Flexible magnetoelastic noise reduction electromagnetic-acoustic detection probe based on ferrofluid and method thereof

Abstract

The flexible magnetic field focusing and shear wave noise reducing electromagnetic-acoustic detection probe based on ferromagnetic fluid and method thereof, the probe comprises a flexible coil, a ferromagnetic fluid device, a permanent magnet and a shell device. The flexible coil is located at the bottom of the probe and is used for exciting and receiving signals; the ferromagnetic fluid device is located above the coil and has the characteristics of high magnetic permeability, difficult propagation of shear waves, fluid flow and incompressibility, thereby playing the roles of focusing the surface magnetic field, reducing the shear wave noise in the permanent magnet and pressing the coil to adhere to the surface of the pipeline; the flexible coil and the ferromagnetic fluid used in the application enhance the sensitivity of the pulse eddy current detection and the amplitude of the electromagnetic ultrasonic detection signal by enhancing the induced eddy current density and the bias magnetic field strength on the surface of the pipeline; on the other hand, the detection signal-to-noise ratio of the probe is improved by reducing the shear wave noise in the permanent magnet; meanwhile, the purpose and effect of synchronous detection of the inner and outer wall defects of the pipeline are achieved through a signal separation method.

Description

Technical Field

[0001] This invention relates to an electromagnetic nondestructive testing probe and its testing method, specifically to a flexible magnetic noise-reducing electromagnetic ultrasonic-pulse eddy current testing probe based on ferrofluid and an integrated electromagnetic nondestructive testing method combining pulse eddy current and electromagnetic ultrasonic. Background Technology

[0002] Pipelines are widely used in many industrial sectors, including oil and natural gas. However, because pipelines often operate in harsh natural environments, they are prone to developing surface cracks, corrosion and thinning defects, and complex defects involving both cracks and corrosion and thinning on the inner wall due to long-term oil and gas impact corrosion, wind erosion, rainwater chemical corrosion, and stress. These defects typically expand over time, ultimately affecting the mechanical properties and overall integrity of the pipeline structure. Pipeline failure can lead to incalculable safety problems. Therefore, how to quickly, efficiently, accurately, and cost-effectively detect these damages and defects is a pressing research need and a hot topic in the oil and natural gas pipeline transportation industry.

[0003] Currently, the detection and evaluation methods for the aforementioned defects include offline detection and online monitoring. Firstly, traditional offline detection methods (such as electromagnetic ultrasonic testing, pulsed eddy current testing, and magnetic particle testing) can typically only detect one type of defect in the pipeline. For the aforementioned combined defects on the inner and outer walls of the pipeline, two or more methods are required for detection, resulting in low efficiency. Secondly, current online monitoring methods mainly involve placing electromagnetic ultrasonic sensors on the surface of the industrial pipeline for continuous, real-time wall thickness monitoring. However, this monitoring method cannot detect stress cracks caused by uneven surface stress and also suffers from blind zones due to the inherent mechanism of electromagnetic ultrasonic testing.

[0004] However, given the harsh operating conditions of pipelines, although the aforementioned composite detection / monitoring methods can effectively detect / monitor complex defects on the inner and outer walls of pipelines, the accuracy of the detection / monitoring results is significantly affected by substantial noise in actual operating conditions. Furthermore, the traditional permanent magnets that provide the bias magnetic field within the composite detection / monitoring sensors suffer from drawbacks such as fixed shape and lack of flexibility, which also lead to unstable detection / monitoring results and poor signal-to-noise ratios when detecting / monitoring pipelines.

[0005] Therefore, for electromagnetic ultrasonic or electromagnetic ultrasonic-pulse eddy current integrated detection / monitoring of pipelines, it is necessary to develop a flexible probe with noise reduction and magnetic focusing capabilities. Summary of the Invention

[0006] To address the problems of the existing technology, the present invention aims to provide a flexible, magnetically focused, noise-reducing electromagnetic ultrasonic-pulse eddy current detection probe based on ferrofluid for pipes with arbitrary curvature or curved plates with multiple curvature variations. This probe enables electromagnetic ultrasonic detection / monitoring or electromagnetic ultrasonic-pulse eddy current integrated detection / monitoring on curved surfaces. Not only can the intensity of the induced eddy current and the bias magnetic field be increased through the flexible coil and the high permeability of the ferrofluid, thereby improving the signal detection amplitude and sensitivity, but the characteristic that ferrofluid has difficulty propagating shear waves can also be utilized to reduce the influence of shear wave noise within the permanent magnet on the detection signal, improving the signal-to-noise ratio. Furthermore, due to the mechanical design and material selection of the probe, it can detect not only pipes and curved surfaces with curvature but also plates without curvature, achieving the effect of a single probe applicable to multiple scenarios.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A flexible magnetic noise-reducing electromagnetic-acoustic detection probe based on ferrofluid includes a flexible self-excited self-testing coil 1, a ferrofluid device 2, a permanent magnet 3, and a housing device 4 that covers the flexible self-excited self-testing coil 1, the ferrofluid device 2, and the permanent magnet 3, adaptable to pipes with various curvatures and diameters. The flexible self-excited self-testing coil 1 is a flexible circuit board coil. The ferrofluid device 2 is located above the flexible self-excited self-testing coil 1 and is composed of a ferrofluid 5 and a flexible sealing protective sleeve 6. The ferrofluid 5 is a water-based or oil-based ferrofluid, and the flexible sealing protective sleeve 6 is made of transparent silicone material, possessing elasticity and extensibility. The permanent magnet 3 is located above the ferrofluid device 2. The housing device 4 includes a housing body 7, a sliding baffle 8, a spring 9, and a top cover 10. The spring 9 is fixed to the top cover 10, the top cover 10 is fixed to the housing body 7, and the sliding baffle 8 is located on the lower side of one side of the housing body 7.

[0009] When not under test, the flexible self-excited self-testing coil 1 and the ferrofluid device 2 are not deformed, and are respectively in the shape of a plane and a cuboid. The spring is not compressed and deformed, and the sliding baffle 8 slides to the bottom of the probe 12. The probe 12 is in a stable state with balanced forces. When the probe 12 is placed on the surface of the pipe 11, the sliding baffle 8 slides upward, and the outer shell 7 on both sides of the bottom of the probe 12 is in complete contact with the surface of the pipe 11. The flexible self-excited self-testing coil 1 is deformed into a curved surface and fits against the surface of the pipe 11. The bottom of the ferrofluid device 2 is compressed and deformed into a curved surface and fits against the flexible self-excited self-testing coil 1. Due to the constraints of the outer shell 7 and the sliding baffle 8, as well as the fluidity and incompressibility of the ferrofluid 5, the ferrofluid device 2 expands upward, pushing the permanent magnet 3 to move upward within the outer shell 4, thereby compressing the spring 9. The spring 9 is deformed by force. When the spring 9 is deformed to a certain extent, the forces on the components inside the probe 12 are balanced, and no more displacement or deformation occurs. The probe 12 returns to a stable state.

[0010] Because the ferrofluid 5 exhibits magnetism under the influence of an external magnetic field, it will adhere to the surface of the permanent magnet 3 without constraint. This will not provide sufficient pressure for the deformation of the flexible self-excited self-test coil 1, affecting the surface fit of the flexible self-excited self-test coil and thus reducing the detection sensitivity of the probe 12. Therefore, by designing a flexible sealing protective sleeve 6 and an outer shell device 4 to constrain the ferrofluid 5, it can provide sufficient pressure for the flexible self-excited self-test coil 1 to adhere to the surface of the pipe 11, ensuring that the flexible self-excited self-test coil 1 is completely attached to the pipe 11 and improving the detection sensitivity of the probe 12.

[0011] Preferably, the flexible circuit board coil is composed of a flexible substrate and a copper-clad coil, the copper-clad coil being spiral-shaped or loop-shaped. This allows the flexible substrate to allow the coil to fit perfectly onto the surface of a pipe with arbitrary curvature, increasing the density of induced eddy currents excited on the pipe surface, thereby improving detection sensitivity and the amplitude of the detection signal. Furthermore, the use of flexible circuit board printing technology allows for more refined design and optimization of the coil configuration, reducing design errors caused by manufacturing processes.

[0012] Preferably, the water-based ferromagnetic fluid has a magnetic solid particle volume percentage concentration of 0.5%-3.6% and a remanence of 30-200 Gauss, while the oil-based ferromagnetic fluid has a magnetic solid particle volume percentage concentration of 0.9%-16.1% and a remanence of 50-900 Gauss. Thus, because the ferromagnetic fluid possesses both the fluidity of a liquid and the magnetism of a solid magnetic material, it can effectively press a coil to completely adhere to the pipe surface, while also achieving a magnetizing effect. Furthermore, since the ferromagnetic fluid is a colloidal liquid, it is difficult to transmit shear ultrasonic waves, thus also reducing shear wave noise within the permanent magnet.

[0013] Preferably, the permanent magnet 3 is cuboid in shape and made of N45 neodymium iron boron.

[0014] Preferably, the spring 9 is a stainless steel compression spring, and the outer shell 7, the sliding baffle 8, and the top cover 10 are all made of 8200 resin. This prevents the outer shell from being magnetized by the permanent magnet and avoids the outer shell from affecting the magnetic field distribution of the permanent magnet.

[0015] The method for defect detection using a flexible magnetic noise-reducing electromagnetic-acoustic detection probe based on ferrofluid utilizes the high magnetic permeability and the difficulty in propagating shear waves of the ferrofluid 5 to increase the amplitude of the ultrasonic shear wave signal propagating within the pipe 11 and reduce the amplitude of the shear wave noise propagating within the permanent magnet 3, ultimately achieving the goal of enhancing the detection signal of the probe 12 and improving the signal-to-noise ratio of the probe 12. Specifically, it includes the following steps:

[0016] Step 1: Construct an electromagnetic ultrasound-pulse eddy current composite detection experimental system. This system includes: an excitation signal generating device consisting of a pulse signal generator and a power amplifier, a probe 12, a duplexer, a filter, and a data acquisition device consisting of an oscilloscope and a data acquisition system. The excitation signal generating device is connected to the duplexer and the data acquisition device, the duplexer is connected to the filter, and the filter is connected to the data acquisition device.

[0017] Step 2: Place the probe 12 on the surface of the pipe 11. At this placement point, there are both cracks on the outer wall surface and thinning defects due to corrosion on the inner wall. First, the pulse signal generator in the excitation signal generator generates a sinusoidal pulse excitation signal for one cycle. Then, the sinusoidal pulse excitation signal is amplified by the power amplifier in the excitation signal generator and transmitted to the probe 12 through the duplexer.

[0018] Step 3: First, according to the law of electromagnetic induction, the flexible self-excited self-testing coil 1 inside the probe 12, after passing an excitation current, will induce eddy currents on the surface of the pipe 11 and inside the ferrofluid 5. Cracks on the outer wall surface of the pipe 11 will affect the distribution of these eddy currents, influencing the coupled dynamic electromagnetic field and ultimately becoming part of the composite detection signal. Second, because the probe 12 is flexible and can completely conform to the surface of the pipe 11, it can induce eddy currents of higher intensity and a wider range on the surface of the pipe 11, thereby improving the sensitivity and efficiency of the probe 12 in detecting surface defects. Then, the magnetic lines of force of the permanent magnet 3 pass through the permanent magnet metal coating, the ferrofluid device 2, the flexible self-excited self-testing coil 1, the air, and the pipe 11, providing a bias magnetic field for the excitation of electromagnetic ultrasonic shear waves. According to the Lorentz force law F = J × B, the induced eddy currents on the surface of the pipe 11 generate shear Lorentz force under the action of the bias magnetic field, and subsequently excite shear ultrasonic waves that propagate towards the inner wall of the pipe 11. When the shear waves propagate to the inner wall of the pipe 11, they bounce back and... When the signal is transmitted back to the surface of pipe 11, a dynamic electromagnetic field is generated under the coupling effect of the bias magnetic field and the shear wave, and is detected by the flexible self-excited self-testing coil 1, generating a composite detection signal. This composite detection signal simultaneously contains information on cracks on the outer wall of the pipe, pipe thickness, and corrosion thinning defects on the inner wall. Furthermore, since ferrofluid 5 is added between the permanent magnet metal coating and the flexible self-excited self-testing coil 1, the induced eddy currents in the metal coating are reduced, and the shear ultrasonic noise of the permanent magnet 3 originally generated by the induced eddy currents in the metal coating is reduced. Although the induced eddy currents in the ferrofluid 5 generate Lorentz force under the action of the bias magnetic field, the shear wave propagating in the permanent magnet 3 is reduced because the ferrofluid is difficult to propagate. The shear wave propagating in the permanent magnet 3 is a noise signal for ultrasonic detection signals, so the ferrofluid 5 can improve the detection signal-to-noise ratio of the probe 12. In addition, since the ferrofluid 5 has high permeability, the probe 12 can focus magnetic lines of force, increase the intensity of the bias magnetic field, and enhance the ultrasonic detection signal.

[0019] Step 4: After receiving the composite detection signal in the pipeline 11, the probe 12 transmits it to the filter through a duplexer. Based on the characteristic that the pulse eddy current detection signal and the electromagnetic ultrasonic detection signal have different frequencies, the filter uses the spectrum filtering method to filter the received composite detection signal. The data acquisition device separates, extracts and analyzes the detection signal to finally obtain the pulse eddy current detection signal and the electromagnetic ultrasonic detection signal in the original detection signal, that is, to obtain the composite defect information contained in the pipeline 11.

[0020] The frequency of the sinusoidal pulse excitation signal in step 2 is 2MHz.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. This invention utilizes the fluidity and high permeability of ferrofluids to design a ferrofluid device suitable for pipes of arbitrary curvature. This device not only adapts its shape to different pipe curvatures, allowing the flexible self-excited self-testing coil to be completely pressed and adhered to the pipe surface, reducing the lift-off height of the coil and improving its sensitivity; it also functions as a fluid magnetic yoke focusing bias magnetic field in curved surfaces, thereby increasing the amplitude of the electromagnetic ultrasonic signal.

[0023] 2. This invention utilizes the characteristic that ferrofluids are less prone to propagating shear waves. By designing a ferrofluid device, the influence of shear wave noise within the permanent magnet on the ultrasonic detection signal is reduced, thereby improving the signal-to-noise ratio of the ultrasonic detection signal.

[0024] 3. The flexible sealing protective sleeve and outer shell device design used in this invention effectively constrains the ineffective displacement of the ferrofluid and allows the bottom of the probe to be properly fitted onto the surface of pipes with different curvatures, thereby improving the detection stability of the probe.

[0025] 4. The electromagnetic ultrasonic-pulse eddy current composite detection method of the present invention separates and extracts the pulse eddy current signal and the electromagnetic ultrasonic signal by means of spectrum filtering, and successfully solves the problem of simultaneous detection of cracks on the outer wall surface of the pipeline and thinning defects on the inner wall of the pipeline. Attached Figure Description

[0026] Figure 1 This is an exploded view of the assembly of the flexible magnetic noise reduction electromagnetic-acoustic detection probe based on ferrofluid of the present invention.

[0027] Figures 2(a), 2(b), 2(c) and 2(d) are respectively the front view, side view, bottom view and oblique view of the planar state of the flexible magnetic noise reduction electromagnetic-acoustic detection probe based on ferrofluid of the present invention.

[0028] Figures 3(a), 3(b), 3(c), and 3(d) are the front view, side view, bottom view, and oblique view of the curved surface state of the flexible magnetic noise reduction electromagnetic-acoustic detection probe based on ferrofluid of the present invention.

[0029] Figure 4 This is a schematic diagram of the detection system and detection method used in this invention.

[0030] Figure 5 This is a schematic diagram illustrating the detection principle of the flexible magnetic noise reduction electromagnetic-acoustic detection probe and method based on ferrofluids according to the present invention.

[0031] Figure 6 This is a schematic diagram of the target defect for pipeline detection according to the present invention. Detailed Implementation

[0032] The present invention will now be described in further detail with reference to the accompanying drawings.

[0033] like Figure 1 As shown, the flexible magnetic noise reduction electromagnetic-acoustic detection probe 12 based on ferrofluid provided by the present invention includes a flexible self-excited self-testing coil 1, a ferrofluid device 2, a permanent magnet 3, and a shell device 4 adaptable to pipes of various curvatures and diameters. The coil 1 is located at the bottom of the probe 12 and is an FPC (Flexible Printed Circuit) coil, composed of a flexible substrate and a copper-clad coil, the copper-clad coil being spiral-shaped or loop-shaped. The ferrofluid device 2 is located above the coil 1 and is composed of a ferrofluid 5 and a flexible sealing protective sleeve 6. The ferrofluid 5 is a water-based ferrofluid or an oil-based ferrofluid. The water-based ferrofluid has a magnetic solid particle volume percentage concentration of 0.5%-3.6% and a remanence of 30-200 Gauss. The oil-based ferrofluid has a magnetic solid particle volume percentage concentration of 0.9%-16.1% and a remanence of 50- The 900 Gauss flexible sealing protective sleeve 6 is made of transparent silicone material, which has a certain elasticity and extensibility; the permanent magnet 3 is located above the ferrofluid device 2, and is rectangular in shape and made of N45 neodymium iron boron; the outer shell device 4 is the outer shell of the probe 12, including the outer shell body 7, the sliding baffle 8, the spring 9 and the top cover 10. Except for the spring, which is a stainless steel compression spring, the rest are made of 8200 resin. The spring 9 is fixed on the top cover 10, the top cover 10 is fixed on the outer shell body 7, and the sliding baffle 8 is located on the lower side of one side of the outer shell body 7.

[0034] As shown in Figures 2(a), 2(b), 2(c), and 2(d), when not in use or when the test plate is in use, the bottom flexible self-excited self-testing coil 1 is in a planar state; the ferromagnetic fluid 5 and the flexible sealing protective sleeve 6 are in a regular hexahedral shape and are located directly above the coil 1; the permanent magnet 3 is located directly above the ferromagnetic fluid 5; the sliding baffles 8 on both sides of the outer shell 7 are located at the bottom, and the spring 9 is also in an undeformed state.

[0035] As shown in Figures 3(a), 3(b), 3(c), and 3(d), when testing pipe 11, the flexible self-excited self-testing coil 1 is fitted to the surface of pipe 11 in a curved manner. Due to their deformability, the bottom surface of the ferromagnetic fluid 5 and the flexible sealing protective sleeve 6 becomes curved along with the flexible self-excited self-testing coil 1. Due to their incompressibility and the constraint of the outer shell 7, the ferromagnetic fluid 5 and the flexible sealing protective sleeve 6 expand upward, thereby pushing the permanent magnet 3 to move upward and compressing the spring 9. Since the top cover 10 is fixed on the outer shell 7, the spring 9 begins to deform. When the spring 9 deforms to a certain extent, the internal components of the probe 12 are subjected to balanced forces, and the probe 12 is in a stable state.

[0036] like Figure 4 , Figure 5 , Figure 6 As shown, the method for defect detection using a flexible magnetic noise-reducing electromagnetic-acoustic detection probe based on ferrofluids in this invention is as follows:

[0037] Step 1, as follows Figure 4 As shown, an electromagnetic ultrasound-pulse eddy current composite detection experimental system is constructed, mainly including: an excitation signal generating device consisting of a pulse signal generator and a power amplifier, a probe 12, a duplexer, a filter, and a data acquisition device consisting of an oscilloscope and a data acquisition system; the excitation signal generating device is connected to the duplexer and the data acquisition device, the duplexer is connected to the filter, and the filter is connected to the data acquisition device.

[0038] Step 2, place probe 12 on the surface of pipe 11. At this placement point, both external wall surface cracks and internal wall corrosion thinning defects exist (e.g., Figure 6 (As shown). First, the pulse signal generator in the excitation signal generator generates a one-cycle sinusoidal pulse excitation signal with a frequency of 2MHz. Then, the sinusoidal pulse excitation signal is amplified by the power amplifier in the excitation signal generator and transmitted to the probe 12 through a duplexer.

[0039] Step 3: First, according to the law of electromagnetic induction, the flexible self-excited self-testing coil 1 inside the probe 12, after passing an excitation current, will induce eddy currents on the surface of the pipe 11 and inside the ferrofluid 5. Cracks on the outer wall surface of the pipe 11 will affect the distribution of these eddy currents, influencing the coupled dynamic electromagnetic field and ultimately becoming part of the composite detection signal. Second, compared to conventional probes, because the probe 12 has higher flexibility and can completely conform to the surface of the pipe 11, it can induce higher intensity and a wider range of eddy currents on the surface of the pipe 11, thereby improving the sensitivity and detection efficiency of the probe 12 in detecting surface defects. Then, the magnetic field lines of the permanent magnet 3 pass through the permanent magnet metal coating, the ferrofluid device 2, the flexible self-excited self-testing coil 1, the air, and the pipe 11, providing a bias magnetic field for the excitation of electromagnetic ultrasonic shear waves. According to the Lorentz force law F=J×B, the induced eddy currents on the surface of the pipe 11 generate shear Lorentz force under the action of the bias magnetic field, and subsequently excite shear ultrasonic waves to propagate to the inner wall of the pipe 11. When the shear wave propagates to the inner wall of the pipe 11, bounces back and propagates back to the surface of the pipe 11, a dynamic electromagnetic field is generated under the coupling effect of the bias magnetic field and the shear wave, and is detected by the flexible self-excited self-testing coil 1, generating a composite detection signal. This composite detection signal simultaneously contains information on cracks on the outer wall surface of the pipe, pipe thickness, and information on corrosion thinning defects on the inner wall. Secondly, the addition of ferrofluid 5 between the permanent magnet metal coating and the flexible self-excited self-testing coil 1 reduces the induced eddy currents within the metal coating. This reduces the shear ultrasonic noise of the permanent magnet 3 originally generated by the induced eddy currents within the metal coating. Although the induced eddy currents within the ferrofluid 5 generate Lorentz force under the bias magnetic field, the shear wave propagating within the permanent magnet 3 is smaller due to the difficulty of propagating shear waves in the ferrofluid. Since the shear wave propagating within the permanent magnet 3 is considered noise in ultrasonic testing, the ferrofluid 5 improves the signal-to-noise ratio of the probe 12. Furthermore, due to the high permeability of the ferrofluid 5, compared to conventional probes, the probe 12 can focus magnetic lines of force, increase the bias magnetic field strength, and enhance the ultrasonic testing signal.

[0040] Step 4, as follows Figure 4 As shown, after receiving the composite detection signal in the pipeline 11, the probe 12 transmits it to the filter through a duplexer. Based on the characteristic that the pulse eddy current detection signal and the electromagnetic ultrasonic detection signal have different frequencies, the filter uses the spectrum filtering method to filter the received composite detection signal. The data acquisition device separates, extracts and analyzes the detection signal, and finally obtains the pulse eddy current detection signal and the electromagnetic ultrasonic detection signal in the original detection signal, that is, obtains the composite defect information contained in the pipeline 11.

Claims

1. A flexible magnetoelastic noise reduction electro-magnetic-acoustic detection probe based on ferrofluids, characterized by: The application relates to a flexible self-excitation self-checking coil (1), a ferromagnetic fluid device (2), a permanent magnet (3), a flexible self-excitation self-checking coil (1) covering device (4) which can adapt to various curvature pipe diameters, and a pipe (11); the flexible self-excitation self-checking coil (1) is a flexible circuit board coil, the ferromagnetic fluid device (2) is located above the flexible self-excitation self-checking coil (1) and is composed of ferromagnetic fluid (5) and a flexible sealing protective sleeve (6), wherein the ferromagnetic fluid (5) is water-based ferromagnetic fluid or oil-based ferromagnetic fluid, the flexible sealing protective sleeve (6) is transparent silica gel material and has elasticity and ductility; the permanent magnet (3) is located above the ferromagnetic fluid device (2); the covering device (4) comprises a covering main body (7), a slidable baffle (8), a spring (9) and a top cover (10), wherein the spring (9) is fixed on the top cover (10), the top cover (10) is fixed on the covering main body (7), and the slidable baffle (8) is arranged below one side of the covering main body (7). When not detected, the flexible self-excitation self-checking coil (1) and the ferromagnetic fluid device (2) are not deformed and respectively present a plane and a cuboid shape, the spring is not compressed and deformed, the slidable baffle (8) slides to the bottom of a probe (12), and the probe (12) is balanced and stable as a whole; when the probe (12) is placed on the surface of the pipe (11), the slidable baffle (8) slides upwards, the two sides of the covering main body (7) at the bottom of the probe (12) completely contact the surface of the pipe (11), the flexible self-excitation self-checking coil (1) is deformed into a curved surface and is attached to the surface of the pipe (11), the bottom of the ferromagnetic fluid device (2) is compressed and deformed into a curved surface and is attached to the flexible self-excitation self-checking coil (1), due to the constraint of the covering main body (7) and the slidable baffle (8) and the flowability and incompressibility of the ferromagnetic fluid (5), the ferromagnetic fluid device (2) expands upwards, the permanent magnet (3) is pushed to move upwards in the covering device (4), the spring (9) is compressed, the spring (9) is deformed under stress, when the spring (9) is deformed to a certain extent, the components in the probe (12) are balanced under stress and do not displace and deform any more, and the probe (12) returns to a stable state. Due to the magnetism of the ferromagnetic fluid (5) under the action of an external magnetic field, the ferromagnetic fluid (5) is adsorbed on the surface of the permanent magnet (3) under the premise of no constraint, cannot provide sufficient pressure for the deformation of the flexible self-excitation self-checking coil (1), influences the surface attachment degree of the flexible self-excitation self-checking coil, and further reduces the detection sensitivity of the probe (12); therefore, the flexible sealing protective sleeve (6) and the covering device (4) are designed to constrain the ferromagnetic fluid (5), so that the ferromagnetic fluid (5) can provide sufficient pressure for the flexible self-excitation self-checking coil (1) to attach to the surface of the pipe (11), the flexible self-excitation self-checking coil (1) can be completely attached to the pipe (11), and the detection sensitivity of the probe (12) is improved. The volume ratio concentration of the magnetic solid particles of the water-based ferromagnetic fluid is 0.5%-3.6%, the residual magnetism is 30-200 Gauss, the volume ratio concentration of the magnetic solid particles of the oil-based ferromagnetic fluid is 0.9%-16.1%, and the residual magnetism is 50-900 Gauss.

2. The flexible magnetoelastic noise-cancelling electro-magneto-acoustic detection probe based on ferrofluids according to claim 1, characterized in that: The flexible circuit board coil is composed of a flexible substrate and a copper-coated coil, and the copper-coated coil is in the shape of a spiral circle or a loop.

3. The flexible magnetoelastic noise cancellation electro-magneto-acoustic detection probe based on ferrofluids according to claim 1, characterized in that: The permanent magnet (3) is in the shape of a cuboid and is made of N45 neodymium iron boron.

4. The ferrofluid-based flexible magnetoacoustic noise cancellation electromagnetic- acoustic detection probe of claim 1, wherein: The spring (9) is a stainless steel compression spring, and the materials of the shell body (7), the slidable baffle (8), and the top cover (10) are all 8200 resin.

5. The method of defect detection using the flexible poly-magnetic noise cancellation electro-magnetic-acoustic transducer probe based on ferro-fluidic as claimed in any one of claims 1 to 4, wherein: The ferromagnetic fluid (5) has high magnetic permeability and is difficult to propagate shear waves, so that the amplitude of the shear wave ultrasonic signal propagating in the pipeline (11) is improved, the amplitude of the shear wave noise propagating in the permanent magnet (3) is reduced, and finally the detection signal of the probe (12) is enhanced and the signal-to-noise ratio of the probe (12) is improved; the specific steps include the following steps: Step 1, build an electromagnetic ultrasonic-pulsed eddy current composite detection experiment system, which includes: an excitation signal generating device composed of a pulse signal generator and a power amplifier, a probe (12), a duplexer, a filter, and a data acquisition device composed of an oscilloscope and a data acquisition system; the excitation signal generating device is connected to the duplexer and the data acquisition device, the duplexer is connected to the filter, and the filter is connected to the data acquisition device; Step 2, place the probe (12) on the surface of the pipeline (11), and there are both outer wall surface cracks and inner wall corrosion and thinning defects at the placement point; first, the pulse signal generator in the excitation signal generating device generates a periodic sinusoidal pulse excitation signal, then the sinusoidal pulse excitation signal is amplified by the power amplifier in the excitation signal generating device, and then the amplified signal is transmitted to the probe (12) through the duplexer; Step 3, first, according to the law of electromagnetic induction, the flexible self-excitation and self-checking coil (1) in the probe (12) will excite induced eddy current on the surface of the pipeline (11) and inside the ferromagnetic fluid (5) after passing through the excitation current, and the surface crack of the outer wall of the pipeline (11) will affect the distribution of the induced eddy current, affect the coupled dynamic electromagnetic field, and finally become part of the composite detection signal; secondly, since the probe (12) is flexible and can completely adhere to the surface of the pipeline (11), a higher intensity and larger range of induced eddy current can be excited on the surface of the pipeline (11), thereby improving the sensitivity and detection efficiency of the probe (12) in detecting surface defects; then, the magnetic force line of the permanent magnet (3) passes through the permanent magnet metal plating layer, the ferromagnetic fluid device (2), the flexible self-excitation and self-checking coil (1), the air and the pipeline (11), and provides a bias magnetic field for the excitation of the electromagnetic shear wave; according to the law of Lorentz force F=J×B , the induced eddy current on the surface of the pipeline (11) generates shear Lorentz force under the action of the bias magnetic field, and excites shear ultrasonic waves to propagate to the inner wall of the pipeline (11); when the shear wave propagates to the inner wall of the pipeline (11) and bounces back to the surface of the pipeline (11), a dynamic electromagnetic field is generated under the coupling action of the bias magnetic field and the shear wave, and is detected by the flexible self-excitation and self-checking coil (1), generating a composite detection signal; the composite detection signal contains information of the surface crack of the outer wall of the pipeline, the thickness of the pipeline and the corrosion and thinning defect information of the inner wall; thirdly, since the ferromagnetic fluid (5) is added between the permanent magnet metal plating layer and the flexible self-excitation and self-checking coil (1), the induced eddy current in the metal plating layer is reduced, and the shear ultrasonic noise of the permanent magnet (3) originally generated by the induced eddy current in the metal plating layer is reduced; although the induced eddy current in the ferromagnetic fluid (5) generates Lorentz force under the action of the bias magnetic field, since the ferromagnetic fluid is difficult to propagate shear waves, the shear wave propagating in the permanent magnet (3) is reduced; and the shear wave propagating in the permanent magnet (3) is a noise signal for the ultrasonic detection signal, so the ferromagnetic fluid (5) can improve the signal-to-noise ratio of the probe (12); in addition, since the magnetic permeability of the ferromagnetic fluid (5) is high, the probe (12) can focus the magnetic force line, improve the strength of the bias magnetic field, and enhance the ultrasonic detection signal. Step 4, after the probe (12) receives the composite detection signal in the pipeline (11), the signal is transmitted to the filter through the duplexer, and according to the characteristics that the pulsed eddy current detection signal and the electromagnetic ultrasonic detection signal have different frequencies, the filter uses the frequency spectrum filtering method to filter the received composite detection signal, and the data acquisition device separates, extracts and analyzes the detection signal, and finally obtains the pulsed eddy current detection signal and the electromagnetic ultrasonic detection signal in the original detection signal, that is, the composite defect information contained in the pipeline (11).

6. The method of defect detection according to claim 5, wherein: The frequency of the sinusoidal pulse excitation signal in step 2 is 2 MHz.

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