AE09.pdf

Environmental noise acquired during short term

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Environmental noise, acquired during short term monitoring, can be manually excluded by reviewing the observations made during waveform recording. For long term monitoring, manual filtering becomes difficult because of the quantity of raw data. Therefore, before transducers are installed on a slope, factors influencing the evaluation of long term acoustic emission monitoring should be studied. Environmental noise is the biggest concern for slope monitoring and is often caused by precipitation. Each form of precipitation can generate continuous emission but the behavior of each provides a unique acoustic signature to help identify it. In general, raindrops larger than 100 μm (0.004 in.) in diameter are called water droplets whereas smaller ones are called cloud particles because the dropping velocity becomes smaller than that of the upward current of air. A water droplet larger than 0.5 mm (0.02 in.) forms an ellipse because of the interaction between air friction and surface tension. A raindrop that becomes larger than 3.5 mm (0.14 in.) in diameter divides into two droplets — no raindrop is more than 3.5 mm (0.14 in.) in diameter. A typical raindrop has a diameter of 1 mm (0.04 in.) and a terminal velocity from 4 to 6 m·s –1 (9 to 13 mi·h –1 ); a larger raindrop has a diameter of 3 mm (0.12 in.) and a terminal velocity of 9 m·s –1 (20 mi·h –1 ). Experimental Procedure Figure 14 shows an experimental configuration used for the raindrop tests. Three prism specimens consisting of ordinary portland cement and aggregate were prepared, 150 mm (6 in.) deep × 150 mm (6 in.) wide × 500 mm (20 in.) tall. The ratio of sand to total aggregate was 60 percent and the maximum size G max of the aggregate was 15 mm (0.6 in.). The specimen was cured for one day in tap water and subsequently two specimens were cut into several pieces with lengths of 100, 200, 300 and 400 mm (4, 8, 12 and 16 in.). In this way, five lengths of specimen at 10, 20, 30, 40 and 50 mm (0.4, 0.8, 1.2 and 1.6 in.) were prepared for the test. The specimen was subjected to the impact of large water droplets from a syringe at a height of 2 m (80 in.) above the top surface of the test object. The diameter of one water droplet was standardized to 2.75 mm (0.11 in.), 315 Acoustic Emission Testing of Infrastructure P ART 3. Evaluation of Slope Stability by Acoustic Emission Testing
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measured by averaging the weight of ten water droplets. The test was performed after 1 day, 3 days and 15 days. Table 1 shows Young’s moduli and the compression strength corresponding to those ages. On the bottom surface of the specimen, two types of acoustic emission transducers (15 and 60 kHz resonance) were placed with a constant contact pressure applied by springs. Acoustic emission waves generated by water droplets impacting on the surface traveled to the bottom of specimen and were detected by both types of transducers. The acoustic emission signals detected were subsequently amplified at 40 dB by preamplifiers and were processed and acquired by the acoustic emission instrument.
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  • Fall '19
  • The Land, Nondestructive testing, Acoustic Emission

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