Acoustic emission testing principles acoustic

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Acoustic Emission Testing Principles. Acoustic emissions are mechanical waves produced by sudden movement in stressed materials. The classic sources of acoustic emission are discontinuity related deformation processes such as crack growth and plastic deformation. Sudden movement at the source produces a stress wave that radiates out into the structure and excites a sensitive piezoelectric sensor. As the stress in the material is raised, emissions are generated. The signals from one or more sensors are amplified and measured to produce data for display and interpretation. 10 Electromagnetic Testing F IGURE 8. Classic setups for ultrasonic testing: (a) longitudinal wave technique; (b) shear wave technique. Transducer Crack Bolt Time Crack Back surface Crack Entry surface Crack Skip distance where a = b (a) (b) a b F IGURE 9. Leakage measurement dynamic leak testing using vacuum pumping: (a) pressurized system mode for leak testing of smaller components; (b) pressurized envelope mode for leak testing of larger volume systems. (a) (b) Leak detector Envelope Source of tracer gas Source of tracer gas Envelope Leak detector System under test System under test
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The source of acoustic emission energy is the elastic stress field in the material. Without stress there is no emission. Therefore, an acoustic emission test (Fig. 10) is usually carried out during a controlled loading of the structure. This can be a proof load before service; a controlled variation of load while the structure is in service; a fatigue, pressure or creep test; or a complex loading program. Often a structure is going to be loaded hydrostatically anyway during service and acoustic emission testing is used because it gives valuable additional information about the expected performance of the structure under load. Other times, acoustic emission testing is selected for reasons of economy or safety and a special loading procedure is arranged to meet the needs of the acoustic emission test. Applications. Acoustic emission is a natural phenomenon occurring in a wide range of materials, structures and processes. The largest scale events observed with acoustic emission testing are seismic and the smallest are small dislocations in stressed metals. The equipment used is highly sensitive to any kind of movement in its operating frequency (typically 20 to 1200 kHz). The equipment can detect not only crack growth and material deformation but also such processes as solidification, friction, impact, flow and phase transformations. Therefore acoustic emission testing is also used for in-process weld monitoring; detecting tool contact and tool wear during automatic machining; detecting wear and loss of lubrication in rotating equipment; detecting loose parts and loose particles; detecting and monitoring leaks, cavitation and flow; preservice proof testing; inservice weld monitoring; and leak testing.
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  • Fall '19
  • The Land, Nondestructive testing, electromagnetic testing

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