WO2020057270A1

WO2020057270A1 - Ultrasonic nondestructive detection method for expanded size of micro crack of material

Info

Publication number: WO2020057270A1
Application number: PCT/CN2019/098888
Authority: WO
Inventors: 项延训; 轩福贞; 涂善东; 朱武军; 刘长军; 马聪云
Original assignee: 华东理工大学
Priority date: 2018-09-20
Filing date: 2019-08-01
Publication date: 2020-03-26
Also published as: CN109283247A

Abstract

An ultrasonic nondestructive detection method for the expanded size of a micro crack, comprising: constructing a measurement system; fabricating multiple reference samples, and performing repeated fatigue test on each reference sample respectively; performing metallographic observation after each fatigue test so as to obtain the length of micro cracks in each reference sample, and performing ultrasonic nonlinear Lamb wave measurement to obtain a nonlinear parameter β₀ of each reference sample; obtaining a nonlinear parameter-fatigue life curve, and marking micro crack sizes corresponding to various points on the curve; and performing nonlinear ultrasonic Lamb wave measurement on a sample to be tested, so as to obtain a nonlinear parameter β₀ thereof, finding a point corresponding to the nonlinear parameter β₀ in the curve, and finding the length of a micro crack corresponding to the point. Thus, the relationship between a micro crack size and a nonlinear parameter is used for determining the length of a micro crack and representing a fatigue damage, which will not damage in-service equipment, thereby quickly detecting the micro crack state of an active material at a low cost.

Description

Ultrasonic non-destructive testing method for microcrack propagation size of materials

Technical field

The invention relates to a method for measuring the length of material micro-cracks, and in particular to an ultrasonic non-destructive testing method for the size of material micro-cracks.

Background technique

Planar layered solid structures are widely used in many fields such as aviation, aerospace and information science. During service, due to the internal factors and the external environment, the degradation of material properties will occur. The early degradation of materials accounts for most of the fatigue life. Under the influence of alternating load, temperature and other factors, there are microcracks initiating, which may rapidly expand and suddenly break, causing serious hidden dangers to the safe operation of engineering components. Therefore, effective detection and evaluation methods for the early microcrack propagation of materials have been developed. It seems very important.

Lamb waves are widely used as an effective detection method for plates because of their flexible excitation and detection methods, and strong interaction with plate defects. The early degradation of materials is usually accompanied by the evolution of the internal microstructure. Nonlinear ultrasound can overcome the shortcomings of linear ultrasound and effectively characterize the changes in the internal microstructure of the material. It is expected to become an effective early damage detection method for materials.

Using the nonlinear effect of Lamb wave to detect the plate, it is expected to detect the early micro-crack damage of the plate. Scholars at home and abroad have done a lot of fruitful research and work on the nonlinear Lamb wave mechanism and nondestructive testing of plate-like structures. In the "Research on the Generation and Propagation of Lamb Wave Second Harmonics in Planar Solid Structures", Professor Deng Mingxi of the Logistics Engineering Institute of the PLA gave theoretically the first analytical solution of the Lamb wave cumulative second harmonic sound field. Experiments have proved the strong nonlinear effect of Lamb waves. Deng and Pruell C, etc. respectively studied in `` Analysis, Second-harmonic, Generation, Lamb, Waves, Propagating, Layered, Plan, Structures, and Imperfect Interfaces, '' `` Evaluation, of fatigue, Damaging, Using, Nonlinear, Linear, Guided, Waves, Smart, Materials, and `` Structures ''. The cumulative effect of the second harmonic is analyzed for the feasibility of fatigue damage detection in the plate. Pruell et al. In "Evaluation of Plasticity Driven Materials, Damaging, Using Lamb Waves, Applied Physics Letters", analyzed the fatigue damage in the plate by nonlinear Lamb waves. In the "Nonlinear Ultrasonic Lamb Wave Method for Nondestructive Evaluation of Fatigue Damage of Solid Sheets", Deng Mingxi put forward the concept of stress wave factor to study the fatigue damage of aviation aluminum alloy under cyclic loading. The second harmonic of ultrasonic Lamb wave The change of the stress wave factor with the number of cycles is very obvious and shows a clear monotonic correspondence.

However, these existing literatures are focused on the characterization and evaluation of material damage. There is less research on the propagation of microcracks by non-linear ultrasonic signals, and the microcrack sizes in the literature are mostly in the visible range. There are studies using non-linear ultrasonic Lamb waves to quantitatively evaluate and evaluate microcrack size.

Summary of the Invention

The purpose of the present invention is to provide an ultrasonic non-destructive testing method for the size of micro-crack propagation, in order to detect the micro-crack propagation and the size of the in-service material in real time.

For contact-type structural damage such as closed micro-cracks and delaminations, some non-linear phenomena such as high-order harmonic generation, sub-harmonic generation, acoustic resonance frequency drift, and mixed-frequency sound field modulation will occur when ultrasonic encounters such damage. It cannot be explained by the traditional linear ultrasound theory, so the ultrasound theory based on contact nonlinearity has gradually developed. Respiration crack models have been used in a small number of studies on Lamb waves to explain higher-order harmonic breeding. Therefore, the micro-crack propagation size of the material can be detected by measuring the second harmonic that is generated after the ultrasonic nonlinear Lamb wave passes through the sample.

In order to achieve the above objective, the present invention provides the following technical solutions based on the above principles:

The invention provides an ultrasonic non-destructive testing method for microcrack propagation size for measuring a sample to be tested, including: S1: selecting a suitable excitation mode, excitation frequency, and excitation angle to build a nonlinear ultrasonic Lamb wave measurement system, The measurement system includes a linear ultrasonic excitation system and an excitation probe which are electrically connected to each other, and a receiving probe and an oscilloscope which are electrically connected to each other. The ends of the excitation probe and the receiving probe are provided with wedges; S2: making a plurality of prefabricated holes with a material The reference sample with the same thickness as the sample to be tested; S3: Perform multiple fatigue tests on each of the multiple reference samples, and perform metallographic observation after each fatigue test to obtain the microstructure of the reference sample. The crack length and ultrasonic nonlinear Lamb wave measurement to calculate the nonlinear parameter β _{0 of the} reference sample; S4: Obtain the nonlinear parameter-fatigue life curve according to the nonlinear parameter β ₀ of the reference sample under different fatigue cycles. , And then mark the microcrack size corresponding to each point on the non-linear parameter-fatigue life curve; S5: perform non-linear ultrasonic La on the test sample mb wave measurement to obtain the non-linear parameter β _{0 of the} sample to be tested, find the point corresponding to the non-linear parameter β ₀ in the non-linear parameter-fatigue life curve described in S4, and find the point corresponding to the point The length of the microcrack; wherein the non-linear parameter β ₀ of the reference sample or the sample to be tested is a signal collected by an oscilloscope, and the signal is obtained by a short-time Fourier transform or a fast Fourier transform to obtain a fundamental frequency The amplitude and the amplitude of the second harmonic are calculated according to the formula.

The step S1 includes: S11: providing a plate-shaped material having the same material and thickness as the sample to be tested, and calculating the dispersion curve of the material by using the software to calculate the thickness, transverse wave velocity, and longitudinal wave velocity of the plate-like material; S12: On the dispersion curve of the plate-shaped material, find the point where the phase velocity is equal and the frequency is twice the relationship, select the frequency of the fundamental frequency as the estimated value of the excitation frequency, and calculate the excitation angle θ by Snell's theorem according to the phase velocity corresponding to the excitation frequency; S13: The non-linear ultrasonic excitation system and the excitation probe are electrically connected to each other, and the receiving probe and the oscilloscope are electrically connected to each other; subsequently, wedges and couplants are respectively set at the ends of the excitation probe and the receiving probe, and the excitation probe and the receiving probe are installed to The surface of the plate-like material; the wedge is set so that the excitation probe and the receiving probe contact the surface of the plate-like material at the excitation angle described in step S12; S14: the excitation frequency of the linear ultrasonic excitation system is at the excitation frequency estimated value Adjust nearby, perform mode selection mode frequency selection experiment, screen to get the excitation frequency with accumulation effect, and get the non-linear Ultrasonic Lamb wave measurement system.

The material of the plate-like material is A17075 aluminum alloy with a thickness of 2mm; the excitation frequency of the nonlinear ultrasonic Lamb wave measurement system is 2MHz, the excitation probe is a narrow-band commercial probe with a center frequency of 2.25MHz, and the receiving probe is a center frequency It is a 5MHz wideband commercial probe, and the wedge angle corresponding to the excitation probe and the receiving probe is 24.5 °.

The pre-made holes in S2 are non-penetrating circular holes obtained by laser drilling.

The non-linear ultrasonic Lamb wave measurement on the reference sample or the sample to be tested includes: firstly placing the excitation probe at 30 mm at the left end of the pre-made hole of the reference sample, and receiving the probe at 30 mm at the right end of the pre-made hole of the reference sample. ; Then start the nonlinear ultrasound excitation system.

The metallographic observation of the reference sample is performed by observing the surface condition near the prefabricated hole under an optical microscope, observing the initiation and propagation of microcracks nearby, and saving the picture.

The S3 includes: S31: metallographic observation of one of the reference samples to obtain its microcrack length, and non-linear ultrasonic Lamb wave measurement to calculate its non-linear parameter β ₀ ; S32: using a fatigue test The machine performs a tensile fatigue interruption test on the reference specimen through a stress control mode. The reference specimens are removed under different fatigue cycles for metallographic observation and nonlinear ultrasonic Lamb wave measurement to calculate the reference specimen. The microcrack length and non-linear parameter β _{0 of the} sample under different fatigue cycles. S33: Select other reference samples and repeat the above S31-S32 until all the reference samples are measured.

The loading waveform of the interruption test of the tensile fatigue is a sine wave, and its maximum stress is lower than the yield stress of the reference sample (31).

The S4 includes: S41: taking an average of a plurality of non-linear parameters β ₀ corresponding to a plurality of reference samples 31 under each fatigue cycle week; S42: according to the reference sample 31 under different fatigue cycle weeks The average value of the non-linear parameter β ₀ is used to obtain the non-linear parameter-fatigue life curve. The position of the inflection point is marked on the non-linear parameter-fatigue life curve. The length of the microcrack at the inflection point is obtained according to the result of metallographic observation. The length of microcracks corresponding to each fatigue cycle is marked on the linear parameter-fatigue life curve.

The non-linear ultrasonic Lamb wave detection method of the present invention combines Lamb wave non-linear ultrasonic measurement with optical microscope observation and measurement to obtain the relationship between the micro-crack size and non-linear parameters before performing the detection. The non-linear parameters of the in-service equipment are measured by waves. The relationship between the micro-crack size and the non-linear parameters is used to determine the length of the material's micro-cracks and characterize the fatigue damage at the same time. This kind of non-linear ultrasonic Lamb wave measurement has fast detection speed. Will not cause damage, the detection cost is low, and the detection covers the surface and the interior of the metal member; the detection accuracy is high, and the error can meet the engineering requirements. In addition, the present invention is particularly suitable for tracking the expansion of microcrack sizes caused by early and medium-term fatigue damage of plate-like structural materials. The size of the microcracks in the plate can be measured by the inflection point of a unimodal curve that varies with the fatigue cycle. Accurate detection and evaluation, timely replacement when the material of the in-service equipment reaches the inflection point, to avoid the rapid expansion of the crack length after a certain length causes the fracture to seriously affect the safety of the in-service equipment. The invention can judge the size of microcracks.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow chart of detecting micro-crack size by nonlinear ultrasonic Lamb wave method for plate specimen

2 is a system block diagram of an ultrasonic non-destructive detection system for a material micro-crack propagation size according to an embodiment of the present invention;

FIG. 3 is a micro-crack propagation diagram, and FIG. 3 (a), (b), (c), and (c) respectively show the micro-crack propagation under 150K, 180K, 230K, and 250K fatigue cycles;

Figure 4 is a non-linear parameter-fatigue life curve diagram;

Specific implementation method

The following describes preferred embodiments of the present invention in combination with the accompanying drawings, and describes them in detail so as to better understand the functions and features of the present invention.

As shown in FIG. 1, the present invention provides an ultrasonic non-destructive testing method for microcrack propagation size of a material. The ultrasonic non-destructive testing method adopts a nonlinear ultrasonic Lamb wave measurement system as shown in FIG. 2, and the method is used for measuring The test sample 32 includes the following steps:

S1: Establish a nonlinear ultrasonic Lamb wave measurement system. The measurement system includes a linear ultrasonic excitation system 1 and an excitation probe 2 which are electrically connected to each other, and a receiving probe 4 and an oscilloscope 5 which are electrically connected to each other. A wedge is provided at the end. This step S1 specifically includes:

S11: Provide a plate-shaped material with the same metal material and thickness as the sample to be tested 32. The dispersion curve of the material is calculated by using the disperse software based on the thickness, shear wave velocity, and longitudinal wave velocity of the plate-shaped material. In this embodiment, the material of the plate-like material is A17075 aluminum alloy, and the thickness is 2 mm.

S12: Select the excitation mode from the dispersion curve of the plate-shaped material, obtain the approximate excitation frequency range, and calculate the excitation angle. Specifically, find the point on the dispersion curve of the plate-shaped material that has the same phase velocity and twice the frequency. , Select the frequency of the fundamental frequency as the estimated value of the excitation frequency, and calculate the excitation angle θ through the Snell theorem according to the phase velocity corresponding to the excitation frequency. The excitation angle θ is calculated by the following formula:

sinθ = V _wedge / V _{material phase velocity} ,

Among them, the _{phase velocity} of the V _{material is the phase velocity} obtained from the dispersion curve, and the V _wedge is the longitudinal wave velocity of the wedge, and the θ excitation angle can be calculated.

S13: As shown in FIG. 2, the non-linear ultrasonic excitation system 1 and the excitation probe 2 are electrically connected to each other through a connection line, and the receiving probe 4 and the oscilloscope 5 are electrically connected to each other; subsequently, the ends of the excitation probe 2 and the receiving probe 4 are respectively A wedge and a couplant are provided, and the excitation probe 2 and the receiving probe 4 are mounted to the surface of the plate-like material for contacting the surface of the plate-like material, wherein the wedge is arranged so that the excitation probe 2 and the receiving probe 4 is in contact with the surface of the plate-like material at the excitation angle described in step S12. This installation guarantees the integrity of the entire process from the excitation of the signal to the propagation in the board to the acquisition of the final signal.

S14: Adjust the excitation frequency of the linear ultrasonic excitation system 1 around the estimated value of the excitation frequency, perform a mode excitation mode frequency selection experiment, select an excitation frequency with a cumulative effect, and obtain a nonlinear ultrasonic Lamb wave measurement system.

Among them, the mode excitation mode frequency selection experiment adjusts the excitation frequency of the linear ultrasonic excitation system 1 near the estimated value of the excitation frequency, uses the excitation probe 2 of the measurement system to contact the plate-shaped material, and makes the receiving probe 4 along the plate-like shape. The surface of the material moves, keeping the excitation probe 2 stationary and moving the receiving probe 4. If the measured distance increases, the value of the measured non-linear parameter needs to increase with the increase of the propagation distance, which indicates that the excitation frequency has a non- Linear accumulation effect. If the value of the non-linear parameter fluctuates with the increase of the propagation, the excitation frequency does not meet the measurement requirements. The excitation frequency needs to be changed and measured again. The non-linear parameter increases as the propagation distance increases. (That is, satisfying the accumulation effect).

In this embodiment, the excitation frequency of the nonlinear ultrasonic Lamb wave measurement system is 2 MHz, the excitation probe 2 is a narrow-band commercial probe with a center frequency of 2.25 MHz, and the receiving probe 4 is a broadband commercial probe with a center frequency of 5 MHz. The wedge angle corresponding to the receiving probe 4 is 24.5 °.

S2: making a plurality of reference samples 31 with prefabricated holes and the same material and thickness as the sample to be tested 32, wherein the prefabricated holes are obtained through laser drilling and have a diameter of about 150am. Through circular holes, this allows micro-cracks to develop near the prefabricated holes, which is good for light microscope observation. The number of reference samples 31 is preferably three.

S3: Perform multiple fatigue tests on the plurality of reference samples 31 with a metal fatigue tester, and perform metallographic observation after each fatigue test to obtain the microcrack length of the reference sample 31 at this time, and perform Ultrasonic nonlinear Lamb wave measurement to calculate the nonlinear parameter β _{0 of the} reference sample 31 at this time,

Wherein, the metallographic observation of the reference sample 31 is performed by observing the surface condition near the prefabricated hole under an optical microscope, observing the initiation and propagation of microcracks nearby, and saving the picture for later comparison.

The S3 specifically includes:

S31: metallographic observation of one of the reference samples 31 to obtain the length of its microcracks, and non-linear ultrasonic Lamb wave measurement of it to calculate its non-linear parameter β ₀ ;

The non-linear ultrasonic Lamb wave measurement on the reference sample 31 specifically includes: firstly placing the excitation probe 2 on the left end of the preformed hole of the reference sample 31 at 30mm, and the receiving probe 4 on the right end of the preformed hole of the reference sample 31 at 30mm The interval between the two probes is 60mm; then the non-linear ultrasonic excitation system 1 is started again. Thus, the activation of the non-linear ultrasonic excitation system 1 allows the signal to reach the excitation probe through the data line, and the Lamb wave signal from the excitation probe 4 through the wedge and The couplant enters the sample, interacts with the reference sample 31, carries a large amount of information to be tested, and then the signal with the information required for detection enters the receiving probe 4 through the wedge and the couplant, and the receiving probe 4 vibrates through the piezoelectric effect The signal is converted into an electrical signal, which is then transmitted to the oscilloscope 5.

The non-linear parameter β ₀ of the reference sample 31 is to acquire a signal through the oscilloscope 5 and obtain the fundamental frequency amplitude A ₁ and quadratic through the signal by short-time Fourier transform or fast Fourier transform (STFT or FFT). The harmonic amplitude A ₂ is calculated according to the formula. Among them, the calculation formula of the non-linear parameter β ₀ of the reference sample 31 is as follows:

β ₀ = A ₂ / A ₁ ²

The non-linear parameter β ₀ can be used as a reference for the non-linear parameter normalization process.

S32: A fatigue tester is used to perform a tensile fatigue interruption test on the reference sample 31 in a stress control mode at room temperature. The reference sample 31 is removed for metallographic observation and non-linear ultrasound under different fatigue cycles. Lamb wave measurement to calculate the micro-crack length and non-linear parameter β _{0 of} the reference sample 31 under different fatigue cycles. Therefore, micro-cracks of different sizes can be prefabricated, the reference sample 31 can be removed at different fatigue cycle times, and the samples that have undergone a certain cycle of fatigue cycle during the interruption test can be observed again with an optical microscope to observe the micro-cracks, and photos of the same location can be saved Observe whether there are microcracks initiating and expanding.

Among them, the interruption test refers to loading once to a predetermined week, then removing it for measurement, loading again, and then removing it for measurement until the final sample breaks. Interrupting the test can eliminate material non-linear differences between different samples, and obtain microcracks with different sizes of material expansion. In this embodiment, the loading waveform of the interruption test of the tensile fatigue is a sine wave, and its maximum stress is far lower than the yield stress of the reference sample 31 to avoid non-linear parameter changes caused by plastic deformation.

The interval between the different fatigue cycle weeks is preferably 2000 cycles until the final break. Thus, the measurement of multiple different fatigue cycle cycles of one of the reference samples 31 is called a group, because each reference sample The life of 31 is different, so the total number of measurements for each sample to be tested is not necessarily the same. The measurement of each reference sample 31 in the same fatigue cycle was repeated 3 times to reduce the error. In addition, a calibration sample can be set to perform the same nonlinear ultrasonic Lamb wave measurement and normalize the result with the first measurement. The error of the measurement system can be obtained. In the case, a calibration sample can be used for system calibration.

The calculation process of the non-linear parameter β ₀ of the reference sample 31 is completely consistent with the calculation process described in step S31 above.

S33: Select other reference samples 31 and repeat the above S31-S32. By repeating the above-mentioned experiment multiple times, there are multiple nonlinear Lamb wave measurements on different reference samples 31 at the data points of each fatigue cycle. This can avoid accidental results of a single reference sample 31.

S4: Obtain the nonlinear parameter-fatigue life curve according to the nonlinear parameter β ₀ under different fatigue cycles, and then mark the microcrack size corresponding to each point on the nonlinear parameter-fatigue life curve.

S4 includes:

S41: Take the average of the multiple non-linear parameters β ₀ corresponding to the multiple reference samples 31 under each fatigue cycle, calculate the standard deviation, and make the error bar. As described above, at each fatigue There are multiple measurements of the reference sample 31 at the data points of the cycle. Therefore, this error calculation can be used to indicate the stability of the three experiments measured at this microcrack length, indicating that this measurement is not accidental.

S42: According to the average value of the non-linear parameter β ₀ of the reference sample 31 under different fatigue cycles, a non-linear parameter-fatigue life curve is obtained, and the position of the inflection point is marked on the non-linear parameter-fatigue life curve. As a result, the length of the microcrack at the inflection point is obtained, and the length of the microcrack corresponding to each fatigue cycle is marked on the nonlinear parameter-fatigue life curve.

S5: Non-linear ultrasonic Lamb wave measurement is performed on the sample to be tested 32 to obtain the non-linear parameter β _{0 of the} sample to be tested 32. Since the sample to be tested 32 and the thickness and material are the same as those of the sample to be tested 32 The number of fatigue cycles and the size of the microcracks corresponding to each non-linear parameter β _{0 of} the reference sample 31 are known, so the non-linear parameter corresponding to the non-linear parameter can be found in the non-linear parameter-fatigue life curve described in S4. β ₀ point, and find out the length of the microcrack corresponding to this point.

The calculation process of the non-linear parameter β ₀ of the sample to be tested 32 is completely consistent with the calculation process of the non-linear parameter β ₀ of the reference sample 31 described above.

Experimental results

As shown in Fig. 3 (a)-Fig. 3 (d), the metallographic observation results of reference sample 31 are shown. The arrow in the figure indicates the crack direction. As the length of the microcrack increases, the normalized value of the nonlinear parameter β is It appears to increase first and then decrease. The simulation results in the existing literature show that the initial nonlinear parameters in the nonlinear ultrasonic Lamb wave experiment increase with the increase of the microcrack length, and the increase of the later crack width leads to the decrease of the later nonlinear parameters. . Therefore, the micro-crack length in the plate can be well detected by the nonlinear ultrasonic Lamb wave method.

The measurement results of multiple reference samples 31 of the same material show that the microcrack length is close to 300am at the inflection point, and there is no exception, and the inflection point information is used to judge the ease of use. There is no need to consider the specificity of each sample, only the nonlinear parameters The real-time measurement results are used to roughly judge the material micro-crack size information.

As shown in Figure 4, the excitation signal is determined according to the time-frequency analysis method, and the generated modal is analyzed and judged. The generated nonlinear second-harmonic mode is obtained by calculating the group velocity of the sound wave in the material. The horizontal curve indicates that the fundamental frequency signal amplitude A ₁ measured during the entire fatigue life of the sample remains basically unchanged, indicating that the linear ultrasound is basically unchanged, and the length of the microcrack cannot be obtained by measuring the linear ultrasound; The single-peak curve in the figure shows that the non-linear parameter (that is, the normalized second harmonic signal amplitude) A ₂ / A ₁ ² shows a single-peak change, which proves that the change of the non-linear parameter is mainly caused by the change of the second harmonic signal. Caused by, the sensitivity of the non-linear parameter β ₀ to the early damage of the material is verified, and the normalized value of the non-linear ultrasonic parameter A ₂ / A ₁ ² increases first and then decreases with the fatigue life, so the non-linear parameter can be increased from The size of the microcrack is judged at the reduced inflection point, and the length of the microcrack of the sample at the inflection point of the non-linear parameter is about a certain value (300 μm in this embodiment).

Therefore, through the above-mentioned literature review and simulation, it was found that the nonlinear response (ie, the nonlinear parameter) of the ultrasonic Lamb wave increases with the increase of the length of the microcrack, and decreases with the increase of the width of the microcrack. Therefore, in the nonlinear ultrasonic Lamb wave experiment, the increase of the initial nonlinear ultrasonic parameter and the increase of the crack width in the later period lead to the decrease of the nonlinear ultrasonic parameter in the later period. Therefore, at the inflection point, the effects of length and width on non-linear parameters reach a balance, which is suitable for judging the length of material microcracks.

In summary, the present invention proposes a non-linear ultrasonic Lamb wave detection method to detect and evaluate the size of micro-cracks in a plate. This method performs ultrasonic Lamb waves on a test plate under the condition that the ultrasonic Lamb wave has a strong nonlinear effect. For the measurement of the second harmonic, the received signal is subjected to STFT or FFT analysis and processing, and the fundamental frequency signal and the second harmonic signal are extracted, and the nonlinear parameters are calculated. Finally, based on the obtained unimodal inflection point information of the non-linear parameter, the micro-crack size under the non-linear parameter is obtained.

The above description is only the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. The above embodiments of the present invention can also make various changes. That is, any simple and equivalent changes and modifications made according to the claims of the present application and the contents of the description fall within the protection scope of the claims of the present invention. What is not described in detail in the present invention is conventional technical content.

Claims

An ultrasonic non-destructive testing method for microcrack propagation size of a material for measuring a sample to be tested (32), characterized in that it includes:

S1: Set up a nonlinear ultrasonic Lamb wave measurement system, the measurement system includes a linear ultrasonic excitation system (1) and an excitation probe (2) electrically connected to each other, and a receiving probe (4) and an oscilloscope (5) electrically connected to each other, Wedges are provided at the ends of the excitation probe (2) and the receiving probe (4);

S2: making a plurality of reference samples (31) with prefabricated holes and the same material and thickness as the sample to be tested (32);

S3: Perform multiple fatigue tests on the multiple reference samples (31), and perform metallographic observation after each fatigue test to obtain the microcrack length of the reference sample (31), and perform ultrasonic nonlinear Lamb Wave measurement to calculate the non-linear parameter β 0 of the reference sample (31);

S4: Obtain a nonlinear parameter-fatigue life curve according to the nonlinear parameter β 0 of the reference sample (31) under different fatigue cycles, and then mark the corresponding microcrack size at each point on the nonlinear parameter-fatigue life curve;

S5: Non-linear ultrasonic Lamb wave measurement is performed on the sample to be tested (32), and the nonlinear parameter β 0 of the sample to be tested (32) is obtained, which is found in the nonlinear parameter-fatigue life curve described in S4. A point corresponding to the non-linear parameter β 0 , and find the length of the microcrack corresponding to the point;

Wherein, the non-linear parameter β 0 of the reference sample (31) or the sample to be tested (32) is a signal collected by an oscilloscope (5), and the signal is subjected to short-time Fourier transform or fast Fourier transform. Transform to obtain the fundamental frequency amplitude A 1 and the second harmonic amplitude A 2 , and then calculate according to the formula β 0 = A 2 / A 1 2 .
The method for ultrasonic nondestructive testing of material microcrack propagation size according to claim 1, wherein the step S1 comprises:

S11: Provide a plate-like material with the same material and thickness as the sample to be tested (32), and use the software to calculate the dispersion curve of the material based on the thickness, transverse wave velocity, and longitudinal wave velocity of the plate-like material;

S12: Find the point on the dispersion curve of the plate-like material where the phase velocity is equal and the frequency is twice the relationship, select the frequency of the fundamental frequency as the estimated value of the excitation frequency, and calculate the excitation angle θ by Snell's theorem according to the phase velocity corresponding to the excitation frequency;

S13: electrically connect the non-linear ultrasonic excitation system (1) and the excitation probe (2) to each other, and electrically connect the receiving probe (4) and the oscilloscope (5) to each other; subsequently, the excitation probe (2) and the receiving probe (4) are electrically connected to each other; Wedges and couplants are respectively set at the ends, and the excitation probe (2) and the receiving probe (4) are installed on the surface of the plate-shaped material; the wedges are arranged so that the excitation probe (2) and the receiving probe (4) ) Contacting the surface of the plate-like material at the excitation angle described in step S12;

S14: The excitation frequency of the linear ultrasonic excitation system (1) is adjusted near the estimated value of the excitation frequency, and a mode selection mode frequency selection experiment is performed to select an excitation frequency with a cumulative effect to obtain a nonlinear ultrasonic Lamb wave measurement system.
The method for ultrasonic non-destructive testing of material micro-crack propagation size according to claim 2, characterized in that the material of the plate-like material is A17075 aluminum alloy with a thickness of 2 mm; and the nonlinear ultrasonic Lamb wave measurement system The excitation frequency is 2MHz, the excitation probe (2) is a narrow-band commercial probe with a center frequency of 2.25MHz, the receiving probe (4) is a broadband commercial probe with a center frequency of 5MHz, and the excitation probe (2) and the receiving probe (4) are The corresponding wedge angle is 24.5 °.
The method for ultrasonic non-destructive testing of material micro-crack propagation size according to claim 1, wherein the pre-made holes in S2 are non-penetrating circular holes obtained by laser drilling.
The method for ultrasonic non-destructive testing of material micro-crack propagation size according to claim 1, wherein the nonlinear ultrasonic Lamb wave measurement performed on a reference sample (31) or a test sample (32) comprises : First place the excitation probe (2) on the left end of the prefabricated hole of the reference sample (31) 30mm, and the receiving probe (4) on the right end of the prefabricated hole of the reference sample (31) 30mm; then start the non-linear ultrasonic excitation System (1).
The method for ultrasonic non-destructive testing of material micro-crack propagation size according to claim 1, characterized in that the metallographic observation of the reference sample (31) is performed by observing the surface near the prefabricated hole under an optical microscope, Observe the nearby microcracks for initiation and propagation, and save the picture to proceed.
The method for ultrasonic nondestructive testing of material microcrack propagation size according to claim 1, wherein the S3 comprises:

S31: Metallographically observe one of the reference samples (31) to obtain the length of the microcracks, and perform a nonlinear ultrasonic Lamb wave measurement on it to calculate its nonlinear parameter β 0 ;

S32: A fatigue tester is used to perform a tensile fatigue interruption test on the reference sample (31) in a stress control mode, and the reference sample (31) is removed for metallographic observation and non-linearity under different fatigue cycles. Ultrasonic Lamb wave measurement to calculate the microcrack length and non-linear parameter β 0 of the reference sample (31) under different fatigue cycles.

S33: Select other reference samples (31) and repeat the above S31-S32 until the measurement of all the reference samples (31) is completed.
The method for ultrasonic nondestructive testing of material microcrack propagation size according to claim 7, characterized in that the loading waveform of the interruption test of the tensile fatigue is a sine wave, and its maximum stress is lower than the reference sample (31) Yield stress.
The method for ultrasonic non-destructive testing of material microcrack propagation size according to claim 1, wherein the S4 comprises:

S41: Take the average of the multiple non-linear parameters β 0 corresponding to the multiple reference samples 31 under each fatigue cycle.

S42: According to the average value of the non-linear parameter β 0 of the reference sample 31 under different fatigue cycles, a non-linear parameter-fatigue life curve is obtained, and the position of the inflection point is marked on the non-linear parameter-fatigue life curve. As a result, the length of the microcrack at the inflection point is obtained, and the length of the microcrack corresponding to each fatigue cycle is marked on the nonlinear parameter-fatigue life curve.