Electromagnetic acoustic based method and system for detecting and distinguishing damages of underwater steel structure

Abstract

The present application relates to a kind of electromagnetic ultrasonic-based underwater steel structure damage detection and distinguishing method, using electromagnetic ultrasonic detection equipment to detect underwater steel structure with non-conductive covering layer on surface.The method comprises: electromagnetic acoustic transducer (EMAT) probe is arranged above the surface of underwater steel structure;According to lift-off distance, configure excitation parameter;Apply excitation pulse to emit ultrasonic wave;Receive echo signal and extract transit time feature, echo amplitude feature and initial wave amplitude feature;At least one change based on the above features distinguishes abnormal type, the abnormal type includes at least one of corrosion thinning, surface opening crack and simple lift-off increase.By the above design, the present application realizes the accurate detection and reliable distinction of underwater steel structure damage state under the condition of non-conductive covering layer.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic ultrasound, and in particular to a method and system for detecting and differentiating damage to underwater steel structures. Background Technology

[0002] Underwater steel structures, such as offshore platforms, subsea pipelines, and offshore wind power foundations, are exposed to the marine environment for extended periods and are generally threatened by the attachment of marine organisms. Barnacles, mussels, and other organisms form non-conductive, uneven, and porous deposits on the structural surface. These deposits not only accelerate the corrosion process of the steel structure but also render conventional non-destructive testing methods ineffective: visual inspection is completely obscured; piezoelectric ultrasonic testing, while applicable, relies on coupling agents, and the sound waves experience severe scattering and attenuation within the deposited layer, resulting in a sharp decrease in the signal-to-noise ratio; magnetic particle or eddy current testing can only detect near-surface defects and cannot achieve quantitative measurement of wall thickness.

[0003] The emergence of Electromagnetic Acoustic Transducer (EMAT) technology offers a new solution to the aforementioned problems. EMAT directly excites ultrasonic waves (such as transverse waves) within the skin layer of metallic materials through the Lorentz force mechanism, without the need for acoustic coupling agents. For non-conductive marine biofilm deposits, the effect is merely a physical lift-off in the sense of electromagnetic coupling, rather than an obstruction of acoustic propagation. This, in principle, makes in-situ inspection of underwater steel structures feasible without removing the biofilm.

[0004] Recently, a high-performance shear wave EMAT technique has been proposed, enabling thickness mapping of corroded areas through a 3 mm simulated adhesion layer. However, this technique has significant limitations: firstly, its function is limited to identifying areas of wall thinning (i.e., corrosion), and it does not address the detection of surface cracks; secondly, and more importantly, this method fails to establish effective distinguishing criteria between three typical operating conditions: corrosion thinning, surface cracks, and lift-off increase simply due to adhesion layer thickening. In real marine environments, the lift-off effect caused by biofilm adhesion often coexists with material damage. If the detection method cannot accurately distinguish between the two, it can easily lead to misjudgment of the structural state (e.g., misjudging lift-off increase as damage) or missed detection (e.g., ignoring the actual damage hidden by lift-off), thus posing significant safety hazards.

[0005] Therefore, there is an urgent need in this field to develop an innovative and systematic method that can not only effectively detect corrosion thinning and surface cracks in underwater steel structures without removing the marine organism attachment layer, but also reliably distinguish between damage types and simple lift-off effects, so as to ensure the safe operation of underwater structures.

[0006] It should be noted that the information disclosed in the background section above is only for understanding the background of this application, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0007] The technical problem this application aims to solve is "how to address the industry challenge of the coupling and difficulty in distinguishing between damage identification and lift-off effect in the non-destructive testing of underwater steel structures covered by marine organism attachment layers, so as to achieve an accurate assessment of the true health status of the structure."

[0008] The technical solution adopted in this application to solve the above-mentioned technical problems is as follows.

[0009] This application proposes a method for damage detection and differentiation of underwater steel structures based on electromagnetic ultrasound. The method uses electromagnetic ultrasound testing equipment to detect underwater steel structures with a non-conductive coating, and includes the following steps: The electromagnetic ultrasonic transducer (EMAT) probe of the electromagnetic ultrasonic testing equipment is placed above the surface of the underwater steel structure. The excitation parameters of the electromagnetic ultrasonic testing equipment are configured based on the lift-off distance between the electromagnetic ultrasonic transducer (EMAT) probe and the surface of the underwater steel structure. Using the configured excitation parameters, an excitation pulse is applied to the electromagnetic ultrasonic transducer (EMAT) probe through the electromagnetic ultrasonic testing equipment to emit ultrasonic waves; The system receives echo signals returned from an underwater steel structure and extracts transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics from the echo signals. The anomaly type is distinguished based on changes in at least one of the transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics. The anomaly type includes at least one of corrosion thinning, surface opening cracks, and simple lift-off increase.

[0010] In some embodiments, the step of distinguishing the type of anomaly includes: if the transit time characteristics show that the arrival time of the echo signal is systematically advanced, then it is determined that corrosion thinning exists.

[0011] In some embodiments, the pre-calibrated normalized time shift Δt is used. norm The linear relationship between Δt and the thinning depth is used to calculate the corrosion thinning depth; where Δt norm =(t0–t) / t0, where t0 is the transit time characteristic of the undamaged reference state, and t is the transit time characteristic of the current measurement.

[0012] In some embodiments, the step of distinguishing the anomaly type includes: if the transit time characteristics do not change but the echo amplitude characteristics show attenuation and the initial wave amplitude characteristics do not change, then it is determined that there is a surface opening crack.

[0013] In some embodiments, the width of the surface-opening crack is estimated by using a pre-calibrated monotonic mapping relationship between the echo amplitude attenuation rate and the crack width.

[0014] In some embodiments, the step of distinguishing the anomaly type includes: if the transit time characteristic does not change but the echo amplitude characteristic shows attenuation and the initial amplitude characteristic shows an increase, then it is determined to be a simple increase in lift-off.

[0015] In some embodiments, when extracting transit time features, echo amplitude features, and initial wave amplitude features from the echo signal, if the first bottom echo B1 of the echo signal is aliased with the transmitted aftershock, rendering its features unusable, then the second bottom echo B2 of the echo signal is used instead to extract transit time features and echo amplitude features; if the features of the second bottom echo B2 are also unusable, then the third bottom echo B3 of the echo signal is used instead to extract transit time features and echo amplitude features, and so on, until the transit time features and echo amplitude features are successfully extracted; the initial wave amplitude features are always extracted from the beginning portion of the echo signal.

[0016] In some embodiments, the excitation parameters of the electromagnetic ultrasonic testing equipment are configured according to the lift-off distance between the electromagnetic ultrasonic transducer (EMAT) probe and the surface of the underwater steel structure, including: when the lift-off distance is less than 0.5 mm, the selected excitation parameters include: a center frequency of 3.5-4.0 MHz and a pulse count of 1; when the lift-off distance is 0.5-1.5 mm, the selected excitation parameters include: a center frequency of 2.9-3.1 MHz and a pulse count of 2-3; when the lift-off distance is greater than 1.5 mm, the selected excitation parameters include: a center frequency of 2.7-2.9 MHz and a pulse count of 3.

[0017] In some embodiments, the excitation parameters further include a gain parameter: when the lift-off distance is less than 0.5 mm, the gain parameter is configured to be 20-25 dB; when the lift-off distance is 0.5-1.5 mm, the gain parameter is configured to be 28-32 dB; and when the lift-off distance is greater than 1.5 mm, the gain parameter is configured to be 30-38 dB.

[0018] In some embodiments, an underwater steel structure damage detection and differentiation system based on electromagnetic ultrasound is also provided to implement the underwater steel structure damage detection and differentiation method of the present invention, comprising: an electromagnetic ultrasound testing device including an electromagnetic ultrasonic transducer (EMAT) probe for placement above the surface of the underwater steel structure to be tested; a parameter configuration module configured to configure excitation parameters according to the lift-off distance between the electromagnetic ultrasonic transducer probe and the surface of the underwater steel structure; a signal excitation and acquisition module configured to apply an excitation pulse to the electromagnetic ultrasonic transducer probe using the configured excitation parameters to emit ultrasonic waves and receive echo signals returned by the underwater steel structure; a feature extraction module configured to extract transit time features, echo amplitude features, and initial wave amplitude features from the echo signals; and a damage discrimination module configured to distinguish abnormality types based on changes in at least one of the transit time features, echo amplitude features, and initial wave amplitude features.

[0019] The present invention has the following beneficial effects: The underwater steel structure damage detection and differentiation method based on electromagnetic ultrasound provided by this invention brings significant beneficial effects through a series of technical steps. First, by employing electromagnetic ultrasound testing equipment to inspect underwater steel structures with non-conductive coatings and by placing the electromagnetic ultrasonic transducer (EMAT) probe of the electromagnetic ultrasonic testing equipment above the surface of the underwater steel structure, the non-contact coupling characteristics of electromagnetic ultrasound testing technology are utilized. This establishes the basic condition for detection without cleaning the non-conductive coating, providing an application scenario and technical foundation for subsequent steps. Second, by configuring the excitation parameters of the electromagnetic ultrasonic testing equipment according to the lift-off distance between the EMAT probe and the surface of the underwater steel structure, adaptive adjustment of the detection parameters is achieved. This ensures effective excitation and reception of ultrasonic signals under non-conductive coating conditions of varying thicknesses, guaranteeing the acquisition of effective echo signals. Next, by employing the technical characteristics of "applying excitation pulses to the electromagnetic ultrasonic transducer (EMAT) probe using configured excitation parameters to emit ultrasonic waves" and "receiving echo signals returned from the underwater steel structure," the excitation and echo signal acquisition process of the ultrasonic waves was completed, providing a data source for subsequent feature extraction. Furthermore, by "extracting transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics from the echo signals," three characteristic parameters reflecting different physical phenomena were obtained from the echo signals, providing multi-dimensional data for distinguishing different types of anomalies. Finally, by employing the technical characteristic of "distinguishing anomaly types based on changes in at least one of transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics, where the anomaly type includes at least one of corrosion thinning, surface-opening cracks, and simple lift-off increase," a comprehensive discrimination framework was constructed. This technical feature, by analyzing the synergistic relationship between transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics, can reveal whether the root cause of signal changes is structural damage or a simple lift-off effect. This allows for the effective differentiation of three anomaly types: corrosion thinning, surface crack opening, and simple lift-off increase. All these technical features work together synergistically to achieve the beneficial effect of accurate detection and reliable differentiation of underwater steel structure damage states under complex conditions with non-conductive coatings.

[0020] In summary, this invention systematically solves the fundamental technical problem of "the coupling and difficulty in distinguishing between damage identification and lift-off effects" through the organic combination and synergy of the aforementioned technical features. The solution path is as follows: the two technical features of "using electromagnetic ultrasonic testing equipment to inspect underwater steel structures with non-conductive coatings" and "arranging the electromagnetic ultrasonic transducer (EMAT) probe of the electromagnetic ultrasonic testing equipment above the surface of the underwater steel structure" jointly establish the physical basis and technical route for solving this problem, ensuring that the detection can cope with the presence of non-conductive coatings. The technical feature of "configuring the excitation parameters of the electromagnetic ultrasonic testing equipment according to the lift-off distance between the EMAT probe and the surface of the underwater steel structure" is a feedforward optimization mechanism. It ensures that the subsequent steps of "applying excitation pulses to the EMAT probe using the configured excitation parameters to emit ultrasonic waves" and "receiving the echo signals returned by the underwater steel structure" can obtain valuable signals under various conditions, laying the foundation for accurate feature extraction. This is an important prerequisite for solving the "difficulty in distinguishing" problem. Subsequently, the technical feature of "extracting transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics from the echo signal" is a crucial step in "decoupling" the coupled physical effects. It decomposes the complex composite signal into three independently measurable dimensions, each corresponding to different physical principles. Finally, the technical feature of "distinguishing anomaly types based on changes in at least one of the transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics" uniquely maps different anomaly root causes to different combinations of these three characteristics, thereby achieving the separation and differentiation of coupling effects at the information level. The entire scheme is interconnected, progressing step by step from ensuring signal acquisition to information decomposition and pattern recognition, ultimately achieving an accurate assessment of the true health status of the structure.

[0021] Other beneficial effects of the present invention will be further described below. Attached Figure Description

[0022] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein: Figure 1 This is a block diagram of the EMAT detection system as an example. Figure 2 Here is a flowchart illustrating the logic of the multidimensional damage type criterion system in an example. Figure 3 This is a geometric model diagram of the simulation model in Example 1; Figure 4 This is a mesh partitioning diagram of Example 1; Figure 5This is a diagram of the internal ultrasonic echo signal of Example 1; Figure 6 This is a cloud diagram showing the internal stress distribution of Example 1; Figure 7 This is a time-shift diagram showing corrosion of a 5mm steel plate in Example 2. Figure 8 This is a time-shift diagram showing corrosion of a 10mm steel plate in Example 2. Figure 9 The normalized time shift and thinning graph for the 5mm steel plate in Example 2 is shown. Figure 10 The normalized time shift and thinning graph for a 10mm steel plate in Example 2 is shown. Figure 11 The following diagrams illustrate the comparison and application of waveform characteristics and criteria for three typical working conditions in Example 4. (a) is a schematic diagram of the non-destructive reference waveform; (b) is a schematic diagram of the time-domain criterion response for corrosion thinning of 2.0 mm (B1 / B2 / B3 are all proportionally offset); and (c) is a schematic diagram of the distinction between crack vs. lift-off and the combined criteria of amplitude domain and initial wave (ToF remains unchanged). Figure 12 This is a calibration curve of the initial wave amplitude and the secondary bottom wave amplitude measured in Example 4; Figure 13 The diagram shows a comparison of the attenuation modes of the echo amplitude under two working conditions in Example 4. (a) is a schematic diagram of linear attenuation of the amplitude under the crack condition; (b) is a schematic diagram of double exponential attenuation of the amplitude under the lift-off condition. Detailed Implementation

[0023] The embodiments of the present invention will be described in detail below. It should be emphasized that the following description is merely exemplary and not intended to limit the scope and application of the present invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0025] This invention recognizes the technical problem of existing technologies being unable to simultaneously distinguish between corrosion thinning and surface cracks in underwater steel structures without removing marine organism deposits. This is because they fail to understand the fundamentally different physical effects of corrosion thinning, surface cracks, and simple lift-off on ultrasonic signals under the Lorentz force mechanism of electromagnetic ultrasonic transducers (EMAT): corrosion thinning causes changes in sound path, reflected as transit time shift; surface cracks cut off the eddy current loop, leading to linear attenuation of echo amplitude while stabilizing the initial wave; and simple lift-off increases electromagnetic coupling efficiency with a double exponential decrease and an increase in initial wave amplitude. Therefore, this invention, through the discovery and profound understanding of this differentiated physical mechanism, proposes a scheme to obtain effective signals using layered excitation parameter configuration and to establish a three-dimensional joint criterion system of time domain, amplitude domain, and initial wave. By analyzing the correlation characteristics among transit time shift, amplitude attenuation mode, and initial wave change, the three working conditions can be accurately distinguished, thus solving the technical challenge of synchronously detecting and reliably identifying multiple types of damage under non-conductive covering layers.

[0026] In some embodiments, this invention is based on the EMAT Lorentz force transverse wave mechanism. The penetration mechanism is verified through multi-physics finite element simulation. An experimental matrix covering steel plates of varying thicknesses, adhesion layer thicknesses, and damage types is designed. A three-dimensional joint criterion system of time domain, amplitude domain, and initial wave is established to achieve synchronous detection and reliable differentiation of three typical working conditions. It should be noted that this invention can also use longitudinal wave EMAT or Rayleigh wave EMAT instead of transverse wave EMAT; no specific limitation is made here.

[0027] In some embodiments, the detection method includes the following steps: ①Detection system ( Figure 1 Setup and parameter configuration.

[0028] The shear wave EMAT probe is positioned above the surface of the steel structure to be tested, with a lifting distance of 0–5 mm. The material being tested is Q235B low-carbon steel (5 mm or 10 mm thick). The adhesion layer is barnacle calcareous shell, mussel, algal biofilm, or a simulation thereof (a mixture of barnacle debris and epoxy resin, cured and polished to the target thickness, with a 0.1 mm PVC film covering the probe for protection). The EMAT probe uses a butterfly coil with a permanent magnet, a center frequency of 2.7–4.0 MHz, and a nominal shear wave velocity of 3255 m / s.

[0029] To ensure the comparability of subsequent experimental data including the adhesion layer, a single-parameter sensitivity analysis was performed under non-lift-off conditions (probe in close contact with the bare steel plate) to determine the optimal combination of excitation and reception parameters. The experimental sample was a defect-free Q235B steel plate, and the ambient temperature was controlled at 25℃. Specific variable settings are shown in Table 1. Each test was repeated twice to evaluate repeatability.

[0030] Table 1. Comparison matrix of single-parameter experiments under zero-lift conditions

[0031] Based on the above-mentioned parameter influence patterns, the parameter optimization principle shown in Table 2 is adopted. In actual engineering, users need to manually select parameter configurations according to the actual situation based on the parameter selection principle of this invention.

[0032] Table 2. Hierarchical Excitation Parameter Configuration Strategy

[0033] The system controls high-voltage pulse excitation, a programmable gain amplifier (10–50 dB), anti-aliasing filtering, and an analog-to-digital converter (ADC) for data acquisition (sampling rate 25 MSa / s) via RS-485 bus. The system automatically shortens the blocking window to reduce near-surface dead zones; the digital bandpass filter employs a linear-phase finite-impulse response (FIR) structure, with its center frequency automatically adjusting to follow the pulse frequency to avoid group delay errors.

[0034] ②Echo feature extraction.

[0035] Three core features are extracted from the EMAT waveform: Time of Flight (ToF): arrival times t of B1 (first bottom echo), B2 (second bottom echo), and B3 (third bottom echo). B1 t B2 t B3 This reflects the round-trip propagation time of the ultrasonic wave within the steel plate; echo amplitude: extract the amplitude values ​​A of B1, B2, and B3. B1 A B2 A B3 Reflects the echo energy level; Initial wave amplitude: Extract the initial wave amplitude A. main This reflects the electromagnetic coupling state of the transmitter.

[0036] It should be noted that the "initial wave amplitude A" defined here... main "This is not an ultrasonic signal, but rather electromagnetic crosstalk between the excitation coil and the receiving coil. This signal appears before the arrival of the ultrasonic echo, and its timing coincides almost exactly with the excitation pulse. In the EMAT system, the echo amplitude (A)..." B1 A B2 A B3 Indeed, it decreases monotonically with increasing lift-off, which is consistent with the general law of weakening electromagnetic coupling. However, the initial wave amplitude A... main The changing trend depends on the matching relationship between the equivalent impedance of the receiving coil and the front-end circuit, rather than simply the acoustic excitation efficiency.

[0037] If a large lift-off causes B1 to overlap with the aftershock of the launch, abandon B1 and use the transit time of B2–B3 and A instead. B2 A B3 Perform the analysis.

[0038] ③ Multidimensional damage type discrimination criteria.

[0039] Establish a three-dimensional joint criterion system of time domain, amplitude domain, and initial wave, and perform discrimination according to the following logic ( Figure 2 ): (1) Time-domain criterion - corrosion thinning judgment: if the transit time of B2 or B3 is systematically shortened relative to the reference waveform, and the normalized time shift Δt norm If Δt = (t0 – t) / t0 monotonically increases with the thinning depth, it is considered corrosion thinning. Within the thinning range of 0–2.0 mm, Δt norm It has a linear relationship with the thinning depth (R) 2 >0.95), and the thinning depth is calculated based on this calibration relationship. This criterion is not affected by the type or thickness of the adhesion layer medium. Where t is the measured transit time of B2 or B3 at the current detection point; t0 is the reference transit time under the same conditions without damage.

[0040] In practical engineering, t0 can be obtained in the following two ways: Method 1: Reference point calibration method (preferred).

[0041] Select a reference point with known wall thickness and no damage on the pipe section to be inspected, and collect the EMAT signal at this point. Measure the transit time of B2 or B3 as t0. For long-distance inspection, multiple reference points can be set at intervals along the pipeline, and the nearest reference point t0 is used for each inspection section.

[0042] Method 2: Acquiring knowledge through engineering drawings.

[0043] The reference transit time t0 without thinning is set by using the design drawings of the corresponding steel structure and the general engineering thickness of similar steel structure standard components.

[0044] (2) Amplitude Criterion – Surface Crack Judgment (Executed when ToF is not offset): If the amplitude of each bottom wave decreases approximately linearly and monotonically with the increase of crack width, and ToF remains unchanged, the echo shape and phase remain constant, and the initial wave amplitude is basically stable, then it is judged as a surface opening crack. The physical essence of linear attenuation is: the crack cuts off the closed loop of the eddy current, and the area of ​​the effective Lorentz force region decreases linearly with the crack width. This is a spatially selective blocking, which is fundamentally different from the double exponential attenuation of the lift-off effect (the lift-off amplitude follows...). That is, as the lift-off distance increases, an exponential decay of amplitude occurs in both the electromagnetic ultrasonic excitation and reception stages, resulting in an overall double exponential decay. This indicates the echo amplitude in the lift-off state. This indicates the echo amplitude when there is no lift-off and the tested steel structure is undamaged. Indicates the distance to lift off. It is related to the attenuation coefficient, coil size, and the material being tested.

[0045] (3) Amplitude-Initial Wave Joint Criterion – Simple Lift-off Judgment (Executed when ToF is not offset and amplitude decays nonlinearly): If the amplitude of each bottom wave follows a double exponential decay with the increase of lift-off, and is accompanied by a monotonically increasing initial wave amplitude, it is determined to be a simple increase in lift-off or thickening of the adhesion layer. The direction of rise and fall of the initial wave amplitude thus becomes a robust auxiliary criterion for distinguishing between "increased lift-off" and "crack existence". The physical mechanism that causes the initial wave to rise is that lift-off leads to an increase in load impedance. The output efficiency of the power amplifier at the transmitting end increases slightly due to load mismatch. Although the excited acoustic wave energy decreases, the intensity of the electromagnetic crosstalk signal (i.e., the initial wave) directly coupled to the receiving end increases due to the increase in the Q value of the resonant circuit.

[0046] (4) Identification under combined working conditions: When corrosion thinning, lift-off and cracks coexist, the time-domain criterion takes precedence—corrosion thinning is identified as long as the wall thickness changes and the ToF shift occurs. If the ToF remains unchanged, cracks and lift-off are identified based on the amplitude attenuation mode (linear vs. exponential) and the direction of the initial wave rise and fall.

[0047] (5) Quantization discrimination method for amplitude decay mode: In the aforementioned amplitude domain criteria, the "linear monotonically decreasing amplitude" caused by cracks and the "double exponential decay" caused by lift-off have different physical essences: the former stems from the crack's selective spatial blocking of the eddy current closed loop, with the effective Lorentz force region area decreasing linearly with the crack width; the latter stems from the global attenuation of electromagnetic coupling efficiency caused by increased lift-off, with sound wave energy loss accumulating exponentially with propagation distance. Their mathematical models are as follows: Linear crack model: A(w) = A0(1 - k·w) Lift-off exponential model: A(l) = a·e^ (-b·l) + c·e^ (-d·l) Where w is the crack width, l is the lift-off distance, and A0, k, a, b, c, and d are all pre-calibrated parameters of the system. k is the crack loss coefficient, which is related to the coil size and the material under test; a and c are the loss ratios during the transmission and reception stages, which are preset by the model; b and d are the attenuation coefficients, which are related to the coil size and the material under test.

[0048] To reliably distinguish between the two attenuation modes, this invention employs the following quantization discrimination process: Step 1: Feature extraction.

[0049] For the current detection point P, extract and calculate the following features: ① ΔToF = ToF P - ToF R (Transit time difference with reference point, where R is the reference detection point, typically a point without damage); ②ΔA = (A R - A P ) / A R (Relative attenuation rate of amplitude); ③ ΔA main = (A mainP - A mainR ) / A mainR (Relative rate of change of the initial wave); Step 2: Time-domain criterion (highest priority).

[0050] If |ΔToF|>th tof (where th) tof This is the threshold for determining whether the initial wave amplitude remains unchanged. It can be adjusted according to specific circumstances. In this system, it is set to th. tof If the wall thickness change is 0.1 mm, then it is defined as corrosion thinning, and the process ends.

[0051] Step 3: Amplitude-initial wave joint criterion.

[0052] If |ΔToF| ≤ th tof Then, a preliminary judgment is made based on the direction of the initial wave change: (a) If |ΔA main | ≤ δ th (where δ) th This refers to the threshold for determining whether the initial wave amplitude remains unchanged; it can be adjusted according to specific circumstances. The measured amplitude A P By matching the crack linear calibration curve A(w) with the inverse function, the crack width w is estimated. est = (A0- A P ) / (A 0·k ), calculate the confidence level C of the linear model. lin If C lin >C th If it is determined to be a surface crack, then w is output. est , where w est The crack width is estimated based on the amplitude variation; A P It is the measured amplitude; A 0·k This corresponds to the rate of change in crack amplitude of the material, which needs to be measured in advance in actual engineering; C lin It is a parameter used to describe the degree of fit between the amplitude change and the calibration curve; C th It is a linear threshold, when Clin If the threshold is exceeded, it can be considered that there is a linear relationship between the crack width and amplitude changes, and it is judged that a crack has appeared.

[0053] (b) If Δ|A main >δ th (Initial wave rise): The measured amplitude A P By matching the liftoff index calibration curve A(l), the liftoff distance l is estimated numerically. est (make A(l) est ) = A P ), calculate the confidence level C of the exponential model. exp If C exp >C th If it is determined to be a simple lift-off, output l. est .

[0054] Step 4: Boundary augmentation (executed when the confidence level in Step 3 is insufficient) If C lin and C exp All are lower than C th If the amplitudes of multiple echoes from the same detection point are close to each other, then: {A} B1 A B2 A B3 Perform linear model A = α·w + β and exponential model A = γ·e respectively. (-δ·n) The least-squares fit of +ε (where n is the echo number, w is the crack width, and α, β, γ, and δ are parameters adjusted according to the fit) yields the goodness-of-fit R. 2 lin and R 2 exp If R 2 lin - R 2 exp >ΔR 2 th If R 2 exp - R 2 lin >ΔR 2 th If the result is positive, it is considered a lift-off; otherwise, it is marked as low confidence, and it is recommended to adjust the parameters and retest.

[0055] ④ Quantitative assessment and result output Corrosion thinning: through pre-calibration of "Δ tnorm – Outputs the residual wall thickness based on the linear relationship between the “thinning depth” and the actual wall thickness. Crack: Outputs an estimated crack width by pre-calibrating a monotonic mapping of "amplitude attenuation rate – crack width"; Simple removal: Outputs the conclusion "Adhesive layer thickened, no obvious structural damage", indicating the need for cleaning.

[0056] The following will further describe specific embodiments of the present invention. These embodiments are merely illustrative and do not mean that the present invention is limited to the following examples.

[0057] Example 1: Finite element simulation verification of the penetration mechanism.

[0058] Two-dimensional axisymmetric model constructed using COMSOL Multiphysics ( Figures 3-6 The geometric domain, from top to bottom, includes the EMAT probe domain (permanent magnet + excitation coil), the air domain, the simulated attachment layer domain (2–5 mm variable), and the Q235B steel substrate domain. The permanent magnet simulates N52 neodymium iron boron, the coil has an equivalent rectangular cross-section of 22 turns, the bottom surface of the coil is 2 mm from the substrate, and a Hanning window modulated sinusoidal pulse excitation (center frequency 2 MHz) is applied.

[0059] Simulation results show that under a 5 mm equivalent hard adhesion layer, the bottom echo packet retains identifiable characteristics, and the echo arrival time is strictly consistent with the unattached state. The adhesion layer mainly causes the signal amplitude to decay exponentially with lift-off, but does not change the propagation time of the ultrasonic wave inside the metal. This simulation mechanistically confirms that the adhesion layer mainly acts as an acoustic barrier rather than an electromagnetic barrier in EMAT detection, providing a theoretical basis for subsequent experiments.

[0060] Example 2: Corrosion thinning detection and quantitative calibration.

[0061] The tested materials were 5 mm and 10 mm thick Q235B steel plates. Corrosion thinning was simulated by machining flat-bottomed holes on the bottom surface, with depths set to 0, 1.0, 1.5, and 2.0 mm, respectively. The adhesion layer thickness was set to 0, 1.5, 2.0, 3.5, and 5.0 mm, respectively, and the media types were air and simulated biological adhesion. A total of 18 complete working conditions were tested.

[0062] Under conditions of 5 mm steel plate and 0 mm lift-off, data were acquired at 2.9 MHz / 1 pulse / 20 dB / 100 average acquisitions. As the thinning depth increased, the arrival times of B2 and B3 continuously shifted towards the direction of the initial wave. Under the condition of 5.0 mm adhesion layer, B1 was mixed with residual vibration and could not be extracted. By using B2–B3 transit time analysis, the changes in thinning depth were still stably reflected.

[0063] Figures 7-10 The relationship between B2 / B3 normalized time-shift and thinning depth is shown for 5 mm and 10 mm steel plates under various lift-off conditions: linearity R in the range of 0–2.0 mm. 2>0.95; the overall migration of 5 mm plates is higher than that of 10 mm plates (thin-walled plates are more sensitive); the difference between bioattachment and air lift-off is <5%, and the monotonic dependence is unaffected.

[0064] Example 3: Detection of surface crack openings.

[0065] The materials under inspection were 5 mm and 10 mm thick Q235B steel plates. Cracks were machined by wire EDM, with widths set to 0, 1, and 2 mm respectively. Lift-off thicknesses were set to 0, 1.0, 1.5, and 2.5 mm, simulating a biofilm layer of equal thickness. A total of 12 working conditions were tested.

[0066] Under the conditions of 5 mm steel plate and 1.5 mm biofilm: a 1 mm crack reduces the B2 amplitude by approximately 30% (the reference benchmark is the average B2 amplitude of a crack-free sample (crack width = 0 mm) under the same lift-off conditions), and a 2 mm crack reduces it by approximately 40%. The ToF of each bottom wave remains strictly unchanged, the echo morphology and phase are constant, and the initial wave amplitude remains essentially unchanged. In the 10 mm steel plate, B3 attenuates more significantly due to crossing the crack region twice, and is submerged in noise first under the 2 mm crack, providing additional sensitivity for crack severity quantification.

[0067] Example 4: Comparison of waveform characteristics and application of criteria for three typical operating conditions.

[0068] Reference Figures 11-13 Under conditions of 10 mm steel plate and 1.5 mm simulated biofilm layer, three states were compared: no damage, corrosion thinning (2.0 mm), and cracks (2.0 mm): Corrosion thinning: The transit time of B2 is shortened by about 12% compared to the undamaged state; the amplitude is reduced to about 55%; the initial wave amplitude remains unchanged.

[0069] Surface cracks: B2 transit time remains unchanged; amplitude decreases to approximately 60%; initial wave amplitude remains unchanged. Amplitude exhibits linear decay.

[0070] Simple lift-off: B2 transit time remains unchanged; amplitude decreases to approximately 35%; initial amplitude increases by approximately 8%. Amplitude exhibits exponential decay.

[0071] It should be noted that the comparison benchmarks for the above amplitude attenuation percentages are all under the condition of "no damage, no lift-off". This set of data shows that the signal attenuation caused by the simple lift-off effect (reduced to 35%) is much greater than the attenuation caused by the simple crack effect (reduced to 60%). This phenomenon stems from the physical mechanism of EMAT technology—increased lift-off leads to an exponential decrease in electromagnetic coupling efficiency, and its influence is dominant; while surface cracks only linearly truncate part of the eddy current loop, and their additional attenuation effect on the signal amplitude is relatively small. These data results experimentally verify that the two modes of "exponential amplitude attenuation" and "linear amplitude attenuation" in this invention differ significantly in magnitude, providing a basis for distinguishing between simple lift-off increase and surface cracks.

[0072] Application logic of the criterion: ① Extract the transit time of B2 or B3, if Δt norm If the threshold is exceeded, it is judged as corrosion thinning, and the thinning depth is output (the thresholds involved in the above criteria are determined through experimental calibration, using the 3σ criterion to ensure a false judgment rate of less than 0.3% under non-destructive conditions. The calibration process is as follows: repeatedly measure the B2 transit time N>50 times on a non-destructive standard sample and calculate the standard deviation σ). t ≈ 8 ns, with a transit time offset threshold of 3σ. t That is, the transit time offset threshold is 0.10mm. ② If the threshold is not exceeded, the amplitude attenuation rate and the initial wave change rate are extracted: if the attenuation is linear and the initial wave is stable, it is judged as a crack (estimate the width); if the attenuation is exponential and the initial wave rises, it is judged as a simple lift-off (output is undamaged).

[0073] In summary, this invention establishes a three-dimensional joint criterion system of time domain, amplitude domain, and initial wave, utilizing the essential differences in the physical responses of corrosion thinning (ToF offset), crack (linear amplitude decay), and lift-off increase (exponential amplitude decay + initial wave rise) to achieve synchronous detection and reliable differentiation of the three working conditions.

[0074] Specifically, the three-layer working mechanism of the present invention is as follows: (1) Penetration layer - EMAT Lorentz force directly excites transverse waves in the metal skin layer (an extremely thin layer inducing eddy currents on the metal surface by an alternating magnetic field, usually only tens to hundreds of micrometers thick). The non-conductive attachment layer only exhibits lift-off and does not block the propagation of sound waves. ToF is not affected by the attachment layer; (2) Response layer - corrosion thinning shortens the sound path → ToF shifts; crack cuts off the eddy current loop → amplitude decreases linearly and ToF or initial wave remains unchanged; increased lift-off leads to double exponential decay and initial wave rises; (3) Criterion layer - ToF shift identifies corrosion; linear amplitude + stable initial wave identifies cracks; exponential amplitude + rising initial wave identifies lift-off.

[0075] The beneficial effects of this invention compared to the prior art are as follows: 1. For the first time, in-situ multi-damage detection without removing the adhesion layer has been achieved. The EMAT detection mechanism generates ultrasonic waves directly within the metal, and the non-conductive adhesion layer is simply lifted off, overcoming the bottleneck of traditional piezoelectric ultrasonic testing that requires the removal of the adhesion layer.

[0076] 2. For the first time, a three-dimensional joint criterion system of time domain, amplitude domain, and initial wave was established to achieve simultaneous differentiation of three working conditions: corrosion thinning, cracking, and lift-off. The criterion is based on the differences in physical essence, not empirical thresholds, and has a clear theoretical basis and engineering interpretability.

[0077] 3. The layered parameter strategy ensures that effective echoes can be obtained within the 0–5 mm adhesion layer range, and the high-order echo substitution strategy further expands the detection capability for thin plate lifting conditions.

[0078] 4. Experimental verification of performance indicators: Under an adhesion layer of ≤5 mm, the minimum identifiable corrosion thinning is 1.0 mm, Δt norm linearity with thinning depth R 2 The crack detection index was >0.95; it could identify 1 mm cracks and distinguish 2 mm cracks under a 2.5 mm adhesion layer. These indicators remained consistent under both air and simulated biological adhesion media.

[0079] 5. Compared with the prior art, the present invention not only has the ability to quantitatively assess wall thickness, but also establishes a synchronous differentiation mechanism between corrosion thinning and cracks, filling the gap in the prior art in the identification of multiple damage types.

[0080] The background section of this invention may include background information about the problems or environment in which the invention is being developed, and is not necessarily a description of prior art. Therefore, the content included in the background section does not constitute an admission of prior art by the applicant.

[0081] The above description provides a further detailed explanation of the present invention in conjunction with specific or preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the concept of the present invention, and all such substitutions or modifications should be considered within the scope of protection of the present invention. In the description of this specification, the reference to terms such as "an embodiment," "some embodiments," "preferred embodiment," "example," "specific example," or "some examples," etc., indicates that the specific features, structures, materials, or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made herein without departing from the scope of protection of the patent application.

Claims

1. A method for underwater steel structure damage detection and differentiation based on electromagnetic ultrasound, characterized in that, The underwater steel structure with a non-conductive coating was inspected using electromagnetic ultrasonic testing equipment, including the following steps: The electromagnetic ultrasonic transducer (EMAT) probe of the electromagnetic ultrasonic testing equipment is arranged above the surface of the underwater steel structure. The excitation parameters of the electromagnetic ultrasonic testing equipment are configured according to the lift-off distance between the electromagnetic ultrasonic transducer (EMAT) probe and the surface of the underwater steel structure. Using the configured excitation parameters, an excitation pulse is applied to the electromagnetic ultrasonic transducer (EMAT) probe via the electromagnetic ultrasonic testing device to emit ultrasonic waves; The system receives the echo signal returned by the underwater steel structure and extracts the transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics from the echo signal. The anomaly type is distinguished based on changes in at least one of the transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics, and the anomaly type includes at least one of corrosion thinning, surface opening cracks, and simple lift-off increase. The steps for distinguishing exception types include: If the transit time characteristics show that the echo signal arrival time is systematically advanced, then corrosion thinning is determined to exist; If the transit time characteristic remains unchanged while the echo amplitude characteristic shows attenuation and the initial wave amplitude characteristic remains unchanged, then it is determined that a surface opening crack exists. If the transit time characteristic remains unchanged while the echo amplitude characteristic shows attenuation and the initial wave amplitude characteristic shows an increase, then it is determined to be a simple increase in lift-off.

2. The underwater steel structure damage detection and differentiation method according to claim 1, characterized in that, By using the pre-calibrated normalized time shift Δt norm The linear relationship between Δt and the thinning depth is used to calculate the corrosion thinning depth; where Δt norm =(t0–t) / t0, where t0 is the transit time characteristic of the undamaged reference state, and t is the transit time characteristic of the current measurement.

3. The underwater steel structure damage detection and differentiation method according to claim 1, characterized in that, The width of the surface-opening crack is estimated by using the pre-calibrated monotonic mapping relationship between the echo amplitude attenuation rate and the crack width.

4. The method for detecting and differentiating underwater steel structure damage according to any one of claims 1 to 3, characterized in that, When extracting the transit time characteristics, echo amplitude characteristics, and initial wave amplitude characteristics from the echo signal, if the first bottom echo B1 of the echo signal is superimposed with the transmitted residual vibration, resulting in its characteristics being unrecognizable... If the second bottom echo B2 of the echo signal is unavailable, the third bottom echo B3 of the echo signal is used to extract the transit time feature and the echo amplitude feature. If the feature of the second bottom echo B2 is also unavailable, the third bottom echo B3 of the echo signal is used to extract the transit time feature and the echo amplitude feature, and so on, until the transit time feature and the echo amplitude feature are successfully extracted. The initial amplitude feature is always extracted from the beginning portion of the echo signal.

5. The method for detecting and differentiating damage to underwater steel structures according to any one of claims 1 to 3, characterized in that, Based on the lift-off distance between the electromagnetic ultrasonic transducer (EMAT) probe and the surface of the underwater steel structure, the excitation parameters of the electromagnetic ultrasonic testing equipment are configured as follows: when the lift-off distance is less than 0.5 mm, the selected excitation parameters include: a center frequency of 3.5-4.0 MHz and a pulse count of 1; when the lift-off distance is 0.5-1.5 mm, the selected excitation parameters include: a center frequency of 2.9-3.1 MHz and a pulse count of 2-3; when the lift-off distance is greater than 1.5 mm, the selected excitation parameters include: a center frequency of 2.7-2.9 MHz and a pulse count of 3.

6. The underwater steel structure damage detection and differentiation method according to claim 5, characterized in that, The excitation parameters also include gain parameters: when the lift-off distance is less than 0.5 mm, the gain parameter is configured to be 20-25 dB; when the lift-off distance is 0.5-1.5 mm, the gain parameter is configured to be 28-32 dB; when the lift-off distance is greater than 1.5 mm, the gain parameter is configured to be 30-38 dB.

7. A damage detection and differentiation system for underwater steel structures based on electromagnetic ultrasound, used to implement the damage detection and differentiation method for underwater steel structures as described in claim 1, characterized in that, include: An electromagnetic ultrasonic testing device includes an electromagnetic ultrasonic transducer (EMAT) probe for placement above the surface of an underwater steel structure to be tested; a parameter configuration module configured to configure excitation parameters based on the lift-off distance between the EMAT probe and the surface of the underwater steel structure; and a signal excitation and acquisition module configured to apply excitation pulses to the EMAT probe using the configured excitation parameters to emit ultrasonic waves and to receive echo signals returned by the underwater steel structure. The feature extraction module is configured to extract transit time features, echo amplitude features, and initial wave amplitude features from the echo signal; the damage discrimination module is configured to distinguish the anomaly type based on at least one change in the transit time features, echo amplitude features, and initial wave amplitude features.

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