1. A structure surface defect detection method based on high-frequency Lamb wave frequency domain information is characterized by comprising the following steps:

step 1: taking a sample without surface defects of a structure to be detected, and arranging a group of exciters and receivers on the surface of the sample;

step 2: acquiring discrete Lamb wave signal r generated by an exciter and received by a receiver when a sample has no surface defects_nR (N Δ t), N being 0,1,.., N-1, N being the total number of sample points; Δ t is the sampling interval;

and step 3: constructing surface defects with different depths on a sample, wherein the surface defects are positioned between an exciter and a receiver, calculating a damage index beta, and drawing a damage depth d-damage index beta curve;

the calculation method of the damage index beta comprises the following steps:

step 3.1: acquiring discrete Lamb wave signal x received by a receiver and generated by an exciter_n＝x(nΔt)；

Step 3.2: for discrete Lamb wave signal x_nBand-pass filtering is carried out to eliminate the influence of non-target frequency components and obtain a filtered discrete signal y_n；

Step 3.3: for signal y_nPerforming discrete Fourier transform to obtain Y_k；

Step 3.4: calculating a damage index beta;

wherein R is_kIn order to solve the problem that when the sample has no surface defect, the receiver receives a discrete Lamb wave signal r_nPerforming discrete Fourier transform to obtain a result;

and 4, step 4: arranging a row of exciters on one side of the surface of the structure to be detected, and arranging a row of receivers on the other side of the surface of the structure to be detected, wherein the receivers correspond to the exciters one to one; and detecting whether surface defects exist on the connecting line of each group of the exciter and the receiver by calculating the damage index beta of each group of the exciter-receiver, and acquiring the estimated value of the damage depth d according to a damage depth d-damage index beta curve.

Background

Nondestructive testing techniques based on ultrasonic guided waves can identify and monitor damage in structures to track and assess structural accidents and anomalies. The technique allows monitoring of hidden structures, coated structures, underwater structures or soil structures, as well as structures sealed in sealing layers and concrete, such as railway tracks, pipes and even aircraft skins. In thin-walled structures, this technique is also referred to as an active Lamb wave based acoustic emission monitoring method. The theoretical basis of the method is the propagation mechanism of Lamb waves in the waveguide. Therefore, application of an excitation signal, usually by one or more exciters, activates a guided wave in a thin-walled structure to propagate at the free surface of the structure. The guided wave amplitude and modal changes are recorded by receiving sensors arranged at different locations of the structure. The existence of the damage can change the guided wave mode and the propagation track, so the damage can be detected and positioned by comparing the echo signal with the original signal of the excitation point.

In recent years, many damage detection methods based on active Lamb wave acoustic emission technology have been established. These methods can be classified into damage detection methods based on Lamb wave linear features or nonlinear features according to differences in extracted features. However, the detection method based on Lamb wave linear characteristics is often limited to detect the damage with the same magnitude as the wavelength, because the damage with small scale does not cause the linear characteristics of the ultrasonic waves to change obviously, so the method is inefficient in detecting the micro-cracks. The damage detection method based on Lamb wave nonlinear characteristics is more sensitive to small-scale damage, but most of the methods extract nonlinear information related to the damage based on the excitation phenomenon of guided wave higher harmonics, and the method is obstructed in the practical application process because the higher harmonic signal source energy generated by damage reflection is weak, and unless complex signal processing is carried out, the higher harmonic is difficult to accurately separate and extract from a plurality of low-frequency signals and interference signals. Furthermore, most of the current research, whether linear or non-linear acoustic, is directed to the detection of penetrating damage, such as holes, and relatively little research is directed to the detection of structural surface defects and characterization of the extent of damage. Many penetrating type damages develop from surface defects, and if the damages can be successfully detected in the early stage of the damage development, namely the surface defect period, and then the maintenance and replacement of components are carried out in time, great engineering significance is achieved.

Disclosure of Invention

The invention aims to provide a structure surface defect detection method based on high-frequency Lamb wave frequency domain information.

The purpose of the invention is realized by the following technical scheme: the method comprises the following steps:

step 1: taking a sample without surface defects of a structure to be detected, and arranging a group of exciters and receivers on the surface of the sample;

step 2: acquiring discrete Lamb wave signal r generated by an exciter and received by a receiver when a sample has no surface defects_nR (N Δ t), N being 0,1,.., N-1, N being the total number of sample points; Δ t is the sampling interval;

and step 3: constructing surface defects with different depths on a sample, wherein the surface defects are positioned between an exciter and a receiver, calculating a damage index beta, and drawing a damage depth d-damage index beta curve;

the calculation method of the damage index beta comprises the following steps:

step 3.1: acquiring discrete Lamb wave signal x received by a receiver and generated by an exciter_n＝x(nΔt)；

Step 3.2: for discrete Lamb wave signal x_nBand-pass filtering is carried out to eliminate the influence of non-target frequency components and obtain a filtered discrete signal y_n；

Step 3.3: for signal y_nPerforming discrete Fourier transform to obtain Y_k；

Step 3.4: calculating a damage index beta;

wherein R is_kIn order to solve the problem that when the sample has no surface defect, the receiver receives a discrete Lamb wave signal r_nPerforming discrete Fourier transform to obtain a result;

and 4, step 4: arranging a row of exciters on one side of the surface of the structure to be detected, and arranging a row of receivers on the other side of the surface of the structure to be detected, wherein the receivers correspond to the exciters one to one; and detecting whether surface defects exist on the connecting line of each group of the exciter and the receiver by calculating the damage index beta of each group of the exciter-receiver, and acquiring the estimated value of the damage depth d according to a damage depth d-damage index beta curve.

The invention has the beneficial effects that:

the invention provides a nonlinear index beta for surface defect detection based on the time domain information change rule of high-frequency Lamb waves, the index is based on the nonlinear characteristics of Lamb waves and can be used for detecting the damage smaller than the excitation wave wavelength of the Lamb waves, and the calculation of the index does not depend on the excitation and extraction of higher harmonics, so that the invention has good engineering applicability and stability. The invention can detect the surface defect in the component and can represent the depth information of the surface defect; when surface damage exists on the monitoring path, the value of the damage index beta is obviously increased and is obviously distinguished from the indexes on the lossless path, and the value of the damage index beta is increased along with the deepening of the damage depth, so that the depth information of the surface damage can be effectively reflected. The invention has stronger anti-noise capability and can still obtain stable monitoring results in stronger noise environment.

Drawings

Fig. 1 is a flowchart of calculation of the damage index β in the present invention.

FIG. 2(a) is a group velocity dispersion plot of a 4mm thick steel plate in an example of the present invention.

FIG. 2(b) is a phase velocity dispersion curve of a 4mm thick steel plate in the example of the present invention.

Fig. 3 is a diagram showing the arrangement of sensors on a steel plate in an embodiment of the present invention.

Fig. 4 is an experimental test piece diagram of different damage depths in the embodiment of the present invention.

Fig. 5 is a statistical chart of the damage indicators on each monitoring path according to the embodiment of the present invention.

FIG. 6 is a graph of lesion depth d versus lesion index β in an example of the present invention.

Fig. 7 is a material property table of a Q235 steel plate in an example of the invention.

Detailed Description

The invention is further described below with reference to the accompanying drawings.

The invention relates to the field of engineering structure health monitoring. The invention provides a structure surface defect detection method based on high-frequency Lamb wave frequency domain information, which aims at the defects of plate-shaped metal structures widely applied to large-scale projects such as aerospace, ships, bridges and the like and aims at the defects of surface cracks and the like of the structures.

The invention provides a nonlinear index beta for surface defect detection based on the time domain information change rule of high-frequency Lamb waves, the index is based on the nonlinear characteristics of Lamb waves and can be used for detecting the damage smaller than the wavelength of an excitation wave of the Lamb waves, and the calculation of the index does not depend on the excitation and extraction of higher harmonics, so that the invention has good engineering applicability and stability, can detect the surface defects in components and can represent the depth information of the surface defects.

The invention aims to overcome the detection problem of surface defects with different depths in a structure, and provides a structure surface defect detection method based on high-frequency Lamb wave frequency domain information, which comprises the following steps:

step 1: taking a sample without surface defects of a structure to be detected, and arranging a group of exciters and receivers on the surface of the sample;

step 2: acquiring discrete Lamb wave signal r generated by an exciter and received by a receiver when a sample has no surface defects_nR (N Δ t), N being 0,1,.., N-1, N being the total number of sample points; Δ t is the sampling interval;

and step 3: constructing surface defects with different depths on a sample, wherein the surface defects are positioned between an exciter and a receiver, calculating a damage index beta, and drawing a damage depth d-damage index beta curve;

the calculation method of the damage index beta comprises the following steps:

step 3.1: acquiring discrete Lamb wave signal x received by a receiver and generated by an exciter_n＝x(nΔt)；

Step 3.2: for discrete Lamb wave signal x_nBand-pass filtering is carried out to eliminate the influence of non-target frequency components and obtain a filtered discrete signal y_n；

Step 3.3: for signal y_nPerforming discrete Fourier transform to obtain Y_k；

Step 3.4: calculating a damage index beta;

wherein R is_kIn order to solve the problem that when the sample has no surface defect, the receiver receives a discrete Lamb wave signal r_nPerforming discrete Fourier transform to obtain a result;

and 4, step 4: arranging a row of exciters on one side of the surface of the structure to be detected, and arranging a row of receivers on the other side of the surface of the structure to be detected, wherein the receivers correspond to the exciters one to one; and detecting whether surface defects exist on the connecting line of each group of the exciter and the receiver by calculating the damage index beta of each group of the exciter-receiver, and acquiring the estimated value of the damage depth d according to a damage depth d-damage index beta curve.

Fig. 1 shows a calculation flow of the damage index β based on the frequency domain information of the high-frequency Lamb wave. Firstly, band-pass filtering is carried out on an original signal to eliminate the influence of non-target frequency components on the signal, then discrete Fourier transform is carried out on the filtered signal, and the maximum frequency component amplitude values of a test signal and a reference signal are extracted to calculate beta. And then, detecting surface defects at different depths in the steel plate by using the damage index, and researching the relation between the damage depth and the damage index.

Given a set of discrete lamb wave signals x_nX (N Δ t), where N is 0,1,.., N-1, N is the total number of sample points, and Δ t is the sampling interval. Firstly, band-pass filtering is carried out on the signal to eliminate the influence of non-target frequency components, and the filtered signal is used as y_nAnd (4) showing. Then y_nDiscrete fourier transform of (d):

Y_kinverse discrete fourier transform of (d):

as can be seen from equation (2), after the discrete fourier transform, the amplitude of the signal in the frequency domain changes by N times. Thus, the maximum frequency component amplitude of the test signal can be expressed as:

in order to eliminate the influence of uncertain factors such as manual operation, material physical property change and the like, the standard signal is used for calculating the damage index. The reference signal is a discrete Lamb signal of the structure under the health condition and is denoted as r_nR (n Δ t), whose discrete fourier transform is denoted R_k. The damage index formula of the invention is as follows:

tests prove that compared with the prior art, the invention has the beneficial effects that:

the method can effectively detect the surface damage in the steel plate, when the surface damage exists on the monitoring path, the value of the damage index beta is obviously increased, and the damage index beta is obviously distinguished from the indexes on the lossless path. The method can effectively reflect the depth information of the surface damage, and the value of the damage index beta is increased along with the deepening of the damage depth. The invention has stronger anti-noise capability and can still obtain stable monitoring results in stronger noise environment.

Example 1:

the feasibility of the invention for detecting surface defects and characterizing defect depth was verified using the detection of surface defects at different depths in a 4mm steel plate as an example. The theoretical basis of the invention is the propagation mechanism of Lamb waves in the waveguide, and the method needs to apply an excitation signal through one or more exciters to activate guided waves in a thin-wall structure so as to enable the guided waves to propagate on the free surface of the structure. Therefore, firstly, the frequency dispersion equation of Lamb in a 4mm steel plate needs to be solved, and the proper excitation frequency is determined by analyzing the Lamb wave frequency dispersion curve. The exciter signals are then applied to the exciter in sequence to obtain corresponding received signals. And finally, extracting damage characteristic information, and calculating a damage index according to a formula (4)) of beta.

1. Determining an excitation frequency

The characteristic equation of Lamb waves in a free state is as follows:

and (3) symmetrical model:

an antisymmetric model:

where k is the component of the angular wave on the cartesian axis. p is a radical of²＝(ω)²/c_L ²-(ω/c_P)²,q²＝(ω)²/c_T ²-(ω/c_P)².c_LAnd c_TRepresenting the wave velocities of longitudinal and transverse waves, respectively, propagating in a solid medium. c. C_pRepresenting the phase velocity of Lamb waves, group velocity c of Lamb waves_gCan be expressed as:

the material properties of the 4mm steel plate are shown in fig. 7, and the characteristic equation equations (5) and (6) of Lamb waves are solved according to the material properties, so that dispersion curves of group velocity and phase velocity are obtained and are shown in fig. 2(a) and 2(b), respectively. As can be seen from the figure, except for the low order mode S₀And A₀In addition, the cut-off frequency exists in the rest high-order modes. And will therefore be higher than a₁The band of modal cut-off frequencies (500kHz) is called the high band. Calculating formula lambda according to the shortest wavelength_min＝c_TThe higher the excitation frequency f, the shortest wavelength λ_minThe smaller. Lamb waves are more sensitive to damage of the same magnitude as the excitation wavelength, so that the higher the excitation frequency is, the smaller the damage can be detected. However, since the higher the excitation frequency, the lower the Lamb wave energy, if the excitation frequency is selected to be too high, the energy of the received signal will be too low. The excitation frequency is set to 1800kHz in this case.

2. Experimental test piece

The geometrical model of the steel plate, the placement of sensors and damage is shown in fig. 3. Experimental specimens of different damage depths are shown in fig. 4. The planar size of the steel plate was 300X 150X 4mm, the sensor diameter was 10mm, the damage length and width were 10mm and 1mm, respectively, and the damage depth was set to 1, 2, and 3mm, respectively. On the steel plate, 10 sensors were arranged, with PZT1 to PZT5 as actuators and PZT6 to PZT10 as receivers. These sensors form five monitoring paths: route 1: PZT1-PZT6, path 2: PZT2-PZT7, path 3: PZT3-PZT8, path 4: PZT4-PZT9, path 5: PZT5-PZT 10. The lesion is located on pathway 1 with its central coordinate (-80mm, 0).

3. Damage detection result

The damage index β calculated on each propagation path is shown in fig. 5. Since there is only a damage on path 1, path 1 is referred to as a damaged path, and the remaining paths are referred to as lossless paths. In this case, the noise immunity of β is also studied, that is, the monitoring results of the damage index β are considered under the conditions that the signal-to-noise ratio is 10 dB, 15 dB, and 20dB, respectively. As shown in fig. 5, β on the lossless path is close to 0 and β on the lossless path is greater than 0.3 at 3 different lesion depths (1, 2, 3mm) and different signal-to-noise ratios (10, 15, 20dB) in 3, which indicates that when there is a lesion on the monitoring path, the lesion index β on the path can be clearly distinguished from the lesion index on the lossless path. This demonstrates that the damage index proposed in the present invention can successfully detect surface defects at different depths in a structure. Analyzing the performance of beta under different signal-to-noise ratios shows that beta on a damage path can be successfully distinguished from beta on a lossless path under the three signal-to-noise ratios. The result proves that beta has stronger anti-noise performance and can obtain accurate and stable monitoring results in a strong noise environment. In addition, the invention also provides a prediction formula of the damage depth:

the depth of the detected surface defect can be predicted according to equation (8). According to the formula, the damage depth and the damage index have positive correlation, and the damage curve of the specific material, the structure and the working condition can be obtained by fitting the damage data, as shown in fig. 6. Therefore, it can be known that if the damage index on the monitoring path is increased, the depth of the damage is increased. At this time, the monitored structure needs to be repaired or replaced in time.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.