CN111272882B - Method for detecting defects of scattering features of structure by utilizing ultrasonic guided waves - Google Patents
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Abstract
The invention relates to a method for detecting defects of scattering features of a structure by utilizing ultrasonic guided waves, which comprises the following steps: respectively calculating guided wave mode wave structures in the free waveguide part and the scattering feature part; calculating a matching coefficient between a guided wave mode wave structure of the free waveguide part and a guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing a guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the change of frequency so as to obtain a first frequency response; calculating scattering characteristic guided wave reflection coefficients in detection signals when different center frequencies are used, and drawing a curve of the guided wave reflection coefficients along with the frequency change to obtain a second frequency response; by comparing the first frequency response with the second frequency response, it is determined whether a defect exists in the detected scattering feature.
Description
Technical Field
The invention belongs to the technical field of nondestructive testing, relates to an ultrasonic guided wave nondestructive testing technology, and particularly relates to a method for detecting defects of scattering features of a structure by utilizing ultrasonic guided waves.
Background
The guided wave detection technology is an efficient structure detection technology, and has the advantages of non-contact, rapidness, long detection distance, capability of realizing detection of the whole structure volume and the like. However, due to scattering features such as an elbow, a clamp and a support in the structure, reflection and transmission of guided waves can occur at the scattering features, if defects occur in the scattering feature area, defect reflection echoes can be overlapped with the scattering feature reflection echoes to be difficult to distinguish, and therefore whether defects exist in the structure or not is difficult to judge from echo signals.
In order to realize detection of defects in scattering feature areas, various detection methods are currently presented. For example, patent document US7565252B2 discloses a method for distinguishing between a pipeline geometrical characteristic reflected signal and a defect reflected signal in ultrasonic guided wave detection, which judges whether a defect exists or not by comparing the phase of a known characteristic reflected signal with the phase information of the reflected signal in the detected signal. There are also many academic papers that mention the basic subtraction, i.e. subtracting the perfect characteristic reflected signal from the detected signal, with defects if there are any residuals and with defects if there are no residuals.
The method for detecting the defects of the scattering feature region by using the phase contrast and the basis subtraction can realize the detection of the defects of the scattering feature region, and avoid missed detection to a certain extent. However, the method needs to have a reflection signal with perfect scattering characteristics as a reference, and in practical application, the reference signal is sometimes difficult to obtain, so that the application of the method is limited, the problem of omission is still caused, and the detection effect is poor.
Disclosure of Invention
Aiming at the problems of missed detection, poor detection effect and the like existing in the existing scattering feature region defect detection, the invention provides a method for detecting the scattering feature region defect of a structure by utilizing ultrasonic guided waves, which can effectively detect the scattering feature region defect and avoid missed detection.
In order to achieve the above object, the present invention provides a method for detecting defects of a scattering feature of a structure by using ultrasonic guided waves, comprising the following steps:
respectively calculating guided wave mode wave structures in the free waveguide part and the scattering feature part;
calculating a matching coefficient between the guided wave mode wave structure of the free waveguide part and the guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing the guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the frequency change;
calculating reflection coefficients of scattering characteristic guided waves in detection signals when different center frequencies are calculated, and drawing a curve of the reflection coefficients of the guided waves along with the frequency change;
performing linear fitting on the curve of the reflection coefficient of the guided wave of the intact scattering feature part along with the frequency change to obtain a first response frequency, and performing linear fitting on the curve of the reflection coefficient of the guided wave of the scattering feature part along with the frequency change in the detection signal to obtain a second response frequency;
by comparing the first frequency response with the second frequency response, it is determined whether a defect exists in the detected scattering feature.
Preferably, a half-analytic finite element method is adopted to calculate guided wave mode wave structures in the free waveguide part and the scattering feature part according to the size and material parameters of the structure to be detected, a matching coefficient between the two guided wave mode wave structures is calculated by using a mode confidence criterion through a formula (1), and the formula (1) is expressed as follows:
wherein ρ (·) represents a matching coefficient, m represents a guided wave mode, MAC (·) represents a mode confidence criterion, a i I-mode, B representing the waveguide region of the free waveguide section j J-mode, ψ, representing scattering feature waveguide region i Represents the mode vector, ψ, of the i-mode guided wave structure of the free waveguide part j A j-mode guided wave structure mode vector of the scattering feature part is represented;
the reflection coefficient of the guided wave of the perfect scattering feature part is expressed by adopting R_MAC, and is calculated by a formula (2), wherein the formula (2) is expressed as follows:
R_MAC＝1-MAC_sum-MAC_mc (2)
wherein, the MAC_sum is the sum of the matching coefficients of a mode wave structure in the free waveguide part and each mode wave structure in the scattering feature part; the mac_mc is the sum of the mode conversion mode matching coefficients generated by a certain mode wave structure in the free waveguide part and the scattering feature part, and represents the mode conversion mode part reflected by the mode encountering the scattering feature part.
Preferably, the free waveguide portion is a structure body having a constant waveguide cross-sectional area, constant material properties in the waveguide axial direction, constant boundary conditions in the waveguide axial direction, and constant wave impedance.
Preferably, the scattering feature is a structure having a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, and different wave impedance from the free waveguide.
Preferably, the scattering features include, but are not limited to, scattering feature tube segments of tubing, scattering features of plates, and scattering features of rods.
Preferably, the scattering features tube sections include, but are not limited to, structural bends, supported structures, and clamp-containing structures.
Preferably, the specific step of judging whether a defect exists in the detected scattering feature is as follows: comparing the first frequency response with the second frequency response, wherein the scattering feature is not defective if the linear angle difference between the first frequency response and the second frequency response is less than or equal to 15 degrees, and the scattering feature is defective if the linear angle difference between the first frequency response and the second frequency response is more than 15 degrees.
Preferably, the first response frequency is the slope of a straight line linearly fitted by a least square method of a curve of the reflection coefficient of the guided wave of the sound scattering feature part along with the change of frequency, and the second response frequency is the slope of a straight line linearly fitted by a least square method of the curve of the reflection coefficient of the guided wave of the scattering feature part along with the change of frequency in the detection signal.
In order to achieve the above object, another aspect of the present invention provides a method for detecting defects of scattering features of a structure by using ultrasonic guided waves, comprising the steps of:
respectively calculating guided wave mode wave structures in the free waveguide part and the scattering feature part;
calculating a matching coefficient between the guided wave mode wave structure of the free waveguide part and the guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing the guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the frequency change;
calculating reflection coefficients of scattering characteristic guided waves in detection signals when different center frequencies are calculated, and drawing a curve of the reflection coefficients of the guided waves along with the frequency change;
performing linear fitting on the curve of the reflection coefficient of the guided wave of the intact scattering feature part along with the frequency change to obtain a first response frequency, and performing linear fitting on the curve of the reflection coefficient of the guided wave of the scattering feature part along with the frequency change in the detection signal to obtain a second response frequency;
by comparing the first frequency response with the second frequency response, it is determined whether a defect exists in the detected scattering feature.
Compared with the prior art, the invention has the advantages and positive effects that:
according to the invention, according to the difference between the change trend of the guided wave reflection coefficient of the sound scattering feature part along with the frequency and the change trend of the guided wave reflection coefficient of the defect scattering feature part along with the frequency, the guided wave reflection frequency response (namely the first frequency response) of the sound scattering feature part is calculated based on the difference, and is compared with the guided wave reflection frequency response (namely the second frequency response) of the scattering feature part obtained by the detection signal, when the two are similar (namely the linear angle difference between the two is less than or equal to 15 degrees), the defect exists, and when the two are different (namely the linear angle difference between the two is more than or equal to 15 degrees), the defect exists, the interference caused by the detection of the guided wave defect of the scattering feature part is overcome, the omission of the defect is avoided, the detection capability of the guided wave in the detection of the actual scattering feature part is improved, and the method is not only suitable for pipeline detection, but also suitable for the defect detection of structures such as plates and rods, and the like, and the application range is wide.
Drawings
FIG. 1 is a flow chart of a method for detecting defects of scattering features of a structure using ultrasonic guided waves according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of the location of a defective elbow according to an embodiment of the present invention;
FIGS. 3a-3e are graphs showing the comparison of reflection coefficients of a defective elbow and reflection coefficients of an intact elbow at different circumferential positions when the defect is located at different axial positions;
FIG. 4 is a graph showing the comparison of the theoretical calculated elbow reflection and the numerical simulation calculated perfect elbow reflection frequency response in accordance with an embodiment of the present invention;
FIGS. 5a-5e are graphs comparing reflection frequency response of a bend with defects at different circumferential positions with reflection frequency response of a theoretically calculated perfect bend when defects obtained by numerical simulation in the embodiment of the present invention are located at different axial positions;
FIG. 6 is a graph showing the comparison of the theoretical calculated elbow reflection frequency response and the experimentally obtained elbow reflection frequency response in accordance with an embodiment of the present invention;
fig. 7a-7c are graphs showing the reflection frequency response of a defective elbow at different circumferential positions compared with the reflection frequency response of a theoretically calculated perfect elbow when the axial position of the defect is 45 degrees according to the embodiment of the present invention.
1. Back of arch, 2, soffit, 3, side of arch I, 4, side of arch II.
Detailed Description
The present invention will be specifically described below by way of exemplary embodiments. It is to be understood that elements, structures, and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.
In the guided wave nondestructive testing process, taking a detected pipeline as an example, pipelines used in various industries are not simple free straight pipes, often contain scattering features such as elbows, supports and clamps, and the existence of the scattering features in the pipelines brings great trouble to ultrasonic guided wave defect detection. The ultrasonic guided wave technology is used for realizing defect detection by receiving and analyzing a defect reflection echo, the scattering feature itself can reflect guided waves to form an echo, the scattering feature area is the place where defects are easy to generate, and when the defects are located in the scattering feature area, the defect reflection echo and the scattering feature reflection echo are overlapped and are difficult to distinguish, so that great difficulty is brought to defect detection. The invention provides a method for detecting defects of scattering features of a structure by utilizing ultrasonic guided waves, which is characterized in that the degree of reflection of sound scattering features is represented by the matching range degree of different part guided wave modal wave structures on the basis of calculating the different part guided wave modal wave structures, the slope of a linear fitting straight line of guided wave reflection along with a frequency change curve is used for representing frequency response, and whether the scattering features have defects is judged by comparing the frequency response. The method of the present invention will be specifically described below.
An embodiment of the present invention provides a method for detecting defects of scattering features of a structure using ultrasonic guided waves, comprising the steps of:
s1, calculating the guided wave mode wave structures in the free waveguide part and the scattering feature part respectively.
Specifically, a half-analytic finite element method is adopted to calculate the guided wave modal wave structures in the free waveguide part and the scattering feature part according to the size and the material parameters of the object to be measured.
S2, calculating a matching coefficient between the guided wave mode wave structure of the free waveguide part and the guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing the guided wave reflection coefficient of the good scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the change of frequency so as to obtain a first frequency correspondence.
Specifically, the first response frequency is the slope of a linear fit straight line by a least square method of a curve of the reflection coefficient of the guided wave of the sound scattering feature part along with the change of frequency.
Specifically, a half-resolution finite element method is adopted to calculate the guided wave modal wave structures in the free waveguide part and the scattering feature part, a matching coefficient between the two guided wave modal wave structures is calculated by using a modal confidence criterion through a formula (1), and the formula (1) is expressed as follows:
wherein ρ (·) represents a matching coefficient, m represents a guided wave mode, MAC (·) represents a mode confidence criterion, a i I-mode, B representing the waveguide region of the free waveguide section j J-mode, ψ, representing scattering feature waveguide region i Represents the mode vector, ψ, of the i-mode guided wave structure of the free waveguide part j And the scattering characteristic part j modal guided wave structure mode vector is represented.
The larger the matching coefficient between different modal vectors is, the smaller the guided wave reflection is, and the larger the transmission is; whereas the greater the reflection, the less the transmission. Considering the limit condition, when the sum of the wave structure matching coefficients of a certain mode in the incident waveguide A and all modes in the transmission waveguide B is 0, the incident guided wave mode is orthogonal to all modes in the transmission waveguide B, and then the incident guided wave mode is totally reflected; when the value is 1, the guided wave is completely transmitted into the waveguide B, and no reflection occurs; the matching degree of the guided wave mode is inversely related to reflection and positively related to transmission. The guided wave undergoes mode conversion at the scattering feature, the mode conversion is related to the axisymmetric change condition of the waveguide, when the axisymmetry of the waveguide is unchanged, even if the discontinuous scattering feature is encountered, the mode conversion cannot occur, and therefore the change degree of the axisymmetry of the waveguide can be used for measuring the mode conversion degree. As in waveguide A A Reflection and transmission at scattering features, resulting in mode conversion, resulting in mode j in waveguide A A And j is A Mode j in mode and waveguide B B Most similar, j A Modality size and (i) A ， j B ) Correlation of matching coefficients, taking into account limit conditions, if
Then j B The mode should be mode j in waveguide A A Because different modes in the same waveguide are orthogonal; that is to say when (i) A ， j B ) The matching coefficient approaches 0, the symmetry of waveguide axes approaches the same, and the modal transformation approaches 0; thus (i) A ，j B ) The matching coefficient is positively correlated with the degree of modal transformation.
Based on the theory, the R_MAC is adopted to represent the reflection coefficient of the guided wave of the perfect scattering feature part, and the reflection coefficient is calculated by a formula (2), wherein the formula (2) is expressed as follows:
R_MAC＝1-MAC_sum-MAC_mc (2)
wherein, the MAC_sum is the sum of the matching coefficients of a mode wave structure in the free waveguide part and each mode wave structure in the scattering feature part; the mac_mc is the sum of the mode conversion mode matching coefficients generated by a certain mode wave structure in the free waveguide part and the scattering feature part, and represents the mode conversion mode part reflected by the mode encountering the scattering feature part.
S3, calculating reflection coefficients of scattering characteristic guided waves in the detection signals when different center frequencies are used, and drawing a curve of the reflection coefficients of the guided waves along with the change of the frequencies, so that the second frequency is obtained correspondingly.
Specifically, the second response frequency is the slope of a linear fit straight line of the scattering feature guided wave reflection coefficient in the detection signal along with the frequency change curve by using a least square method.
S4, judging whether the detected scattering feature part has defects or not by comparing the first frequency response with the second frequency response.
Specifically, the specific steps of judging whether a defect exists in the detected scattering feature are as follows: comparing the first frequency response with the second frequency response, wherein the scattering feature is not defective if the linear angle difference between the first frequency response and the second frequency response is less than or equal to 15 degrees, and the scattering feature is defective if the linear angle difference between the first frequency response and the second frequency response is more than 15 degrees.
In the above method, the free waveguide part is a structure body with a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, and constant wave impedance. The scattering feature is a structure with a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, but different wave impedance from the free waveguide.
In particular, the scattering features include, but are not limited to, scattering feature tube segments of tubing, scattering features of plates, and scattering features of rods. Wherein the scattering features include, but are not limited to, structural bends, supported structures, and clamp-containing structures, such as: pipe bends, supported pipes, pipes with clamps, etc.
The method overcomes the interference of the scattering feature part on the detection of the guided wave defect, avoids the omission of the defect, improves the detection capability of the guided wave in the detection of the actual scattering feature part, is suitable for the detection of pipelines, and is also suitable for the detection of the defect of structures such as plates, rods and the like, and the application range is wide.
Another embodiment of the present invention provides a method for detecting defects of scattering features of a structure using ultrasonic guided waves, comprising the steps of:
s1, calculating the guided wave mode wave structures in the free waveguide part and the scattering feature part respectively.
Specifically, a half-analytic finite element method is adopted to calculate the guided wave modal wave structures in the free waveguide part and the scattering feature part according to the size and the material parameters of the object to be measured.
S2, calculating a matching coefficient between the guided wave modal structure of the free waveguide part and the guided wave modal structure of the scattering feature part by using a modal confidence criterion, representing the guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the frequency.
Specifically, a half-resolution finite element method is adopted to calculate the guided wave modal wave structures in the free waveguide part and the scattering feature part, a matching coefficient between the two guided wave modal wave structures is calculated by using a modal confidence criterion through a formula (1), and the formula (1) is expressed as follows:
wherein ρ (·) represents a matching coefficient, m represents a guided wave mode, MAC (·) represents a mode confidence criterion, a i I-mode, B representing the waveguide region of the free waveguide section j J-mode, ψ, representing scattering feature waveguide region i Represents the mode vector, ψ, of the i-mode guided wave structure of the free waveguide part j And the scattering characteristic part j modal guided wave structure mode vector is represented.
The larger the matching coefficient between different modal vectors is, the smaller the guided wave reflection is, and the larger the transmission is; whereas the greater the reflection, the less the transmission. Considering the limit condition, when the sum of the wave structure matching coefficients of a certain mode in the incident waveguide A and all modes in the transmission waveguide B is 0, the incident guided wave mode is orthogonal to all modes in the transmission waveguide B, and then the incident guided wave mode is totally reflected; when the value is 1, the guided wave is completely transmitted into the waveguide B, and no reflection occurs; the matching degree of the guided wave mode is inversely related to reflection and positively related to transmission. The guided wave undergoes mode conversion at the scattering feature, the mode conversion is related to the axisymmetric change condition of the waveguide, when the axisymmetry of the waveguide is unchanged, even if the discontinuous scattering feature is encountered, the mode conversion cannot occur, and therefore the change degree of the axisymmetry of the waveguide can be used for measuring the mode conversion degree. As in waveguide A A Reflection and transmission at scattering features, resulting in mode conversion, resulting in mode j in waveguide A A And j is A Mode j in mode and waveguide B B Most similar, j A Modality size and (i) A ， j B ) Correlation of matching coefficients, taking into account limit conditions, if
Then j B The mode should be mode j in waveguide A A Because different modes in the same waveguide are orthogonal; that is to say when (i) A ， j B ) The matching coefficient approaches 0, the symmetry of waveguide axes approaches the same, and the modal transformation approaches 0; thus (i) A ，j B ) The matching coefficient is positively correlated with the degree of modal transformation.
Based on the theory, the R_MAC is adopted to represent the reflection coefficient of the guided wave of the perfect scattering feature part, and the reflection coefficient is calculated by a formula (2), wherein the formula (2) is expressed as follows:
R_MAC＝1-MAC_sum-MAC_mc (2)
wherein, the MAC_sum is the sum of the matching coefficients of a mode wave structure in the free waveguide part and each mode wave structure in the scattering feature part; the mac_mc is the sum of the mode conversion mode matching coefficients generated by a certain mode wave structure in the free waveguide part and the scattering feature part, and represents the mode conversion mode part reflected by the mode encountering the scattering feature part.
S3, calculating reflection coefficients of scattering characteristic guided waves in detection signals when different center frequencies are achieved, and drawing a curve of the reflection coefficients of the guided waves along with the frequency change.
S4, performing linear fitting on the curve of the reflection coefficient of the guided wave of the perfect scattering feature part along with the frequency change to obtain a first response frequency, and performing linear fitting on the curve of the reflection coefficient of the guided wave of the scattering feature part along with the frequency change in the detection signal to obtain a second response frequency.
Specifically, the first response frequency is the slope of a linear fit straight line by a least square method of a curve of the reflection coefficient of the guided wave of the sound scattering feature part along with the change of frequency. And the second response frequency is the slope of a linear fitting straight line of the scattering feature guided wave reflection coefficient in the detection signal along with the frequency change curve by using a least square method.
S5, judging whether the detected scattering feature part has defects or not by comparing the first frequency response with the second frequency response.
Specifically, the specific steps of judging whether a defect exists in the detected scattering feature are as follows: comparing the first frequency response with the second frequency response, wherein the scattering feature is not defective if the linear angle difference between the first frequency response and the second frequency response is less than or equal to 15 degrees, and the scattering feature is defective if the linear angle difference between the first frequency response and the second frequency response is more than 15 degrees.
In the above method, the free waveguide part is a structure body with a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, and constant wave impedance. The scattering feature is a structure with a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, but different wave impedance from the free waveguide.
In particular, the scattering features include, but are not limited to, scattering feature tube segments of tubing, scattering features of plates, and scattering features of rods. Wherein the scattering features tube segments include, but are not limited to, structural bends, supported structures, and clamp-containing structures, such as: pipe bends, supported pipes, pipes with clamps, etc.
The method overcomes the interference of the scattering feature part on the detection of the guided wave defect, avoids the omission of the defect, improves the detection capability of the guided wave in the detection of the actual scattering feature part, is suitable for the detection of pipelines, and is also suitable for the detection of the defect of structures such as plates, rods and the like, and the application range is wide.
In order to illustrate the effectiveness of the method according to the invention, a further description of the method according to the invention will be given below with reference to the accompanying drawings, taking as an example the detection of bends in pipes.
Referring to fig. 1, a method for detecting defects of a pipe elbow by using ultrasonic guided waves comprises the following steps:
s1, respectively calculating guided wave mode wave structures in the straight pipe section and the bent pipe section by using a semi-analytic finite element method.
It should be noted that, before calculating the guided wave mode structure, the parameters of the pipe, that is, the dimensions and material parameters of the pipe, need to be obtained.
S2, calculating wave structure matching coefficients among guided wave modes of different pipe sections under each frequency, obtaining an elbow reflection degree representation parameter R_MAC, and drawing a curve of R_MAC along with frequency change;
s3, calculating reflection coefficients of reflection signals in the detection signals under each frequency, and drawing a curve of the reflection coefficients along with the frequency change;
s4, performing linear fitting on the R_MAC curve obtained in the previous step along with the frequency change curve and the reflection coefficient curve along with the frequency change curve by using a least square method to obtain two different fitting straight lines;
s5, comparing angles of the two fitting straight lines, if the angle difference of the two fitting straight lines is less than or equal to 15 degrees, the slope is considered to be similar, namely, the frequency response is similar, the defect is avoided, and if the angle difference is more than 15 degrees, the defect is avoided; and outputting the defect detection result.
In order to ensure the applicability to defects of different elbow positions, the elbow defect positions are divided into an axial position 5 and a circumferential position 4, namely 20 positions, and referring to fig. 2, the length of a pipeline is 3m, the outer diameter is 20mm, the wall thickness is 3mm, the bending radius of the elbow is 100mm, and the bending angle is 90 degrees.
In this embodiment, by using ANSYS finite element simulation software, L (0, 1) mode guided waves are excited and received in the pipeline, and simulation is performed on the detection of guided waves without defects and with defects at different elbow positions, and the reflection coefficient thereof is calculated, so as to obtain a curve of the reflection coefficient with frequency, see fig. 3a-3e. In fig. 3a-3e, in most cases, no matter where the defect is located, the frequency is what, when the defect is located in the extrados 1, the signal reflection coefficient is greater than when the defect is located in other positions, and is greater than the straight tube defect reflection coefficient; when the reflection coefficient is the smallest in the arch 2; when the reflection coefficients are equal at two arch sides. The straight tube defect reflection coefficient tends to increase linearly with increasing frequency, and the defect-containing elbow reflection coefficient tends to increase but has no linear characteristic except for the case that the axial position of the defect is 45 degrees (see fig. 3 c); when the axial position of the defect is 45 degrees (see fig. 3 c), the trend of the reflection coefficient of the bend with the defect along with the frequency change is more gentle, the reflection coefficient of the defect at each circumferential position is larger than the reflection coefficient of the defect of the straight pipe at low frequency, and the reflection coefficient of the defect at the arch web and the arch side is smaller than the reflection coefficient of the defect of the straight pipe at high frequency. In fig. 3a and 3e, when the axial position of the defect is 0 degree and 90 degrees, the difference of the reflection coefficient when the defect is positioned on the back arch and the soffit respectively does not change greatly with the increase of the frequency, which indicates that the increase of the frequency fails to cause the increase of the non-uniformity of the circumferential energy distribution of the two parts; and except for individual cases (0 degree 40kHz,90 degrees 35kHz and 40kHz, but not quite different), the reflection coefficient of the defect at different circumferential positions is larger than that of the straight pipe defect. In fig. 3b and 3d, the difference in reflection coefficient between the defect at the back and the soffit increases with increasing frequency when the defect is at 22.5 degrees and 67.5 degrees, respectively, indicating that more and more guided wave energy is concentrated at the back, which is not apparent when the defect is at other axial positions; in most cases, the straight tube defect reflectance is less than the extrados defect reflectance and greater than or similar to the intrados and intrados defect reflectance. The variation trend of the elbow reflection coefficient along with the frequency when the elbow is defective and the variation trend of the elbow reflection coefficient along with the frequency when the elbow is not defective have obvious difference. Obtaining the trend of the reflection coefficient of the elbow along with the frequency variation when the elbow is free of defects by using a theoretical or numerical calculation method, comparing the trend of the reflection coefficient along with the frequency variation obtained by an actual detection signal, and judging that the elbow is defective if the reflection coefficient and the reflection coefficient have obvious differences; if there is no obvious difference, it can determine that the bending head is defect-free.
In this embodiment, the parameter r_mac is represented by calculating the elbow reflection table coefficients under different frequencies, drawing a curve of r_mac along with the frequency change, comparing with a curve of the reflection coefficient of the perfect elbow along with the frequency change obtained by finite element simulation, and then performing linear fitting to obtain a fitting straight line, see fig. 4. If the angle difference of the fitting straight line is calculated to be 8.5 degrees, the theoretically calculated reflection frequency response of the perfect elbow (namely the slope of the fitting straight line) in fig. 4 is considered to be similar to the frequency response obtained by numerical simulation, and the accuracy of theoretical calculation is demonstrated.
The frequency response of the perfect bend reflection is then represented by R_MAC and then compared to the frequency response of the reflection of the defect-containing bend represented by the reflection coefficient obtained by numerical modeling, as shown in FIGS. 5a-5 e. The angle difference of each fitting straight line was calculated as shown in table 1.
TABLE 1
As can be seen from Table 1, no matter where the defect is located in the bend, the angle difference between the frequency response straight line of the bend with the defect obtained by numerical simulation and the frequency response straight line of the bend reflected by the perfect bend represented by R_MAC is obviously more than 15 degrees, so that the defect in the bend can be judged.
The guided wave detection system is used for detecting stainless steel pipes with the outer diameter of 20mm, the wall thickness of 3mm and the length of 3m, an elbow with the bending radius of 100mm is arranged in the middle, and the bending angle is 90 degrees. The perfect bend is first inspected to obtain a comparison of the theoretically calculated bend reflection frequency response and the bend reflection frequency response obtained with the experimental inspection signal, as shown in fig. 6a-6 c. The calculated fit straight line angle difference is 12.1 degrees, and then the theoretical calculation in fig. 6a-6c is considered to be similar to the elbow reflection frequency response obtained by experiment, and the correctness of elbow reflection represented by the matching range degree among different pipe section modes is further verified.
Fig. 7 shows the reflection frequency response of the elbow with the defect obtained by experiment when the elbow contains the defect and the reflection frequency response of the perfect elbow calculated by theory, wherein the axial position of the defect is 45 degrees, the circumferential position comprises the arch back, the arch side and the arch abdomen, the fitting straight line angle difference is 64.9 degrees, 57.1 degrees and 51.8 degrees respectively, and the fitting straight line angle difference is obviously greater than 15 degrees, and the reflection frequency response of the elbow with the defect is considered to be obviously different from the reflection frequency response of the perfect elbow represented by r_mac, so that the defect is judged.
The above-described embodiments are intended to illustrate the present invention, not to limit it, and any modifications and variations made thereto fall within the spirit of the present invention and the scope of the appended claims.
Claims (7)
1. A method for detecting defects in scattering features of a structure using ultrasonic guided waves, comprising the steps of:
respectively calculating guided wave mode wave structures in the free waveguide part and the scattering feature part;
calculating a matching coefficient between a guided wave mode wave structure of the free waveguide part and a guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing a guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the change of frequency so as to obtain a first frequency response; the reflection coefficient of the guided wave of the perfect scattering feature part is expressed by adopting R_MAC, and is calculated by a formula (2), wherein the formula (2) is expressed as follows:
R_MAC＝1-MAC_sum-MAC_mc (2)
wherein, the MAC_sum is the sum of the matching coefficients of a certain modal wave structure in the free waveguide part and each modal wave structure in the scattering feature part; the MAC_mc is the sum of mode conversion mode matching coefficients generated by a certain mode wave structure and a scattering feature in the free waveguide part, and represents a mode conversion mode part reflected by the mode when encountering the scattering feature;
calculating scattering characteristic guided wave reflection coefficients in detection signals when different center frequencies are used, and drawing a curve of the guided wave reflection coefficients along with the frequency change to obtain a second frequency response;
judging whether a defect exists in the detected scattering feature by comparing the first response frequency with the second response frequency; the specific steps for judging whether the detected scattering feature has defects are as follows: comparing the first response frequency with the second response frequency, wherein if the linear angle difference between the first response frequency and the second response frequency is less than or equal to 15 degrees, the scattering feature part is not defective, and if the linear angle difference between the first response frequency and the second response frequency is more than 15 degrees, the scattering feature part is defective;
the first response frequency is the slope of a straight line linearly fitted by a least square method of the curve of the reflection coefficient of the guided wave of the scattering feature part with the frequency change, and the second response frequency is the slope of a straight line linearly fitted by a least square method of the curve of the reflection coefficient of the guided wave of the scattering feature part with the frequency change in the detection signal.
2. The method for detecting defects in scattering features of a structure by using ultrasonic guided waves according to claim 1, wherein the guided wave mode wave structures in the free waveguide and the scattering features are calculated by using a semi-analytic finite element method according to the size and material parameters of the structure to be detected, and the matching coefficient between the two guided wave mode wave structures is calculated by using a mode confidence criterion according to formula (1), wherein formula (1) is expressed as:
wherein ρ (& gt)) Represents the matching coefficient, m represents the guided wave mode, MAC (·) represents the mode confidence criterion, A i I-mode, B representing the waveguide region of the free waveguide section j J-mode, ψ, representing scattering feature waveguide region i Represents the mode vector, ψ, of the i-mode guided wave structure of the free waveguide part j And the mode shape vector of the j-mode guided wave structure of the scattering feature part is shown.
3. The method for detecting defects of scattering features of a structure by using ultrasonic guided waves according to claim 1 or 2, wherein the free waveguide part is a structure body with a constant waveguide sectional area, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide and constant wave impedance.
4. A method of detecting defects in a scattering feature of a structure using ultrasonic guided waves as claimed in claim 3, wherein the scattering feature is a structure having a constant cross-sectional area of the waveguide, constant material properties in the axial direction of the waveguide, constant boundary conditions in the axial direction of the waveguide, but different wave impedance from the free waveguide.
5. The method of detecting structural scattering features defects using ultrasonic guided waves of claim 4, wherein the scattering features include, but are not limited to, scattering features tube segments of tubing, scattering features of plates, and scattering features of rods.
6. The method of detecting structural scattering features defects using ultrasonic guided waves of claim 5, wherein the scattering features include, but are not limited to, structural bends, supported structures, and clamp-containing structures.
7. A method for detecting defects in scattering features of a structure using ultrasonic guided waves, comprising the steps of:
respectively calculating guided wave mode wave structures in the free waveguide part and the scattering feature part;
calculating a matching coefficient between the guided wave mode wave structure of the free waveguide part and the guided wave mode wave structure of the scattering feature part by using a mode confidence criterion, representing the guided wave reflection coefficient of the perfect scattering feature part by using the matching coefficient, and drawing a curve of the guided wave reflection coefficient along with the frequency change; the reflection coefficient of the guided wave of the perfect scattering feature part is expressed by adopting R_MAC, and is calculated by a formula (2), wherein the formula (2) is expressed as follows:
R_MAC＝1-MAC_sum-MAC_mc (2)
wherein, the MAC_sum is the sum of the matching coefficients of a certain modal wave structure in the free waveguide part and each modal wave structure in the scattering feature part; the MAC_mc is the sum of mode conversion mode matching coefficients generated by a certain mode wave structure and a scattering feature in the free waveguide part, and represents a mode conversion mode part reflected by the mode when encountering the scattering feature;
calculating reflection coefficients of scattering characteristic guided waves in detection signals when different center frequencies are calculated, and drawing a curve of the reflection coefficients of the guided waves along with the frequency change;
performing linear fitting on the curve of the reflection coefficient of the guided wave of the good scattering feature part along with the frequency change to obtain a first response frequency, and performing linear fitting on the curve of the reflection coefficient of the guided wave of the scattering feature part along with the frequency change in the detection signal to obtain a second response frequency;
judging whether a defect exists in the detected scattering feature by comparing the first response frequency with the second response frequency; the specific steps for judging whether the detected scattering feature has defects are as follows: comparing the first response frequency with the second response frequency, wherein if the linear angle difference between the first response frequency and the second response frequency is less than or equal to 15 degrees, the scattering feature part is not defective, and if the linear angle difference between the first response frequency and the second response frequency is more than 15 degrees, the scattering feature part is defective;
the first response frequency is the slope of a straight line linearly fitted by a least square method of the curve of the reflection coefficient of the guided wave of the scattering feature part with the frequency change, and the second response frequency is the slope of a straight line linearly fitted by a least square method of the curve of the reflection coefficient of the guided wave of the scattering feature part with the frequency change in the detection signal.
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