National Physical Laboratory

Guide on Acoustic Emission Sensor Couplants

Correct coupling of an acoustic emission sensor to the surface of a structural material is very important for obtaining a good measurement. This webpage provides information on the requirement for a sensor couplant, and provides guidance on the types of couplants available and their use and application.

1. Why use couplants?

When attaching an AE sensor to a measurement surface, a couplant material is primarily used to remove any air from the interface, introduced due to the microstructure of the two contacting surfaces. The reason for doing this is that the acoustic impedance of air is much lower than that of the sensor face or material surface and will cause considerable loss in transmission.

PZT crystal (commonly used for AE sensors) has a relatively high acoustic impedance of the order of 106 N.s.m-3, with steel (a typical measurement surface) having an acoustic impedance of the same order. This makes them a relatively good match.

Air has a very low acoustic impedance of around 412 N.s.m-3, depending on temperature and pressure. Introducing a couplant material with an acoustic impedance higher than that of air, which displaces the air between the two surfaces, can increase transmission substantially. An increase of several orders of magnitude is not uncommon. If couplant cannot be used in a particular application then a very high force can be used to improve the transmission, but a couplant will always provide better transmission.

There are a number of couplant types to choose from, typically liquid, gel or grease. Other types of couplants based on compounds or adhesives can be used which provide bonding of the sensor to the surface. The advantages and disadvantages of each type are considered in this guide.

2. The use of couplants

Before selecting a couplant, one needs to consider the application and how the sensor will be used. There are several issues which will dictate the suitability of a given couplant, some of which are listed below for consideration:

  • How long will the measurement take?
  • Is long-term stability important?
  • How often is the sensor removed?
  • Could the sensor be permanently bonded?
  • Is there any source of movement or vibration which could displace the sensor?
  • What are the environmental conditions i.e. temperature, humidity?
  • Could the couplant react with the measurement surface or cause corrosion?
  • What type of wave is being measured? (Is good in-plane displacement transmission required?)
  • Is the use of a magnetic mounting fixture possible?

3. Choosing the right couplant

Generally, acoustic emission sensors have a dominant response to particle motion normal to the surface. For this type of measurement the viscosity of the couplant is not necessarily important for good transmission. However, if detecting particle motion in the plane of the sensor face (shear motion) then a more viscous couplant or rigid bond will provide greater sensitivity. This section considers the different couplants available and discusses the advantages and disadvantages of each type. This should aid the user in deciding which type of couplant is best for their application. A list of common couplants is listed in Table 1.

Liquid – generally suited to smooth surfaces
Liquid couplants generally provide lower acoustic impedance but often offset this with the ease of application and the ease with which air can be forced out. On a smooth surface they can offer good longitudinal wave transmission, comparable to gel type couplants and adhesives. The main disadvantages of liquid based couplants are that they have a low viscosity and have a tendency to drip, run out or dry up with time. They are not particularly suitable for vertical mounting as this encourages running and drying up of couplant layer. Liquid couplants do require a mounting fixture and should not be used if the measurement will exceed a few hours and require stability during this period. Propylene Glycol has the advantage that it will not cause corrosion of the measurement surface and liquid couplants are usually extremely easy to clean off the surface after use.

Gel - generally better for rougher surfaces
xGel type couplants will usually provide a slightly higher acoustic impedance than liquid based couplants, the most common ones being ultrasonic gel or glycerin. These are also extremely easy to apply and clean off after use and are less likely to drip than liquid based couplants making them suitable for vertical mounting. The higher viscosity of gels over liquids does make them more appropriate on rougher surfaces where the filling of gaps is required. Many gel type couplants will dry out over time, particularly around the edge of the sensor. This limits gels to measurements which do not exceed a few hours. Due to their relatively low viscosity they are very good at forcing out trapped air from the contact region with a small amount of force on the sensor. Powder based gels such as thixotropic gelatine do require preparation. However, they can be made with varying viscosities by varying the amount of water added. Corrosion inhibitors can also be added to powder based gels. Glycerin offers the highest acoustic impedance of the most common liquid and gel couplants therefore producing better transmission in most cases. A clamping fixture is required for all gel-based couplants.

Grease - generally better for rougher surfaces
Grease-based couplants have a much higher viscosity than gels or liquids and are therefore generally very good on rougher surfaces. The high viscosity does also mean that a high application force is required on the sensor to remove all the trapped air but enables the sensor to be mounted vertically. Grease based couplants sometimes benefit from a small amount of lateral movement during application of the sensor to encourage displacement of the trapped air, particularly on rough surfaces. If well clamped, grease offers better long-term stability than gels or liquids and generally does not damage the surface. Cleaning the surface after use is more difficult, particularly for silicon based greases. Grease couplants offer comparable transmission to liquids/gels if all the air is removed and also provide slightly better transmission of shear displacements on the surface. They are generally more popular for application based AE measurements than gels or liquids due to their versatility and stability on a range of surface conditions.

Dry couplants
Elastomer couplants are more popular for ultrasonic inspection than they are for AE measurement and are sometimes integrated into ultrasonic transducers. They are generally used to avoid the drawbacks of wet couplants such as time variability, drying up, residue etc and are available as pads for use with conventional contact transducers. These have varying transmission performance properties; elastomers with acoustic impedance close to that of water offering the closest performance to wet type couplants. Some elastomer pads come with pre-applied adhesive on both sides; otherwise the sensor should be clamped.

The use of standard double-sided adhesive tape is not recommended due to poor transmission and an insufficient bond strength to hold the sensor in place.

The use of putty like materials or plastiscines is not advisable for use as a dry couplant due to their extremely poor ultrasonic transmission. The use of such a material will compromise signal detection.

Adhesive couplants - no clamp required
Bonding agents can be used as an acoustic couplant that physically attach the sensor to the measurement surface. These are ideal for applications where the sensor will not be removed very often, if at all, and for measurements where absolute stability in the coupling or the sensor position is required.

Rubber compound
The most popular adhesive used, as an AE couplant for industrial applications is silicon rubber compound which can be applied as a fluid to achieve a thin, bubble free couplant layer and then held in place until cured to provide a permanent bond. This type of couplant works very well on rough surfaces. If a thin layer is obtained, silicon rubber compound can provide excellent transmission (comparable to that of wet couplants) with a relatively strong bond. A rubber compound bond provides good stability in applications where surface vibration might be present and good resistance to bond failure where surface movement might occur. This type of bond also provides substantially greater transmission of shear displacements than wet couplants (typically greater than twice the shear transmission). It is possible to remove the sensor after use by using a gradually increasing shear force, but cleaning the rubber residue off both the sensor and the surface is not easy. There is risk of damage to the sensor if it is not removed carefully. The cure time should be checked as the acoustic properties of the compound may change during the curing process.

Rigid bond
On a clean, flat surface, a rigid bond with an adhesive such as cyanoacrylate can provide the best transmission of both longitudinal and shear displacements if applied correctly. Force should be applied to the sensor during the short cure time to ensure a thin adhesive layer is achieved with no air bubbles. Once cured, the bond might still change with time, particularly the shear modulus. This can change the acoustic performance of the bond and the manufacturers specifications for long term stability should be checked. This type of couplant should only be used if the sensor is to remain in place as there is a relatively high risk of sensor damage during removal. The safest way to remove the sensor is to apply a rapid shear force. Do not attempt to pull the sensor off the surface. Acetone can be used to soften cyanoacrylate but this will only penetrate the edges of the sensor. Once removed, the sensor and surface can be cleaned with acetone to remove residue. When using rigid bonds, the user must always consider any vibration or thermal expansion which might cause the bond to crack or fail. Dental cement can be used in place of of cyanoacrylate if a less permant bond is required.

Special couplants
Shear wave:
For good transmission of shear waves, a high viscosity couplant should be used to provide a good coupling of transverse forces. Specialist shear wave couplants can be purchased which provide a highly viscous contact and good transmission of in-plane/shear surface motion. Honey is a more convenient alternative, which offers comparable transmission of shear waves to that of the more expensive specialist couplants. These couplants can also provide good transmission of longitudinal waves. They do require a mounting fixture and a large applied force to achieve an even layer with no trapped air. The sensors are easy to remove but the couplant is very difficult to clean from the sensor and the surface.

High temperature:
Many standard couplants are not suitable for measurements at elevated temperatures as they will exhibit large changes in physical and acoustic properties. Specialist couplants are available which are suitable for over 500 °C*. This type of couplant comes as a semi-solid paste that liquefies at high temperature and therefore requires temperatures of above 250 °C before it begins to operate correctly. For temperatures below this, medium temperature range couplants are widely available which are gel based and work from ambient temperature up to over 300 °C. These high and medium temperature couplants will dry out at elevated temperatures, which does limit their useable time period. Clamping will also be necessary.

* Care must be taken not to expose the sensor to high temperatures approaching the Curie temperature of the piezoelectric crystal. This will result in de-poling of the crystal and will result in a permanent loss in sensitivity of the sensor.

Table 1: Examples of common couplants used in AE applications

Couplant

Couplant type

Clamp required?

Viscosity

Temperature range

Silicone oil

Liquid

Yes

V Low

Low

Propylene glycol

Liquid

Yes

V Low

Medium

Glycerin

Liquid/Gel

Yes

Medium

Medium

Ultrasonic gel

Gel

Yes

Medium

Low

Brown grease

Grease

Yes

High

Low

Silicone grease

Grease

Yes

High

Medium

Petroleum jelly

Grease

Yes

High

Low

Honey

Sticky paste/gel

Yes

High

Low

Silicone compound

Elastomer adhesive

No

Elastic solid

Medium

Hot melt glue

Elastomer adhesive

No

Elastic solid

Low

Cyanoacrylate

Adhesive

No

Rigid

Low

Dental cement

Adhesive

No

Rigid

Low

Wax beads

Dry adhesive

No

V High

Low


4. Surface preparation

To maximise ultrasonic transmission, the surface should be prepared as best the application allows. Loose paint and oxidisation layers can reduce the transmission significantly. If air is trapped in layers below the surface then the application of a couplant will offer little improvement. The flatter and smoother the surface, the better the couplant will work. If a smooth surface cannot be achieved then a couplant more suited to rough surfaces should be selected.

5. Mounting the sensor

Sensor mounting fixtures are strictly only necessary with non-adhesive couplants and are used to hold the sensor in a fixed position and with a constant force. Mounting fixtures should be chosen which do not significantly block out or create false acoustic emission signals.

The most common mounts compress the sensor to the surface with either a spring or a screw thread. The mounts can be either rigid bonded, welded or clamped with magnets to the surface of the structure. For adhesive couplants, a mounting fixture is not usually necessary except whilst the adhesive cures.

The sensor is usually best coupled by achieving the thinnest layer possible. This can be done by applying a small amount of couplant to the centre of the sensor face and pushing the sensor down with a uniform force such that the couplant spreads out towards the edge of the sensor. The sensor should be held down until no excess couplant is being ejected from around the edge of the sensor face.

The coupling should then be tested with a reference source or a pencil lead fracture and checked periodically to ensure it has not changed. It is always worth repeating the coupling exercise to establish the typical level of transmission achievable on the structure under test. For further guidance on mounting AE sensors refer to ASTM E650-97(2007) Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors.

6. Useful reading

  • ASTM E650-97(2007) 'Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors'
  • Colombo, S., A. Giannopoulos, et al. (2005). "Frequency response of different couplant materials for mounting transducers." NDT&E International 38: 187-193
  • Cros, B., N. Brunet, et al. (2000). "Increase in performances of focused microacoustic sensors by couplant adjustment." Eur. Phys. J. AP 9: 81-85
  • Culjat, M. O., R. S. Singh, et al. (2005). "Evaluation of gallium-indium alloy as an acoustic couplant for high-impedance, high-frequency applications." Acoustics Research Letters Online 6: 125-129
  • Dugmore, K., D. Jonson, et al. (2002). "A comparison of the signal consistency of common ultrasonic couplants used in the inspection of composite structures." Composite Structures 58: 601-603
  • Habeger, C. C., W. A. Wink, et al. (1988). "Using neoprene-faced, PVDF transducers to couple ultrasound into solids." J. Acoust. Soc. Am. 84: 1388-1396
  • Hill, R. and S. M. A. El-Dardiry (1980). "A theory for optimisation in the use of acoustic emission transducers." J. Acoust. Soc. Am. 67: 673-682
  • Mak, D. K. (1993). "Ultrasonic phase velocity measurement incorporating couplant correction." British Journal of NDT 35(8): 443-449
  • Na, W-B. and T. Kundu (2002). "A combination of PZT and EMAT transducers for interface inspection." J. Acoust. Soc. Am. 111(5): 2128-2139
  • Sadler, J., B. O'Neill, et al. (2005). "Ultrasonic wave propagation across a thin nonlinear anisotropic layer between two half-spaces." J. Acoust. Soc. Am. 118(1): 51-59

For further information, please contact Dr Pete Theobald

Last Updated: 1 May 2012
Created: 6 Oct 2008

Registration

Please note that the information will not be divulged to third parties, or used without your permission

Login