AE04.pdf

However two assumptions are made 1 all transducer to

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However, two assumptions are made: (1) all transducer-to-amplifier chains are adjusted to identical sensitivities; (2) ambient noise, either electronic or mechanical, is completely absent. Although it may be feasible to acceptably adjust sensitivities, it is rarely possible to eliminate noise. In cases where noise is present, it may be necessary to measure the noise at some point where the operator is certain that there is no leak signal. With the noise level determined, the true signal level can be determined from the following: (1) where n is the root mean square of noise, S m is the root mean square of the measured signal and S t is the root mean square of the true signal. In the case of discrete signals, the signal difference technique may be used as well. Especially in the case where transducer spacing is very large and the possibility of nearly simultaneous events is high, multiple signals from two or more transducers must be sorted out to determine which go together to define an event and a source location. The signal S S n m t + = 2 2 124 Acoustic Emission Testing F IGURE 5. Attenuation pattern for several leak sites with a 10.5 m (34 ft) transducer separation. This chart is bounded on the left and right by the positions of transducers 3 and 4, respectively (see Table 1). 21 23 25 27 29 31 (64) (70) (76) (82) (88) (94) Distance from transducer 1, m (ft) Signal measured by transducer 3 (dB) Signal measured by transducer 4 (dB) Transducer 3 Transducer 4 0 –4 –8 –12 –16 –20 –24 A 1 D 2 C 2 B 2 A 2 B 1 C 1 D 1 Leak site Legend A 1 , B 1 , C 1 , D 1 = points where signal curves from leak sites (stipulated as A , B , C and D ) intersect the vertical axis at position of transducer 3 A 2 , B 2 , C 2 , D 2 = points where signal curves from leak sites (stipulated as A , B , C and D ) intersect the vertical axis at position of transducer 4 0 –4 –8 –12 –16 –20 –24 F IGURE 6. Signal difference over distance for two transducers. 0 –4 –8 –12 –16 –20 –24 21 23 25 27 29 31 (64) (70 (76) (82) (88) (94) Position, m (ft) Signal difference (dB) Transducer 3 Transducer 4 Transducer 3 Transducer 4 Transducer 4 Transducer 3 1.9 + 1.9 = 3.8 dB·m –1 8.1 + 1.9 = 10 dB·m –1 T ABLE 1. Signals at transducers for test with constant 200 mV noise field. Signal ________________ Position True Measured _____________ Transducer m (ft) (mV) (mV) 1 0 (0) 41 200 2 10.5 (413) 61 209 3 21.0 (827) 1000 1020 4 31.5 (1240) 251 321 5 42.0 (1654) 15 201 6 52.5 (2067) 1 200 7 63.0 (2480) 0 200 8 73.5 (2894) 0 200 9 84.0 (3307) 0 200
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difference can be measured and compared with an attenuation model to determine if the right signals have been chosen for defining the event. Timing Technique When acoustic emission signals are transient (have a well defined beginning and end), it is much more apparent that timing techniques can form the basis of a useful source location methodology. If it is possible to identify a particular characteristic of the signal as it is detected by a transducer array, then timing techniques can be used.
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