engineer working on structure

Acoustic Emission in Engineering

Building materials have big roles to fill; they not only have to meet budget demands, but they need to perform well and sometimes look good while doing so. 

What is Acoustic Emission?

In civil engineering, it’s important to understand the phenomena of acoustic emission. These are the elastic waves experienced by solid materials during mechanical loading. As the term “acoustic” implies, these stress waves are ultrasound and are measured in kilohertz (kHz) and hertz (Hz). 

A structure, whether a building or a piece of machinery, needs to be designed with acoustic emissions in mind so proper materials are used in the first place. If it’s determined that a structure has experienced elastic waves and deformation, there is a process of evaluating the results.

Acoustic Emission Testing (AET)

To check for acoustic emissions, there’s a nondestructive test (NDT) that uses sensors and electrical signals to detect and record any AE waves. 

Methods Used for Nondestructive Tests

  • Global screening
  • Failure testing
  • Fatigue testing
  • Monitor during normal operation
  • Proof testing

Determine the Source

Acoustic emissions are caused by movement. In nature, it’s the sound of a tree falling and hitting the ground. In a building, acoustic emissions can be generated by a structure settling. Whatever the source, the emissions can leave behind a mark in the form of fissures or small cracks. Sometimes the results are so minute they cannot be easily identified. That’s when acoustic emission testing (AET) needs to be performed. 

Causes of Acoustic Emission

  • Debonding
  • Cracking when a material cools
  • Crystalline distortion
  • Fiber breakage
  • Matric cracking
  • Thermal stress

The results yielded by the AE testing help triangulate the source of the event and locate any flaws in the material.

Evlauate Material Performance

From a mechanical standpoint, an engineer needs to determine how the materials withstood the impact of the elastic waves. What type of impact did the stress of the event have on a concrete foundation or pillars, for example? What defects, if any, are now apparent in the material? 

Materials Affected by Acoustic Emission

  • Concrete
  • Metal
  • Plastic
  • Polymer
  • Wood

The type of event, and the type of material, will determine the outcome and the steps for remediation.

Long-term Monitoring

After an acoustic emission has been identified, and the building’s material health evaluated, there should be long-term monitoring of the structure. Can the bridge, for example, continue to be used regularly after undergoing stress? Or, is it time to administer repairs to strengthen the structure and help it withstand future friction?

Why Acoustic Emission Testing is Important

As with any problem, early detection is often key to preventing it from getting bigger. Running an AET can help locate micro-defects in building materials so they can be addressed before the damage gets more serious.

Benefits of AET

  • An AET can be conducted for an entire system, while everything is still in operation. 
  • AETs detect problems that are actively growing, not latent ones.
  • AETs are noninvasive, so structures don’t have to be penetrated to be tested.
  • Local, state, and federal regulations often accept AET results to ensure compliance.
  • Results are immediate.

Is AET Right for You?

Not every structure is going to benefit from AET, or AET alone. AETs are commonly conducted on bridges and heavy-duty machinery. It’s also an important test for structures made of composite materials. Acoustic emission tests are often carried out in power plants and heating facilities (like THIS ONE Anderson Engineering surveyed for in Utah), Here are some of the limitations of AET:

AET Provides Qualitative Results

AET does not provide quantitative results. So, you will only know that the problems exist, and not necessarily their extent. For specifics such as the depth of a crack, or its length, more specialized testing is required.

AET Doesn’t Find Latent Flaws

If there’s a pre-existing crack in your structure, an AET probably won’t detect it. There need to be acoustic emissions ranging between 20 kHz and 1 Hz for the AET to register it.

AET Can be Inaccurate in Loud Environments

If the test is going to be administered while systems are still running, it can sometimes impact the results. However, the AET can be modified with signal discrimination and noise reduction practices.
Acoustic emission tests should only be carried out and evaluated by a professional. Get in touch with Anderson Engineering if you have questions about your construction engineering needs.


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