AE technology uses NDE techniques for detecting, locating, and monitoring fatigue cracks in steel bridges. AE can be used on a wide variety of metal structures, including airframes, pipelines, and pressure vessels. AE is implemented by mounting one or more acoustic sensors on the surface of the material being tested to detect transient elastic waves produced by the evolution of damage in the material; for example, when a crack increases in length due to the application of load, acoustic waves are produced. These waves spread through the material and are detected by the acoustic sensors mounted on the surface. The waveform signals detected by the acoustic sensor(s) are analyzed to determine the rate at which AE signals produced from damage evolution are occurring. This analysis helps locate the damage and estimate the rate at which the damage is progressing.
AE signals are produced by the progression of damage such as the increase in length of a fatigue crack; therefore, loading must be applied to cause the damage to progress. For crack detection in highway bridges, a structure is monitored over time as traffic loads are applied to the bridge, which may result in crack growth. If a fatigue crack is not increasing in length, AE signals generally are not produced. Consequently, AE is not an effective method for detecting arrested cracks or other damage not progressing under load application.
The position of the AE source can be found by triangulating based on the arrival time of the acoustic waves and the wave velocity in the material. Additional sensors may be employed as ;guard; sensors to detect AE signals generated from sources outside an area of interest.
AE technology is commonly used to monitor crack growth in steel bridges or determine if a retrofit intended to arrest cracking has been successful. AE can also be used to detect a crack that is progressing; for example, AE may be used to monitor a structural detail that is susceptible to crack growth to detect the development of a new crack.