Acoustic burst

Kramer and co-workers (7) reported that acoustic emission occurred in polystyrene immersed in diflFerent swelling liquids only when the crazes ruptured but not during their formation and growth. As long as the bridging by filamental elements is still intact, the deformation at the craze tip and in the craze is still so slow that no acoustic bursts are generated. It is the final fracture of these elements which is abrupt enough to cause the emission of a detectable acoustic burst. 

The time dependence of local displacement c and displacement velocity dc/dt at the craze or crack initiation is shown schematically in Figure 1. The velocity drops to one half of its maximum value at ti/2 If one puts t = 0 at the maximum and assumes a symmetric time dependence of displacement velocity, the half width of velocity distribution at such an elementary act is 2ti/2. The Fourier transform (FT) of velocity yields the spectral distribution of the emitted acoustic burst. The intensity is a maximum at zero frequency and drops to half this value at (01/2 = 2mv /2> In the first approximation the product of the half width of velocity and frequency distribution of acoustic emission, 2ti/2 X 2vi/2, equals 0.8825. To have a substantial amount of energy available in the frequency range of 1 MHz ( —1 1/2) the displacement velocity curve vs. time must have a half width of 0.5 /xsec, i.e., the major part of the local displacement must occur within 0.5 /i.sec. The square of the maximum value of velocity times... 

The evolution of the defects is only acceptable if the materials are sufficiently ductile. Prior study of the acoustic emission of the materials used should show that detection of defects and their evolution take place at pressures with a sufficiently wide margin relative to the burst pressure. 

Afterwards, the vessels were pressurized according a proof test and a burst test with acoustic emission measurements. Acoustic emission data recorded during these tests are analysed in order to determine assessment criteria. The criteria are mainly based on ... 

In order to confirm the capability of acoustic emission to detect the evolution of a defect during a pneumatic test, a vessel with a lack of thickness due to the action of corrosion was pressurized pneumatically until burst. 

The high level of amplitude (> 60 dB) of these AE events indicated the presence of a evolving defect. After this proof test, the vessel was pressurized until burst. All along this test the activity of acoustic emission was very important, and divided in two phasis. 

For special applications such as air coupled testing a special programmable transmitter board was developed. This transmitter generates rectangular and burst signals, which increase the acoustical power in an optimized frequency range, and provides a superior signal-to-noise ratio. 

The solid lines in Figure 4.5 represent extrapolations of experimental data to full-scale vessel bursts on the basis of dimensional arguments. Attendant overpressures were computed by the similarity solution for the gas dynamics generated by steady flames according to Kuhl et al. (1973). Overpressure effects in the environment were determined assuming acoustic decay. The dimensional arguments used to scale up the turbulent flame speed, based on an expression by Damkohler (1940), are, however, questionable. 

Acoustic cavitation (AC), formation of pulsating cavities in a fluid, occurs when a powerful ultrasound is applied to a non-viscous fluid. The cavities are formed when the variable acoustic pressure in the rarefaction phase exceeds the cohesive strength of the fluid. Under acoustic treatment (AT), cavities grow to resonance dimensions conditioned by frequency, amplitude of oscillations, stiffness properties and external conditions, and start to pulsate synchronously (self-consistently) with acoustic pressure in the medium. The cavities undergo significant strains (compared to their dimensions) and their size decreases under compression up to collapsing. This nonlinear behavior determines the active, destructional character of the cavities near which significant shear velocities, local pressure and temperature bursts occur in the fluid. Cavitation determines the specific character of acoustic treatment of the fluid and effects upon objects resident in the fluid, as well as all consequences of these effects. 

Godsill and Rayner, 1992] Godsill, S. J. and Rayner, P. J. W. (1992). A Bayesian approach to the detection and correction of bursts of errors in audio signals. In Proc. IEEE Int. Conf. Acoust., Speech and Signal Proc, volume 2, pages 261-264, San Francisco, CA. 

Dispersion Technology DT100 acoustic spectrometer. This instrument is a modern example of commerciai equipment which, in its basic design, foiiows the transmission tone-burst variabie gap technique pioneered by Andreae [101-103]. Figure 9.16A shows a photograph of a typicai unit. 

Fig. 3.21. Cumulative distribution of acoustic emission bursts with respect to their amplitudes for three different samples. The inset shows a typical experimental record of the acoustic emission signal time series, with the propagation of fracture. The five stronger bursts correspond to increases in the stress (from Petri et al 1994).

A major effort has been put recently in the development of simulations of explosions that go beyond the one-dimensional approximation. This is motivated not only by the difficulty of obtaining successful CCSNe in onedimensional simulations, but also by the mounting observational evidence that SN explosions deviate from spherical symmetry, not to mention the possible connection between the so-called soft long-duration gamma-ray bursts, and grossly asymmetric explosions accompanied with narrow jets of relativistic particles, referred to as JetSNe. The multi-dimensional extension of the simulations opens the potentiality to treat in a proper way different effects that may turn out to be essential in the CCSN or JetSNe process. As briefly reviewed by e.g. [26], they include fluid instabilities, or rotation and magnetic fields on top of the neutrino transport already built into the one-dimensional models. Acoustic power may be another potential trigger of CCSNe [27] (see also [24] for a brief review of multidimensional simulations). 

The sound transmission through the coal/toluene/benzene slurry was examined in the frequency range of 0.1-1 MHz. Measurements were made by pulsing an AE transducer with a sine tone burst. The tone burst travels through the medium and is received by another AE transducer directly across the pipe. Acoustic emission transducers were chosen for this study because of their flat... 

The basic design of an ultrasonic cross-correlation flowmeter (Sheen and Raptis, 1983), as shown by Fig. 5-8, consists of two pairs of transducers positioned in parallel and separated a known distance. The transducers can be mounted by either a clamp-on arrangement or use of special windows that provide a better impedance match to the flow so that more acoustic energy can be transmitted. Pitch-catch is the typical mode of operation. The transducers on one side generate ultrasonic waves in the form of sine tone bursts and the... 

The exposed tubes exhibited a 20% reduction in burst strength, compared to dry tubes. The acoustic emission response suggested that damage started in the wet tubes at a much lower pressure. Stress rupture tests, that is, pressurize and hold, were conducted at three pressures (four were claimed in the text, but only three levels were stated) with the highest pressure being close to the rupture pressure of the wet tubes. There was no indication of moisture-induced degradation, even for the worst-case scenario , that is simultaneous application of a constant stress and immersion... 

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