Polyethylene gas pipeline hot melting joint cold welding defect detection method based on microwave resonance

Abstract

The invention discloses a method for detecting cold welding defects of a hot-melt joint of a polyethylene gas pipeline based on microwave resonance, which comprises the following steps of: S1, placing a scanning seat loaded with a microwave resonance probe on the outer surface of a to-be-detected pipeline, so that the distance between a point focusing antenna and the pipe wall of the pipeline is equal to the focal point of the antenna; s2, the probe conducts two-dimensional scanning in the circumferential direction and the axial direction of the pipeline, so that a laser positioner on a scanning base obtains coordinate information of the position of the probe. The method for detecting the cold welding defect of the polyethylene gas pipeline hot-melting joint based on microwave resonance has the beneficial effects that whether the change degree of the resonance curve exceeds the preset interval or not is judged specially aiming at the polyethylene gas pipeline hot-melting joint, so that whether the polyethylene gas pipeline hot-melting joint has the cold welding defect or not is accurately judged; and the specific position where the cold welding defect exists can be further indicated.

Description

Technical Field

[0001] The invention belongs to the field of metal surface defect detection, and in particular relates to a method for detecting cold welding defects of hot-melt joints of polyethylene gas pipelines based on microwave resonance. Background Art

[0002] The invention patent application with the publication number CN102253058A and the subject name of the invention patent application is "Microwave nondestructive detection device for metal surface defects and detection method thereof", and its IPC classification number is G01N22 / 02. Its technical solution discloses "feeding the swept-frequency microwave signal generated by the microwave sweeper 1 into a "T"-shaped tube, and then the "T"-shaped tube divides the microwave input signal into a first signal and a second signal of equal magnitude and phase, and feeds them to the corresponding input ends of the directional coupler 3 respectively. When the main waveguide receives the signal, the first waveguide and the second waveguide arms excite microwave signals of equal amplitude and phase; wherein The first waveguide signal is fed to a rectangular waveguide with a first terminal opening. A waveguide probe 4 is provided at the waveguide terminal opening. The gap between the waveguide probe 4 and the metal surface to be measured can be equivalent to a uniform waveguide short circuit with equivalent electrical length, which short-circuits the metal surface to be measured, within a certain range, i.e., an "equivalent short circuit device". The incident signal terminates at the metal surface to be measured, and the incident signal and the reflected signal are superimposed to form a standing wave. When the crack enters the waveguide opening, the standing wave changes its position, and the "equivalent short circuit device" makes the reflection coefficient phase angle of the measuring point linearly related to the electrical length, while the electrical length varies with the surface defects of the metal 6 to be measured".

[0003] It can be seen that the above invention patent application has disclosed one of the technical solutions of the microwave non-destructive detection method for metal surface defects. However, the technical solution disclosed in the above invention patent application focuses on the direct detection of defects on the metal surface through a non-metallic covering material at a certain distance, and does not further solve the problem of how to accurately determine whether there is a cold welding defect in the hot-melt joint of the polyethylene gas pipeline, which needs to be further improved. Summary of the invention

[0004] In view of the status of the prior art, the present invention overcomes the above defects and provides a method for detecting cold welding defects of hot-melt joints of polyethylene gas pipelines based on microwave resonance.

[0005] The present invention adopts the following technical scheme, and the method for detecting cold welding defects of hot-melt joints of polyethylene gas pipelines based on microwave resonance comprises the following steps:

[0006] Step S1: placing a scanning base loaded with a microwave resonant probe on the outer surface of a pipeline to be inspected, so that the distance between the point focusing antenna and the pipe wall is equal to the focus of the antenna;

[0007] Step S2: The probe performs two-dimensional scanning along the circumference and axial direction of the pipeline, so that the laser locator on the scanning seat obtains the coordinate information of the probe position;

[0008] Step S3: the vector network analyzer records the S21 amplitude corresponding to the resonance peak of each coordinate information to draw the resonance curve of the outer surface of the pipeline to be detected in real time;

[0009] Step S4: determine whether the change degree of the resonance curve exceeds the preset interval. If it exceeds the preset interval, the outer surface of the pipeline to be inspected is determined to have a cold welding defect; otherwise, the outer surface of the pipeline to be inspected is determined to have no cold welding defect.

[0010] As a preferred technical solution of the above technical solutions, the outer surface of the pipeline to be inspected is the central weld of the polyethylene gas pipeline hot-melt joint.

[0011] As a preferred technical solution of the above technical solution, step S4 is specifically implemented as the following steps:

[0012] Step S4.1: In S 21 Add two dashed lines N dB apart to the amplitude curve, with the upper dashed line at S 21 The maximum amplitude;

[0013] Step S4.2: When S 21 When the amplitude curve is between the two dotted lines, the S 21 There is no cold welding defect in the joint fusion area corresponding to the amplitude curve. On the contrary, when S 21 When the amplitude curve is outside the two dotted lines, the S 21 There is a cold welding defect in the joint fusion area corresponding to the amplitude curve.

[0014] As a preferred technical solution of the above technical solution, in step S4.1, N is 0.5-1.0dB.

[0015] As a preferred technical solution of the above technical solutions, the microwave resonant probe includes a point-focusing antenna and a resonant cavity. The resonant cavity is provided with a strong coupling port and a weak coupling port. The strong coupling port is connected to the point-focusing antenna through a rectangular-circular transition device, and the weak coupling port is connected to the vector network analyzer through a microwave cable.

[0016] As a preferred technical solution of the above technical solutions, the point focusing antenna is composed of a conical horn and a dielectric lens, and the dielectric lens is arranged at the aperture of the conical horn.

[0017] As a preferred technical solution of the above technical solution, the microwave resonance frequency of the microwave resonance probe is any frequency within the range of 5-26.5 GHz.

[0018] As a preferred technical solution of the above technical solution, the dielectric lens is a biconvex lens.

[0019] As a preferred technical solution of the above technical solutions, the focal length of the dielectric lens is 10-100 mm.

[0020] The microwave resonance-based cold welding defect detection method for polyethylene gas pipeline hot-melt joints disclosed in the present invention has the beneficial effect of specifically targeting polyethylene gas pipeline hot-melt joints to determine whether the degree of change of the resonance curve exceeds a preset interval, thereby accurately determining whether there is a cold welding defect in the polyethylene gas pipeline hot-melt joint, and being able to further indicate where the cold welding defect exists. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1A It is a schematic diagram of the microwave resonant probe scanning the HDPE pipe and the resonance curve.

[0022] Figure 1B yes Figure 1A A local enlarged view of the resonance curve of the selected area.

[0023] Figure 2A It is a schematic diagram of the resonance curves of the hot melt joint and the defect-free position of the HDPE pipe.

[0024] Figure 2B yes Figure 2A A partial enlarged view of .

[0025] Figure 3A It is a schematic diagram of the resonance curve of the HDPE pipe welded at 230℃.

[0026] Figure 3B It is a schematic diagram of the resonance curve of the HDPE pipe welded at 210℃.

[0027] Figure 3C It is a schematic diagram of the resonance curve of the HDPE pipe welded at 190°C.

[0028] Figure 3D It is a schematic diagram of the resonance curve of the HDPE pipe welded at 180°C.

[0029] Figure 3E It is a schematic diagram of the resonance curve of the HDPE pipe welded at 170°C.

[0030] Figure 3F yes Figure 3E A local enlarged view of the resonance curve of the selected area. DETAILED DESCRIPTION

[0031] The present invention discloses a method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance. Figures 1A to 3F , the specific implementation modes of the present invention are further described.

[0032] Those skilled in the art should note that the “microwave resonant probe”, “resonant probe” and “probe” that may be involved in various embodiments of the present invention are all the same concept and are no longer distinguished.

[0033] Example 1.

[0034] Preferably, the method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance comprises the following steps:

[0035] Step S1: placing a scanning base loaded with a microwave resonant probe on the outer surface of a pipeline to be inspected, so that the distance between the point focusing antenna and the pipe wall is equal to the focus of the antenna;

[0036] Step S2: the probe performs two-dimensional scanning along the circumferential direction (x direction) and axial direction (z direction) of the pipeline, so that the laser positioner on the scanning seat obtains the coordinate information of the probe position;

[0037] Step S3: The vector network analyzer records the S corresponding to the resonance peak of each coordinate information 21 Amplitude, so as to draw the resonance curve of the outer surface of the pipeline to be inspected in real time;

[0038] Step S4: (manual or preset automatic judgment method) determine whether the degree of change of the resonance curve exceeds the preset interval. If it exceeds the preset interval, the outer surface of the pipeline to be inspected is judged as having a cold welding defect; otherwise, the outer surface of the pipeline to be inspected is judged as not having a cold welding defect.

[0039] The outer surface of the pipeline to be inspected is preferably the central weld of the polyethylene gas pipeline hot-melt joint.

[0040] Wherein, step S4 is specifically implemented as the following steps:

[0041] Step S4.1: S (at each temperature) 21 Add two dashed lines N dB apart to the amplitude curve, with the upper dashed line at S 21 The maximum amplitude;

[0042] Step S4.2: When S 21 When the amplitude curve is between the two dotted lines, the S 21 There is no cold welding defect in the joint fusion area corresponding to the amplitude curve (current temperature). On the contrary, when S 21 When the amplitude curve is outside the two dotted lines, the S 21 There is a cold welding defect in the joint fusion area (at the current temperature) corresponding to the amplitude curve.

[0043] Wherein, in step S4.1, N is 0.5-1.0dB, and N is preferably 0.66dB.

[0044] Among them, the microwave resonant probe includes a point-focusing antenna and a resonant cavity. The resonant cavity is provided with a strong coupling port and a weak coupling port. The strong coupling port is connected to the point-focusing antenna through a rectangular-circular transition device, and the weak coupling port is connected to a vector network analyzer through a microwave cable.

[0045] Among them, the point focusing antenna is composed of a conical horn and a dielectric lens. The dielectric lens is set at the aperture of the conical horn. Different dielectric lenses can be selected according to needs to adjust the focal length.

[0046] Among them, the microwave resonant probe can operate in the Ka, K and Ku bands.

[0047] The microwave resonance frequency of the microwave resonance probe is any frequency within the range of 5-26.5 GHz.

[0048] Among them, the dielectric lens is preferably a biconvex lens.

[0049] Among them, the focal length of the dielectric lens is 10-100mm.

[0050] When two-dimensional scanning is performed along the circumferential direction (x direction) and axial direction (z direction) of the pipeline, the distance between the probe and the pipeline surface is 10-100 mm.

[0051] The following is a technical overview of the cold welding defect detection method for polyethylene gas pipeline hot-melt joints based on microwave resonance disclosed in this embodiment.

[0052] HDPE pipes (high-density polyethylene) have many excellent properties, such as strong toughness, crack resistance, corrosion resistance, good weather resistance, excellent scratch resistance and crack transmission resistance. HDPE pipes are widely used in the construction of urban natural gas pipeline networks. In the pipeline system, the connection between pipes is the weak link. Therefore, the safe service of the HDPE pipeline system is closely related to the quality of the pipeline welding joints. Hot melt connection and electric fusion connection are the main connection methods for polymer pipes, among which hot melt connection is more widely used and suitable for the connection of pipes of various specifications.

[0053] At present, the mainstream detection method for HDPE pipes (HDPE pipe joints) is ultrasonic nondestructive testing. Ultrasonic testing can directly obtain the internal state information of the material and realize the identification of macroscopic defects. However, polyethylene is a polymer material that absorbs and loses more ultrasonic energy, and due to its complex structure, it will enhance the scattering of sound waves. These factors greatly increase the difficulty of ultrasonic testing of HDPE pipe welding joints. Therefore, ultrasonic testing technology is very difficult to detect process defects in HDPE pipes.

[0054] Microwave nondestructive testing is a non-contact detection technology that uses the changes in electromagnetic wave characteristics when microwaves propagate in the object to detect the internal structure and properties of the object. When microwaves propagate in the object, they are affected by the electromagnetic characteristics of the material, and their amplitude, phase, frequency and other parameters will change. By measuring and analyzing the changes in these parameters, the internal structure and properties of the object being tested can be determined. HDPE is a dielectric material with a low relative dielectric constant and weak absorption capacity for microwaves. Microwaves have strong penetration and reflection capabilities in it. Changes in the internal structure of HDPE will cause microwaves passing through it to reflect, refract, scatter, etc., causing microwave parameters to change. Changes in microwave parameters can be measured by instruments such as vector network analyzers.

[0055] Cold welding is a typical process defect of HDPE pipe joints. During the welding process, improper welding parameters (temperature, pressure, etc.) are the main cause of cold welding defects. Unlike macro defects such as holes, cracks, and inclusions, cold welding has no obvious appearance characteristics and is easily overlooked, and it is difficult to detect it with existing conventional non-destructive testing methods. Cold welding poses a great threat to the safety of HDPE pipe systems, because the polyethylene molecules in the interface layer of the two pipes at the cold welding joint are not fully diffused and entangled, and the penetration depth between the molecules is insufficient. Although the connection between the materials is achieved, the connection strength is not developed, and the mechanical properties are lower than those of normally welded pipe joints.

[0056] The working principle of the cold welding defect detection method for hot-melt joints of polyethylene gas pipelines based on microwave resonance disclosed in this embodiment is explained below.

[0057] In order to detect the cold welding defects of HDPE pipe joints, a point-focused antenna loaded with a resonant cavity was designed as a microwave resonant probe for detecting HDPE pipes. The microwave resonant probe and the vector network analyzer form a microwave detection system. The microwave resonant probe converts the influence of the sample under test (HDPE pipe joint) on the antenna radiation into the change of the resonant parameters of the resonant cavity, and has high detection sensitivity. The cold welding defect detection method of polyethylene gas pipeline hot-melt joints based on microwave resonance focuses on the detection of cold welding defects of HDPE pipe hot-melt joints. The experimental results show that cold welding defects of different degrees can be identified.

[0058] The microwave resonant probe includes a point-focusing antenna and a resonant cavity. The resonant cavity is provided with a strong coupling port and a weak coupling port. The strong coupling port is connected to the point-focusing antenna through a rectangular-circular transition device, and the weak coupling port is connected to the vector network analyzer through a microwave cable. The point-focusing antenna is composed of a conical horn and a dielectric lens (the dielectric lens is preferably a biconvex lens). The dielectric lens is arranged at the aperture of the conical horn. Different dielectric lenses can be selected as needed to adjust the focal length. The microwave resonant probe can operate in the Ka, K and Ku bands.

[0059] When testing HDPE pipes, place the scanning seat loaded with microwave resonant probes on the outer surface of the pipe so that the distance between the point focusing antenna and the pipe wall is equal to the focus of the antenna. The probe performs two-dimensional scanning along the circumference (x direction) and axial direction (z direction) of the pipe, and the laser locator on the scanning seat can obtain the coordinate information of the probe position. When testing the hot-melt joints of HDPE pipes, the preset microwave resonant frequency is any frequency in the range of 5-26.5GHz. The above frequencies match well with the probe and are highly sensitive to HDPE materials. The focal length of the dielectric lens is 10-100mm, and the distance between the probe and the pipe surface during testing is 10-100mm.

[0060] During the test, the point focusing antenna gathers the energy radiated from the resonant cavity at the focal position. The radiated microwaves are reflected after encountering the sample under test. The reflected signal interacts with the electromagnetic field inside the resonant cavity, causing the resonant parameters of the resonant cavity to change. The resonance curve, i.e., the S parameter S, is obtained in real time through the vector network analyzer. 21 The amplitude of the resonant cavity changes with frequency in the resonant frequency range. The changes in the resonance parameters of the resonant cavity caused by the measured sample are reflected in the changes in the resonance curve, such as the resonance peak (S 21 The resonance parameters are very sensitive to the changes in the sample being tested, which provides the prerequisite for the detection of cold welding defects in the hot-melt joints of HDPE pipes.

[0061] The following describes the experimental process and experimental results of the method for detecting cold welding defects of hot-melt joints of polyethylene gas pipelines based on microwave resonance disclosed in this embodiment.

[0062] Experimental process: holes were punched in the wall of the HDPE pipe, with diameters of 2 mm and 3 mm, and depths of 5 mm, as shown in the attached figure. Figure 1A The above HDPE pipe was scanned and tested using a microwave resonant probe. During the entire scanning process, a constant distance was maintained between the probe and the pipe surface. At the end of the scan, three resonance curves were recorded, corresponding to the probe being located at one of the normal positions of the pipe, at the 2mm hole, and at the 3mm hole, as shown in the attached figure. Figure 1B . The resonance peaks of the three resonance curves are located in the frequency range of 23.135-23.140GHz. Compared with the normal position, the resonance peak value obtained at the 2mm hole is reduced, and the resonance peak has a small frequency shift. At the 3mm hole, the resonance peak is smaller than that at the 2mm hole, and the peak position moves slightly further toward the high frequency direction. The presence of the 2mm hole and the 3mm hole causes changes in the resonance curve. The larger the hole size, the more obvious the curve change, the more the resonance peak drops, and the greater the frequency deviation of the resonance peak.

[0063] In the resonance cavity of the microwave resonance probe, microwaves resonate in the preset mode. At the same time, the point focusing antenna emits microwaves to the sample under test, and part of the microwaves are reflected back by the sample. When the probe is located at a defect-free position of the HDPE pipe, the reflected microwaves and the microwaves in the resonance cavity form a stable resonance state, which is manifested as an S 21 The resonance curve of the versus frequency. When the probe is moved along the surface of the HDPE pipe, the resonance curve remains almost unchanged at various locations without defects. When the probe moves to the location of the hole, the reflected microwave signal changes, causing the microwave resonance state in the resonance cavity to change, causing the resonance curve to change significantly compared with the defect-free case. When the size of the hole increases, the microwave resonance state in the resonance cavity is more affected. The experimental results show that the probe is very sensitive to changes in the reflected signal and can detect changes in the state of the HDPE pipe in real time.

[0064] The ends of two HDPE pipes are connected together in a heated and molten state, and a hot melt joint is formed after cooling. In the molten state and under a certain pressure, the polyethylene molecular chains diffuse and entangle with each other, and then combine together during the cooling process. During the hot melt connection process, the molecular chain orientation and crystallinity of the pipe will change, resulting in a change in the dielectric constant. Therefore, the response of the HDPE pipe joint and the pipe body to microwave signals is different.

[0065] The microwave resonance probe tests the hot melt joint and the defect-free position of the HDPE pipe respectively. The resonance curves are as follows: Figure 2A and Figure 2B The resonance curve of the hot melt joint is different from that of the pipe body. The resonance peak of the resonance curve of the hot melt joint corresponds to S 21 The amplitude is small, and the difference between the two is 1.04dB. Therefore, the influence of welding on the structure of HDPE material can be detected by the probe, which further confirms the feasibility of microwave resonance probe to detect HDPE pipe hot melt joints.

[0066] See Figures 3A to Figure 3F , HDPE pipes were welded at 170℃, 180℃, 190℃, 210℃ and 230℃ to obtain five hot-melt joints. The standard temperature for hot-melt connection of HDPE pipes is 210℃~230℃. Cold welding defects may occur if the temperature is lower than 210℃. The microwave resonant probe is positioned at the weld in the center of the hot-melt joint, and a 360° scan is performed along the circumference of the pipe. During the scanning process, the S corresponding to the resonance peak at each position is recorded. 21 Amplitude. The circumferential scanning distance is the x-axis, S 21 The amplitude is the y-axis, and the scanning results of the five joints are as follows FIG. 3A to FIG. 3F shown.

[0067] At various positions in the circumferential direction, the S of the heat-melt joint welded at 230°C is 21 The amplitude fluctuation is small, and the maximum and minimum values ​​differ by 0.66dB (please note that 0.66dB is a typical value, but is not limited to the typical value, the same below), that is, Figure 3A The distance between the two dotted lines in the figure. 230℃ is the standard temperature for hot melting. The hot melt joint formed at this temperature has a tight structure and a relatively uniform fusion surface. The S of the joint welded at 210℃ is 21 The fluctuation range of the amplitude curve is similar to that at 230℃. 21 The difference between the maximum and minimum amplitudes is 0.69dB, which is very close to 0.66dB. When the welding temperature is lower than 210℃, the resulting joint may have cold welding defects. FIG. 3C to FIG. 3E It can be seen that the S of the joints formed at 190°C, 180°C and 170°C 21 The amplitude curve fluctuates more than at 230℃ and 210℃, and the fluctuation increases as the temperature decreases. 21 Two dotted lines with a distance of 0.66 dB are added to the amplitude curve. The upper dotted line is located at S 21 In other words, when S 21 When the amplitude curve is between the two dotted lines, there is no defect in the corresponding position of the joint fusion area; 21 When the amplitude curve is outside the two dotted lines, it indicates that there is a cold welding defect in the corresponding area. For the joint welded at 190℃, S 21 The amplitude falls outside the range of the two dashed lines in the 0-10 cm region, indicating that there is a lack of fusion in the 0-10 cm region ( Figure 3C ). For the 180℃ welded joint, in most areas within 0-22cm, S 21 The amplitudes all fall outside the range of the two dashed lines, indicating that there is insufficient fusion in this area ( Figure 3D ). It can be seen that the under-fusion area of ​​the 180℃ weld joint is larger than that of the 190℃ weld joint. In addition, the S 21 The difference between the maximum and minimum amplitudes is 1.45 dB, while the S 21 The maximum and minimum amplitudes differ by 1.07 dB, indicating that the degree of insufficient fusion of the 180°C weld joint is greater than that of the 190°C weld joint. For the 170°C weld joint, although the insufficient fusion area is between 0 and 15 cm, which is smaller than that of the 180°C weld joint, S 21 The maximum and minimum amplitudes differ by 1.79 dB ( Figure 3E ), the degree of insufficient fusion is greater than that of a 180℃ weld joint.

[0068] Figure 3F yes Figure 3E Medium S 21 The corresponding resonance curves of the three points on the amplitude curve. Compared with the points located on the two dotted lines, that is, the locations of the defect-free areas, the peaks of the resonance curves of the points in the insufficient fusion area are significantly smaller. From the above results, it can be seen that the S 21 The amplitude is greater than S at the defect-free position. 21 The amplitude is small. Insufficient fusion is a manifestation of cold welding defects. 21 The amplitude also shows a similar trend. From the above results, it can be seen that using a resonant probe to scan the center weld of a hot melt joint in a circumferential direction can detect whether there is insufficient fusion in the joint and the area of ​​insufficient fusion, so as to determine whether there is a cold weld defect and the degree of the cold weld defect. Hot melt joints formed at a temperature lower than the standard welding temperature will have cold weld defects, and the lower the welding temperature, the more serious the defect.

[0069] It is worth mentioning that the specific structure and other technical features of the vector network analyzer involved in the patent application of this invention should be regarded as the prior art. The specific structure, working principle and possible control method and spatial layout method of these technical features can be selected by conventional methods in the field, and should not be regarded as the inventive point of the patent of this invention. The patent of this invention will not be further elaborated.

[0070] For those skilled in the art, it is still possible to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance, characterized in that: The following steps are involved: Step S1: placing a scanning base loaded with a microwave resonant probe on the outer surface of a pipeline to be inspected, so that the distance between the point focusing antenna and the pipe wall is equal to the focus of the antenna; Step S2: The probe performs two-dimensional scanning along the circumference and axial direction of the pipeline, so that the laser locator on the scanning seat obtains the coordinate information of the probe position; Step S3: the vector network analyzer records the S21 amplitude corresponding to the resonance peak of each coordinate information to draw the resonance curve of the outer surface of the pipeline to be detected in real time; Step S4: determine whether the change degree of the resonance curve exceeds the preset interval. If it exceeds the preset interval, the outer surface of the pipeline to be inspected is determined to have a cold welding defect; otherwise, the outer surface of the pipeline to be inspected is determined to have no cold welding defect.

2. The method for detecting cold welding defects of polyethylene gas pipeline hot melt joints based on microwave resonance according to claim 1 is characterized in that: The outer surface of the pipeline to be tested is the center weld of the polyethylene gas pipeline hot-melt joint.

3. The method for detecting cold welding defects of polyethylene gas pipeline hot melt joints based on microwave resonance according to claim 1 is characterized in that: Step S4 is specifically implemented as the following steps: Step S4.1: In S 21 Add two dashed lines N dB apart to the amplitude curve, with the upper dashed line at S 21 The maximum amplitude; Step S4.2: When S 21 When the amplitude curve is between the two dotted lines, the S 21 There is no cold welding defect in the joint fusion area corresponding to the amplitude curve. On the contrary, when S 21 When the amplitude curve is outside the two dotted lines, the S 21 There is a cold welding defect in the joint fusion area corresponding to the amplitude curve.

4. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 3 is characterized in that: In step S4.1, N is 0.5-1.0 dB.

5. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 1 is characterized in that: The microwave resonant probe comprises a point-focusing antenna and a resonant cavity. The resonant cavity is provided with a strong coupling port and a weak coupling port. The strong coupling port is connected to the point-focusing antenna through a rectangular-circular transition device, and the weak coupling port is connected to a vector network analyzer through a microwave cable.

6. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 5 is characterized in that: The point focusing antenna consists of a conical horn and a dielectric lens, and the dielectric lens is arranged at the aperture of the conical horn.

7. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 1 is characterized in that: The microwave resonance frequency of the microwave resonant probe is any frequency within the range of 5-26.5 GHz.

8. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 6 is characterized in that: The dielectric lens is a biconvex lens.

9. The method for detecting cold welding defects of polyethylene gas pipeline hot-melt joints based on microwave resonance according to claim 6 is characterized in that: The focal length of the dielectric lens is 10-100mm.

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