Ultrasonic thickness measurement

In the field of industrial ultrasonic testing, ultrasonic thickness measurement (UTM) is a method of performing non-destructive measurement (gauging) of the local thickness of a solid element (typically made of metal, if using ultrasound testing for industrial purposes) basing on the time taken by the ultrasound wave to return to the surface. This type of measurement is typically performed with an ultrasonic thickness gauge.

Ultrasonic waves have been observed to travel through metals at a constant speed characteristic to a given alloy with minor variations due to other factors like temperature. Thus, given this information, called celerity, one can calculate the length of the path traversed by the wave using this simple formula:

$l_m = c t / 2$
where
$l_m$ is the thickness of the sample
$c$ is the celerity of sound in the given sample
$t$ is 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.

The wave is usually emitted by a piezoelectric cell or EMAT Sensor that is built into the measurement sensor head and the same sensor is used to record the reflected wave. The sound wave has a spherical pattern of propagation and will undergo different phenomena like multipath reflection or diffraction. The measurement does not need to be affected by these since the first recorded return will normally be the head of the emitted wave traveling at the shortest distance which is equivalent to the thickness of the sample. All other returns can be discarded or might be processed using more complicated strategies.

Advantages

Non-destructive technique
Does not require access to both sides of the sample
Can be engineered to cope with coatings, linings, etc.
Good accuracy (0.1 mm and less) can be achieved using standard timing techniques
Can be easily deployed, does not require laboratory conditions
Relatively cheap equipment
EMAT does not require the use of couplant.
EMAT can conduct thickness measurements through corrosion and other surface coatings on metals
No need to remove the coating of the metal.

Disadvantages

Usually requires calibration for each material
Requires good contact with the material
Cannot take measurement over rust (Does not apply to EMAT)
Requires coupling material between the measured surface and the probe. (Does not apply to EMAT)
Interpretation needs experience

Typical usage

UTM is frequently used to monitor metal thickness or weld quality in industrial settings. NDE Technicians equipped with portable UTM probes reach steel plating in sides, tanks, decks and the superstructure. They can read its thickness by simply touching the steel with the measurement head (transducer). Contact is usually assured by first removing visible corrosion scale and then applying petroleum jelly or another couplant before pressing the probe against metal. However, when UTM is used with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required. These testing methods are used to inspect metal to determine quality and safety without destroying or compromising its integrity. It is a requirement of many classification societies

The techniques and technologies associated with UTM are closely related to the use of ultrasound in other contexts, such as the various other industrial ultrasonic measurements, as well as medical ultrasonography and preclinical imaging micro-ultrasound.

Classification Requirements for UTM Hull Surveys

Classification societies have detailed requirements for the thickness measurement of hull structures. These requirement depend greatly on vessels type, age and length. All IACS member classifications have similar requirements since they need to comply with IACS guidelines.^[1] The allowable diminution thickness depends on the building rules of each classification. Also depending on type the societies request 1 or 2 operators. The UTM operators need to be Level II certified according with SNT-TC-1A^[2] or similar standard. Also the company that performs the ultrasonic thickness measurement survey must be approved by the classification that the vessel is registered with. The classification society review the documented procedures of the UTM company and audit them on board in order to issue an approval certificate.^[3] Finally the equipment used need to be type approved by the classifications.

Standards

For ISO and CEN standards relating to UTM and related techniques, see the article Ultrasonic testing.

References

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