How to Inspect a Structure Using Acoustic Emission Testing?

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Acoustic emission (AE) is defined as the physical phenomenon in which a material generates elastic waves when subjected to stress or deformation. These signals are registered using a non-destructive test method (NDT) called the acoustic emission test. An acoustic emission test is conducted to monitor a member's structural health to assess its retrofitting and repairing possibilities.

Acoustic emission testing is an NDT method with widespread application in structural integrity test, monitoring bridges, pipelines, wind turbines, and leak detection.

This article explores the principle and features of acoustics emission testing in construction.

Principle of Acoustic Emission Test

An acoustic emission test locates the yield point of the material by representing cracks, fractures, leakage points, or similar problems. The yield point is responsible for the generation of stress waves that propagate through the structure. The waves that reach the surface of the structure are used to stimulate a piezoelectric sensor, which converts these stress waves into an electrical signal for further analysis.

An AE test is sensitive to internal damages. These damages have parameters that correlate with the internal load.

An AE test on concrete helps to relate the mechanical behavior of concrete with the crack propagation within the structure. As shown in Figure-2 below, the propagation of cracks and the localization of micro-cracks are studied using the AE test. Highly localized stresses are developed at the crack tip, which releases stress waves. These waves are monitored by the piezoelectric sensors, where they are converted from mechanical signals to electrical signals.

The characteristics of damages and cracks in new and historical structures are studied based on the obtained AE parameters. The AE parameters are AE event, AE count, AE energy, AE arrival time, and AE source location.

Acoustic Emission Test Methods

In the simplest terms, acoustic emission in a structure can be defined as tiny earthquakes occurring within the material. These emissions are developed when the test material is subjected to external stimulus like loads or temperature.

This technique globally monitors the defects within large structures or machines with minimum disruption to the material. AE test method can be transient or continuous.

Transient Acoustic Emission Test

In this test, the acoustic emission exceeding the threshold is captured by the sensors. Here, features like signal energy, duration of emitted waves, and peak amplitude are determined. This method of AE testing is suited for studying the defects like cracks.

Continuous Acoustic Emission Test

In this method, the sensors capture the acoustic emission within a fixed time period, for example, 1/10th of a second. From the obtained data, the average signal level and the root-mean-squared (RMS) value is obtained. This method is suited for areas where the amplitude of emission is low, like testing gearboxes or leak detection.

Acoustic emission testing can be conducted in the laboratory as well as in field conditions.

Results of Acoustic Emission Results

An AE testing provides the following test information depending on the method and the recipient being tested:

1. Inspection of Structure

Proper location of the sensor during the AE test helps to inspect the entire structure even with a smaller number of sensors. This is best applied for large structures like heat exchangers where the inspection is difficult.

2. Determination of Significant Flaws

Acoustic emission tests can distinguish abnormalities developed and growing within the structure even under actual loading conditions.

3. Measurement of Structural Severity

AE tests measure the status of the structure and provide the details regarding the inspections that need to be conducted on the structure. The test is conducted without requiring any access inside the materials.

Advantages of Acoustic Emission (AE) Tests

The main advantages of acoustic emission test are:

1. Acoustic emission test detects damage mechanisms, friction, impacts, delamination, corrosion, fiber breakages, and cracking of materials and structures.
2. The test locates and differentiates the sources of damage.
3. It performs global monitoring of the structure.
4. The structures are assessed and studied during real operational conditions.
5. The test can be conducted remotely.
6. Acoustic emission test detects damages in structures that are difficult to detect by other conventional testing techniques.

Disadvantages of Acoustic Emission (AE) Tests

The disadvantages of acoustic emission tests are:

1. Acoustic emission test cannot detect the existing defects that do not move or grow with time.
2. The method is slower than other NDT test methods.
3. The test ends its objective by assessing structural integrity or machine health by detecting the issue. Further inspection must be performed by other methods.

Applications of Acoustic Emission Tests

Acoustic emission tests have the following applications:

1. To test the condition of concrete structures like bridges and buildings.
2. To test metallic structures like pipelines, aircraft structures, steel cables, storage tanks, etc.
3. To test composite structures like composite beams, aircraft structures, etc.
4. To determine early wear in bearings and gearboxes of rotating machinery.
5. To detect leaks in pipelines and pressure systems

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