the gate, quantitatively characterize the damage degree of each point, is not interfered by the complex structure of the gate, has no blind area in detection, and improves the detection accuracy.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
a nonlinear wave detection method of an underwater gate comprises the following steps:
(1) Wave velocity calculation
Determining the length, width, thickness, elastic modulus and poisson ratio of the gate panel by referring to engineering data, field measurement and inversion by combining monitoring data; solving the wave equation of the lamb wave:
Wherein ω is angular frequency, c L is transverse wave velocity, c T is longitudinal wave velocity, h is half plate thickness, k is wave number, and a dispersion curve of lamb wave is drawn to determine the relation between the phase velocity and the product of frequency and plate thickness of lamb wave;
based on the relationship of phase velocity and group velocity:
Determining the relation between the group velocity of the lamb wave and the product of the frequency plate thickness;
(2) Coupling optimization
Modulating a sine function by using a window function m (t), and generating an excitation signal:
the window function modulation is to ensure that the excitation signal has a peak of energy at the center frequency omega 0, and the propagation speed of the excitation signal is obtained by inquiring omega 0 in the dispersion curve of the lamb wave.
Decomposing displacement of gate panel into scalar potential functions by Helmholtz decompositionAnd the vector potential function ψ, assuming that water cannot transfer shear force, its displacement is represented by scalar potential only, resulting in the following equation of motion:
the non-water side of the gate panel is a free surface, the stress is 0, and the boundary condition is obtained:
the water blocking side of the gate panel has stress balance and out-of-plane displacement continuity conditions:
the combined type (4) to (6) are used for solving the displacement of the water body and representing the energy of the lamb wave leaking into the water body; since the wave numbers in the metal plate and the water body have the following geometrical relationship:
Solving a conditional extremum of water body displacement by using the geometric relation (7) as a limiting condition through a Lagrangian method, searching for optimal wave numbers kx and ky, reducing the energy of the lamb wave leaking into water, and increasing the energy of the lamb wave in a gate plate;
(3) Single point focusing
On the water blocking surface, a linear array consisting of 7 transducers is stuck on the gate panel, and the serial numbers are 1-7 in sequence. Each transducer has equal distance and can independently excite and receive lamb wave signals; selecting the center of the array as the origin of coordinates O, marking the coordinates of the focusing point as O', and having
Wherein Δt is the time when the i-transducer is excited earlier than the 4-transducer, and if Δt is negative, it means that the i-transducer is delayed by Δt than the 4-transducer. Setting excitation time of each transducer according to the criterion, ensuring that excitation waves of each transducer reach a focusing point at the same time, and generating maximum scattered waves at the focusing point;
(4) Signal to noise ratio enhancement
After the excitation is completed, the transducers collect the excitation of the adjacent 6 transducers; taking two adjacent transducers as an example, the distance between the two transducers is denoted as Deltax; the waveform of the excitation signal after the transmission of Δx is as follows:
When k 0,k1 takes the first two-order Taylor expansion coefficients of the A-mode wave number, the A-mode direct wave excited by the transducer a is obtained;
When k 0,k1 takes the first two-order Taylor expansion coefficients of the S-mode wave number, the S-mode direct wave excited by the transducer a is obtained;
removing direct waves of A and S modes from the signal received by the transducer b to obtain a detection signal of the transducer b;
(5) Damage product calculation
The detection signal contains two components, wherein one component is a linear wave, the frequency is equal to the excitation frequency, the two components are from damage linear scattering, structural scattering and edge scattering, and the other component is a nonlinear wave, the frequency is an integer multiple of the excitation frequency, and the two components are from damage nonlinear scattering; extracting integer frequency multiplication frequency components from the frequency spectrum, and adding the frequency spectrum energy of the nonlinear wave to be used as the damage degree of the focusing point;
(6) Lesion visualization
And continuously changing the focusing point, and repeating the step 3-5 to obtain the damage degree of each point so as to form a damage image of the whole gate.
The beneficial effects of the invention are as follows:
(1) The detection can be carried out under the condition that the water blocking of the gate is not influenced;
(2) The method is not interfered by the complex structure of the gate, and the blind area is detected;
(3) The integral damage visualization of the gate can be realized, and the damage degree of each point can be quantitatively represented;
(4) Contact damage such as corrosion, bolt loosening, fatigue and the like can be detected;
(5) The probe does not need to be moved during detection.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a graph showing the relationship between the phase velocity of lamb wave and the product of the thickness of the frequency plate in the step (1) of the present invention;
FIG. 3 is a graph showing the relationship between the group velocity of the lamb wave and the product of the frequency and plate thickness in the step (1) of the present invention;
FIG. 4 is a schematic diagram showing the optimization of the excitation wave number in the step (2) of the present invention;
FIG. 5 is a phase difference calculation chart of single-point focusing in the step (3) of the present invention;
FIG. 6 is a single-point focusing schematic diagram of the step (3) of the present invention;
FIG. 7 is a diagram of the raw signals received by transducer b in step (4) of the present invention;
FIG. 8 shows the A-mode direct wave from transducer a in step (4) of the present invention;
FIG. 9 shows an S-mode direct wave emitted by the transducer a in step (4) of the present invention;
FIG. 10 is a graph of the detection signal of the transducer b after the signal-to-noise ratio is increased in step (4) of the present invention;
FIG. 11 is a frequency spectrum of the detection signal of transducer b in step (5) of the present invention;
FIG. 12 is an image of the damage to the gate as a whole in step (6) of the present invention;
FIG. 13 is a schematic diagram of the A-scan, B-scan, and C-scan methods in the background art.
Detailed Description
The present invention is further illustrated in the following drawings and detailed description, which are to be understood as being merely illustrative of the invention and not limiting the scope of the invention.
As shown in the figure, the nonlinear wave detection method of the underwater gate comprises the following steps of:
(1) Wave velocity calculation
The length, width, thickness, elastic modulus and poisson ratio of the gate panel are determined by means of consulting engineering data, field measurement, inversion combining monitoring data and the like. Solving the wave equation of the lamb wave:
Where ω is angular frequency, c L is transverse wave velocity, c T is longitudinal wave velocity, h is half-plate thickness, and k is wave number. And (3) drawing a dispersion curve of the lamb wave, and determining the relation between the phase velocity and the product of the frequency plate thickness of the lamb wave, as shown in fig. 2:
based on the relationship of phase velocity and group velocity:
The relation between the group velocity of lamb wave and the product of the frequency and plate thickness is determined as shown in fig. 3:
(2) Coupling optimization
Modulating a sine function by using a window function m (t), and generating an excitation signal:
The purpose of the window function modulation is to ensure that the excitation signal has an energy peak at the center frequency ω0, and in the dispersion curve of the lamb wave, the propagation speed of the excitation signal can be obtained by querying ω0, as shown in fig. 4.
Decomposing displacement of gate panel into scalar potential functions by Helmholtz decompositionAnd the vector potential function ψ, assuming that water cannot transfer shear force, its displacement is represented by scalar potential only, resulting in the following equation of motion:
the non-water side of the gate panel is a free surface, the stress is 0, and the boundary condition is obtained:
the water blocking side of the gate panel has stress balance and out-of-plane displacement continuity conditions:
the combined type (4) to (6) are used for solving the displacement of the water body, and representing the energy of the lamb wave leaked into the water body, as shown in fig. 4, the wave numbers in the metal plate and the water body have the following geometric relationship:
And solving a conditional extremum of water body displacement by using the geometric relation (7) as a limiting condition through a Lagrangian method, searching for optimal wave numbers kx and ky, reducing the energy of the lamb wave leaking into water, and increasing the energy of the lamb wave in a gate plate.
(3) Single point focusing
On the water-retaining surface, a linear array composed of 7 transducers (such as piezoelectric transducer and electromagnetic transducer) is stuck on the gate panel, and the serial numbers are 1-7 in sequence. Each transducer has equal distance and can independently excite and receive lamb wave signals. Selecting the center of the array as the origin of coordinates O, marking the coordinates of the focusing point as O', and having
As shown in fig. 5, where Δt is the time when the i-transducer is excited earlier than the 4-transducer, if Δt is negative, it means that the i-transducer is delayed by Δt than the 4-transducer. Setting the excitation timings of the respective transducers according to the criterion can ensure that the excitation waves of the respective transducers arrive at the focus point at the same time, and the maximum scattered wave is generated at the focus point, as shown in fig. 6.
(4) Signal to noise ratio enhancement
After the excitation is completed, the transducers will acquire excitation of the adjacent 6 transducers. Because of the multi-mode phenomenon of lamb waves, the excitation of one transducer comprises two wave packets A and S, and the detection signal is quite complex. Taking two adjacent transducers as an example, the received signal of one transducer is shown in FIG. 7
Let the distance between the two transducers be deltax. The waveform of the excitation signal after the transmission of Δx is as follows:
When k 0,k1 takes the first two-order Taylor expansion coefficients of the A-mode wave number, the A-mode direct wave excited by the transducer a is obtained;
when k 0,k1 takes the first two-order Taylor expansion coefficients of the S-mode wave number, the S-mode direct wave excited by the transducer a is obtained; the direct wave of the mode A and the mode S is removed from the signal received by the transducer b, and a detection signal of the transducer b is obtained, as shown in fig. 10.
Since the direct wave (linear wave) is removed, the energy ratio of the nonlinear wave in the detection signal is further increased. In the existing nonlinear wave extraction process, one major difficulty is that in the spectrum, the peak value of the linear wave is too large, the energy is too high, so that the spectrum peak of the nonlinear wave is not obvious, and the nonlinear wave cannot be extracted. The present invention can highlight the spectrum peak of nonlinear wave through the operation of this step.
(5) Damage product calculation
The detection signal contains two components, namely a linear wave, the frequency is equal to the excitation frequency, the linear wave is from damage linear scattering, structural scattering and edge scattering, and the nonlinear wave is from damage nonlinear scattering, and the frequency is an integral multiple of the excitation frequency. Frequency components of integral multiples are extracted from the frequency spectrum, and the spectral energy of the nonlinear wave is added as the degree of damage of the focal point, as shown in fig. 11.
(6) Lesion visualization
Continuously changing the focus point and repeating the steps 3-5 to obtain the damage degree of each point. A lesion image shown in fig. 12 is formed, and a point with a high pixel is a lesion position.
According to the invention, the linear wave scattered by the structure is removed, and only the nonlinear wave scattered by damage is used for detecting the gate, so that the lamb wave energy can be used for detecting the metal plate with a complex structure, and the detection can be carried out without affecting the water retaining of the gate; the overall damage visualization of the gate can be realized, and the damage degree of each point can be quantitatively represented.
Several points are explained:
(1) The phased array may be not only linear, but also circular, annular, rectangular, etc. The directional detection can be realized not only by a phase modulation method, but also by other methods such as frequency modulation, or by adopting a transducer which can be directionally excited in hardware.
(2) By replacing Δx in step 4 with the path length scattered by the edge of the restrictor, the scattered wave at the edge of the restrictor can be removed, thereby further improving the component ratio of the nonlinear wave. The method for improving the signal-to-noise ratio also comprises a plurality of signal processing methods such as wavelet transformation, fourier transformation, dufen oscillator and the like, and can replace the method of the step 4.
(3) The invention is not only suitable for gates, but also suitable for thin-wall structures such as airplanes, ship shells, bridge decks, gas tanks, pipelines and the like, the structures can be made of metal materials, can be made of composite materials, can be filled with liquid, and can be wrapped by soil, concrete and water (such as submarine petroleum pipelines and underground buried pipes).
The technical means disclosed by the scheme of the invention is not limited to the technical means disclosed by the embodiment, and also comprises the technical scheme formed by any combination of the technical features.
Claims (1)
1. A nonlinear wave detection method of an underwater gate is characterized in that: the method comprises the following steps:
(1) Wave velocity calculation
Determining the length, width, thickness, elastic modulus and poisson ratio of the gate panel by referring to engineering data, field measurement and inversion by combining monitoring data; solving the wave equation of the lamb wave:
Wherein ω is angular frequency, c L is transverse wave velocity, c T is longitudinal wave velocity, h is half plate thickness, k is wave number, and a dispersion curve of lamb wave is drawn to determine the relation between the phase velocity and the product of frequency and plate thickness of lamb wave;
based on the relationship of phase velocity and group velocity:
Determining the relation between the group velocity of the lamb wave and the product of the frequency plate thickness;
(2) Coupling optimization
Modulating a sine function by using a window function m (t), and generating an excitation signal:
The window function modulation is to ensure that the excitation signal has energy wave peaks at the center frequency omega 0, and the propagation speed of the excitation signal is obtained by inquiring omega 0 in the dispersion curve of the lamb wave;
the displacement of the gate panel is decomposed into the sum of a scalar potential function phi and a vector potential function ψ through Helmholtz decomposition, and the displacement is only represented by a scalar potential assuming that water cannot transfer shear force, so that the following motion equation is obtained:
the non-water side of the gate panel is a free surface, the stress is 0, and the boundary condition is obtained:
the water blocking side of the gate panel has stress balance and out-of-plane displacement continuity conditions:
the combined type (4) to (6) are used for solving the displacement of the water body and representing the energy of the lamb wave leaking into the water body; since the wave numbers in the metal plate and the water body have the following geometrical relationship:
Solving a conditional extremum of water body displacement by using the geometric relation (7) as a limiting condition through a Lagrangian method, searching for optimal wave numbers kx and ky, reducing the energy of the lamb wave leaking into water, and increasing the energy of the lamb wave in a gate plate;
(3) Single point focusing
On the water blocking surface, a linear array consisting of 7 transducers is stuck on a gate panel, and the serial numbers are 1-7 in sequence; each transducer has equal distance and can independently excite and receive lamb wave signals; selecting the center of the array as the origin of coordinates O, marking the coordinates of the focusing point as O', and having
Wherein Δt is the time for which the i-transducer is excited earlier than the 4-transducer, if Δt is negative, it means that the i-transducer is delayed by Δt than the 4-transducer; setting excitation time of each transducer according to the criterion, ensuring that excitation waves of each transducer reach a focusing point at the same time, and generating maximum scattered waves at the focusing point;
(4) Signal to noise ratio enhancement
After the excitation is completed, the transducers collect the excitation of the adjacent 6 transducers; taking two adjacent transducers as an example, the distance between the two transducers is denoted as Deltax; the waveform of the excitation signal after the transmission of Δx is as follows:
When k 0,k1 takes the first two-order Taylor expansion coefficients of the A-mode wave number, the A-mode direct wave excited by the transducer a is obtained;
When k 0,k1 takes the first two-order Taylor expansion coefficients of the S-mode wave number, the S-mode direct wave excited by the transducer a is obtained;
removing direct waves of A and S modes from the signal received by the transducer b to obtain a detection signal of the transducer b;
(5) Damage product calculation
The detection signal contains two components, wherein one component is a linear wave, the frequency is equal to the excitation frequency, the two components are from damage linear scattering, structural scattering and edge scattering, and the other component is a nonlinear wave, the frequency is an integer multiple of the excitation frequency, and the two components are from damage nonlinear scattering; extracting integer frequency multiplication frequency components from the frequency spectrum, and adding the frequency spectrum energy of the nonlinear wave to be used as the damage degree of the focusing point;
(6) Lesion visualization
Continuously changing the focusing point, and repeating the steps (3) - (5) to obtain the damage degree of each point so as to form a damage image of the whole gate.
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| CN202010795436.XA CN112147225B (en) | 2020-08-10 | 2020-08-10 | Nonlinear wave detection method for underwater gate |
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| CN202010795436.XA CN112147225B (en) | 2020-08-10 | 2020-08-10 | Nonlinear wave detection method for underwater gate |
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