Method for determining laser ablation threshold of material based on ultrasonic signal amplitude

Abstract

The present application relates to laser ultrasonic nondestructive testing technical field, specifically to a kind of based on ultrasonic signal amplitude judgment material laser ablation threshold method.The present application uses pulsed laser to irradiate the surface of material to be measured multiple times with energy lower than reference ablation threshold, obtains reference signal by laser vibrometer acquisition ultrasonic displacement time domain signal and average, extracts reference amplitude;According to preset energy step, gradually increase the energy of pulsed laser to form multiple energy levels, corresponding ultrasonic displacement time domain signal is collected;After band-pass filtering, detrending and normalization processing to signal, extract maximum peak-peak value, calculate the drop coefficient of each energy level average amplitude relative to reference amplitude;When first appear continuously three energy levels drop coefficient are all greater than preset threshold, the laser energy density corresponding to the minimum energy level among them is determined as laser ablation threshold.The present application only needs conventional laser vibrometer, without destructive observation afterwards, calibration is fast and accurate.

Description

Technical Field

[0001] This invention relates to the field of laser ultrasonic nondestructive testing technology, specifically a method for determining the laser ablation threshold of materials based on the amplitude of ultrasonic signals. Background Technology

[0002] Post insulators play a crucial role in power systems by isolating live conductors, primarily serving in substations and power plants. Their unique awning structure effectively increases the current leakage path and maintains insulation strength even in rain, fog, and polluted environments. To ensure the normal operation of post insulators, their service status needs to be monitored online.

[0003] Laser-ultrasonic nondestructive testing (LDPT) combines the advantages of high sensitivity and non-contact measurement, and is gradually becoming the mainstream method for detecting insulator defects. The excitation mechanism of laser-ultrasonic testing is divided into thermoelastic and ablation mechanisms, the difference being whether the incident laser power density exceeds the material's damage threshold. LDT requires the laser to operate under a thermoelastic mechanism; therefore, the laser damage threshold of the material must first be obtained. Existing threshold calibration methods mainly rely on high-speed imaging, spectral detection, or post-procedure microscopic observation, which suffers from high equipment costs, the need for destructive observation, and slow calibration speed, making it difficult to meet the needs of rapid on-site calibration. Summary of the Invention

[0004] This invention provides a method for determining the laser ablation threshold of materials based on the amplitude of ultrasonic signals, which enables rapid threshold calibration without the need for destructive post-procedure observation, thereby reducing detection costs.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention discloses a method for determining the laser ablation threshold of materials based on ultrasonic signal amplitude, comprising:

[0007] S100: The surface of the material under test is irradiated multiple times with a pulsed laser at an energy lower than the reference ablation threshold. Multiple ultrasonic displacement time-domain signals are collected by a laser vibrometer and averaged to obtain a reference signal. The maximum peak-to-peak value of the reference signal within the evaluation time window is extracted as the reference amplitude.

[0008] S200: The pulsed laser energy is gradually increased according to the preset energy step to form multiple energy levels. The surface of the material under test is irradiated multiple times at each energy level, and the corresponding ultrasonic displacement time domain signal is collected.

[0009] S300: Performs bandpass filtering, detrending and normalization on the acquired ultrasonic displacement time domain signal, and extracts the maximum peak-to-peak value of each ultrasonic displacement time domain signal within the preset evaluation time window after processing.

[0010] S400: Average the peak-to-peak values ​​obtained from multiple measurements at the same energy level to obtain the average amplitude of that energy level, and calculate the drop coefficient of the average amplitude of each energy level relative to the reference amplitude using the reference amplitude as a reference.

[0011] S500: When the drop coefficients of three consecutive energy levels are all greater than the preset threshold for the first time, the laser energy density corresponding to the lowest energy level is determined as the laser ablation threshold of the material under test.

[0012] As a preferred embodiment of the present invention, the irradiation of the surface of the material to be tested multiple times specifically includes:

[0013] The initial laser energy is fixed within the range of 0.2 to 0.3 times the reference ablation threshold, and the surface of the material to be tested is irradiated no less than 5 times;

[0014] The laser wavelength, pulse width, and spot diameter of the pulsed laser are selected based on the light absorption coefficient and thermal diffusivity of the material under test.

[0015] As a preferred embodiment of the present invention, the evaluation time window includes the time period from the arrival time of the longitudinal wave to 1.5 μs after the arrival of the surface wave.

[0016] As a preferred embodiment of the present invention, the step of gradually increasing the pulsed laser energy according to a preset energy step size specifically includes:

[0017] Set the initial energy and preset energy step size;

[0018] For each energy level, the initial energy is added to the product of the current energy level number and the preset energy step size to obtain the laser energy of the current energy level;

[0019] The energy levels are sequentially increased according to their energy level numbers, forming multiple energy levels that increase in a step-like manner.

[0020] As a preferred embodiment of the present invention, the bandpass filter has a frequency band of 0.5MHz to 15MHz.

[0021] As a preferred embodiment of the present invention, the calculation steps of the drop coefficient include:

[0022] Obtain the average amplitude and reference amplitude for each energy level;

[0023] Calculate the difference between the average amplitude of each energy level and the reference amplitude;

[0024] Divide the difference by the baseline amplitude to obtain the relative drop ratio;

[0025] The relative drop ratio is converted into a percentage form as the drop coefficient.

[0026] As a preferred embodiment of the present invention, the preset threshold is a threshold value of the drop coefficient, and the value range is 10% to 20%.

[0027] As a preferred embodiment of the present invention, the step of determining the laser ablation threshold includes:

[0028] Obtain the laser energy and laser spot diameter corresponding to the minimum energy level;

[0029] Calculate the irradiated area of ​​the laser spot based on its diameter;

[0030] The laser energy density is obtained by dividing the laser energy by the irradiated area.

[0031] The laser energy density is determined as the laser ablation threshold of the material under test.

[0032] As a preferred technical solution of the present invention, a damage verification step is also included: using a confocal microscope to scan the surface morphology of the irradiated area corresponding to the determined laser ablation threshold, and confirming the effectiveness of the laser ablation threshold when microcracks or ablation pits appear.

[0033] The beneficial effects of this invention are:

[0034] 1. This invention determines material damage by monitoring the drop characteristics of ultrasonic signal amplitude relative to a non-destructive reference, eliminating the need for post-treatment microscopic observation or surface morphology scanning of the irradiated area. This method requires only a conventional laser vibrometer to determine the ablation threshold in real time, avoiding the high costs of high-speed imaging and spectral detection equipment, and achieving rapid and low-cost threshold calibration.

[0035] 2. This invention employs a criterion that the drop coefficients of three consecutive energy levels all exceed a preset threshold. Combined with a statistical averaging of multiple measurements, this effectively avoids misjudgments caused by fluctuations in single-point measurements and localized material inhomogeneities. This determination mechanism improves the reliability and repeatability of ablation threshold calibration, making it particularly suitable for online safety parameter calibration of brittle materials such as ceramics.

[0036] 3. This invention achieves precise capture of the critical point of transition from thermoelastic to ablation mechanisms based on the drop characteristics of ultrasonic signal amplitude, organically combining non-destructive testing with threshold calibration. This method can complete the entire threshold calibration process without damaging the material, providing accurate and safe energy parameters for subsequent laser ultrasonic non-destructive testing, ensuring the true non-destructive nature of the testing process. Attached Figure Description

[0037] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0038] Figure 1 This is a flowchart illustrating a method for determining the laser ablation threshold of materials based on ultrasonic signal amplitude according to the present invention.

[0039] Figure 2 This is a schematic diagram of the experimental system of the present invention;

[0040] Figure 3 This is a comparison diagram of the ultrasonic displacement time-domain signals obtained by simulation at different energy levels according to the present invention;

[0041] Figure 4 This is a comparison diagram of the ultrasonic displacement time-domain signals collected in experiments at different energy levels according to the present invention. Detailed Implementation

[0042] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0043] Example 1: As Figure 1 As shown, the present invention provides a method for determining the laser ablation threshold of materials based on the amplitude of ultrasonic signals, comprising:

[0044] S100: The surface of the material under test is irradiated multiple times with a pulsed laser at an energy lower than the reference ablation threshold. Multiple ultrasonic displacement time-domain signals are collected by a laser vibrometer and averaged to obtain a reference signal. The maximum peak-to-peak value of the reference signal within the evaluation time window is extracted as the reference amplitude.

[0045] Assuming the laser beam has a Gaussian distribution and considering symmetry, cylindrical coordinates are used. Without considering the changes in thermodynamic parameters with temperature, the temperature distribution inside the material Satisfies the classical heat conduction equation:

[0046] ;

[0047] In the formula, The density of the material; Specific heat capacity of the material; The thermal conductivity coefficient; For heat source items; This represents the time variable of the laser action process.

[0048] For post insulator materials, laser energy absorption mainly occurs at the material surface. Therefore, the laser source can be considered equivalent to a heat source boundary condition applied to the material surface. In the above equation, let... The boundary conditions are changed to:

[0049] ;

[0050] The spatial and temporal distributions of the laser are as follows:

[0051] ;

[0052] ;

[0053] In the formula, The maximum power density of the incident laser; The light absorption rate of the material surface; The radius of the light spot; This represents the laser pulse width.

[0054] The aforementioned heat conduction equations demonstrate that the selection of laser parameters (wavelength, pulse width, and spot diameter) needs to match the material's optical absorption coefficient and thermal diffusivity to ensure that the laser action operates within a thermoelastic mechanism when establishing the reference signal, thus preventing damage to the material. This provides a theoretical basis for the rational selection of laser parameters in subsequent steps.

[0055] Furthermore, the irradiation of the surface of the material to be tested multiple times specifically includes:

[0056] The initial laser energy is fixed within the range of 0.2 to 0.3 times the reference ablation threshold, and the surface of the material to be tested is irradiated no less than 5 times;

[0057] The laser wavelength, pulse width, and spot diameter of the pulsed laser are selected based on the light absorption coefficient and thermal diffusivity of the material under test.

[0058] Furthermore, the evaluation time window includes the period from the arrival time of the longitudinal wave to 1.5 μs after the arrival of the surface wave.

[0059] Specifically, in this step, a non-destructive excitation reference is first established. Based on the optical absorption coefficient and thermal diffusivity of the material under test, the laser wavelength, pulse width, and spot diameter of the pulsed laser are selected. The initial laser energy is then... Fixed at the reference ablation threshold Within the range of 0.2 to 0.3 times, of which This is a reference threshold obtained from previous experiments. This energy range ensures that the laser action is within a thermoelastic mechanism and will not damage the material surface.

[0060] The surface of the material under test was irradiated multiple times with a pulsed laser, and the ultrasonic displacement time-domain signal was collected after each irradiation using a laser vibrometer. Preferably, the number of irradiations No less than 5 times, the first The signal collected in this measurement is denoted as The collected data The arithmetic mean of the ultrasound signals was used to obtain a non-destructive reference signal. :

[0061] ;

[0062] The evaluation time window needs to include complete ultrasonic information. Specifically, the evaluation time window starts from the arrival time of the P-wave. Start, until the preset time after the surface wave arrives End, i.e., time window is Preferably, the preset time after the surface wave arrives is 1.5 μs, which is sufficient to capture the complete waveform characteristics of the surface wave. Longitudinal wave arrival time. Based on the longitudinal wave velocity of the material and the distance from the laser irradiation point to the measurement point The calculation yielded:

[0063] ;

[0064] The arrival time of surface waves can be determined based on the surface wave velocity. Calculations show that the surface wave velocity is typically about 0.9 times that of the longitudinal wave velocity. For the C130 post insulator material used in this embodiment, the material parameters, including density, are set according to the C130 column in the national standard GB8411.3-2009. Elastic modulus Compared to Poisson The longitudinal wave velocity can be calculated:

[0065] ;

[0066] After substituting the parameters, the calculated longitudinal wave velocity is approximately 6200 m / s, and the corresponding surface wave velocity is approximately 5580 m / s. In this embodiment, a laser wavelength of 532 nm, a pulse width of 10 ns, and a spot diameter of 0.6 mm are selected.

[0067] Extracting the reference signal During the evaluation time window The maximum peak-to-peak value within the range is used as the reference amplitude. :

[0068] ;

[0069] The benchmark amplitude The ultrasonic response characteristics of the material in a non-destructive state serve as a reference for subsequent judgment on whether laser damage has occurred. Since the laser energy is within the thermoelastic mechanism range at this time, no ablation damage occurs on the material surface, and the amplitude of the excited ultrasonic signal remains stable.

[0070] S200: The pulsed laser energy is gradually increased according to the preset energy step to form multiple energy levels. The surface of the material under test is irradiated multiple times at each energy level, and the corresponding ultrasonic displacement time domain signal is collected.

[0071] Furthermore, the stepwise increase of pulsed laser energy according to a preset energy step size specifically includes:

[0072] Set the initial energy and preset energy step size;

[0073] For each energy level, the initial energy is added to the product of the current energy level number and the preset energy step size to obtain the laser energy of the current energy level;

[0074] The energy levels are sequentially increased according to their energy level numbers, forming multiple energy levels that increase in a step-like manner.

[0075] Specifically, after establishing the reference signal, a stepped scanning of the laser pulse energy begins. First, the initial energy is set. and preset energy step size Initial energy Typically, the same energy value as in step S100 is used, i.e., it is at the reference ablation threshold. Within the range of 0.2 to 0.3 times. Preset energy step size. The selection of [the appropriate method] needs to consider both detection accuracy and efficiency; preferably, The range is 1 mJ to 3 mJ.

[0076] For each energy level, the initial energy is added to the product of the current energy level index and the preset energy step size to obtain the laser energy of the current energy level. Specifically, the energy of the i-th energy level... The calculation formula is:

[0077] ;

[0078] in, Energy level number The energy levels are sequentially numbered, forming a stepped increase in number. The total number of energy levels... Based on the set maximum energy Determined, preferably, Set to 50 mJ to ensure coverage of the full energy range of the material, from undamaged to damaged.

[0079] At each energy level Under these conditions, the same or adjacent locations on the surface of the material under test are irradiated multiple times. Preferably, the number of irradiations per energy level is... At least three irradiations were performed to obtain statistically reliable data. After each irradiation, the corresponding ultrasonic displacement time-domain signal was synchronously acquired using a laser vibrometer. ,in Indicates the energy level number. This indicates the first energy level. Second measurement, .

[0080] The time window for acquiring the ultrasonic displacement time-domain signal is consistent with the evaluation time window in step S100, i.e., from the arrival time of the longitudinal wave. 1.5 μs after the surface wave arrives In this embodiment, the following is set: The initial energy is 2mJ. The maximum energy is 8 mJ (approximately 0.25 times the reference threshold). The energy level was 50 mJ, thus forming a total of 21 energy levels. Each energy level was irradiated five times, and five sets of ultrasonic displacement time-domain signals were collected. .

[0081] This stepped energy scanning method allows for the systematic observation of how the amplitude of the ultrasonic signal changes with increasing laser energy, thereby accurately capturing the critical point at which a material transitions from a thermoelastic mechanism to an ablation mechanism.

[0082] S300: Performs bandpass filtering, detrending and normalization on the acquired ultrasonic displacement time domain signal, and extracts the maximum peak-to-peak value of each ultrasonic displacement time domain signal within the preset evaluation time window after processing.

[0083] Furthermore, the bandpass filter operates in the frequency range of 0.5MHz to 15MHz.

[0084] Specifically, in this step, the ultrasonic displacement time-domain signal acquired in step S200 is... Preprocessing is performed to improve signal quality and extract effective features. Preprocessing includes three processes: bandpass filtering, detrending, and normalization.

[0085] First, bandpass filtering is performed. Since the ultrasonic signal excited by a laser is distributed within a certain frequency range, low-frequency drift and high-frequency noise are inevitably introduced during the acquisition process. Bandpass filtering can preserve the main frequency components of the ultrasonic signal while suppressing out-of-band noise. The bandpass filtering frequency needs to be determined based on material properties and ultrasonic propagation characteristics; preferably, the frequency band is set to 0.5MHz to 15MHz. The lower limit of 0.5MHz effectively filters out low-frequency interference introduced by environmental vibrations and electronic equipment, while the upper limit of 15MHz covers the main frequency components of the laser ultrasonic signal. In this embodiment, a digital bandpass filter is used, with the frequency band set to 0.5MHz to 15MHz.

[0086] Next, detrending processing is performed. Detrending refers to removing low-frequency trend terms or DC components from the signal, which are usually caused by zero-point drift or baseline offset of the measurement system. Through detrending processing, the baseline of the signal can be made to approach zero, which facilitates the accurate extraction of subsequent amplitude characteristics. Common detrending methods include subtracting the signal mean or fitting and removing low-order polynomial trends.

[0087] Finally, normalization is performed. Normalization maps the signal amplitude to a uniform numerical range, eliminating amplitude differences under different measurement conditions. A common normalization method is to divide the signal by its maximum absolute value, so that the normalized signal amplitude range is [-1, 1]. Normalization is beneficial for comparative analysis of signals at different energy levels.

[0088] After the above three preprocessing steps, the processed ultrasonic displacement time-domain signal is obtained. For each processed signal, within a preset evaluation time window... Internal extraction of maximum peak-to-peak value The peak-to-peak value is defined as the difference between the maximum and minimum values ​​of the signal within that time window.

[0089] ;

[0090] The peak-to-peak value reflects the energy intensity of ultrasound and is a key characteristic parameter for determining whether laser damage has occurred in a material. When ablation damage occurs on the material surface, the excitation efficiency and propagation characteristics of ultrasound change, leading to a significant decrease in the peak-to-peak value.

[0091] S400: Average the peak-to-peak values ​​obtained from multiple measurements at the same energy level to obtain the average amplitude of that energy level, and calculate the drop coefficient of the average amplitude of each energy level relative to the reference amplitude using the reference amplitude as a reference.

[0092] Furthermore, the calculation steps for the drop coefficient include:

[0093] Obtain the average amplitude and reference amplitude for each energy level;

[0094] Calculate the difference between the average amplitude of each energy level and the reference amplitude;

[0095] Divide the difference by the baseline amplitude to obtain the relative drop ratio;

[0096] The relative drop ratio is converted into a percentage form as the drop coefficient.

[0097] Specifically, in this step, the maximum peak-to-peak value extracted in step S300 is statistically analyzed to calculate the average amplitude of each energy level and its drop coefficient relative to the reference amplitude.

[0098] First, the peak-to-peak values ​​obtained from multiple measurements at the same energy level are arithmetically averaged. For the i-th energy level, a total of k measurements are performed at that energy level, resulting in k peak-to-peak values. Calculate the average amplitude of this energy level. :

[0099] ;

[0100] Averaging multiple measurements can reduce random errors from a single measurement and improve the reliability of the results. In this embodiment, five measurements are taken at each energy level, i.e., k=5.

[0101] Next, using the reference amplitude obtained in step S100 For reference, calculate the drop factor of the average amplitude of each energy level relative to the reference amplitude. The drop factor is used to quantify the degree of decrease in ultrasonic signal amplitude relative to a non-destructive reference, and is a core criterion for determining whether laser damage has occurred in a material.

[0102] The calculation of the drop coefficient specifically includes the following steps: First, obtain the average amplitude of the i-th energy level. Compared with the reference amplitude Then calculate the difference between the average amplitude and the reference amplitude. This difference represents the absolute decrease in signal amplitude; divide this difference by the reference amplitude. This yields the relative drop ratio; finally, the relative drop ratio is converted to a percentage form, which is then multiplied by 100% to obtain the drop coefficient. :

[0103] ;

[0104] Drop coefficient This reflects the degree of attenuation of the ultrasonic signal amplitude relative to the undamaged state. Under the thermoelastic mechanism, the laser energy does not exceed the material damage threshold, the material surface remains intact, and the ultrasonic excitation efficiency is stable, resulting in a small drop coefficient, typically below 5%. When the laser energy exceeds the damage threshold and enters the ablation mechanism, microcracks or ablation pits appear on the material surface. The ultrasonic excitation efficiency decreases, and the propagation path changes, leading to a significant decrease in the amplitude of the ultrasonic signal received at the measurement point and a sharp increase in the drop coefficient. This amplitude abrupt change phenomenon is the core principle of this invention for determining the ablation threshold.

[0105] S500: When the drop coefficients of three consecutive energy levels are all greater than the preset threshold for the first time, the laser energy density corresponding to the lowest energy level is determined as the laser ablation threshold of the material under test.

[0106] Furthermore, the preset threshold is the threshold of the drop coefficient, and its value ranges from 10% to 20%.

[0107] Furthermore, the step of determining the laser ablation threshold includes:

[0108] Obtain the laser energy and laser spot diameter corresponding to the minimum energy level;

[0109] Calculate the irradiated area of ​​the laser spot based on its diameter;

[0110] The laser energy density is obtained by dividing the laser energy by the irradiated area.

[0111] The laser energy density is determined as the laser ablation threshold of the material under test.

[0112] Specifically, in this step, the drop coefficients of each energy level calculated in step S400 are used. To determine whether the material has suffered laser damage and to determine the laser ablation threshold.

[0113] First, set the preset threshold. As a criterion for determining material damage. Preset threshold. The threshold value for the drop coefficient is used to distinguish between the undamaged and damaged states of the material. When the drop coefficient exceeds this threshold, it indicates a significant decrease in the ultrasonic signal amplitude, suggesting that the material may have undergone laser damage. The value of the preset threshold needs to comprehensively consider measurement error and material characteristics; a value that is too low may lead to misjudgment, while a value that is too high may miss actual damage. Preferably, the preset threshold value is... The value range is 10% to 20%. In this embodiment, a preset threshold is used. Set to 15%.

[0114] Damage assessment uses a continuity criterion, which is that when the drop coefficients of three consecutive energy levels are all greater than a preset threshold for the first time... At that time, laser damage is determined to have occurred in the material. The specific determination process is as follows: the drop coefficient of each energy level is checked one by one. When the first Each energy level satisfies , No. Each energy level satisfies , No. Each energy level satisfies When the drop coefficients of three consecutive energy levels are all greater than a preset threshold, the material is determined to have started to show damage at the i-th energy level. Using a criterion of three consecutive energy levels instead of a single energy level can effectively avoid misjudgments caused by measurement fluctuations or local inhomogeneities, thus improving the reliability of threshold calibration.

[0115] After determining that material damage has occurred, record the sequence number of the lowest energy level that meets the conditions. The corresponding laser energy Let be the critical energy of the material being tested. The specific steps for determining the laser ablation threshold include: first, obtaining the laser energy corresponding to the minimum energy level. And the laser spot diameter d; then calculate the irradiation area of ​​the spot based on the laser spot diameter. For a circular spot, the irradiation area is... Next, the laser energy is divided by the irradiated area to obtain the laser energy density. Finally, the laser energy density was determined as the laser ablation threshold of the material under test.

[0116] ;

[0117] In the formula, Laser ablation threshold, in units of ; The critical energy is expressed in J. The laser spot diameter is measured in cm. In this embodiment, when the 13th energy level (32 mJ) first exhibits a drop factor greater than 15% for three consecutive energy levels, it is determined that... Spot diameter The laser ablation threshold was calculated. .

[0118] To verify the accuracy of the calibrated laser ablation threshold, after determining the critical energy, a damage verification step is also included: using a confocal microscope to scan the surface morphology of the irradiated area corresponding to the determined laser ablation threshold, and confirming the validity of the laser ablation threshold when microcracks or ablation pits appear.

[0119] Specifically, confocal microscopy is used to study the critical energy... The surface morphology of the corresponding laser-irradiated area is scanned. Confocal microscopy has high spatial resolution and three-dimensional imaging capabilities, enabling clear observation of microscopic morphological changes on the material surface.

[0120] After scanning the irradiated area, check the surface for microcracks or ablation pits. Microcracks typically appear as a network of fine cracks on the surface, while ablation pits appear as depressions or traces of material removal. The presence of microcracks or ablation pits confirms that laser damage has indeed occurred on the material surface, and the calibrated laser ablation threshold is effective. If no obvious damage characteristics are observed on the surface morphology, the preset threshold needs to be readjusted. Alternatively, check the measurement system to ensure the accuracy of the threshold calibration.

[0121] The damage verification step allows the ablation threshold determined based on the ultrasonic signal amplitude to be correlated with actual material damage, thus verifying the effectiveness and reliability of the method of this invention. In this embodiment, confocal microscopy scanning was performed on the irradiation point corresponding to the critical energy of 32 mJ, and shallow ablation pits with a diameter of approximately 50 μm and radial microcracks were observed on the surface, confirming that laser damage did indeed occur in the material and verifying the accuracy of the calibrated ablation threshold.

[0122] Example 2: To verify the effectiveness of the method of the present invention in practical applications, this example uses C130 post insulator material as the object and conducts a calibration experiment on the laser ablation threshold.

[0123] like Figure 2 As shown, the experimental system mainly includes an Nd:YAG tunable pulsed laser (wavelength 532nm, pulse width 10ns), a laser vibrometer, an oscilloscope, and the material under test. The pulsed laser light generated by the laser is focused by an optical lens onto the surface of the material under test on a cylindrical mirror. The laser vibrometer measures the ultrasonic displacement signal from the back of the material, and the oscilloscope displays and acquires the time-domain waveform of the ultrasonic displacement in real time. The material under test is C130 aluminum-ceramic post insulator material conforming to the national standard GB8411.3-2009, with a thickness of 5mm. Based on previous experiments, the reference ablation threshold of this material is approximately 30-35 mJ.

[0124] Threshold calibration was performed according to the method of this invention. First, the initial laser energy was set to 8 mJ (approximately 0.25 times the reference threshold) to establish a non-destructive excitation reference. Subsequently, the laser energy was increased stepwise in 2 mJ increments, with each energy level irradiated 5 times and ultrasonic displacement time-domain signals were acquired.

[0125] To verify the relationship between the amplitude of the ultrasonic signal and the laser energy, a theoretical analysis was first conducted through numerical simulation. Figure 3 This figure shows a comparison of time-domain ultrasonic displacement signals at different energy levels obtained from simulations based on the heat conduction equation. The horizontal axis represents time, and the vertical axis represents displacement amplitude. The figure uses… Indicates the reference ablation threshold In the illustration The values ​​represent the energy levels corresponding to 0.3, 0.4, 0.5, and 0.6 times the reference ablation threshold, respectively. Simulation results show that as the laser energy increases, the ultrasonic signal amplitude exhibits a trend of first stabilizing and then decreasing, which provides a theoretical basis for experimental verification.

[0126] Figure 4The experiment showcases typical ultrasonic displacement time-domain signals acquired at different energy levels. The figures show that at lower energies, the ultrasonic signal waveforms are clear and complete with large amplitudes. As the energy increases, the signal amplitude decreases significantly, consistent with the simulation trend, directly demonstrating the material's transition from a thermoelastic to an ablation mechanism. Based on material parameters, the arrival time of the longitudinal wave is calculated to be approximately 0.8 μs, and the evaluation time window is set from 0.8 μs to 2.3 μs (1.5 μs after the surface wave arrival). After bandpass filtering, detrending, and normalization of the acquired signals (0.5 MHz–15 MHz), the maximum peak-to-peak value of each signal within the evaluation time window is extracted.

[0127] By calculating the average amplitude of each energy level and its drop coefficient relative to the baseline amplitude, it was found that the drop coefficient was less than 8% in the energy range of 10mJ to 30mJ, indicating that the material was in a damage-free thermoelastic mechanism. When the energy reached 32mJ, the drop coefficient suddenly increased to 17.3%, exceeding the preset threshold of 15% for the first time. Further increasing the energy to 34mJ and 36mJ, the drop coefficients were 22.8% and 28.3%, respectively. All three energy levels exceeded the preset threshold, meeting the damage assessment criteria.

[0128] Therefore, the critical energy is determined. The value is 32 mJ. The laser ablation threshold is calculated based on a spot diameter of 0.6 mm. It is 11.3 J / cm².

[0129] Using confocal microscopy to study critical energy Surface morphology observation of the corresponding irradiated area revealed shallow ablation pits with a diameter of approximately 50 μm, confirming that the material did indeed suffer laser damage and verifying the accuracy of the calibration results of this invention.

[0130] Compared with the traditional "point-by-point irradiation-microscopic observation" method, the method of this invention has the following significant advantages: First, this invention can determine the occurrence of damage by monitoring the changes in the amplitude of the ultrasonic signal in real time, eliminating the need for post-operative microscopic observation, thus improving calibration efficiency by approximately 3 times; Second, this method can detect damage as soon as it begins to occur in the material, and the calibrated threshold is closer to the true critical state; Third, by using a quantitative drop coefficient as the criterion and employing the "three consecutive points" criterion, the influence of subjective judgment in traditional microscopic observation methods is effectively avoided. In five repeated experiments, the relative standard deviation of the calibration threshold of this method was only 6.2%, demonstrating good repeatability; Finally, this method can accurately capture the critical point of damage occurrence, controlling the necessary damage location within a minimal range, avoiding the problem of damage traces appearing at multiple locations in traditional methods.

[0131] This embodiment demonstrates that the method of the present invention can quickly and accurately calibrate the laser ablation threshold of C130 post insulator material, providing a reliable and safe parameter basis for subsequent online non-destructive testing of this type of material using laser ultrasonic technology. In practical applications, the calibrated threshold can be used as a reference. Choosing an appropriate laser energy (typically 60%–80% of the threshold) ensures sufficient ultrasonic signal intensity for defect detection while preventing damage to the material. Furthermore, this method is not only applicable to post insulator materials but also suitable for laser ablation threshold calibration of other brittle materials such as ceramics and composites, demonstrating broad engineering application prospects.

[0132] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the laser ablation threshold of materials based on ultrasonic signal amplitude, characterized in that, The method comprises the following steps: S100: irradiating the surface of the material to be tested with pulsed laser at an energy lower than the reference ablation threshold value multiple times, collecting multiple ultrasonic displacement time domain signals by a laser vibration meter and averaging to obtain a reference signal, and extracting the maximum peak-to-peak value of the reference signal in the evaluation time window as the reference amplitude; S200: increasing the energy of the pulsed laser by a preset energy step to form multiple energy levels, irradiating the surface of the material to be tested multiple times at each energy level, and collecting corresponding ultrasonic displacement time domain signals; S300: band-pass filtering, detrending and normalizing the collected ultrasonic displacement time domain signals, and extracting the maximum peak-to-peak value of each ultrasonic displacement time domain signal in a preset evaluation time window; S400: averaging the maximum peak-to-peak values obtained by multiple measurements at the same energy level to obtain the average amplitude of the energy level, and calculating the drop coefficient of the average amplitude of each energy level relative to the reference amplitude; S500: when the drop coefficients of the first three consecutive energy levels are all greater than a preset threshold value, the laser energy density corresponding to the minimum energy level is determined as the laser ablation threshold value of the material to be tested; The calculation step of the drop coefficient comprises: obtaining the average amplitude of each energy level and the reference amplitude; calculating the difference between the average amplitude of each energy level and the reference amplitude; dividing the difference by the reference amplitude to obtain the relative drop ratio; converting the relative drop ratio to percentage form as the drop coefficient.

2. The method of claim 1, wherein, The multiple irradiations of the surface of the material to be tested specifically comprise: fixing the initial energy of the laser in the interval of 0.2 to 0.3 times the reference ablation threshold value, and irradiating the surface of the material to be tested not less than 5 times; The laser wavelength, pulse width and spot diameter of the pulsed laser are selected according to the optical absorption coefficient and thermal diffusivity of the material to be tested.

3. The method of claim 1, wherein the method is characterized by: The step of increasing the energy of the pulsed laser by a preset energy step specifically comprises: setting the initial energy and the preset energy step; for each energy level, adding the product of the initial energy, the current energy level number and the preset energy step to obtain the laser energy of the current energy level; increasing the energy level number in sequence to form multiple energy levels in a stepwise manner.

4. The method of claim 1, wherein, The frequency band of the band-pass filtering is 0.5MHz to 15MHz.

5. The method of claim 1, wherein the method is based on the amplitude of the ultrasonic signal to determine the laser ablation threshold of the material. The preset threshold value is the threshold value of the drop coefficient, and the value range is 10% to 20%.

6. The method of claim 1, wherein, The determination step of the laser ablation threshold value comprises: obtaining the laser energy and the laser spot diameter corresponding to the minimum energy level; calculating the irradiation area of the spot according to the laser spot diameter; dividing the laser energy by the irradiation area to obtain the laser energy density; determining the laser energy density as the laser ablation threshold value of the material to be tested.

7. The method of claim 1, wherein, Further comprising a damage verification step: scanning the surface morphology of the irradiation area corresponding to the determined laser ablation threshold value by a confocal microscope, and confirming that the laser ablation threshold value is effective when microcracks or ablation pits appear.

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