ASNT

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Time

Back surface

Bolt

Figure 10 Test object demonstrating magnetic particle method.

Transducer

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(a)

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to detect internal discontinuities in most engineer- ing metals and alloys. Bonds produced by welding, brazing, soldering, and adhesives can also be ultrasonically tested. Inline techniques have been developed for monitoring and classifying materials as acceptable, salvageable, or scrap and for process control. Also tested are piping and pressure vessels, nuclear systems, motor vehicles, machinery, railroad stock, and bridges. Acoustic Emission Testing Principles . Acoustic emissions are stress waves produced by sudden movement in stressed materials. The classic sources of acoustic emission are crack growth and plastic deformation. Sudden movement at the source produces a stress wave that radiates out into the test object 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. 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 (Figure 12) is usually carried out during a controlled loading of the test object. 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.

Entry surface

Crack

(b)

Figure 11 Classic setups for ultrasonic testing: (a) longitudinal wave technique; (b) transverse wave technique.

Applications . Acoustic emission is a natural phenomenon occurring in the widest range of materials, structures, and processes. The largest- scale events observed with acoustic emission testing are seismic; the smallest are microscopic 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, f low, and phase transformations. Therefore,

CHAPTER 1

14

Part 1