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Time reversal (TR) is a method of focusing wave energy at a point in space. The optimization of a TR demonstration is described, which knocks over one selected LEGO minifigure among other minifigures by focusing the vibrations within an aluminum plate at the target minifigure. The aim is to achieve a high repeatability of the demonstration along with reduced costs to create a museum exhibit. By comparing the minifigure's motion to the plate's motion directly beneath its feet, it is determined that a major factor inhibiting the repeatability is that the smaller vibrations before the focal event cause the minifigure to bounce repeatedly and it ends up being in the air during the main vibrational focal event, which was intended to launch the minifigure. The deconvolution TR technique is determined to be optimal in providing the demonstration repeatability. The amplitude, frequency, and plate thickness are optimized in a laboratory setting. An eddy current sensor is then used to reduce the costs, and the impact on the repeatability is determined. A description is given of the implementation of the demonstration for a museum exhibit. This demonstration illustrates the power of the focusing acoustic waves, and the principles learned by optimizing this demonstration can be applied to other real-world applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Evidence of the ability to probe depth information of stress corrosion cracking (SCC) are presented using the time reversed elastic nonlinearity diagnostic (TREND). Depth estimation of SCC is important to determine when a stainless steel canister has been breached. TREND is a method to focus elastic energy to a point in space in order to probe that point for damage and its’ depth penetration is used here to study depth information about SCC. High frequencies are used to probe near the surface, while low frequencies are used to probe deeper into a stainless steel section of a cylinder. [ABSTRACT FROM AUTHOR]

In this letter, the equivalence between local and global measures of nonclassical nonlinear elasticity is established in a slender resonant bar. Nonlinear effects are first measured globally using nonlinear resonance ultrasound spectroscopy (NRUS), which monitors the relative shift of the resonance frequency as a function of the maximum dynamic strain in the sample. Subsequently, nonlinear effects are measured locally at various positions along the sample using dynamic acousto elasticity testing (DAET). After correcting analytically the DAET data for three-dimensional strain effects and integrating numerically these corrected data along the length of the sample, the NRUS global measures are retrieved almost exactly. [ABSTRACT FROM AUTHOR]

This letter presents a series of vibrational communication experiments, using time reversal, conducted on a set of cast iron pipes. Time reversal has been used to provide robust, private, and clean communications in many underwater acoustic applications. Here the use of time reversal to communicate along sections of pipes and through a wall is demonstrated to overcome the complications of dispersion and multiple scattering. These demonstrations utilize a single source transducer and a single sensor, a triaxial accelerometer, enabling multiple channels of simultaneous communication streams to a single location. [ABSTRACT FROM AUTHOR]

This Letter presents a series of time reversal experiments conducted on the surface of a fused silica glass block. Four different time reversal techniques are compared using three different imaging conditions. The techniques include two classical time reversal experiments: one with a pulse waveform source and one with an impulse response generated from a chirp signal. The other two techniques utilize the deconvolution, or inverse filter, signal processing methods for obtaining the signals to back propagate using a pulse waveform and an impulse response from a chirp. The max-in-time, symmetry, and energy current imaging conditions are compared. [ABSTRACT FROM AUTHOR]

This paper describes the principle behind a high amplitude non-contact acoustic source based on the principle of time reversal (TR), a process to focus energy at a point in space. By doing the TR in an air filled, hollow cavity and using a laser vibrometer in the calibration of the system, a non-contact source may be created. This source is proven to be more energetic than an off the shelf focused ultrasound transducer. A scaled up version of the proposed source has the potential to allow nondestructive evaluation processes that require high amplitude, such as nonlinear elastic wave spectroscopy (NEWS) techniques. [ABSTRACT FROM AUTHOR]

This study broadens vibration-like techniques developed for osseointegration monitoring to the nonlinear field. The time reversed elastic nonlinearity diagnostic is applied to two mock models. The first one consists of tightening a dental implant at different torques in a mock cortical bone; the second one allows one to follow glue curing at the interface between a dental implant and a mock jaw. Energy is focused near the implant interface using the time reversal technique. Two nonlinear procedures termed pulse inversion and the scaling subtraction method, already used successfully in other fields such as contrast agents and material characterization, are employed. These two procedures are compared for both models. The results suggest that nonlinear elasticity can provide new information regarding the interface, complementary to the linear wave velocity and attenuation. The curing experiment exhibits an overall low nonlinear level due to the fact that the glue significantly damps elastic nonlinearity at the interface. In contrast, the torque experiment shows strong nonlinearities at the focus time. Consequently, a parallel analysis of these models, both only partially reflecting a real case, enables one to envisage future in vivo experiments. [ABSTRACT FROM AUTHOR]

Reverse time migration (RTM) is a commonly employed imaging technique in seismic applications (e.g., to image reservoirs of oil). Its standard implementation cannot account for multiple scattering/ reverberation. For this reason it has not yet found application in nondestructive evaluation (NDE). This paper applies RTM imaging to NDE applications in bounded samples, where reverberation is always present. This paper presents a fully experimental implementation of RTM, whereas in seismic applications, only part of the procedure is done experimentally. A modified RTM imaging condition is able to localize scatterers and locations of disbonding. Experiments are conducted on aluminum samples with controlled scatterers. [ABSTRACT FROM AUTHOR]

Clusters of N thin parallel and identical shells (aligned or not! in water are considered. Assuming a harmonic plane wave is normally incident upon one cluster, the scattered-field classical expression is recalled, and then computed for different types of clusters, along with resonance spectra. The scattering S matrix is defined, and its unitarity property used to check the numerical results. All spectra are compared with that of a single shell, in the frequency range where resonances are due to an A-wave phase matching only. Whatever the cluster, each resonance of the single shell is seen to split into M different ones. The value of M depends on the number of shells, the distance between them, and the symmetries of the cluster. Apart from the very special case of aligned shells (M 52N), no simple law has been found to predict the value of M. [ABSTRACT FROM AUTHOR]