A technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasonic wave

Abstract

The present invention discloses a technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound. The method is based on the nonlinear response effect of ultrasound on microscopic damage of solid medium. The nonlinear ultrasonic signal of cladding layer is collected by means of a frequency-doubled ultrasonic probe group in a transmit-receive mode. The signal-to-noise ratio is used as an evaluation index. The frequency domain signal of nonlinear ultrasound is calculated by wavelet analysis method and fast Fourier transform theory. Then, the correlation between ultrasonic nonlinear coefficient and fatigue cycle is established. Finally, the quantitative relationship between ultrasonic nonlinear coefficient and fatigue cycle is obtained by numerical fitting method, so as to realize nonlinear ultrasonic evaluation of fatigue damage of cladding layer. The present invention provides a non-destructive, convenient, fast, safe and online applicable technology for quantitative evaluation of fatigue damage of cladding layer.

Description

Technical Field

[0001] The invention relates to ultrasonic nondestructive evaluation technology, in particular to a technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasonic waves. Background Art

[0002] With the increasingly harsh service conditions, the probability of overall failure of mechanical equipment due to fatigue damage of key components of mechanical equipment is getting higher and higher, which not only leads to waste of resources and energy, but also increases the difficulty of environmental pollution control and energy conservation and emission reduction. Mechanical equipment remanufacturing provides an effective way to solve this problem. Due to its advantages such as high energy density and high forming accuracy, laser cladding technology has become one of the commonly used technologies for mechanical equipment remanufacturing. However, due to multiple influencing factors, the fatigue performance of laser cladding remanufacturing products is difficult to control. For this reason, domestic and foreign scholars have conducted a lot of theoretical and experimental research on it, mainly including the following three aspects: ① Optimization of cladding process; ② Innovation of cladding technology; ③ Development of cladding materials. Although the above methods can improve the fatigue performance of laser cladding remanufacturing products to a certain extent, it is still difficult to achieve quantitative control of the fatigue performance of laser cladding remanufacturing products, which undoubtedly poses a safety hazard to the service performance and quality of laser cladding remanufacturing products. Therefore, it is urgent to actively explore technologies that can achieve quantitative evaluation of fatigue performance of laser cladding remanufacturing products.

[0003] Generally speaking, fatigue performance evaluation methods can be divided into two categories: destructive evaluation and non-destructive evaluation. The destructive evaluation method mainly obtains the correlation between fatigue cycles and fatigue performance through a large number of fatigue experiments, and then realizes the evaluation of the fatigue performance of laser cladding specimens. However, this type of method belongs to the category of small sample sampling detection, and cannot realize the evaluation of the fatigue performance of the entire specimen, which undoubtedly brings safety hazards to product quality and safety. The non-destructive evaluation method is a method that realizes the non-destructive evaluation of the fatigue performance of the specimen by analyzing the acoustic, electrical, magnetic, optical and other signals without destroying the integrity of the specimen. This method not only has the characteristics of non-destructiveness, but also can realize the detection of the entire product, so it has attracted the attention of many scholars in the field of fatigue performance evaluation. In comparison, due to its advantages such as safety, convenience, speed and online application, nonlinear ultrasonic technology has attracted widespread attention from many scholars in the field of fatigue performance evaluation.

[0004] Nonlinear ultrasonic technology is a method of fatigue damage evaluation based on the nonlinear part of the ultrasonic wave equation responding to micro-damage of solid media. However, this response is extremely weak, so how to improve the accuracy of the nonlinear response to fatigue damage is extremely critical. Related studies have shown that the accurate extraction of nonlinear ultrasonic high-order harmonics is the key to ensuring the accuracy of nonlinear ultrasonic evaluation of fatigue damage of samples. Therefore, based on the wavelet analysis theory and the time-frequency domain signal analysis theory, a set of effective nonlinear ultrasonic technology that can accurately extract the amplitude of nonlinear ultrasonic high-order harmonics and then realize the quantitative evaluation of laser cladding fatigue damage is explored and established. It can not only provide technical support for the service safety and reliability evaluation of laser cladding remanufactured products, but also is crucial to reduce or even avoid the quality risks of remanufactured products. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasonic wave, so as to realize quantitative evaluation of fatigue damage of laser cladding layer non-destructively and rapidly.

[0006] Technical solution: The wave theory of ultrasonic waves in solid media shows that its nonlinear effect becomes more and more obvious with the degree of damage to the solid media, that is, as the degree of fatigue damage increases, the nonlinear coefficient of ultrasonic waves gradually increases. Therefore, the quantitative evaluation of the degree of fatigue damage can be achieved by calculating the nonlinear coefficient of ultrasonic waves. However, the nonlinear response of ultrasonic waves to fatigue damage is extremely weak and is seriously affected by noise signals. Therefore, it is urgent to effectively and accurately extract the ultrasonic nonlinear coefficient that can characterize the degree of fatigue damage. In response to this problem, the present invention is based on the nonlinear effect of ultrasonic waves, and adopts an ultrasonic probe group with a one-transmit-one-receive mode and an integer multiple of the center frequency. Through signal analysis and processing technology, ultrasonic high-order harmonics that can characterize the degree of fatigue damage are extracted, and the relative nonlinear coefficient is calculated. Then, the corresponding relationship between the relative nonlinear coefficient of ultrasonic waves and the degree of fatigue damage is established, and finally the quantitative evaluation of fatigue damage of laser cladding layers by nonlinear ultrasonic wave evaluation is achieved.

[0007] The present invention discloses a technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasound, comprising the following steps:

[0008] (1) Optimize the process specifications for preparing the cladding layer, prepare a cladding layer of a certain thickness on the surface of the base material, and process the cladding layer fatigue specimen in accordance with the national standard for room temperature fatigue specimens of metal materials; the process specifications include energy power, cladding speed, powder feeding method and powder feeding amount;

[0009] (2) optimizing the excitation parameters of the nonlinear ultrasonic wave, and adjusting and fixing the distance between the nonlinear ultrasonic wave excitation probe and the acquisition probe to be constant in combination with the attenuation of the nonlinear ultrasonic wave energy; the excitation parameters include frequency, pulse form and number;

[0010] (3) determining the yield strength of the cladding layer sample in step (1) by static load tensile testing, thereby setting the fatigue load, stress ratio and fatigue cycle program of the fatigue test, and sequentially collecting the nonlinear ultrasonic signals of the cladding layer at different cycle times until the fatigue test is stopped;

[0011] (4) Calculate the signal-to-noise ratio of the nonlinear ultrasonic signal of the laser cladding layer through wavelet analysis and optimize its noise reduction parameters;

[0012] (5) The frequency domain signal of the nonlinear ultrasonic wave of the cladding layer at each fatigue cycle is calculated by the fast Fourier transform theory, and the amplitude of its fundamental wave and higher harmonics is extracted, and then the relative nonlinear coefficient of the ultrasonic wave of the cladding layer at each fatigue cycle is calculated and obtained;

[0013] (6) The corresponding relationship between the ultrasonic relative nonlinear coefficient of the cladding layer and the fatigue cycle number is established, and a polynomial function is used to fit it, and a quantitative relationship between the ultrasonic relative nonlinear coefficient and the fatigue cycle number is obtained, thereby realizing a quantitative evaluation of the fatigue damage degree of the cladding layer; the thickness, initial stress state, loading form, etc. of the cladding layer are the same as those in steps (1) to (3).

[0014] The cladding layer in step (1) is well metallurgically bonded to the substrate, there are no defects in the cladding layer, and the cladding layer fatigue specimen is a composite specimen of the cladding layer + substrate material, and the deviation of the cladding layer thickness is much smaller than the cladding layer thickness.

[0015] In the step (2), the propagation time of the nonlinear ultrasonic wave from the excitation probe to the collection probe is greater than the excitation time of the nonlinear ultrasonic wave, and the thickness of the cladding layer is not greater than the detection depth of the nonlinear ultrasonic wave and depends on its excitation frequency.

[0016] The laser cladding layer in step (3) does not show macro cracks during the fatigue test, and its fatigue load is not higher than its yield strength.

[0017] The noise frequency amplitude of the nonlinear ultrasonic signal after noise reduction in step (4) is much lower than the amplitude of its higher harmonics.

[0018] The fundamental frequency of the nonlinear ultrasound in step (5) is the excitation frequency of the ultrasound wave, and the higher harmonic frequency is an integer multiple of the fundamental frequency, and the multiple is greater than or equal to 2.

[0019] The cladding layer in step (6) has no macro cracks, and the fatigue damage in steps (1) to (6) is damage before macro cracks appear.

[0020] A computer storage medium stores a computer program, which, when executed by a processor, implements the above-mentioned technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasonic waves.

[0021] A computer device comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the above-mentioned technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasonic wave is implemented.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following advantages: the present invention prepares a laser cladding layer sample of a certain thickness, collects nonlinear ultrasonic signals of different fatigue cycle times by means of fatigue test, improves the amplitude accuracy of nonlinear ultrasonic high-order harmonics of the laser cladding layer based on wavelet analysis theory, calculates its ultrasonic nonlinear coefficient, and establishes a quantitative relationship between the coefficient and the fatigue cycle times, thereby obtaining the ultrasonic nonlinear coefficient of any fatigue cycle, and finally realizing the nonlinear ultrasonic evaluation of the fatigue damage degree of the laser cladding layer; the present invention provides a nondestructive evaluation method for fatigue damage of the laser cladding layer, and has the advantages of being fast, convenient, safe, and capable of realizing online evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a flow chart of the steps of the present invention;

[0024] Figure 2 It is the nonlinear ultrasonic signal of the laser cladding layer;

[0025] Figure 3 is the frequency domain signal of nonlinear ultrasonic wave of laser cladding layer;

[0026] Figure 4 This is the corresponding relationship between the ultrasonic relative nonlinear coefficient of the laser cladding layer and the number of fatigue cycles. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0028] like Figure 1 As shown in the figure, a technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasonic wave is shown in the figure. The specific steps are as follows:

[0029] Step 1: According to the selected laser cladding base plate (thickness not less than 10 mm, carbon content (equivalent) not more than 0.6%) and the properties of the cladding powder material, the laser cladding process parameters are optimized, including: laser power, cladding speed, powder feeding amount, etc., to prepare a laser cladding layer sample with good metallurgical bonding, no defects in the cladding layer and balanced thickness, and the thickness deviation of the laser cladding layer is not more than 20% of the cladding layer thickness;

[0030] Step 2: Based on the national standard for room temperature fatigue specimens of metal materials, laser cladding layer specimens are machined by machining methods, with the number of not less than 2 groups. After machining, the surface roughness of the laser cladding layer specimens is ensured to be no greater than Ra1.5, meeting the requirements for nonlinear ultrasonic signal acquisition;

[0031] Step 3, optimize the nonlinear ultrasonic excitation frequency (not more than 10MHz), pulse form, number of wave packets and other parameters, and determine and optimize the distance between the nonlinear ultrasonic excitation probe and the receiving probe according to the number of wave packets and the degree of nonlinear ultrasonic energy attenuation;

[0032] Step 4, determining the yield strength of the laser cladding layer sample in step 2 by a uniaxial static tensile test, setting the fatigue load to be no greater than its yield strength, and the stress ratio to be between -1 and 1, applying the fatigue load, and stopping the loading without unloading after a certain number of cycles (no less than 1000 cycles), collecting the nonlinear ultrasonic signal of the laser cladding layer, until the expected number of fatigue cycles is reached, and then stopping the experiment;

[0033] Step 5: Select a denoising mother wavelet based on the waveform of the nonlinear ultrasonic wave, the decomposition level is not less than 3 layers, the denoising threshold is a soft threshold or a hard threshold, and the signal-to-noise ratio (not less than 10) of the nonlinear ultrasonic signal after denoising is calculated, so as to optimize and clarify the mother wavelet, decomposition level and denoising threshold in the wavelet denoising method;

[0034] Step 6: Calculate the frequency of the nonlinear ultrasonic wave of the laser cladding layer after noise reduction by the fast Fourier transform method, extract the amplitude of the fundamental wave and the higher harmonic frequency (the order of the higher harmonic is between 2 and 4), and bring them into formula (1) to calculate the relative nonlinear coefficient of the ultrasonic wave of the laser cladding layer at each fatigue cycle;

[0035]

[0036] Where: β is the relative nonlinear coefficient of ultrasound, A n is the amplitude of higher harmonics (n is the order), and A1 is the amplitude of fundamental wave.

[0037] Step 7: Establish the corresponding relationship between the relative nonlinear coefficient of the ultrasonic wave of the laser cladding layer and the number of fatigue cycles. Use a polynomial function, see formula (2), and fit it to obtain a quantitative relationship between the relative nonlinear coefficient of the ultrasonic wave and the number of fatigue cycles.

[0038] y=a n x n +a n-1 x n-1 +a n-2 x n-2 +a n-3 x n-3 +......+a (2)

[0039] Where: y is the relative nonlinear coefficient of ultrasonic wave, x is the number of fatigue cycles, a n 、a n-1 、a n-2 , a is the fitting coefficient.

[0040] Step 8: Use the nonlinear ultrasonic excitation parameters in step 3 to collect the nonlinear ultrasonic signal of the laser cladding layer in any damage state, reduce the noise according to step 5, calculate the relative nonlinear coefficient of the ultrasonic signal according to step 6, and substitute it into formula (2) to obtain the cycle number, and then calculate and obtain the fatigue damage degree of the laser cladding layer through step 4.

[0041] Taking Q235 steel as the laser cladding base plate and Fe314 alloy powder as the laser cladding powder to prepare the laser cladding layer sample as an example, a technical method and steps for evaluating the fatigue damage of the cladding layer based on nonlinear ultrasonic wave are described. The specific process is as follows:

[0042] Step 1. Select Q235 steel with a thickness of 30 mm as the base plate material, adopt the side powder feeding method, use argon as the protective gas, optimize and fix the laser cladding process parameters as follows: laser power is 1.7 kW, laser cladding speed is 2.0 mm / s, powder feeding amount is 16.0 g / s, and prepare a Fe314 alloy laser cladding layer with a thickness of (2.0±0.25) mm and no defects;

[0043] Step 2: According to the standard GB / T3075-2008 for the control of axial force in fatigue tests of metal materials, two groups of laser cladding layer specimens with an overall thickness of 8 mm were prepared by mechanical processing (the thickness of the laser cladding layer was 2.0 mm, and the thickness of the base plate was 6.0 mm). To ensure that the surface roughness of the specimens met the requirements of the fatigue test, a grinder was used to process the surface of the laser cladding layer specimens, and the roughness was Ra1.2;

[0044] Step 3, adjust the excitation parameters of the nonlinear ultrasonic wave, select the excitation frequency of 5MHz according to the thickness of the laser cladding layer, fix the excitation frequency signal to 10 sine waves, and calculate and fix the distance between the nonlinear ultrasonic wave excitation probe and the receiving probe to 25mm when the amplitude of the nonlinear ultrasonic wave receiving signal reaches 70% of the full screen of the DPO3034B oscilloscope;

[0045] Step 4: According to GB / T 228-2002 Standard for Room Temperature Tensile Test Methods for Metallic Materials, the yield strength of the laser cladding layer sample in step 2 is 685 MPa, which is set as fatigue strength, and the fatigue load stress ratio is set to -1. Fatigue load is applied to the laser cladding layer sample. Based on this, the fatigue test is stopped every 5000 cycles, and the nonlinear ultrasonic signal of the laser cladding layer is collected after the load is maintained for 60 seconds without unloading. See attached Figure 2 , until the fatigue cycle reaches 55000 times, the laser cladding layer breaks and the experiment is stopped;

[0046] Step 5: Based on the wavelet analysis method, the nonlinear ultrasonic signal of the laser cladding layer at each fatigue cycle is subjected to denoising, and the denoising parameters are optimized and fixed as follows: sym6 wavelet as mother wavelet, 6-layer decomposition, and soft threshold denoising. After denoising, the signal-to-noise ratio of the nonlinear ultrasonic signal of the laser cladding layer at each fatigue cycle is higher than 18.0;

[0047] Step 6: Based on the fast Fourier transform (FFT) theory, the frequency signal of the nonlinear ultrasonic wave of the laser cladding layer at each fatigue cycle is calculated by superimposing the Hanning window on the nonlinear ultrasonic wave signal. Figure 3 , extract its fundamental wave and second harmonic amplitude, and substitute them into formula (1) to calculate its ultrasonic relative nonlinear coefficient, as shown in Table 1.

[0048] Table 1 Ultrasonic nonlinear coefficient of laser cladding layer

[0049]

[0050]

[0051] Step 7: Establish the corresponding relationship between the relative nonlinear coefficient of the ultrasonic wave of the laser cladding layer and the number of fatigue cycles in Table (1), see Appendix Figure 4 , and fit it using formula (2) to obtain the quantitative relationship between the relative nonlinear coefficient of ultrasonic wave and the number of fatigue cycles, as shown in formula (3):

[0052] A=0.0003C 3 -0.0043C 2 +0.0132C 1 +0.1002C+0.6449 (3)

[0053] Where: A is the relative nonlinear coefficient of ultrasound, and C is the number of fatigue cycles.

[0054] Step 8, using the nonlinear ultrasonic excitation parameters in step 2 to collect the nonlinear ultrasonic signal of the Fe314 alloy cladding layer to be evaluated (the laser cladding layer thickness preparation process is the same as the process in step 1). First, the noise reduction parameters in step 5 are used to reduce the noise, and the signal-to-noise ratio after noise reduction is 22.322; secondly, according to step 6, the fundamental wave and second harmonic amplitude of the frequency signal are extracted, and the relative nonlinear coefficient of the ultrasonic wave is calculated as 1.1256185 by substituting into formula (1), and then substituting into formula (3) to obtain the corresponding fatigue cycle number of about 25386. Finally, the ratio of the fatigue cycle number (55000) when the laser cladding layer sample is broken in step 4 is calculated to be about 0.462, that is, the degree of fatigue damage is about 46.2% of the life of the laser cladding layer.

Claims

1. A technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound, characterized in that: The following steps are involved: (1) Optimizing the process specifications for preparing the cladding layer: a cladding layer of a certain thickness is prepared on the surface of the substrate material, the cladding layer and the substrate are well metallurgically bonded, there are no defects in the cladding layer, and the fatigue specimen of the cladding layer is a composite specimen of the cladding layer + substrate material, and the deviation of the thickness of the cladding layer is much smaller than the thickness of the cladding layer; the process specifications include energy power, cladding speed, powder feeding method and powder feeding amount; (2) Optimizing the excitation parameters of nonlinear ultrasound: In combination with the attenuation of nonlinear ultrasound energy, adjusting and fixing the distance between the nonlinear ultrasound excitation probe and the acquisition probe to be constant; the excitation parameters include frequency, pulse form and number; (3) determining the yield strength of the cladding layer sample in step (1) by static load tensile testing, thereby setting the fatigue load, stress ratio and fatigue cycle program of the fatigue test, and sequentially collecting the nonlinear ultrasonic signals of the cladding layer at different cycle times until the fatigue test is stopped; (4) Calculate the signal-to-noise ratio of the nonlinear ultrasonic signal of the laser cladding layer through wavelet analysis and optimize its noise reduction parameters; (5) The frequency domain signal of the nonlinear ultrasonic wave of the cladding layer at each fatigue cycle is calculated by the fast Fourier transform theory, and the amplitude of its fundamental wave and higher harmonics is extracted, and then the relative nonlinear coefficient of the ultrasonic wave of the cladding layer at each fatigue cycle is calculated and obtained; (6) The corresponding relationship between the ultrasonic relative nonlinear coefficient of the cladding layer and the number of fatigue cycles is established, and a polynomial function is used to fit it, and a quantitative relationship between the ultrasonic relative nonlinear coefficient and the number of fatigue cycles is obtained, thereby realizing the quantitative evaluation of the fatigue damage degree of the cladding layer.

2. According to the technical method of evaluating fatigue damage of cladding layer based on nonlinear ultrasound in claim 1, it is characterized in that: In the step (2), the propagation time of the nonlinear ultrasonic wave from the excitation probe to the collection probe is greater than the excitation time of the nonlinear ultrasonic wave, and the thickness of the cladding layer is not greater than the detection depth of the nonlinear ultrasonic wave and depends on its excitation frequency.

3. The technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound according to claim 1 is characterized in that: The laser cladding layer in step (3) does not show macro cracks during the fatigue test, and its fatigue load is not higher than its yield strength.

4. The technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound according to claim 1 is characterized in that: The noise frequency amplitude of the nonlinear ultrasonic signal after noise reduction in step (4) is much lower than the amplitude of its higher harmonics.

5. The technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound according to claim 1 is characterized in that: The fundamental frequency of the nonlinear ultrasound in step (5) is the excitation frequency of the ultrasound wave, and the higher harmonic frequency is an integer multiple of the fundamental frequency, and the multiple is greater than or equal to 2.

6. The technical method for evaluating fatigue damage of cladding layer based on nonlinear ultrasound according to claim 1 is characterized in that: The cladding layer in step (6) has no macro cracks, and the fatigue damage in steps (1) to (6) is damage before macro cracks appear.

7. A computer storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasound as described in any one of claims 1 to 6 is implemented.

8. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the technical method for evaluating fatigue damage of a cladding layer based on nonlinear ultrasound according to any one of claims 1 to 6 is implemented.

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