A circular probe with a diameter of 12mm uses a 2 MHz longitudinal wave in steel

Question

A circular probe with a diameter of 12mm uses a 2 MHz longitudinal wave in steel. Calculate the height of the pulse that is reflected from a circular reflector with a diameter of 0.5 mm at a depth of 25 mm, assuming that the velocity in steel is 5.96mm/us, and the attenuation coefficient is 0.03 If a block of steel, 50 mm thick is required amplification of the back wal echo is found to be 37.6 dB. Now a thick, is tested using the same probes amplification of the back wall echo is found to be 72.4dB. What is the attenuation coefficient of the unknown t an electromagnetic transducer is and how it works. List 2 A 12mm diameter transducer at 2MHz is used to detect a disc-shaped reflector at a depth of 20mm. The amplitude of the echo is -36.3dB. If we assume the velocity in the block is 5.96 diagram to estimate the area of the reflector. Please note that you can the

Explanation / Answer

Electro-Magnetic Acoustic Transducer (EMAT) is an Ultrasonic Testing (UT) technique that generates sound in the component being inspected rather than the transducer. Because the sound is generated in the part inspected instead of the transducer, EMAT is a completely non-contact technique. This gives it significant advantages over more conventional piezoelectric transducers; although it also has some weaknesses, which can limit its applications.

EMAT works by generating ultrasonic waves into a test object using electromagnetic induction with two interacting magnetic fields. A relatively high frequency field generated by electrical coils interacts with a low frequency or static field generated by magnets to generate a Lorentz force in a manner similar to an electric motor. This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit. For ferromagnetic conductors, magnetostriction produces additional stresses that can enhance the signals to much higher levels than could be obtained by the Lorentz force alone.

EMAT is capable of generating all wave modes used in ultrasonic testing, including some modes that are very difficult or impractical with conventional transducers. They are used in a wide variety of applications in the O&G and chemical processing industries such as:

EMAT Inspection of In-Service Piping

One of the most common uses of a field EMAT inspection device is for volumetric inspection of in-service piping. This technique can be used to examine equipment such as large diameter header piping at high temperature, transmission lines for hazardous wastes, and natural gas distribution lines

EMAT Inspection of Tubulars

One of the the most common uses of EMAT inspection in the field is the inspection of tubes. Tubes are tested by generating and propagating ultrasonic Lamb waves circumferentially in the tubular. A Lamb wave is a class of plate waves which means that the entire volume of the pipe is filled with ultrasonic energy.

Remaining Wall Thickness at Pipe Supports

A particular application of employing EMATs on tubulars is for the specific determination of pipe wall thickness at the point of contact with a pipe support. Pipes often suffer localized OD corrosion at the support and a determination of remaining wall thickness is extremely difficult using standard techniques. Our recent work has improved on known analysis techniques.

EMAT Inspection of Vessels

EMAT is proving to be an efficient means of completely inspecting relatively large surface areas of material. The procedure, which has been developed to examine vessels and plates, involves mounting a transmit and a receive EMAT sensor in a fixture, allowing 1 to 2 feet separation. Bulk ultrasonic waves are then generated and transmitted through the material, effecting a volumetric test as the fixture moves across the surface.

As a non-contact UT technique, EMAT has distinct advantages that make it the technique of choice for many applications, including dry inspection capabilities, an imperviousness to surface conditions, and unique wave modes such as shear waves with horizontal polarization (SH waves).  

There are two basic components in an EMAT transducer. One is a magnet and the other is an electric coil. The magnet can be a permanent magnet or an electromagnet, which produces a static or a quasi-static magnetic field. In EMAT terminology, this field is called bias magnetic field. The electric coil is driven with an alternating current (AC) electric signal at ultrasonic frequency, typically in the range from 20 kHz to 10 MHz. Based on the application needs, the signal can be a continuous wave, a spike pulse, or a tone-burst signal. The electric coil with AC current also generates an AC magnetic field. When the test material is close to the EMAT, ultrasonic waves are generated in the test material through the interaction of the two magnetic fields.

Advantages

Compared to piezoelectric transducers, EMAT probes have the following advantages:

No couplant is needed. Based on the transduction mechanism of EMAT, couplant is not required. This makes EMAT ideal for inspections at temperatures below the freezing point and above the evaporation point of liquid couplants. It also makes it convenient for situations where couplant handling would be impractical.

EMAT is a non-contact method. Although proximity is preferred, a physical contact between the transducer and the specimen under test is not required.

Dry Inspection. Since no couplant is needed, the EMAT inspection can be performed in a dry environment.

Less sensitive to surface condition. With contact-based piezoelectric transducers, the test surface has to be machined smoothly to ensure coupling. Using EMAT, the requirements to surface smoothness are less stringent; the only requirement is to remove loose scale and the like.

Easier for sensor deployment. Using piezoelectric transducer, the wave propagation angle in the test part is affected by Snell’s law. As a result, a small variation in sensor deployment may cause a significant change in the refracted angle.

Easier to generate SH-type waves. Using piezoelectric transducers, SH wave is difficult to couple to the test part. EMAT provide a convenient means of generating SH bulk wave and SH guided waves.

Challenges and disadvantages

The disadvantages of EMAT compared to piezoelectric UT can be summaried as follows:

Low transduction efficiency. EMAT transducers typically produce raw signal of lower power than piezoelectric transducers. As a result, more sophisticated signal processing techniques are needed to isolate signal from noise.

Limited to metallic or magnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMAT.

Size constraints. Although there are EMAT transducers as small as a penny, commonly used transducers are large in size. Low-profile EMAT problems are still under research and development. Due to the size constraints, EMAT phased array is also difficult to be made from very small elements.

Caution must be taken when handling magnets around steel products.

Applications

EMAT has been used in a broad range of applications and has potential to be used in many other applications. A brief and incomplete list is as follows.

Thickness measurement for various applications

Flaw detection in steel products

Plate lamination defect inspection

Bonded structure lamination detection

Laser weld inspection for automotive components

Various weld inspection for coil join, tubes and pipe

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.