GB/T 20935: Evolution and historical versions
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Metal materials - Method of electromagnetic acoustic inspection
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GB/T 20935-2025
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Metal materials -- Method of electromagnetic acoustic inspection -- Part 1: Standard guide for electromagnetic acoustic transducers (EMATs)
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| GB/T 20935.1-2007 | English | 479 |
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Method of electromagnetic acoustic inspection for metal materials -- Part 1: Standard guide for electromagnetic acoustic transducers (EMATs)
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GB/T 20935.1-2007
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Basic data | Standard ID | GB/T 20935-2025 (GB/T20935-2025) | | Description (Translated English) | Metal materials - Method of electromagnetic acoustic inspection | | Sector / Industry | National Standard (Recommended) | | Date of Implementation | 2026-03-01 | | Older Standard (superseded by this standard) | GB/T 20935.1-2018, GB/T 20935.2-2018, GB/T 20935.3-2018 | GB/T 20935.1-2018: Metal materials -- Method of electromagnetic acoustic inspection -- Part 1: Standard guide for electromagnetic acoustic transducers (EMATs)
---This is a DRAFT version for illustration, not a final translation. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.) will be manually/carefully translated upon your order.
Metal materials--Method of electromagnetic acoustic inspection--Part 1. Standard guide for electromagnetic acoustic transducers (EMATs)
ICS 77.040.20
H26
National Standards of People's Republic of China
Replace GB/T 20935.1-2007
Electromagnetic ultrasonic testing method for metal materials
Part 1. Guide to Electromagnetic Ultrasound Transducers
Part 1. Standardguideforelectromagneticacoustictransducers (EMATs)
Published on.2018-03-15
2018-12-01 implementation
General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China
China National Standardization Administration issued
Foreword
GB/T 20935 "Electromagnetic Ultrasonic Testing Method for Metal Materials" is divided into the following three parts.
--- Part 1. Guide to electromagnetic ultrasonic transducers;
--- Part 2. Methods for ultrasonic testing using electromagnetic ultrasonic transducer technology;
--- Part 3. Ultrasonic surface detection using electromagnetic ultrasonic transducer technology.
This part is the first part of GB/T 20935.
This part is drafted in accordance with the rules given in GB/T 1.1-2009.
This part replaces GB/T 20935.1-2007 "Electromagnetic ultrasonic testing methods for metallic materials - Part 1. Electromagnetic ultrasonic transducers
South, compared with GB/T 20935.1-2007, the main technical content changes are as follows.
--- Added Chapter 5 application conditions (see Chapter 5);
--- Removed the qualification requirements of Chapter 8 (see Chapter 8 of the.2007 edition);
--- Removed the application content of Chapter 9 (see Chapter 9 of the.2007 edition);
--- Added the typical application example of the informative Appendix A electromagnetic ultrasonic transducer (see Appendix A).
This part was proposed by the China Iron and Steel Association.
This part is under the jurisdiction of the National Steel Standardization Technical Committee (SAC/TC183).
This section drafted by. Iron and Steel Research Institute, Steel Research Institute Testing Technology Co., Ltd., Metallurgical Industry Information Standards Institute.
The main drafters of this section. Fan Hong, Zhang Jianwei, Zhang Ke, Shen Haihong, Xu Lei, Dong Li.
This section replaces the standard previous versions of the release.
---GB/T 20935.1-2007.
Introduction
Ultrasound technology has fully established its position in the field of non-destructive testing. At first, the generation of ultrasonic waves is mainly through the piezoelectric effect.
The conversion of electrical energy and mechanical energy is an effective method for generating ultrasonic waves. But its disadvantage is that in order for the ultrasound to enter smoothly
The material to be inspected requires a liquid as the coupling medium. When using a couplant, it is usually to immerse the test material in a liquid or apply a thin layer on the surface of the material.
liquid.
The electromagnetic ultrasonic transducer can transmit and receive ultrasonic waves thereto without contacting the material to be inspected. However, the pair of electromagnetic ultrasonic testing
The image must be a metallic material (ferromagnetic or non-ferromagnetic). The ultrasonic transmitter of the electromagnetic ultrasonic transducer consists of a metal coil that is placed in gold
In a steady magnetic field on a surface of a material (ferromagnetic or non-ferromagnetic), an alternating current is used to excite ultrasonic waves. Metal material surface according to
The principle of the transformer induces a current, and the current is generated by the Lorentz force in the magnetic field to generate an oscillating stress wave (sometimes magnetic in the ferromagnetic conductive material)
The stretching force and the Lorentz force work together). Upon receiving the ultrasonic waves, the surface of the conductor oscillates in the magnetic field to induce a voltage in the coil. on
The conversion process is carried out in the electromagnetic skin layer of the material. Electromagnetic ultrasonic transducer is a reproducible non-contact ultrasonic
Transmitting and receiving system.
Electromagnetic ultrasonic testing method for metal materials
Part 1. Guide to Electromagnetic Ultrasound Transducers
1 Scope
This part of GB/T 20935 gives the meaning and use of ultrasonic testing using electromagnetic ultrasonic transducers (EMATs), application strips
Guide to parts, calibration, principles, and system configuration.
This section provides a basic reference for confirming the effectiveness of electromagnetic ultrasonic transducers in specific applications.
Note. This section does not contain detailed methods for the special application of electromagnetic ultrasonic transducers. It is not recommended to use electromagnetic ultrasonic transducers for actual testing without sufficient testing.
Some application examples of electromagnetic ultrasonic transducers are briefly introduced in Appendix A.
2 Normative references
The following documents are indispensable for the application of this document. For dated references, only dated versions apply to this article.
Pieces. For undated references, the latest edition (including all amendments) applies to this document.
GB/T 9445 Non-destructive testing personnel qualification and certification
GB/T 11259 Non-destructive testing method and preparation method for steel reference test block for ultrasonic testing
GB/T 12604.1 Non-destructive testing terminology ultrasonic testing
GB/T 12604.6 Non-destructive testing term eddy current testing
JB/T 10062 Ultrasonic flaw detection probe performance test method
3 Terms and definitions
The following terms and definitions as defined in GB/T 12604.1 and GB/T 12604.6 apply to this document.
3.1
Electromagnetic ultrasonic transducer electromagneticacoustictransducer; EMATs
An electromagnetic device that performs electrical energy-acoustic energy conversion in a magnetic field.
3.2
Lorentzforces Lorentzforces
The force that a current is subjected to in a magnetic field.
Note. Lorentz force is perpendicular to the direction of the magnetic field and current, the same principle as the motor.
3.3
Magnetostrictive force magnetostrictiveforces
The force generated by the movement of magnetic domain walls when the ferromagnetic material is magnetized.
3.4
Folding coil meanercoil
Periodically wound, disjoint and evenly spaced electromagnetic ultrasonic transducer coils.
3.5
Flat (spiral) coil pancake(spiral)coil
Spiral wound electromagnetic ultrasonic transducer coils with uniform spacing.
3.6
Body wave bulkwave
Ultrasonic waves used to detect volumetric materials in non-destructive testing can be either longitudinal or transverse.
4 Meaning and use
4.1 Overview
Ultrasonic testing is a widely used non-destructive testing method. Most ultrasonic testing uses piezoelectric wafers to achieve direct electrical energy and acoustic energy.
Conversion. This section describes a technique for achieving electrical-acoustic energy conversion in a ferromagnetic or non-ferromagnetic metal material in a non-contact manner.
Compared with piezoelectric ultrasonic probes, electromagnetic ultrasonic transducers are unique and are an important technical means in some ultrasonic testing applications.
4.2 Advantages
Because electromagnetic ultrasonic transducer technology is non-contact, it does not require liquid coupling and can be used for dynamic detection, remote or
Detection of hazardous locations, detection of high temperatures and rough surfaces. Since this technology does not use potentially contaminated or dangerous chemicals,
It is environmentally friendly. It scans complex geometric objects quickly. Based on the electromagnetic vibration generation mechanism, the signal has a good signal
Repeatability. Electromagnetic ultrasonic transducers produce horizontally polarized transverse waves (SH waves) without pattern switching and scan with SH waves
Detection. Electromagnetic ultrasonic transducers also have the ability to electrically control the direction of the transverse wave.
Note. Conventional ultrasound technology requires the use of epoxy or high viscosity couplant to generate this mode of wave, so traditional ultrasound technology does not easily use SH
Waves are detected.
4.3 Limitations
Electromagnetic ultrasonic transducers are very inefficient. Compared to traditional ultrasonic methods, electromagnetic ultrasonic transducers have insertion losses of up to 40 dB or more.
many. Electromagnetic ultrasonic technology can only be used on ferromagnetic or non-ferromagnetic metal materials. Electromagnetic ultrasonic transducer probe design is more complex than piezoelectric probe
miscellaneous. Due to its low efficiency, electromagnetic ultrasonic transducers require special devices to transmit and receive signals, strong emission currents, low noise receivers and fine
It is true that impedance matching is a must in system design. Electromagnetic ultrasonic transducers have the same range of applications as piezoelectric transducers.
5 application conditions
If the contract requires, the personnel performing the tests in this part shall obtain the technical assets identified by the relevant departments according to GB/T 9445 or equivalent standards.
Grid, and authorized by the employer. The criteria for the qualification basis (including the version number) should be stated in the contract.
6 calibration
6.1 Comparative sample
As with conventional piezoelectric ultrasonic testing, electromagnetic ultrasonic testing should produce a set of comparative tests that show the expected discontinuity of the material being inspected.
For the purpose of verifying the sensitivity, see GB/T 11259 (the manufacturing and testing methods for aluminum reference test blocks can refer to this standard).
6.2 Transducer
The calibration method of the traditional contact piezoelectric transducer is generally applicable to the electromagnetic ultrasonic transducer, see JB/T 10062, and it can be adapted if necessary.
When corrected or modified. This section does not include calibration methods specifically for electromagnetic ultrasonic transducers.
7 Principle
7.1 Non-magnetic conductive materials
7.1.1 The mechanism by which elastic waves are generated in a conductive material depends on the nature of the material. In non-magnetic conductive materials, the generation of sound waves is Loren
The result of the force acting on the material lattice. The role of Lorentz force can be described using a solid free electron model. Free electrons based on conductor
In theory, the outer valence electrons of an atom are detached from the atomic lattice, leaving the positively charged ions in a free electron cloud. In order to produce in the material
The elastic wave is generated and the resultant force acts on the material lattice. If only an eddy current coil is used to generate an electromagnetic field in the conductor, it acts on the material lattice
The resultant force is zero because the force acting on the electrons and the force on the ions are equal and opposite, see equations (1) and (2).
F electron →=-qE
→ (1)
F ion →= qE
→ (2)
In the formula.
q---Electronic charge in units of Coulomb (C);
E→---The electric field strength vector of the electromagnetic ultrasonic transducer in Newtons per Coulomb (N/C).
7.1.2 If the above electromagnetic field is in an applied constant magnetic field, the resultant force on the crystal lattice will produce an elastic wave. This synergy is the effect
Lorentz forces on electrons and ions, see equation (3).
L=qv
×B
(3)
In the formula.
⇀ --- The speed of movement of electrons in meters per second (m/s);
--- Stable magnetic induction vector, in Tesla (T).
7.1.3 The electrons are free to move and the ions are bound by the crystal lattice. Because electrons have speed, the Lorentz force acting on the electrons is very strong.
This force acts on the ions of the crystal lattice by collision.
7.2 Magnetic conductive materials
In magnetic conductive materials, magnetostrictive forces and Lorentz forces simultaneously affect the motion of ions. In magnetic materials, electromagnetic fields can change materials
The magnetostriction coefficient of the material, which in turn produces a periodically varying magnetostrictive force, is superimposed on the Lorentz force. Magnetostrictive force is very complicated, it depends on
The distribution of magnetic domains is also affected by the applied strength and direction of the steady magnetic field. Although theoretically analyzing the magnetic extension in magnetic conductive materials
The force is very complicated, but this additional force is very useful, and this force can greatly increase the strength of the signal compared to the signal generated by Lorentz force alone.
When a strong magnetic field causes the material to become magnetically saturated, Lorentz force becomes the only cause of sound waves. Magnetostrictive force is only weak in the magnetic field
It plays a leading role, and it is significantly stronger than the sound waves generated by Lorentz force at the same field strength. Therefore, in order to make full use of the magnetostrictive effect
It is necessary to carefully verify the above relationship in the magnetic material to excite the ultrasonic wave under the weak magnetic field strength.
7.3 Wave mode
7.3.1 Overview
The electromagnetic ultrasonic transducer can generate longitudinal waves, transverse waves, surface waves, and lamb waves in a suitable combination of magnets and coils. External magnetic field
The direction, the geometry of the coil, and the frequency of the electromagnetic field will determine the mode in which the ultrasonic ultrasonic transducer produces ultrasonic waves.
7.3.2 Longitudinal wave
Figure 1 shows how the direction of the steady magnetic field applied in the conductor and the direction of the generated Lorentz force produce a longitudinal wave. To create vertical
Wave, Lorentz force direction and ion movement direction should be perpendicular to the conductor surface. Longitudinal wave generation compared to other modes in ferromagnetic materials
The rate is very low.
Description.
1---electromagnetic wave;
2---Conductor surface.
Figure 1 Generation of electromagnetic ultrasonic longitudinal waves
7.3.3 Transverse wave
Figure 2 shows how the direction of the steady magnetic field applied in the conductor and the direction of the generated Lorentz force produce a transverse wave. In order to produce a horizontal
Waves, Lorentz force directions and ion displacement directions should be parallel to the conductor surface. Electromagnetic ultrasonic transducers can produce both horizontally polarized transverse waves and
A transversely polarized transverse wave is produced. The difference between the two polarized waves is shown in Figure 3.
Description.
1---electromagnetic wave;
2---Conductor surface.
Figure 2 Electromagnetic ultrasonic transverse wave generation
Description.
1---incident wave;
2---reflected wave;
3---vertical polarization (SV);
4---Horizontal Polarization (SH) (pointing to the page).
Figure 3 Horizontal and vertical polarized transverse wave diagram
7.3.4 Surface waves
In general, the surface wave is generated in the same way as the transverse wave, and the direction of the applied stable magnetic field should be perpendicular to the surface of the conductor. Electromagnetic ultrasonic transducer
The frequency depends on the fold line spacing of the fold-back coil, and by selecting the appropriate frequency, a pure surface wave can be excited. If the thickness of the material is greater than the sound
Surface waves can be generated by five times the wavelength of the wave. The wavelength of the surface wave is determined by the frequency and the wave speed.
7.3.5 Lamb wave
Various modes of Lamb waves (symmetric and antisymmetric) can be produced in a manner similar to surface waves. The frequency of the Lamb wave foldback coil is
The Lamb wave pattern and material thickness to be produced are determined.
8 system configuration
8.1 Transducer
8.1.1 Overview
As with conventional piezoelectric ultrasonic testing, the beam direction of an electromagnetic ultrasonic transducer has two basic forms. a straight beam and a diagonal beam, two
The transducers in the form of sound beams are described in 8.1.2~8.1.3.
8.1.2 Straight beam
The most effective way for an electromagnetic ultrasonic transducer to produce a straight beam is to use a spiral flat coil, with the direction of the steady magnetic field perpendicular to the coil.
Plane, as shown in Figure 4. The steady magnetic field can be provided by a permanent magnet, an electromagnet or a pulsed magnet. Suppose the magnetic field is not parallel to the direction of the coil
The amount produces a transversely polarized transverse wave. Since the magnetic field always has a small gradient in the direction of the parallel coil, there is a small amplitude of longitudinal waves.
This longitudinal wave component can be minimized by the design of the electromagnetic ultrasonic transducer. This approach is equally applicable to placement in vertical alternating
The butterfly flat coil in the magnetic field excites the linearly polarized shear wave.
Description.
1---magnet;
2---Helical coils produce radially polarized transverse waves.
Figure 4 Flat coil type electromagnetic ultrasonic transducer
8.1.3 oblique beam
8.1.3.1 Electromagnetic ultrasonic transducer The folded coil can also excite the oblique beam ultrasound, and the direction of the applied static magnetic field is perpendicular to the coil plane, as shown in Fig. 5.
Shown. The shape of the foldback coil is shown in Figure 6. The periodic stress on the surface of the material is related to the coil size when the following
This stress can excite ultrasonic waves, see equation (4).
Nλ=2L (4)
In the formula.
n---odd integer;
λ---surface wave wavelength in meters (m);
L --- adjacent coil spacing in meters (m).
8.1.3.2 When the projection of the coil spacing in the direction of propagation of the selected body wave mode satisfies equation (5), the excitation of the body wave can be achieved.
Nλ=2Lsinθ (5)
In the formula.
θ---the angle with the surface normal.
8.1.3.3 Equation (5) applies to both transverse and longitudinal waves. Therefore, the folded loop can be used to generate a transverse beam or a longitudinal wave, and the angle of the beam
The degree can be controlled by the frequency of the electromagnetic field. The case of a vertically polarized transverse wave is shown in Figure 3. Due to the different longitudinal and shear wave velocities, these two modes
There is a low cutoff frequency. Selecting the appropriate frequency can excite pure surface waves or pure transverse waves, but it is impossible to excite pure longitudinal waves.
For the longitudinal wave, there is always a component of the transverse wave.
Description.
1---SV wave generated by the folded-back coil (vertically polarized oblique beam);
2---magnet.
Figure 5 Rewinding coil type electromagnetic ultrasonic transducer
Description.
S---line width;
w---coil width;
L---line spacing.
Figure 6 Electromagnetic ultrasonic transducer foldback coil geometry
8.1.4 Frequency
Electromagnetic ultrasonic transducers can be designed either as narrowband frequency response or as broadband frequency response. When the folded loop is excited by a sine wave
A narrowband frequency response is generated within 20% of the center frequency, with a typical center frequency ranging from 0.1 MHz to 10 MHz. And spiral
The wideband frequency response is produced when the ring is excited by a sharp pulse.
8.1.5 Lifting away
8.1.5.1 Although the electromagnetic ultrasonic transducer is non-contact when detecting the material, the proximity of the coil to the material to be inspected is a strong influence signal.
The main factor of degree is shown in equation (6).
S(g)=S0e
-2πg
D( ) (6)
In the formula.
S(g)---the function of the signal strength as a function of the lift-off gap;
S0 --- signal strength at zero gap, in decibels (dB);
g --- the gap of the coil from the surface of the material, in millimeters (mm);
D --- coil wire spacing, in millimeters (mm);
8.1.5.2 It is important to maintain a minimum gap to get the strongest signal. In addition, keeping the gap constant for signal repeatability and signal analysis is also
Important, this is often achieved by adding a thin layer of material between the electromagnetic ultrasonic transducer coil and the workpiece being inspected, such as sticking a high-resistance metal sheet to electricity.
On the magnetic ultrasonic transducer, as long as the thickness of the sheet is smaller than the depth of the electromagnetic skin. Electromagnetic super can also be made from ceramic and carbon fiber reinforced plastics.
The protective layer of the acoustic transducer coil makes it wear-resistant during scanning.
8.2 Transmitter and receiver
Although some pulse transmitters can be used simultaneously for electromagnetic ultrasound, electromagnetic transducers are electrically compared to conventional piezoelectric transducers.
There are obvious differences in the energy. Electromagnetic ultrasonic transducers are generally inductive loads, while piezoelectric transducers are capacitive loads. Therefore, electromagnetic ultrasonic pulses
The flush transmitter is significantly different from conventional ultrasound. When designing an electromagnetic ultrasonic transducer pulse transmitter, another consideration is that with conventional pressure
Compared to electrical devices, the insertion loss of electromagnetic ultrasonic transducers is as high as 40 dB or higher, so the preamplifier needs high gain, so that noise
Sound level and saturation recovery time are important considerations in the design of electromagnetic ultrasonic receivers. For example, in an electromagnetic ultrasonic transducer
In a pulse echo system, the preamplifier should be able to withstand the peak voltage applied to the electromagnetic ultrasonic transducer and be able to recover quickly enough to detect
Defect signal.
8.3 data processor
Computers configured for electromagnetic ultrasound transducer signal processing should have sufficient data processing capabilities. The computer has a universal interface board and
Data conversion board. Used to acquire and store data obtained by electromagnetic ultrasonic transducers and transducer position coordinate data such as in the scan. Signal also
It can be evaluated by conventional ultrasonic methods.
8.4 Materials to be inspected
8.4.1 Since the electromagnetic ultrasonic transducer technology relies on the electromagnetic principle to emit and receive acoustic energy on the material to be inspected, the material to be inspected shall
It is a ferromagnetic or non-ferromagnetic metal material.
8.4.2 Since the electromagnetic ultrasonic transducer technology is non-contact, the surface roughness of the material has a greater impact on sensitivity than conventional piezoelectric ultrasound.
A lot smaller, although this effect cannot be ignored.
8.4.3 Because electromagnetic ultrasonic transducers do not use couplant, they are easier to use in high temperature materials than conventional piezoelectric ultrasonics.
Appendix A
(informative appendix)
Typical application examples of electromagnetic ultrasonic transducer
A.1 Metal base material inspection
A.1.1 Steel rod
Electromagnetic ultrasonic transducers can be used to detect cracks and folds on the surface of steel rods. Electromagnetic ultrasonic transducers use pulsed magnetic fields and retracting coils
The 2MHz surface wave propagates circumferentially along the surface of the steel rod, and can detect cracks and folds of tens of microns. Electromagnetic ultrasonic transducer using pulse magnet
The purpose is to use the skin effect to concentrate the magnetic field on the surface area of the steel bar.
A.1.2 Steel plate
Electromagnetic ultrasonic transducers can be used to detect internal discontinuities in the steel plate. Electromagnetic ultrasonic transducers use permanent magnets and retracting coils to excite
The surface of the 2.5MHz transverse wave vertical steel plate can effectively detect the pores and inclusions inside the plate.
A.2 Weld inspection
A.2.1 Fuel tank weld
The electromagnetic ultrasonic transducer detects the aluminum weld of the liquid fuel tank outside the spacecraft. It detects internal defects as a ray method
The supplement can replace the surface penetration type defect instead of the traditional penetration method. Electromagnetic ultrasound systems use a variety of transduce...
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