AE06.pdf

Structure having the leak undamped ultrasonic

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structure having the leak, undamped ultrasonic piezoceramic transducers having a strong radial response are particularly sensitive to leakage noise. In addition, instruments for most acoustic emission test systems have bandpass filters, which tend to have a slight peaking effect at the edges of the bandpass frequency range when driven with random frequency (noise) signals. If the bandpass is set for the radial resonance of the sensor, a very high sensitivity can be obtained for leakage signals. The contact sensor is frequently much more sensitive to leakage noise than are air coupled sensors. Therefore, leaks that may be classified as passive because the air coupled sensor lacks the sensitivity required for their detection may prove to be violently active with a contact sensor. Coupling Ultrasound Sensors to Structures during Leak Testing With acoustic emission systems, contact sensors coupled indirectly to structures with ultrasonic waveguides will not have the sensitivity of sensors ultrasonically coupled directly to the subject structure. Ultrasonic coupling means that the sensor face is coated with an oil, short fiber grease, resinous material or an adhesive and pressed into intimate contact or affixed to the structure for the purpose of eliminating an air interface between the sensor face and subject structure. However, some coupling fluids may react with the test material, causing corrosion or erosion acoustic emission (noise) that may remotely resemble intermittent leakage noise. The fluid causing the corrosion or erosion noise may be either the couplant or the fluid contained by the structure. Such noise is usually detectable by only the most sensitive vibration sensing instrumentation. Airborne Acoustic Signals for Leak Testing When structures cannot be monitored by direct coupling of sensors to their surfaces, air coupled or water coupled microphones can be used to detect ultrasonic emissions generated by leakage. Air coupled sensors are convenient but are most usable on active, external leaks. The sensitivity and directivity of remote microphonic detectors can be enhanced by addition of parabolic reflectors. Leaks at high velocity generate broad bandwidth amplitudes that center at frequencies of about 40 kHz (40 000 vibration cycles per second). Structures immersed in (or filled with) liquids are also observed to generate about 40 kHz peak signal amplitudes at the onset of high velocity, low volume leaks. Ultrasound leak detectors are often designed to respond to this 30 to 50 kHz signal frequency range and signals at other random frequencies are suppressed. This reduces interference from machinery or other ambient noise sources. Artificial Sound Sources for Leak Testing in Large Containers Large containers can be leak tested without internal pressurization by placing an ultrasound generator in the fluid inside the container. An ultrasound detector is then moved about the outside of the container until a sudden increase in the ultrasonic signal amplitude is observed.
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  • Fall '19
  • Acoustic Emission

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