A b 2 m 80 in 2 m 80 in excavated length 1 m 40 in 1

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(a) (b) 2 m (80 in.) 2 m (80 in.) Excavated length 1 m (40 in.) 1 m (40 in.) Legend = acoustic emission transducer = shear crack = tensile crack 4 m (160 in.) Test tunnel 2 m (80 in.) 2 m (80 in.) 1 m (40 in.) 5 m (200 in.)
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sensitivity depend on test material size and properties. Typically, high frequency (150 kHz) resonant transducers are used for laboratory specimens, low frequency (60 kHz) resonant transducers are used for middle sized structures measuring 3 to 5 m (120 to 200 in.) and very low frequency (15 kHz) resonant transducers are effective for large structures measuring 10 to 15 m (37 to 50 ft). If the test object is a thin plate, such as a certain type of tensile test object, three-dimensional source location is impossible. In such a case, system analysis can be applied for a two-dimensional transducer placement with four transducers. 17 If an area with known discontinuities in a concrete plate or a vessel wall needs to be monitored and there is access only to the outside of the structure, all the transducers must be placed on the outside planes. Moment tensor analysis is possible in such a case. The analysis has been successful with a large concrete block with all six transducers on the top plane. 12 A special algorithm for a three-dimensional source location was necessary in that case. If a displacement transducer that measures surface displacement is available as an additional seventh transducer, it is possible to evaluate a volume of the formed crack and its time history by inverse analysis. Angles, sizes and formation velocities of fracture facets caused by hydrogen assisted cracking of a stainless steel overlay were quantitatively evaluated. 16 Because the cracks were produced inside the material and the location accuracy of the acoustic emission events (solvable waveform sets) is limited, system solutions cannot be easily verified. To estimate the solution errors and to screen out poor solutions, a postanalysis procedure was developed. 18 Synthesized acoustic emission waveforms are first computed from the crack kinematics determined from solutions obtained by experiments. Then, the procedure is applied to the synthesized waveforms. Finally, the solutions obtained from the experiments are compared with those of theoretical solutions determined from the synthesized waveforms. In this way, it is possible to screen out poor solutions and to verify the reliability of solutions determined from experiments. Conclusions Acoustic emission testing has been widely used to evaluate cracking processes in laboratory specimens and engineering structures because of its high sensitivity to crack initiation and growth. Moment tensor analysis is a very promising technique for fracture studies and monitoring. A quantitative comparison between prediction and actual cracks has been made to validate the technique. It has been shown that the analysis can be used in a wide range of applications for evaluation of structural integrity in the field for various types of structures.
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  • Fall '19
  • The Land, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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