Acoustic Emission is referred to the radiation of acoustic waves in solids as a result of damage or any irreversible change in its internal structure. Acoustic emission monitoring is well-known for its application in structural health monitoring of civil infrastructure.
Structural Health Monitoring
Health monitoring is a process of implementing inspection, evaluation, reaction, re-inspection and re-evaluation in order to maintain the functionality of a system. Structural Health Monitoring (SHM) is widely used for assessment and reliability evaluation of major civil infrastructure.
Structural health monitoring normally involves the observation of the structural system over time using periodical inspection, evaluation and reaction in order to reach to target service life. To do so, a maintenance plan and budget is adopted from beginning. All this process is performed over time in order to ensure integrity, safety and reliability of system over the life cycle.
How Does SHM work?
The term of SHM evokes an array of sensors installed in structure for real time collection of data. The collected data is transmitted to a host computer with specified embedded software for further analysis and extracting the present state of structure.
Various sensors and transducers can be used for the purpose of real time data collection. This includes strain gages, displacement sensors, thermocouples, humidity meters, acoustic sensors, etc. All these data are collected in real time with no interruption in routine service of structure.
Analysis is normally done by comparing two system states. SHM can be used to locate the damage; it can also be used to evaluate the extent of the damage.
Acoustic Emission Monitoring
Acoustic Emission Monitoring is a famous form of SHM, and involves the use of the NDT methods for real-time condition assessment of structures. This method includes an array of acoustic sensors installed in the locations that are prone to damage. Unlike traditional NDT methods, The AE concept relies on the acoustic waves generated by incidents (such as cracking, plastic deformation) in the test area. This permanent changes can be because of overloading, environmental loadings such as temperature gradient, etc. These incidents generate stress waves when accumulated elastic energy in material is released rapidly. This stress wave is captured with AE transducers with different range of frequencies. Depending on AE test area, transducers with a specified range of frequency are used to detect the stress wave released from an event.
How Does Acoustic Emission Work?
Different parameters such as energy of received signal, amplitude of major pick, threshold level and number of recorded incident are used to analyze an event in a test area. Depending on the energy of recorded signal by each transducer in an specified array, it is possible to determine the location of incident in test area.
Applications of Acoustic Emission Monitoring
The AE methods is used in different structures for the purpose of health monitoring. The following represents a number of AE application for real time monitoring of structure:
+ Structural members under high stress such as post-tensioning and pre-stressing cables
+ Cable bridges
+ Pipe lines and high pressure vessels
+ Structures under risk of fatigue such as wind turbine, generator foundations, etc.
+ Components of airplanes such as wings, engine connections, etc.
+ Tunnels and mines under risk of slope movement
+ Dams under risk of landslides
The AE method can be used to monitor existing damage and cracks in order to follow up changes over time due to service load and potential overload. Landslide or slope movement in dam structures is of examples of such application.