Non-Destructive Thickness Measurement

A popular and non-destructive thickness measurement technique is generally employed to calculate the thickness of a material from one side. This technique is called ultrasonic thickness gauging.

Non-destructive Testing (NDT) use beams of waves of short wavelength and high-frequency, transmitted from a small probe and detected by the same or other probes. Such waves can travel large distances in fine-grain metal, in the form of a divergent wave with advanced attenuation.

Thickness measurement as part of Non-destructive Testing is one of the most common uses of ultrasonic technologies. Precise wall thickness gauges can also be used to detect damage caused by erosion and corrosion on, for example, ships, storage tanks, pipelines, and cranes.

The wave is usually emitted by a piezoelectric cell or EMAT sensor that is built into the measurement sensor head and the same or other sensors are used to record the reflected wave.

In this article, following NDT measurement techniques are used

Ultrasonic thickness measurement (UTM)
Electromagnetic acoustic transducer (EMAT)

Ultrasonic Thickness Measurement (UTM)

Thickness measuring is essential across many industries to monitor corrosion, erosion and internal hidden discontinuities that may be deep below the surface. Ultrasonic thickness measurement (UTM) is a non-destructive testing method used to measure the metal thickness.

Ultrasonic testing can be done to measure almost any kind of material. Ultrasonic thickness gages can be constructed for glass, plastics, metals, ceramics, fiberglass, and composites.

UTM is commonly used and the method can be applied to a wide range of structures and components that includes ship hulls, piping, large turbine rotors, pressure vessels and structural steel.

The frequency is in the range 0.1 to 20 MHz and the wavelength in the range 1 to 10 mm. The velocity depends on the material and is in the range 1000-6000 m/s.

Construction

UTM is a method measurement of thickness of a solid surface (typically made of metal) based on the time taken by the ultrasonic wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge.

Ultrasonic thickness measurement gauge mainly consists of

Ultrasonic thickness gauge meter
Gauge probe

Gauge Probe

These sensors are piezoelectric sensors. It emits sound waves into the material when excited. Choosing the right gauge and transducer depends on a number of factors such as the material to be measured, geometry, thickness range, accuracy requirements, temperature, and any specific conditions that may exist.

Typically, these transducers use a fixed frequency, however in some thickness gauges allow frequency tuning in order to inspect a wider range of material.

Thickness Gauge

An ultrasonic thickness gauge is a measuring instrument for the non-destructive testing of thickness of material using ultrasonic waves.

A rugged ultrasonic thickness gauge determines sample thickness by measuring the amount of time taken by sound wave to traverse from the transducer through the material to the back end of a workpiece and reflect back. The ultrasonic thickness gauge then calculates the data based on the speed of the sound through the tested sample.

Ultrasonic thickness gauges are generally divided into two types

Corrosion gauges
Precision gauges

Corrosion gauges are specifically designed for measuring the remaining wall thickness of workpiece that are subject to internal corrosion that cannot be seen from the outside. They use signal processing techniques that detect the minimum remaining thickness in a rough, corroded test piece.

The precision gauges are extremely versatile and can measure to higher accuracy than corrosion gauges.

For calculating the time interval, generally following three techniques are available. Specific application needs and the type of transducer will determine the choice of the mode.

The time interval between the excitation pulse that produces sound waves and the first returning echo is measured. Then a small zero offset value is subtracted to offset delays for probe, gauge and cable.
The time interval between the first backwall echo and the returning echo from the test piece’s surface is measured.

3. The time interval between two consecutive backwall echoes is measured.

Couplant

Some ultrasonic coating thickness gauges require that a couplant to eliminate gaps between the transducer and the test piece. It can be in gel, paste or moderately viscous format.

Their use is necessary because sound energy at the ultrasonic frequencies typically used for NDT is not effectively transmitted through air. Even an extremely thin air gap between the transducer and the test piece will prevent efficient sound energy transmission and make conventional testing impossible.

A number of common substances such as water, motor oil, grease, and even some commercial products like hair gel can be used as ultrasonic couplants in many applications. One common couplant is propylene glycol.

However, for specific conditions such as high-temperature and normal incidence shear wave testing, it is necessary to use specially formulated couplants. Following are some of the special purpose couplants.

1. Glycerine (Couplant B2)

It is also a general purpose couplant. The glycerine is viscous and has a high acoustic impedance, making it the preferred couplant for rough surfaces and highly attenuating materials. The maximum recommended temperature for glycerine is nearly 90°C.

2. Gel (Couplant D12)

Gel type couplants are often recommended for rough surfaces where the transducer cannot make smooth contact with the test surface. The high viscosity and relatively high acoustic impedance of gel couplants maximize sound coupling in such situation. It can be used on hot surfaces up to approximately 90 °C.

3. Couplant H-2 – High-Temperature Couplant

Couplant H-2 is a gel that can be used at elevated temperatures. It can be used at temperatures up to 400 °C under certain open environment conditions.

4. Couplant I-2 – High-Temperature Couplant

Couplant I-2 is a gel that can be used at elevated temperatures. It can be used from −40 to 675 °C.

5. Shear Wave Couplant – SWC-2

Normal incidence shear wave transducers require couplants of very high viscosity. SWC-2 is a nontoxic, water-soluble organic substance of very high viscosity that is easy to apply and remove. SWC-2 may be used at temperatures up to approximately 4 ° to 32 °C.

Working

A piezoelectric transducer is stimulated by a short electrical impulse to produce ultrasonic waves. Contact is usually assured by removing visible corrosion scale and then applying couplant before pressing the probe against workpiece.

The sound waves are coupled into test material and reflect back from the inside surface or far wall. Because sound waves reflect from boundaries between dissimilar materials, this measurement is normally made from one side in a “pulse or echo” mode. The reflected waves travel back to the ultrasonic transducer, which changes the sound energy into electrical energy.

Typically, this time interval is only a few millionths of a second. The thickness of the test material (Workpiece) can be calculated using mathematical formula

${\rm{T}} = {\rm{ }}\;{\rm{ }}\frac{{{\rm{V }}\;{\rm{ t}}}}{2}$

where
T = the thickness of the workpiece
V = the velocity of sound in the test material
t = the traverse time

The formula features division by two because usually the instrumentation emits and records the ultrasound wave on the same side of the sample using the fact that it is reflected on the boundary of the element. Thus, the time corresponds to traversing the sample twice.

It is important to note that the velocity of sound in the test material is an essential part of this calculation. The velocity of sound changes in different materials.

Following table indicates magnitude of sound velocity in different materials.

Material	Velocity (m/s)
Aluminum	3040 – 6420
Brick	3600 – 4200
Concrete	3200 – 3700
Copper	3560 – 3900
Glass	3950 – 5000
Iron	3850 – 5130
Lead	1160 – 1320
Steel	4880 – 5050
Wood	3300 – 5000

Sound velocity can change with temperature hence it is important that an ultrasonic thickness gauge must be calibrated to the speed of sound in the material that is being determined.

Advantages of Ultrasonic Thickness Measurement

It is a non-destructive technique.
High penetrating power, which allows the detection of flaws deep in the part.
It needs to access only one surface and does not require access to both sides of the sample.
The detection of extremely small faults is possible.
Good accuracy can be achieved using standard timing techniques.
It is not hazardous to operators or to nearby personnel and has no effect on equipment and materials in the vicinity.
Capable of portable or highly automated operation.
Quick results are obtained.

Disadvantages of Ultrasonic Thickness Measurement

It requires calibration for nearly every material.
Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.
Although paint that is properly bonded to a surface need not be removed before reading.
Cannot take measurement over rust.
Couplant is required between the measured surface and the probe.
The equipment can be expensive.

Applications of Ultrasonic Thickness Measurement

Ultrasonic testing techniques are widely accepted for quality control and materials testing in many industries, including electric power generation, production of steel, aluminium and titanium, in the fabrication of airframes, jet engine manufacture and ship building.

Electro-Magnetic Acoustic Transducer (EMAT)

An Electro-Magnetic Acoustic Transducer (EMAT) is a non-contact, non-destructive testing (NDT) technique.

In ultrasonic thickness gauge the ultrasonic wave is generated by piezoelectric transducer but EMAT generates sound in the component being inspected hence EMAT is a completely non-contact technique.

Construction of EMAT

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.

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.

An EMAT induces ultrasonic waves into a test object with two interacting magnetic fields. One magnetic field is generated by coil and other by magnet as shown in the following figure.

Working of EMAT

A relatively high frequency (RF) 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.

Variations in the electrical conductivity or magnetic permeability of the test object or the presence of flaws will change the flow patterns of the eddy currents and there will be a corresponding change in the phase and amplitude of the measured current.

Various types of waves can be generated using different combinations of RF coils and magnets. Depending on the design and orientation of coils and magnets, shear horizontal (SH) bulk wave mode (norm-beam or angle-beam), surface wave, plate waves such as SH and Lamb waves, and all sorts of other bulk and guided-wave modes can be excited.

Advantages of EMAT

EMAT is a non-contact method.
Since no couplant is needed, the EMAT inspection can be performed in a dry environment.
It is not affected by surface conditions such as coatings, oil, oxide etc.
No need to remove the coating of the metal.
Smooth surface of object is not necessary in EMAT, the only requirement is to remove loose scale and the like.
It provides a convenient means of generating Shear Horizontal (SH) bulk wave and SH guided waves.

Disadvantages of EMAT

Limited to metallic or ferromagnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMAT.
Low transduction efficiency.
Caution must be taken when handling magnets around steel products.
EMAT transducers also require high power and specific electronic equipment that is not widely available.
EMAT instruments are generally more expensive than piezoelectric UT instruments.

Applications of EMAT

EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, railroad, pipeline, boiler and pressure vessel industries, in which they are typically used for non-destructive testing (NDT) of metallic structures.
Thickness Measurement
Corrosion and erosion measurement.
Flaw detection, such as, inclusions, de-laminations and disbond.