Time reversal focusing of elastic waves in plates for an educational demonstration

Willardson

et al. 2019

Self Cite

Time reversal (TR) focusing used for nonlinear detection of cracks relies on the ability of the TR process to provide spatially localized, high-amplitude excitation. The high amplitude improves the ability to detect nonlinear features that are a signature of the motion of closed cracks. It follows that a higher peak focal amplitude than what can be generated with the traditional TR process will improve the detection capability. Modifying the time-reversed impulse response to increase the amplitude of later arrivals in the impulse response, while maintaining the phase information of all arrivals, increases the overall focal signal amplitude. A variety of existing techniques for increasing amplitude are discussed, and decay compensation TR, a technique wherein amplitude is increased according to the inverse of the amplitude envelope of the impulse response decay, is identified as the best modification technique for nonlinear crack detection. This technique increases the focal signal amplitude with a minor introduction of harmonic content, a drawback in two other methods studied, one-bit TR and clipping TR. A final study employs both decay compensation TR and traditional TR, focusing on a rod with stress corrosion cracking, and compares the merits of each in detecting nonlinearity from cracks in a real system. V

“…Clipping TR, a fairly new technique, is very similar to one-bit TR apart from one key difference. 40 A threshold, T CP , is applied to the impulse response, just as with one-bit TR, and…”

Section: Tr Signal Processing Modification Techniquesmentioning

confidence: 99%

Section: Focal Signal Analysis Metricsmentioning

confidence: 99%

A comparison of impulse response modification techniques for time reversal with application to crack detection

Young

Willardson

et al. 2019

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“…It compares the peak focal amplitude, f P , with the total energy of the focal signal f ðtÞ. While different definitions of temporal quality have been used in the past, 18,21,27,28 this paper uses a modified version of the metric defined by Heaton et al 28 n…”

Section: B Temporal Focus Qualitymentioning

confidence: 99%

Time reversal acoustics applied to rooms of various reverberation times

Denison

2018

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Time Reversal (TR) is a technique that may be used to focus an acoustic signal at a particular point in space. While many variables contribute to the quality of TR focusing of sound in a particular room, the most important have been shown to be the number of sound sources, signal bandwidth, and absorption properties of the medium as noted by Ribay, de Rosny, and Fink [J. Acoust. Soc. Am. 117(5), 2866-2872 (2005)]. However, the effect of room size on TR focusing has not been explored. Using the image source method algorithm proposed by Allen and Berkley [J. Acoust. Soc. Am. 65(4), 943-950 (1979)], TR focusing was simulated in a variety of rooms with different absorption and volume properties. Experiments are also conducted in a couple rooms to verify the simulations. The peak focal amplitude, the temporal focus quality, and the spatial focus clarity are defined and calculated for each simulation. The results are used to determine the effects of absorption and room volume on TR. Less absorption increases the amplitude of the focusing and spatial clarity while decreasing temporal quality. Dissimilarly, larger volumes decrease focal amplitude and spatial clarity while increasing temporal quality. V

“…TR focusing has been shown to generate 30 times higher peak amplitude than direct excitation (though a factor of about 10 is typical), per channel, and therefore has sufficient amplitude to allow detection of local nonlinear properties. 3 Analysis of the TR focal signal reveals nonlinear variations in a number of forms including higher signal amplitude, waveform distortions, 4,5 or nonlinear harmonic content. 6,7 For a focus at a cracked location in a medium, all of these indicators and more typically exist.…”

Section: Introductionmentioning

confidence: 99%

Nonlinearity from stress corrosion cracking as a function of chloride exposure time using the time reversed elastic nonlinearity diagnostic

Young

Hogg

et al. 2019

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The Time Reversed Elastic Nonlinearity Diagnostic (TREND) has a long history of successful nondestructive detection of cracks in solids using nonlinear indicators. Recent research implemented TREND to find stress corrosion cracking (SCC) in the heat-affected zone adjacent to welds in stainless steel. SCC development around welds is likely to occur due to the temperature and chemical exposure of steel canisters housing spent nuclear fuel. The ideal SCC detection technique would quantify the size and extent of the SCC, rather than just locating it, as TREND has been used for in the past. The current paper explores TREND's ability to detect an assumed increase in SCC over time using 13 samples exposed to a magnesium chloride (MgCl2) bath for different lengths of time. The samples are then scanned with TREND and nonlinearity is quantified for each scan point and each sample. The results suggest that TREND can be used to not only locate SCC in the heat-affected zone, but also track an increase in nonlinearity, and thereby an increase in damage, in samples exposed to the MgCl2 solution for a longer duration.