Dry-Coupled Ultrasonic Testing: An Effective, Yet Underutilized NDE Method

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing (UT). Although there are non-contact options available for some applications (e.g. EMAT), the most widely used technique involves using a piezoelectric element to generate the ultrasound. Due to the impedance mismatch between the piezoelectric transducer and the material inspected, a liquid couplant is typically used to transmit the vibrations from the transducer into the part and receive the vibrations back into the transducer. While this technique is very efficient and popular, liquid couplants used for ultrasonic inspection have some inherent limitations and disadvantages. The liquid couplant itself is sensitive to extreme weather conditions, can be harmful to the part and environment, can make some inspections cumbersome or impossible, and their use and disposal can be expensive. An alternative method to using liquid couplant is Dry-Coupled Ultrasonic Testing (DCUT).

Applications and Advantages

A new family of piezoelectric transducers has recently been developed to withstand very high voltages and can be efficiently coupled through rubber with no need for liquid couplant. These DCUT transducers can be used to inspect metals, composites, and any material suited for ultrasound propagation with the following advantages over more conventional options:

  • Dry-Coupling - The inspection is cleaner, easier, and does not contaminate the material inspected or the environment.
  • Low-profile and Flexible - DCUT transducers can be made only a few millimeters tall/thick, and can easily bend and adapt to curved and irregular geometries.
  • Less Costly Inspections - DCUT transducers eliminate all the costs associated with couplant; including its purchase, management, and disposal.

DCUT transducers are able to overcome the natural inefficiencies typically associated with other dry coupled piezoelectric methods by using high-power UT instruments that provide the necessary energy to couple the sensors using only a thin rubber layer and very light pressure. Currently the rubber material limits measurements to materials at up to 80°C, but development efforts continue to incorporate materials that will allow DCUT measurements at even higher temperatures. Existing DCUT sensors include flexible and rigid contact sensors, wheel probes, and remote contact sensors designed to generate L waves in normal beam mode for flaw detection, thickness measurement, and weld inspection.

Inspection Methods

Dry-Coupled Flexible Transducers

These low-profile transducers can be used to take measurements and detect flaws in complex geometries without the use of water tanks and multi-axis scanners. Using light pressure on contact, they quickly provide accurate measurements on a variety of materials. Due to their low profile, they can also be inserted into small tubes or pipes and locations with minimal clearance where having to apply and use couplant would be difficult or impossible.

[picture goes here] Figure 1. Dry-Coupled Flexible Tranducers.

Dry-Coupled Broadband Contact Transducers

With a similar form-factor as standard piezoelectric models, these transducers have ultra-wide frequency response and do not require any liquid coupling. They are ideal for high-precision thickness measurement and flaw detection on sensitive components where using couplant could be problematic or damaging to the material. They can be also used with delay lines and dry-coupled wheel scanners for more inspection options.

[picture goes here] Figure 2. Dry-Coupled Broadband Contact Transducer

Dry-Coupled Wheel Transducers

With a transducer installed in the center of the wheel and a ring of rubber around the outer diameter, the wheel allows fast scanning of long areas of pipe, plates, and large areas. With an encoder attached, it allows for accurate thickness measurements to be taken at regular intervals without the need to reapply couplant and realign the transducer. This can allow personnel to take more measurements over an area in less time to provide a more accurate picture of material thickness.

[picture goes here] Figure 3. Dry-Coupled Wheeled Transducer.

Dry-Coupled Remote Contact Transducers

The transducer is installed on an extendable pole with a rubber tip to couple to the material. This allows personnel to measure the thickness of material that would typically require scaffolding to be built or other items to be removed to access the measurement area. The semicircular rubber tip allows accurate thickness measurements even when the tip is not at a 90° angle to the material.

[picture goes here] Figure 4. Dry-Coupled Remote Contact Transducer.

Conclusion

DCUT transducers offer an excellent solution for piezoelectric UT inspection when used with high powered instrumentation. These inspection methods can allow better use of inspectors’ time, decrease equipment costs, and provide inspection options that were currently not available or possible. As the options to perform inspections without the need for couplant increase, the capabilities of inspectors also increase.

Contact us to learn more about our DCUT.