Water-immersion shear wave mixing ultrasonic detection method for near-surface micro-defects

Abstract

The present invention provides a method for detecting near-surface micro-defects using water-immersion shear-wave mixing ultrasonic testing. By optimizing and selecting non-collinear shear-wave mixing nonlinear ultrasonic testing parameters, water-immersion non-collinear shear-wave mixing scanning is performed on the test piece along its depth direction. The sum-frequency longitudinal wave signal amplitude in the scanning data is extracted using an n-p signal processing method. The presence of near-surface micro-defects in the test piece is assessed based on the characteristics of the curve (A-D curve) showing the variation of the sum-frequency longitudinal wave amplitude along the depth direction. This method detects near-surface micro-defects in the test piece, resolving the problem of traditional non-destructive testing methods being unable to detect near-surface micro-defects. Furthermore, the method provides simple and reliable evaluation of the test results, enabling automated scanning and testing, and possesses wide applicability.

Description

Technical Field

[0001] The present invention relates to the field of nondestructive testing of materials, and in particular to a water-immersion shear wave mixing ultrasonic testing method for near-surface micro-defects. Background Art

[0002] When there are no stress concentration sources inside a component, micro-defects on the surface and near the surface often lead to the initiation of cracks in the component. Under the action of a specific load, the initiated cracks will gradually expand, leading to structural fracture. At present, the surface defect detection methods widely used in engineering include penetration, magnetic powder and eddy current methods. However, these detection methods have low sensitivity for micro-defect detection, and penetration testing has certain pollution to the environment and components. Magnetic powder and eddy current testing have requirements for the material of the object being inspected, and all have certain limitations. Therefore, in order to effectively ensure the safe and stable operation of key equipment and related equipment, it is urgent to develop a near-surface micro-defect detection method.

[0003] Nonlinear ultrasonic testing technology has attracted widespread attention due to its high sensitivity to detecting early-stage material damage and micro-defects. This technology utilizes the interaction of finite-amplitude ultrasonic waves with a nonlinear source to detect materials based on nonlinear effects such as newly generated harmonics, frequency mixing, and resonant frequency shifts in the test signal.

[0004] In the field of surface micro-defect detection, domestic and international scholars have conducted extensive research on nonlinear ultrasonic testing. Researchers have also conducted research on surface wave harmonic testing. This research has found that a 5.1% plastic strain in titanium alloy plastic strain specimens can cause a 40% increase in the surface wave nonlinear coefficient, demonstrating the high sensitivity of the surface wave harmonic method to plastic damage. Research on surface wave harmonic detection of pitting damage in 304 stainless steel has shown that the normalized nonlinear coefficient can be used to characterize the extent of pitting damage on the stainless steel surface. However, because the effective detection depth of surface waves is concentrated within a single wavelength range, the detection capability of surface wave nonlinear ultrasonic testing technology in the material depth direction is limited.

[0005] Compared to surface waves, bulk waves can propagate deeply into the test object, offering significant advantages in depth coverage. The nonlinear ultrasonic testing method of noncollinear bulk wave mixing can effectively reduce the impact of the detection system on the test results by adjusting the acoustic wave type, frequency, incident direction, and detection position, allowing for scanning inspection of the test object. Regarding bulk wave mixing nonlinear ultrasonic testing technology, relevant scholars have conducted a systematic theoretical analysis of the mixing of two different modes of bulk waves in isotropic solids, providing a numerical solution for the far-field scattering of the mixing waves and the resonant conditions for significant mixing effects. These include important parameters such as the wave pattern combination of the two fundamental ultrasonic waves, the frequency ratio, and the interaction angle, as well as the scattering angle of the mixing waves. Some relevant experimental studies have been conducted. Experimental studies on the collinear mixing of shear and longitudinal waves in aluminum alloy specimens have found that by adjusting the acoustic wave delay to change the intersection point of the transverse and longitudinal waves, scanning aluminum alloy bars can be performed to detect and locate plastic deformation within the specimen. Non-collinear shear wave fusion testing was used to detect fatigue damage in aluminum alloy specimens. The results showed that the fusion nonlinear coefficient increased significantly with the degree of fatigue damage. Non-collinear shear wave testing was applied to fatigue crack detection in steel and aluminum materials. The results showed that the fatigue crack length can be measured based on the spatial distribution of the fusion nonlinear coefficient.

[0006] In summary, researchers have conducted extensive and fruitful research on bulk-wave mixing nonlinear ultrasonic testing technology, demonstrating its high sensitivity to detecting early-stage material damage and contact-type microdefects. However, existing research has primarily focused on internal structural damage or defects, with limited reports on nonlinear ultrasonic testing of near-surface defects. Furthermore, nonlinear ultrasonic testing has not yet been applied to near-surface defect detection. To effectively address the challenges of nondestructive detection of near-surface microdefects, there is an urgent need to develop an immersion ultrasonic testing method for nondestructive detection of near-surface microdefects. Summary of the Invention

[0007] To address the shortcomings of the aforementioned prior art, the present invention optimizes and selects non-collinear shear-wave mixing nonlinear ultrasonic testing parameters, performs immersion-based non-collinear shear-wave mixing scanning along the depth direction of the test piece, and utilizes NP signal processing to extract the sum-frequency longitudinal wave signal amplitude from the scanning data. Based on the depth-dependent curve (AD curve) of the sum-frequency longitudinal wave amplitude, the presence of near-surface micro-defects in the test piece is assessed. This method detects near-surface micro-defects in the test piece, resolving the difficulty of traditional non-destructive testing methods in detecting near-surface micro-defects. Furthermore, the method provides simple and reliable evaluation of test results, enables automated scanning, and has broad applicability.

[0008] Specifically, the present invention provides a method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection, which comprises the following steps:

[0009] S1. Prepare a normal test block with the same shape, material, molding process, heat treatment status and surface roughness as the test block to be tested;

[0010] S2, optimizing the parameters of the immersion nonlinear ultrasonic testing to obtain the optimal sum frequency longitudinal wave value, which includes the following sub-steps:

[0011] S21, determining the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves;

[0012] S22. Calculate the incident angles of two incident longitudinal waves in water. The specific calculation steps are as follows:

[0013] First, based on the interaction angle α of the two fundamental frequency shear waves, the relationship between the interaction angle α of the two fundamental frequency shear waves and the incident angles of the two incident longitudinal waves in water is obtained as shown in formula (2):

[0014]

[0015] Where, v w is the sound velocity of longitudinal waves in the liquid surrounding the specimen, v s is the sound velocity of the shear wave in the test piece, α is the interaction angle between the two fundamental frequency shear waves in the test piece, θ1 and θ2 are the incident angles of the two incident longitudinal waves in water respectively;

[0016] Taking θ1=θ2=θ, simplifying formula (2) to obtain formula (3), use formula (3) to calculate the incident angle of two incident longitudinal waves in water;

[0017]

[0018] S23. Calculate the sum-frequency longitudinal wave scattering angle ψ and arrange the longitudinal wave straight probe. The specific calculation steps are as follows:

[0019] Two beams of fundamental frequency shear waves are emitted using the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S21 and the incident angles of the two incident longitudinal waves in the water. The angle between the wave vector k3 of the newly generated sum frequency longitudinal wave and the wave vector k1 of the incident shear wave is the sum frequency longitudinal wave scattering angle ψ. The sum frequency longitudinal wave scattering angle ψ is calculated according to the following formula (4). Based on the sum frequency longitudinal wave scattering angle ψ, a longitudinal wave straight probe is arranged to receive the sum frequency longitudinal wave.

[0020]

[0021] S24. Calculate the optimal sum-frequency longitudinal wave using the NP signal processing method: Use voltage signals with four different initial phase combinations to perform non-collinear shear wave detection respectively, analyze and process the four collected detection signals according to the NP signal processing method described in formula (5), and extract the mixing component in the detection signal, which is the sum-frequency longitudinal wave y(t).

[0022]

[0023] Where y pp (t), y nn (t), y np (t) and y pn (t) are the detection signals obtained by the interaction of the fundamental frequency shear waves x1(t) and x2(t) with four different initial phase combinations, as shown in equations (6) to (9) respectively;

[0024]

[0025]

[0026]

[0027]

[0028] Where, the superscripts n and p represent the initial phases of the two fundamental frequency shear waves, respectively, and |np| = 180°;

[0029] S3. Perform water immersion non-collinear shear wave mixing detection on the normal test block and the test block to be tested, and obtain AD curves of the normal test block and the test block to be tested:

[0030] Based on the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S2, an immersion non-collinear shear wave mixing scanning test is performed on the normal test block and the test piece to be tested. The obtained test signals are analyzed and processed using the NP signal processing method. The amplitudes of the sum frequency longitudinal waves at different horizontal detection positions and different depths of the normal test block and the test piece to be tested are extracted respectively. The curve of the change of the sum frequency longitudinal wave amplitude at each horizontal position with depth is plotted as the AD curve;

[0031] S4. The characteristics of the AD curve are divided into feature a and feature b, wherein feature a is that the amplitude of the sum frequency longitudinal wave gradually decreases with increasing depth, and feature b is that the amplitude of the sum frequency longitudinal wave first increases and then gradually decreases with increasing depth;

[0032] S5, near-surface micro-defect judgment, which includes the following sub-steps:

[0033] S51. Extract the features of the AD curve of the normal test block:

[0034] The AD curve of the normal test block is characterized by a feature;

[0035] S52, extracting the features of the AD curve of the test piece to be inspected and determining the near-surface micro-defects of the test piece to be inspected:

[0036] When the AD curve of the test piece to be inspected has feature b, it is determined that there is a micro defect near the surface of the test piece to be inspected;

[0037] When the AD curve features of the test block to be inspected are all feature a, the difference ΔA between the peak value A1 of the AD curve of the test block to be inspected and the peak value A2 of the AD curve of the normal test block is calculated, ΔA = |A1-A2| / A2, and ΔA is compared with the set threshold B. When ΔA is greater than B, it is determined that there are micro-defects on the near surface of the test block to be inspected.

[0038] Preferably, in step S21, the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves are calculated by the following formula (1):

[0039]

[0040] Wherein, r=f2 / f1, f2 and f1 are the frequencies of the two fundamental frequency shear waves respectively.

[0041] Preferably, in step S21 , the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves are optimized by performing multiple water-immersion non-collinear shear wave mixing tests.

[0042] Preferably, the threshold value B is determined by means of a water immersion nonlinear ultrasonic testing test and metallographic analysis.

[0043] Preferably, in step S3, the AD curves of the normal test block and the test block to be detected are plotted on the same graph.

[0044] Preferably, the sum frequency longitudinal wave is two columns of shear waves interacting with a nonlinear source to generate a third column of sum frequency longitudinal waves.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) The present invention provides a method for detecting near-surface micro-defects, which solves the problem that existing conventional non-destructive testing methods cannot realize near-surface micro-defect detection. It can optimize and select non-collinear shear wave mixing nonlinear ultrasonic detection parameters, carry out water immersion non-collinear shear wave mixing scanning detection on the test piece along the depth direction, and use the NP signal processing method to extract the amplitude of the sum frequency longitudinal wave signal in the scanning data. According to the variation curve of the sum frequency longitudinal wave amplitude along the depth direction (AD curve), the situation of the near-surface micro-defects of the test piece to be tested is judged, and the near-surface micro-defects of the test piece to be tested are accurately detected.

[0047] (2) The method of the present invention has wide applicability and is not subject to the requirements of the detection material. It can be used to detect surface defects of various materials such as metals, non-metallic materials, ferromagnetic materials, non-ferromagnetic materials, conductive materials, non-conductive materials, etc., and ensures the accuracy of the detection results.

[0048] (3) The present invention adopts a water immersion coupling method, and the ultrasonic coupling is uniform and stable, which is convenient for water temperature control. The detection results are highly reliable and the judgment of the detection results is simple. There is no need for complex judgment to avoid the probability of error, thus ensuring the accuracy of the results and realizing automated scanning detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a flow chart of a method for detecting near-surface micro-defects using water-immersion non-collinear shear wave mixing nonlinear ultrasonic detection according to an embodiment of the present invention;

[0050] Figure 2 This is a schematic diagram of a method for detecting near-surface micro-defects using water-immersion non-collinear shear wave mixing nonlinear ultrasonic detection according to an embodiment of the present invention;

[0051] Figure 3 Schematic diagram of a water-immersion non-collinear shear wave mixing nonlinear ultrasonic detection system for near-surface micro-defects according to an embodiment of the present invention;

[0052] Figure 4 A test piece to be inspected having near-surface plastic deformation according to an embodiment of the present invention;

[0053] Figure 5 Parameters for water-immersion non-collinear shear wave mixing nonlinear ultrasonic detection of near-surface micro-defects according to an embodiment of the present invention;

[0054] Figure 6 : AD curve of the test piece with near-surface plastic deformation according to the embodiment of the present invention.

[0055] Figure 7 A test piece to be inspected having near-surface fatigue cracks according to an embodiment of the present invention;

[0056] Figure 8 This is an α-r characteristic diagram of a test piece containing a near-surface fatigue crack to be detected according to an embodiment of the present invention;

[0057] Figure 9 It is the AD curve of the test piece containing near-surface fatigue cracks according to the embodiment of the present invention. DETAILED DESCRIPTION

[0058] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0059] The present invention provides a method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection, which comprises the following steps:

[0060] S1. Prepare a test block to be tested and a normal test block of identical shape. To ensure accurate results, the normal test block and the test block to be tested should be identical in material, molding process, heat treatment, and surface roughness. The shape can be determined based on the type of test block to be tested, as long as the normal test block and the test block to be tested have the same shape.

[0061] S2, optimizing the parameters of the immersion nonlinear ultrasonic testing to obtain the optimal sum frequency longitudinal wave value, which includes the following sub-steps:

[0062] S21. Determine the interaction angle α and frequency ratio r of the two fundamental frequency shear waves.

[0063] In some embodiments, the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves in step S21 are calculated by the following formula (1):

[0064]

[0065] Wherein, r=f2 / f1, f2 and f1 are the frequencies of the two fundamental frequency shear waves respectively.

[0066] In other embodiments, especially for contact-type micro-defects such as fatigue cracks, the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves in step S21 are also optimized by performing multiple immersion non-collinear shear wave mixing tests.

[0067] S22. Calculate the incident angles of two incident longitudinal waves in water. The specific calculation steps are as follows:

[0068] First, based on the interaction angle α of the two fundamental frequency shear waves, the relationship between the interaction angle α of the two fundamental frequency shear waves and the incident angles of the two incident longitudinal waves in water is obtained as shown in formula (2):

[0069]

[0070] Where, v w is the sound velocity of longitudinal waves in the liquid surrounding the specimen, v s is the sound velocity of the shear wave in the test piece, α is the interaction angle between the two fundamental frequency shear waves in the test piece, θ1 and θ2 are the incident angles of the two incident longitudinal waves in water respectively;

[0071] Taking θ1=θ2=θ, simplifying formula (2) to obtain formula (3), use formula (3) to calculate the incident angle of two incident longitudinal waves in water;

[0072]

[0073] S23. Calculate the sum-frequency longitudinal wave scattering angle ψ and arrange the longitudinal wave straight probe. The specific calculation steps are as follows:

[0074] Two beams of fundamental frequency shear waves are emitted using the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S21 and the incident angles of the two incident longitudinal waves in the water. The angle between the wave vector k3 of the newly generated sum frequency longitudinal wave and the wave vector k1 of the incident shear wave is the sum frequency longitudinal wave scattering angle ψ. The sum frequency longitudinal wave scattering angle ψ is calculated according to the following formula (4). Based on the sum frequency longitudinal wave scattering angle ψ, a longitudinal wave straight probe is arranged to receive the sum frequency longitudinal wave.

[0075]

[0076] S24. Calculate the optimal sum-frequency longitudinal wave using the NP signal processing method: Use voltage signals with four different initial phase combinations to perform non-collinear shear wave detection respectively. The four collected detection signals are analyzed and processed according to the NP signal processing method of formula (5). The mixed frequency component in the detection signal is extracted as the sum-frequency longitudinal wave y(t).

[0077]

[0078] Where y pp (t), y nn (t), y np (t) and y pn (t) are the detection signals obtained by the interaction of the fundamental frequency shear waves x1(t) and x2(t) with four different initial phase combinations, as shown in equations (6) to (9) respectively;

[0079]

[0080]

[0081]

[0082]

[0083] Where the superscripts n and p represent the initial phases of the two fundamental frequency shear waves, respectively, and |np| = 180°.

[0084] S3. Perform water immersion non-collinear shear wave mixing detection on the normal test block and the test block to be detected, and obtain AD curves of the normal test block and the test block to be detected.

[0085] According to the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S2, water-immersed non-collinear shear wave mixing scanning detection is performed on the normal test block and the test piece to be tested. The obtained detection signals are analyzed and processed using the above-mentioned NP signal processing method, and the sum-frequency longitudinal wave amplitudes at different horizontal detection positions and different depths of the normal test block and the test piece to be tested are extracted respectively. The curve of the change of the sum-frequency longitudinal wave amplitude at each horizontal position with the depth is plotted as the AD curve.

[0086] For the convenience of observation, in a specific embodiment, the AD curves of the normal test block and the test block to be detected can be drawn on the same graph in step S3.

[0087] S4. The characteristics of the AD curve are divided into feature a and feature b. Feature a is that the amplitude of the sum-frequency longitudinal wave gradually decreases with increasing depth, and feature b is that the amplitude of the sum-frequency longitudinal wave first increases and then gradually decreases with increasing depth.

[0088] S5, near-surface micro-defect judgment, which includes the following sub-steps:

[0089] S51. Extract the features of the AD curve of the normal test block:

[0090] The AD curve of the normal test block is characterized by a feature;

[0091] S52, extracting the features of the AD curve of the test piece to be inspected and determining the near-surface micro-defects of the test piece to be inspected:

[0092] When the AD curve characteristics of the test piece to be inspected have feature b, it is determined that there are micro defects near the surface of the test piece to be inspected.

[0093] When the AD curve characteristics of the test piece to be inspected are all feature a, the difference ΔA between the peak value A1 of the AD curve of the test piece to be inspected and the peak value A2 of the AD curve of the normal test piece is calculated, ΔA = |A1-A2| / A2. ΔA is compared with a set threshold value B. When ΔA is greater than B, it is determined that a micro-defect exists near the surface of the test piece to be inspected. In a specific embodiment, threshold value B can be determined based on the required accuracy and parameters such as the material of the test piece to be inspected. Specific embodiments

[0095] The embodiment of the present invention provides a method for detecting near-surface micro-defects by water immersion non-collinear shear wave mixing nonlinear ultrasonic detection. Figure 1 The flowchart of an embodiment of the present invention is shown. The immersion nonlinear ultrasonic testing technology utilizes two longitudinal wave ultrasonic straight probes to generate two incident longitudinal waves. Waveform conversion occurs at the water / test specimen interface. The two shear waves generated in the specimen interact with the nonlinear source to produce a third sum-frequency longitudinal wave. This sum-frequency longitudinal wave is received by a longitudinal wave straight probe. Based on the generation and amplitude of the sum-frequency longitudinal wave, micro-defects in the test specimen are detected. Figure 2 Shown is a schematic diagram of the principle of an immersion-type non-collinear shear wave mixing nonlinear ultrasonic detection method according to an embodiment of the present invention.

[0096] Figure 3 The following is a schematic diagram of a water-immersion, non-collinear, shear-wave mixing, nonlinear ultrasonic detection system for near-surface micro-defects according to an embodiment of the present invention. The detection system comprises an acoustic module, two longitudinal-wave excitation straight probes (T1 and T2), a longitudinal-wave receiving probe (R), a water tank, and necessary fixtures. The ultrasonic module should be able to transmit two excitation signals with adjustable frequency, number of cycles, and amplitude. The fixture can freely adjust the inclination angles and positions of the two longitudinal-wave straight probes and the longitudinal-wave receiving probe.

[0097] This method can optimize and select non-collinear shear wave mixing nonlinear ultrasonic testing parameters, carry out immersion non-collinear shear wave mixing scanning and testing on the test piece along the depth direction, and use the NP signal processing method to extract the amplitude of the sum frequency longitudinal wave signal in the scanning data. According to the variation curve of the sum frequency longitudinal wave amplitude along the depth direction (AD curve), the situation of near-surface micro defects of the test piece can be judged, and the near-surface micro defects of the test piece can be accurately detected.

[0098] In order to demonstrate the applicability of the present invention, it is applied to two typical examples, namely, immersion non-collinear shear wave mixing nonlinear ultrasonic testing of test pieces with near-surface plastic deformation micro-defects and near-surface fatigue cracks.

[0099] Example 1: Immersed non-collinear shear wave mixing nonlinear ultrasonic testing of a test piece with near-surface plastic deformation micro-defects.

[0100] The specific steps include:

[0101] S1: Preparation of comparison test blocks for immersion nonlinear ultrasonic testing;

[0102] The control test block is a micro-defect-free test block. It is made of aluminum, and its molding process, heat treatment status, and surface roughness should be the same as those of the test piece. The control test block has a regular rectangular shape and dimensions of 342 × 31.5 × 41.5 mm (length × width × height).

[0103] Figure 4 This is a test piece with near-surface plastic deformation, as tested in accordance with an embodiment of the present invention. The test piece is made of aluminum and measures 342 × 31.5 × 41.5 mm (length × width × height). Three-point bending fatigue loading was used to create a plastic deformation zone near the surface of the central portion of the test piece.

[0104] S2: Optimization and selection of parameters for immersion nonlinear ultrasonic testing;

[0105] The key parameters of immersion nonlinear ultrasonic testing include the interaction angle α and the frequency ratio r. For different micro-defect types, the two key parameters that can be used for immersion nonlinear ultrasonic testing can be calculated using equations (3) and (4). Figure 5 These are two key parameters that can be used in the embodiment of the present invention for near-surface micro-defect water-immersion non-collinear shear wave mixing nonlinear ultrasonic detection. Figure 5 As well as the existing probe models, an interaction angle of 122° and a frequency ratio of 0.8 were selected as the detection parameters for immersion non-collinear shear wave mixing nonlinear ultrasonic testing.

[0106] S3: Implementation of water-immersion non-collinear shear wave mixing detection;

[0107] According to the interaction angle α and frequency ratio r obtained by S2, a water-immersion non-collinear shear wave mixing xz scanning inspection is carried out on the defect-free comparison block and the test piece to be inspected using a water-immersion non-collinear shear wave mixing xz scanning inspection system.

[0108] S4: Detection data analysis and processing;

[0109] The test data obtained from the immersion non-collinear shear wave mixing test in S3 were analyzed and processed using the polarity reversal signal processing method. The sum frequency longitudinal wave amplitudes at different test positions of the comparison test block and the test block to be tested were extracted to obtain the AD curves at different positions. Figure 6 AD curves of a non-defective control test block and a test piece with near-surface plastic deformation according to an embodiment of the present invention.

[0110] S5: Assessment of near-surface micro-defects.

[0111] Depend on Figure 6 As can be seen, the AD curve for the non-microdefective comparison specimen shows a characteristic of gradually decreasing sum-frequency longitudinal wave amplitude with increasing depth, a Class A characteristic. The AD curve for the test piece with near-surface plastic deformation exhibits similar characteristics to the non-microdefective comparison specimen, with a gradually decreasing sum-frequency longitudinal wave amplitude with increasing depth, also a Class A characteristic. However, the peak value of the AD curve for the test piece with near-surface plastic deformation is greater than that for the non-microdefective comparison specimen, with a specific calculation showing a 54% difference between the two.

[0112] When the threshold value B is 30%, since 54%>B, it can be determined that there are micro defects near the surface of the test piece under inspection.

[0113] Example 2: Immersed non-collinear shear wave mixing nonlinear ultrasonic testing of a test piece with a near-surface fatigue crack.

[0114] The specific steps include:

[0115] S1: Preparation of comparison test blocks for immersion nonlinear ultrasonic testing;

[0116] The control test block is a micro-defect-free test block. It is made of aluminum, and its molding process, heat treatment status, and surface roughness should be the same as those of the test piece. The control test block has a regular rectangular shape and dimensions of 342 × 31.5 × 41.5 mm (length × width × height).

[0117] Figure 7 This is a test piece with a near-surface fatigue crack, as tested in an embodiment of the present invention. The test piece is made of aluminum and measures 342 × 31.5 × 41.5 mm (length × width × height). A fatigue crack approximately 3 mm long was machined near the surface of the central portion of the test piece using three-point bending fatigue loading.

[0118] S2: Optimization and selection of parameters for immersion nonlinear ultrasonic testing;

[0119] Key parameters for immersion nonlinear ultrasonic testing include the interaction angle α and the frequency ratio r. In this embodiment, a fatigue crack is a contact-type microdefect, so the key parameters for immersion nonlinear ultrasonic testing, including the interaction angle α and the frequency ratio r, can be obtained through experimental methods.

[0120] During the test, a comparison specimen containing micro-defects was subjected to water-immersion non-collinear shear wave mixing testing at different interaction angles α and frequency ratios r. The amplitude of the sum-frequency longitudinal wave was extracted using a polarity-reversal signal processing method, and the influence of different interaction angles and frequency ratios on the mixing nonlinear effect was obtained, namely, the mixing α-r characteristic diagram. During the test, the interaction angle varied from [94°, 130°] with a step of 2°. The corresponding longitudinal wave straight probe incident angle varied from [20.05°, 25.15°] with a step of 0.47°. The frequency ratio varied from [0.5, 1.4] with a step of 0.025. In the specific test process, the smaller the step, the higher the test result accuracy. At different frequency and interaction angle combinations, the sum-frequency longitudinal wave scattering angle ψ was calculated according to Equation (5). Based on this, the longitudinal wave straight probe was positioned to receive the sum-frequency longitudinal wave. The amplitude of the sum-frequency longitudinal wave was extracted using a polarity-reversal signal processing method.

[0121] Figure 8 This is the α-r characteristic diagram of the test piece containing near-surface fatigue cracks to be detected in the embodiment of the present invention. Figure 8 It can be seen that the interaction angle between the two fundamental frequency shear waves corresponding to the peak of the sum frequency longitudinal wave is 106° and the frequency ratio is 0.8. Therefore, an interaction angle of 106° and a frequency ratio of 0.8 are selected as the detection parameters for underwater non-collinear shear wave mixing nonlinear ultrasonic testing.

[0122] S3: Implementation of water-immersion non-collinear shear wave mixing detection;

[0123] Based on the interaction angle α and frequency ratio r obtained in step 2, a water-immersion non-collinear shear wave mixing xz scanning inspection is performed on the defect-free comparison block and the test piece to be inspected using a water-immersion non-collinear shear wave mixing xz scanning inspection system.

[0124] S4: Detection data analysis and processing;

[0125] The test data obtained by the water-immersion non-collinear shear wave mixing test in step 3 is analyzed and processed using the polarity reversal signal processing method to extract the sum frequency longitudinal wave amplitudes at different test positions of the comparison test block and the test block to be tested, and obtain the AD curves at different positions. Figure 9 AD curves of a non-defective control test block and a test piece with near-surface fatigue cracks according to an embodiment of the present invention.

[0126] S5: Assessment of near-surface micro-defects.

[0127] Depend on Figure 9 It can be seen that for the comparison specimen without microdefects, its AD curve is characterized by a gradual decrease in the amplitude of its sum-frequency longitudinal wave with increasing depth, which is a Class A characteristic. The AD curve characteristics of the test specimen with near-surface fatigue cracks are significantly different from those of the comparison specimen without microdefects. The AD curve characteristics of the test specimen with near-surface fatigue cracks are characterized by an initial increase and then a gradual decrease in the amplitude of its sum-frequency longitudinal wave with increasing depth, which is a Class B characteristic. Therefore, it can be determined that the test specimen has microdefects near the surface.

[0128] In summary, the results of water-immersion non-collinear shear wave mixing nonlinear ultrasonic testing of near-surface micro-defects in this case show that this method can effectively detect near-surface defects.

[0129] The embodiments described above are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for detecting near-surface micro-defects by immersion shear wave mixing ultrasonic detection, characterized by: It includes the following steps: S1. Prepare a normal test block with the same shape, material, molding process, heat treatment status and surface roughness as the test block to be tested; S2, optimizing the parameters of the immersion nonlinear ultrasonic testing to obtain the optimal sum frequency longitudinal wave value, which includes the following sub-steps: S21, determining the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves; S22. Calculate the incident angles of two incident longitudinal waves in water. The specific calculation steps are as follows: First, the relationship between the interaction angle α of the two fundamental frequency shear waves and the incident angles of the two incident longitudinal waves in water is obtained according to the interaction angle α of the two fundamental frequency shear waves, as shown in formula (2): Where, v w is the sound velocity of longitudinal waves in the liquid surrounding the specimen, v s is the sound velocity of the shear wave in the test piece, α is the interaction angle between the two fundamental frequency shear waves in the test piece, θ1 and θ2 are the incident angles of the two incident longitudinal waves in water respectively; Taking θ1=θ2=θ, simplifying formula (2) to obtain formula (3), use formula (3) to calculate the incident angle of two incident longitudinal waves in water; S23. Calculate the sum-frequency longitudinal wave scattering angle ψ and arrange the longitudinal wave straight probe. The specific calculation steps are as follows: Two beams of fundamental frequency shear waves are emitted using the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S21 and the incident angles of the two incident longitudinal waves in the water. The angle between the wave vector k3 of the newly generated sum frequency longitudinal wave and the wave vector k1 of the incident shear wave is the sum frequency longitudinal wave scattering angle ψ. The sum frequency longitudinal wave scattering angle ψ is calculated according to the following formula (4). Based on the sum frequency longitudinal wave scattering angle ψ, a longitudinal wave straight probe is arranged to receive the sum frequency longitudinal wave. S24. Calculate the optimal sum-frequency longitudinal wave using the NP signal processing method: Use voltage signals with four different initial phase combinations to perform non-collinear shear wave detection respectively, analyze and process the four collected detection signals according to the NP signal processing method described in formula (5), and extract the mixing component in the detection signal, which is the sum-frequency longitudinal wave y(t). Where y pp (t), y nn (t), y np (t) and y pn (t) are the detection signals obtained by the interaction of the fundamental frequency shear waves x1(t) and x2(t) with four different initial phase combinations, as shown in equations (6) to (9) respectively; Where, the superscripts n and p represent the initial phases of the two fundamental frequency shear waves, respectively, and |np| = 180°; S3. Perform water immersion non-collinear shear wave mixing detection on the normal test block and the test block to be tested, and obtain AD curves of the normal test block and the test block to be tested: Based on the interaction angle α and frequency ratio r of the two fundamental frequency shear waves obtained in step S2, an immersion non-collinear shear wave mixing scanning test is performed on the normal test block and the test piece to be tested. The obtained test signals are analyzed and processed using the NP signal processing method. The amplitudes of the sum frequency longitudinal waves at different horizontal detection positions and different depths of the normal test block and the test piece to be tested are extracted respectively. The curve of the change of the sum frequency longitudinal wave amplitude at each horizontal position with depth is plotted, which is the AD curve; S4. The characteristics of the AD curve are divided into feature a and feature b, wherein feature a is that the amplitude of the sum frequency longitudinal wave gradually decreases with increasing depth, and feature b is that the amplitude of the sum frequency longitudinal wave first increases and then gradually decreases with increasing depth; S5, near-surface micro-defect judgment, which includes the following sub-steps: S51. Extract the features of the AD curve of the normal test block: The AD curve of the normal test block is characterized by a feature; S52, extracting the features of the AD curve of the test piece to be inspected and determining the near-surface micro-defects of the test piece to be inspected: When the AD curve of the test piece to be inspected has feature b, it is determined that there is a micro defect near the surface of the test piece to be inspected; When the AD curve features of the test block to be inspected are all feature a, the deviation degree ΔA between the peak value A1 of the AD curve of the test block to be inspected and the peak value A2 of the AD curve of the normal test block is calculated, ΔA = |A1-A2| / A2, and ΔA is compared with the set threshold B. When ΔA is greater than B, it is determined that there are micro-defects on the near surface of the test block to be inspected.

2. The method for detecting near-surface micro-defects by immersion shear wave mixing ultrasonic detection according to claim 1, characterized in that: In step S21, the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves are calculated by the following formula (1): Wherein, r=f2 / f1, f2 and f1 are the frequencies of the two fundamental frequency shear waves respectively.

3. The method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection according to claim 1, characterized in that: In step S21 , the interaction angle α and the frequency ratio r of the two fundamental frequency shear waves are optimized by performing multiple water-immersion non-collinear shear wave mixing tests.

4. The method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection according to claim 1, characterized in that: The threshold value B is determined by means of water immersion nonlinear ultrasonic testing and metallographic analysis.

5. The method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection according to claim 1, characterized in that: In step S3, the AD curves of the normal test block and the test block to be detected are plotted on the same graph.

6. The method for detecting near-surface micro-defects by water-immersion shear wave mixing ultrasonic detection according to claim 1, characterized in that: The sum frequency longitudinal wave is generated by the interaction between two series of transverse waves and a nonlinear source to generate a third series of sum frequency longitudinal waves.

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