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Introduction
The Acoustic Emission Testing (AET) is a technique in which waves are applied for stress redistribution and for analyzing the structure of an object. This approach is one of the nondestructive testing techniques (NDT), meaning that an individual uses non-invasive techniques to determine the properties of the tested material. Though the process of inspection, one can determine if an object was damaged, which is the primary aim of NDT and AET in particular. The concept of AET is connected to the notion of stress waves. This refers to the structural integrity that a solid object has, which is tested by examing the stress waves.
Next, the concept of acoustic emissions is an external stimulus in the form of waves. Therefore, the main objective of AET is to determine if a solid object can remain integral under the impact of stress waves. Any observed object under stress of technical, thermal, or chemical nature produces noise or soundwaves. This noise is an emission connected to the energy release. Therefore, this technique aims to detect and examine sound produced in places where cracks exist within a solid material by analyzing acoustic emissions.
In nature, an object can be subjected to stress through earthquakes or other disasters, while during manufacturing procedures, the machinery can compromise structural integrity. These factors suggest that AET testing is crucial for creating a visual interpretation of a materials structure and examining its integrity. This can be done by analyzing the diagrams of acoustic emissions. One should note that the acoustic emissions are acoustic waves that affect a material as a result of irreversible changes. This report will outline the main principles of AET and procedures that one must know to apply this testing method in practice successfully.
Basic Principles and Theories
In order to perform AET in a practical setting, one must understand the underlying theoretical background and principles that guide the AET. It is assumed that a solid material subjected to the stress of any nature can disintegrate, meaning that cracks in it will open. Other common issues are crack growth, opening, closure, and movements that lead to further dislocation, all of which affect the properties of the examined material. This can lead to issues with further exploitation of an object and subsequent defects, making it necessary to use AET for the timely detection of structural changes.
Basic principles of AET that will be outlined below highlight the approach that AET takes, what can be measured through this method, and necessary equipment. In simple terms, acoustic emissions are the sounds that are derived from the places in a solid object, where deformation occurs. These can be cracks and any alterations of openings, as well as thermal stress, twining, and fiber breakage (Shen, Wu, & Zhang, 2015). As a result of deformation, energy is rapidly released, which results in the distribution of elastic sound waves. The main principle of AET is the need to detect these waves and locate the source of them.
Usually, the range of testing is 100 kHz and 1 MHz. A piezoelectric transducer is a sensor used to measure changes within an object and convert it to energy. This electroacoustic transducer is the main element of equipment necessary to perform AET. These alterations may include pressure, temperature, acceleration force, and strain. Preamplifiers and filters help refine the sound and proceed with further analysis. There are two main types of acoustic emissions continuous and burst. The first one produces a waveform, similar to the Gaussian random noise. However, the amplitude varies, depending on the activity of emissions. This type of acoustic emission (AE) indicates dislocation. The former type is characterized by high amplitude and strain energy, which decreases (Shen, Wu, & Zhang, 2015).
Due to the fact that the sound emission is connected to the energy release, the AET technique allows detecting these structural changes through sensors. As was mentioned, this can be used for any solid object, and in addition, AET is often applied to find leakages in systems. Notably, the Kaiser effect suggests that further damage within an object can occur only if the stress level is more significant to the one that occurred prior, which describes the behavior of metal structures. The main parameters that one should examine are burst, duration of the sound, threshold, and several others.
The application of AET in the context of NDT is varied, for instance, it can help examine pipelines or storage tanks, as well as the bearing of equipment.
Unlike other testing techniques that focus on sound waves, AET only examines objects under stress. Others can focus on conditions in which the object is affected by waves produced outside of it, for instance, in ultrasonic testing methods. Additionally, other NDT methods focus on internal structures, while AET only analyzes inside of an object. Other NDT methods use different approaches to sending and examing energy as part of their testing procedure, while AET only examines sound waves produced by the material itself. The International Organization for Standardization (ISO) is among one of the establishments, which outlined the standards for performing AET. Therefore, the basic principles and methods of AET highlight the need to examine acoustic waves to detect structural damage.
Procedures
This paragraph will focus on highlighting the core elements of AE procedures. As was previously mentioned, a sensor is placed on a surface, which helps track and detect wave emission. Piezoelectric transducers are used because of their convenient size, the convenience of the circuits, and the fact that they do not require an extern energy source, which makes them essential in AE testing ( What is a transducer, n.d.). Some limitations of this approach include the need to have a high impedance cable to establish the sensors connectivity with an interface. However, the primary benefit of performing this test is that it can be done even of an object is operating.
The following AE instruction outlined the steps one must take when conducting an AET:
- Clearly define the goals and objective of the AET as well as the material and the object that will be tested (Shen, Wu, & Zhang, 2015).
- Assess the environmental conditions to ensure that humidity and temperature will not interfere with a sensors work.
- Place a sensor on the surface of the tested object, ensure that it is connected to the high impedance cable.
- Set up preamplifier and connect it to the sensor to ensure that the signal from the latter is enhanced.
- Install hardware for collecting acoustic emissions, in the form of a microprocessor.
- Collecting analog signals.
- Conduct signal processing procedures.
- Carry out a frequency analysis.
- Visualize the data to examine the AE.
- Make conclusions about the structure of the examined solid object.
The AET can be applied in dynamic settings since the testing procedure does not interfere with outside processes. However, as can be seen from the instruction above, the environmental factors can impact the process of AET testing, making it necessary to ensure that the conditions for testing are adequate. The analog signal collected from the sensor in analog form and digitalized to receive a representation of each second. The AET 5500 system can be used to assess and examine the data. Other digital software that is designed to perform AET can be used as a substitute, to aid the analysis.
Several approaches to examining and evaluating the results of AET exist. One of them is acoustic counting that refers to the process of determining how many times the emissions exceeded the threshold over the course of the experiment. This is a simple technique that includes a visual inspection of the waveform. The ring down count or the RDC is the calculation of instances during which the amplitude exceeded the threshold.
Some limitations of AET procedures should be considered, as well. This technique allows one to qualitatively assess the damage or make predictions regarding the prospects of exploitation. Additionally, a stimulus is required for the AET procedure to take place. The signals detected through AE are usually weak, meaning that in noisy environments, AET is impossible to carry out. In addition, some flaws can be unnoticed because the AE produced by them are too weak and cannot be detected through AET ( Overview of acoustic emission testing (AET), n.d.). Overall, this report highlights the main applications of AET, its specifics, some limitations, and provides an outline of the procedure for AET.
References
Overview of acoustic emission testing (AET). (n.d.). Web.
Shen, G., Wu, Z., & Zhang, J. (Eds.). (2015). Advances in acoustic emission technology. Cham, Switzerland: Springer.
Whats a transducer? (n.d.). Web.
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