Acoustic Emission Testing: A Guide

Acoustic emission testing is an inspection method that uses the release of ultrasonic stress waves to identify defects in materials. These ultrasonic waves are not introduced from an external source, as they are in ultrasonic testing, but rather originate from within the material being inspected.

Acoustic Emission Testing is also called Acoustic Emission (AE), Acoustic Testing (AT, Acoustic NDT, or AE Testing.

In this guide, we will use these terms interchangeably.

Acoustic emission testing is one of the most common and useful methods of non-destructive testing (i.e., testing that allows inspectors to collect data on materials without harming them).

The primary advantages of acoustic emission testing are that it allows inspectors to test a material's entire load history without damaging it.

Historically, AE has been used only for inspecting and maintaining expensive structures due to the high costs associated with it. But new developments have helped lower the cost of AE equipment, and it is becoming more accessible for a host of inspection applications.

[Acoustic emission NDT is just one of the non-destructive testing methods that inspectors use. Learn more about NDT in this in-depth guide.]

How Does Acoustic Emission Testing Work?

In an acoustic emission test, an inspector records elastic ultrasonic waves traveling through the surface of a solid material using one or more sensors.

As an acoustic wave travels on or through the surface of an object any defect it encounters can change that wave, both in terms of its speed and in terms of its amplitude. And inspectors look for these changes to identify the presence of defects.

The range of ultrasound typically used for acoustic emission testing is 20 KiloHertZ (KHZ) and 1 MegaHertZ (MHZ). (One KiloHertZ is equal to one one thousand Hertz, or cycles per second; one MegaHertZ is equal to one million Hertz, or cycles per second).

Here are a few definitions of terms we’ll use throughout this article:

Ultrasonic. The terms ultrasonic and ultrasound refer to sound waves that are so high humans can’t hear them.
Acoustic emission. The term acoustic emission refers to the generation of transient waves during the rapid release of energy from localized sources within a material.

Where Do Acoustic Emissions Come From?

Acoustic emissions happen when a material is under stress, either from holding a heavy load or from extremes of temperature.

These emissions typically correspond with some kind of defect or damage being done to the structure emitting them—and this damage is what inspectors are looking for when they do an AE test.

Sources of acoustic emission can include:

Phase transformation
Thermal stress
Cool down cracking
Melting
Bond and/or fibre failure

The History of Acoustic Emission Testing

Compared to other NDT methods like magnetic particle testing or dye penetrant testing, acoustic emission testing is relatively new.

It was first used in the early 1980s as a way for inspectors to test polymer matrix composites (PMCs).

The sensors used to record acoustic emissions use a piezoelectric material. Piezoelectricity is the production of electrical charges by the introduction of mechanical stress. Imagine setting using a crane to set a slab of granite on to the top of a bus.

The heavy granite will push down onto the bus, generating stress and electrical charges. And these charges are a type of piezoelectricity.

Piezoelectricity was first discovered in 1880, by two brothers named Pierre Curie and Paul-Jacques Curie. But it was not used for much of anything until the early 1920s, when an inventor named Walter Cady experimented with using piezoelectricity for stabilizing electronic oscillators.

Around sixty years later, researchers began testing piezoelectricity for identifying defects in polymer matrix composites. Today, the sensors used for acoustic emission testing are called piezoelectric acoustic wave sensors, because they apply an oscillating electric field in order to generate a mechanical wave.

This wave then travels through a material and becomes an electric field, which can be measured by an inspector.

Although AE is a promising NDT method it is still in its infancy, and will require years of research and development before it is a completely reliable, stand-alone inspection technique.

One interesting new application for AE is using it to detect earthquakes before they actually happen, but this application is also just in the early stage of development.

Common Applications and Industries for

Inspectors typically use AE to look for:

Corrosion—on the surfaces of various types of materials
Coating removal—of protective coatings put on materials
Faults/defects—for monitoring welding and for other general flaw detection
Leaks—in pipe systems or storage tanks
Partial discharges—from components subject to high voltage

For fibre specifically, AE is commonly used to test for cracking, corrosion, delamination, and breakages.

Here are some of the most common applications for acoustic testing:

Airplane longevity estimation
Bridge inspections (learn more about bridge inspections)
Concrete corrosion monitoring (learn more about corrosion monitoring)
Mine wall stability inspections
Pressure vessel inspections (learn more about pressure vessel inspections)
Structural integrity inspections
Wind turbine inspections

Acoustic Emission Testing vs. Ultrasonic Testing

Although both acoustic testing and ultrasonic testing use ultrasound they are distinct inspection methods.

In acoustic emission NDT, inspectors “listen” for acoustic emissions from defects present in a material.

AE testing is specifically useful for determining whether a structure is overloaded, and it’s the only NDT method that can be used during manufacturing. It does not require any use of external energy (unlike ultrasonic testing), because the test material or structure itself releases the acoustic emission.

In ultrasonic testing, on the other hand, inspectors send ultrasonic waves through a structure of material from an external source. If the waves are interrupted, this indicates the presence of a defect at the point of interruption.

See our guide to ultrasonic testing to learn how it works.

The Pros and Cons of Acoustic Emission Testing

Acoustic testing is a popular NDT method because it can provide a direct measure of failure mechanisms in action—but that’s just one of the reasons inspectors commonly use it to look for defects in a material.

Here’s a list of pros and cons for acoustic emission NDT:

Pros

It gives you a direct measure of failure mechanisms
It is highly sensitive
It provides data immediately
It is non-destructive to the material being tested
It allows for a structure to be globally monitored
It can be used in hazardous environments, including those that have high pressures, are irradiated, or have high temperatures
It can be done remotely, and can detect defects in materials that might be hard to test using other NDT methods

Cons

One of the drawbacks to AE is that it’s not always reliable, in part because it is still a relatively new NDT method.

Here are the main cons for acoustic testing as an NDT method:

It’s usefulness is generally limited to locating a defect, not describing it in detail—that is, commercial acoustic testing systems can only provide qualitative estimations for the extent of damage found
It cannot detect defects that do not change over time (i.e., defects that don’t move or grow)
It can be slow to implement
It can be hard to use—AE signals can be very weak, making noise reduction and signal discrimination crucial for accurate readings

Acoustic Emission Technique

To use AE testing, inspectors start by thoroughly cleaning the surface of the object they want to inspect.

After cleaning, they will place acoustic emission sensors onto the structure or material that they want to inspect.

Sensors will need to be mounted on the structure with an appropriate couplant—that is, a medium to help the transmission of the acoustic signal. Adhesives or grease are commonly used for this purpose.

Once attached, the sensors will convert any stress waves present in the material into electrical signals so that they can be read by the inspector.

Inspectors feed data from the sensors to a monitor using shielded coaxial cables, displaying the information in the form of both readable results and raw data. Once the data is available, inspectors interpret it to identify where there is stress on the object they are inspecting, and look for the possible locations of defects caused by that stress.

Determinations for the amount of sensors an inspector will need for a given structure are made according to several factors, including:

The complexity of the material or structure
The size of the structure
The type of material being tested

The Kaiser Effect

The Kaiser effect refers to the absence of acoustic emission in an object until the level of stress that was previously applied to it has been exceeded.

The effect was first discovered in 1950, when a researcher named Kaiser found that metals could “remember” the maximum amount of stress to which they had previously been subjected.

Due to the Kaiser effect, a structure could be under damaging stress that inspectors cannot identify using AE if that stress has not exceeded the prior amount of stress the structure has experienced.

Acoustic Emission Testing Equipment

Here are the types of acoustic emission equipment used in acoustic NDT.

Transducers / Sensors / Strain Gauges

These devices collect raw acoustic emission data. They are also called:

Piezoelectric transducers
Piezoelectric sensors
Strain gauges

A transducer in use on an assembly line

The most common set of transducers for AE testing consists of two sets of interdigital transducers, which is a device made of two interlocking, comb-shaped arrays of metallic electrodes arranged like a zipper.

One of the transducers converts electric field energy into mechanical wave energy, and the other transducer converts the mechanical wave energy back into an electric field.

Here are some of the different types of acoustic NDT sensors:

Thickness shear mode resonator. Measures metal deposition rates.
Displacement gauges. A strain gauge that converts the acoustic emission of displacement caused by stress on a structure into electronic readings.
Accelerator gauges. A strain gauge that converts the acoustic emission of velocity caused by stress on a structure into electronic readings.
Bulk acoustic wave device (BAW). A machine that propagates waves through the substrate of a material or structure. surface wave devices
Surface acoustic wave sensor (SH-SAW). A type of BAW device used to detect acoustic emissions on the surface of a material.
Surface transverse wave sensor (STW). A type of BAW device used to detect acoustic emissions on the surface of a material.

Low-Noise Preamplifiers

A low-noise preamplifier amplifies the output from the sensors to make it readable for inspectors.

These devices, combined with the right training, allow inspectors to identify the location of defects in a material that might not be visible to the naked eye.

A low-noise preamplifier (source: Stanford Research Systems)

Acoustic Emission Testing Standards and Codes

Given how inexpensive and easy it is, acoustic emission testing is often used by inspectors for informational purposes—that is, for inspections that do not have to comply with a specific code or set of standards.

But acoustic testing is also commonly used for code-based inspections.

For these inspections, inspectors must follow specific steps in how they conduct the test, including the requirement that they follow a written procedure and that the person conducting the test is certified to do so by the relevant standards body.

Here are some of the more widely used acoustic testing codes:

Acoustic Emission Testing: A Guide