When welding alloy and high carbon steels a

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signals in the presence of scale noise. When welding alloy and high carbon steels, a transformation to martensite is possible. Martensitic transformation is a true and energetic source of acoustic emission and is caused by rapid cooling. Figure 3 is a plot of acoustic emission from spot welding of high carbon steel. The square wave in the upper left portion of the illustration indicates weld power duration. The associated acoustic emission is from nugget formation. When the weld interval ended, the nugget cooled rapidly, causing (1) martensitic transformation and (2) the large envelope of acoustic emission during the immediate postweld interval. 16 Miscellaneous Noise Metals such as aluminum give off considerable acoustic emission when in contact with acids during etching or pickling. Some solution annealed aluminum alloys will emit acoustic emission when heated to 190 °C (375 °F) because of precipitation hardening (see Fig. 4). 43 Fundamentals of Acoustic Emission Testing F IGURE 2. Acoustic emission signals ( 60 dB) from fracturing of mill scale. Acoustic emission (10 3 counts per second) 7 6 5 4 3 2 1 Time (60 s intervals) 0 4 8 12 16 20 F IGURE 3. Postweld acoustic emission signals from spot weld martensitic transformation. Postweld acoustic emission Weld acoustic emission F IGURE 4. Acoustic emission signals from precipitation hardening of aluminum heated to 190 °C (375 °F). Acoustic emission (10 3 counts per second) 10 9 8 7 6 5 4 3 2 1 0 Time (80 s intervals) 1 2 3 4 5 6 7 8
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Noise from Solid State Bonding When clean metallic surfaces are brought into intimate contact, some degree of solid state bonding is possible. This effect increases with temperature and pressure. Such incidental bonding usually does not have much strength and may rupture at the first change of stress, creating a new free surface and a stress wave that is true but undesired acoustic emission. Fretting is an example of this phenomenon. An underwelded spot weld called a sticker causes a similar condition. Even a properly welded spot may be surrounded by solid state bonding and will result in acoustic emission when the structure is first loaded. Such acoustic emission is termed secondary emission 17 and may be an indicator of discontinuity growth. Electromagnetic Interference Electromagnetic interference consists of noise signals coupled to the acoustic emission instrumentation by electrical conduction or radiation. Sources of electromagnetic interference include fluorescent lamps, electric motor circuits, welding machines and turning electrical power on and off by relays with inductive loads. Large spikes of electrical noise are thus created and may contain the highest amplitude and frequencies of any signals detected. The extent to which electromagnetic interference is encountered is a function of the environment, shielding and the transducer design. Hostile environments often require better shielding and differential transducers. Optimum standard shielding can be provided by enclosing the transducers, preamplifiers and main amplifiers in high conductivity
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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