What is Acoustic Emission Testing?
Acoustic emission testing (AET) is a non-destructive testing (NDT) technique that detects flaws in objects by monitoring the pattern of ultrasonic stress waves within the object through an attached set of sensors. These sensors will convert the stress waves into electrical signals and relay those signals to an external device for processing.
The amount of damage a component has sustained is directly proportional to the release of ultrasonic energy detected in the test. That means that if you test the acoustic emissions of a component with extensive fractures, the test results will display this damage as high levels of ultrasonic energy.
But first, why should you use non-destructive testing methods? Destructive testing methods cause irreversible damage to the component undergoing testing, meaning you will need to replace it. On the other hand, NDT methods assess the part without causing damage. As a result, you can reuse that same part afterward.
Types of Acoustic Emissions
The term “acoustic emissions” refers specifically to the transient elastic waves created from events of rapid energy releases within a material. These waves, also known as sound waves, can come from a wide range of events, which include:
- Fiber breakage
- Thermal stress
- Phase changes
- Crystalline distortions like twinning
- Objects cracking under stress
- Dislocation resulting from plastic deformation
There are two primary types of acoustic emissions:
- Burst: A burst is a short emission that results from distinct individual events.
- Continuous: Continuous acoustic emissions are sustained signals resulting from one or more overlapping or successive sources
There are also several different acoustic emission testing methods you can use. Each technique serves a different purpose:
- Proof testing: Proof testing demonstrates that a component can withstand a certain degree of pressure by gradually increasing the load on the component. The absence of significant emissions determines a successful result.
- Fatigue testing: This testing method estimates the maximum useful lifespan of an object by applying a cyclic load and monitoring the emissions produced within the object.
- Failure testing: Failure tests determine the load at which an object’s structural integrity begins to fail. Like a proof test, failure tests involve gradually increasing the load on the object. However, failure tests go a step further by continuously increasing the load until the emissions readings indicate imminent failure.
- Global screening: This method evaluates the conditions of all system components under increased stress levels. The test involves a gradual increase of thermal or pressure stress to levels slightly above normal and monitoring how the system functions under these conditions. Global screening aims to reveal any unknown flaws within the system that could worsen over time.
- Emission monitoring during normal operating conditions: By testing under normal conditions, you can keep an eye on the damage progression of the system without needing to artificially increase stress levels. This method can help you keep an eye on any previously discovered flaws.
Applications of Acoustic Emission Testing
Because acoustic emission testing can detect many different flaws, it’s versatile across many industries, including nuclear power, automotive, aerospace, infrastructure and manufacturing. Key applications of AET include the following:
- Machines: AET is useful for testing the function and condition of machinery across multiple industries. For example, it’s a valuable technique for facilities that use rotating machinery, as it can help to detect early wear in gearboxes and bearings. It’s also helpful in detecting partial discharges in electrical machinery components like transformers and bushings.
- Structures: AET is a useful way to evaluate structural integrity because it allows you to detect weaknesses before they become more significant problems. You can use it for composite, concrete and metallic structures.
- Processes: You can use AET to evaluate the efficiency and success of your processes as well. One common application is using this technique to assess build quality in additive manufacturing. Since AET is a non-destructive testing method, you can even conduct this test during regular operation. Other examples of using AET to evaluate operational processes include detecting leaks in pipes and other high-pressure components, evaluating particle impact and determining friction levels within systems.
Acoustic Emission Testing Advantages
AET is a practical testing choice for a variety of reasons:
- Flexibility: You can conduct AET in a lab or in-field, depending on the needs of your operation. You can even conduct AET testing remotely, which allows you to test in even hazardous environments like high temperatures, high pressure and even corrosive or nuclear environments.
- Reduced downtime: Acoustic emission testing is non-destructive and non-invasive, so you can conduct tests while the component is in normal operation. You can keep your operation running as usual even while testing is underway.
- Versatility: AET can detect many different types of damage, including cracking, corrosion, delamination, friction, fiber breakage and more, before they disrupt your operation.
- Readability: You can differentiate between the different sources of damage you detect based on their unique acoustic signatures. So if there are multiple flaws within your component, you’ll be able to tell how severe each one is.
- Reliability: AET allows you to detect damages that other NDT techniques, like dye penetration or leak testing, may not have the capacity to find.
Acoustic Emission Testing Limitations
The main limitation of AET is that it can only produce qualitative results. The test determines that flaws exist, but it cannot determine any quantitative characteristics of those flaws. You will need to conduct further testing to determine the full extent of any imperfections that may appear.
Similarly, AET can only detect active flaws. Even though stagnant defects may be present, they will not produce acoustic emissions, which means AET will not pick them up.
Getting clear results in loud environments is difficult because background noise can affect your readings. To ensure precise results, you need to filter out all excess noise using noise reduction technologies and signal discrimination techniques.
Lastly, performing AET requires operators to be experienced in using the technology. You’ll also need the right hardware and software which can be costly.
Meet Your Non-Destructive Testing Needs With Fujifilm
We at Fujifilm have decades of experience in non-destructive testing, so you can count on us to deliver innovative, effective imaging products for use in any industry. We offer a range of digital radiography devices in addition to conventional X-ray imaging products.
Additionally, if you’re looking to develop your workforce’s skill set, we provide comprehensive Digital Radiography NDT training courses to prepare your employees for certification. We offer scheduled regional classes for employees at multiple experience levels, but on-site training is also available for operations that wish to provide a more individualized, hands-on experience.