Laser ultrasonics in a multilayer structure: Semi-analytic model and simulated examples.

Laser-generated elastic waves have been the subject of numerous experimental, theoretical, and numerical studies to describe the opto-acoustic generation process, involving electromagnetic, thermal, and elastic fields and their couplings in matter. Among the numerical methods for solving this multiphysical problem, the semi-analytic approach is one of the most relevant for obtaining fast and accurate results, when analytic solutions exist. In this paper, a multilayer model is proposed to successively solve electromagnetic, thermal, and elastodynamic problems. The optical penetration of the laser line source, as well as thermal conduction and convection, are accounted for. Optical, thermal, and mechanical coupling conditions are considered between the upper and lower media of the multilayer. The simulation of laser-generated ultrasounds in multilayer structures is of interest for the development of nondestructive evaluation methods of complex structures, such as bonded assemblies in aeronautics [as discussed in Hodé et al., J. Acoust. Soc. Am. 150, 2076 (2021)]. The developed Python code is provided for free at https://doi.org/10.5281/zenodo.4301720.

Laser ultrasonics in a multilayer structure: Plane wave synthesis and inverse problem for nondestructive evaluation of adhesive bondings.

A laser ultrasonic method is proposed for the nondestructive evaluation of bonded assemblies based on the analysis of elastic plane waves reflected from the bonding interface. Plane waves are numerically synthesized from experimentally detected cylindrical waves. Several angles of incidence with respect to the bonding interface are achieved by varying the delay in the synthesis step. An inverse problem using these plane waves is then solved to identify the normal and transverse interfacial stiffnesses that model the mechanical coupling between two bonded media. The semi-analytic model developed and detailed in Hodé et al. [J. Acoust. Soc. Am. 150, 2065 (2021)] is used to create the database that contains simulated laser-generated ultrasounds required to solve the inverse problem. The developed method is first validated with semi-analytic simulated input data where Gaussian noise has been added. Next, the method is applied using signals acquired on an aluminum alloy plate and on assemblies (with and without adhesion defects) made of two aluminum alloy plates bonded by an aeronautical structural epoxy adhesive film. Differences between the identified values of interfacial stiffnesses distinguish the three samples and obtain quantitative values to characterize the adhesive bonding.

Dynamic analysis of cardiac R-R intervals in normal infants and in infants who subsequently succumbed to the sudden infant death syndrome.

Infants who subsequently succumb to the sudden infant death syndrome (SIDS) have higher heart rates and reduced heart rate variation compared with other infants. We examined dynamic changes in cardiac interbeat intervals to explore these differences in cardiac control. Recordings of electrocardiographic activity and respiratory movement were acquired from 13 SIDS victims before their deaths. Moment-to-moment changes in R-R intervals during quiet sleep, rapid eye movement sleep, and waking were compared with values of 13 matched control infants. For each sleep-waking state, every R-R interval was plotted against the previous interval (Poincaré plots), and each change in interbeat interval was plotted against the previous change. Dispersion of interbeat intervals at different heart rates was reduced in SIDS victims, resulting in Poincaré plots markedly different from those of controls. The dispersion, sampled at the 10th and 90th percentiles of heart rates, was reduced across all sleep-waking states in SIDS victims. At high heart rates, the difference between groups disappeared after correcting for basal rate; however, the reduced range at low heart rates was independent of basal rate. SIDS victims also showed smaller beat-to-beat changes in heart rate and fewer sustained runs of consistent heart rate changes during waking relative to controls. The differences in cardiac rate dynamics suggest altered autonomic control in infants who succumb to SIDS. We speculate that the autonomic disturbance may lead to cardiac instability or may indicate CNS alterations with the potential to affect other vital functions.

Heart rate dynamics after acute cocaine administration.

We examined heart rate (HR) patterns after a bolus intravenous (i.v.) administration of a high (10 mg/kg) dose of cocaine in unrestrained cats. Mean R-R intervals, SD, and other measures of variability were assessed in three periods: waking baseline, early postcocaine administration, and later recovery periods. Cocaine resulted in initial tachycardia and reduced HR variability. This reduction in variability was independent of changes in the average rate: during the recovery period, HR returned to baseline values, but the reduced variability persisted. Nonlinear methods of assessment yielded additional results: Cocaine introduces a high correlation between one beat and the next and a tendency for cardiac accelerations to be followed immediately by decelerations and vice versa. The overall effect of the drug is to restrict deviation from a fixed rate.

Dynamic characteristics of cardiac R-R intervals during sleep and waking states.

Traditional assessment of cardiac R-R intervals during sleep-wake states uses summary statistics such as mean and interquartile range. Such summary statistics are less able to provide information about instantaneous or dynamic aspects of beat-to-beat control. We examined state-dependent beat-to-beat patterning by plotting each cardiac R-R interval against the previous interval (Poincaré plots). This procedure provides an indication of the probability of occurrence of one interval from its predecessor and allows assessment of dynamic properties of R-R interval variation. Cardiac R-R intervals were determined over 5-12-minute periods of wakefulness (AW), quiet sleep (QS) and rapid eye movement sleep (REM) from six cats; each interval was plotted on the y-axis against the previous value on the x-axis resulting in an RRn + 1 vs. RRn plot. These plots provided evidence that the correlation between an interval and its successor was greater in REM than in QS, even though the overall range was greater in REM. The results suggest that short-term cardiac control during REM may be influenced by more closely regulated mechanisms than suggested by classic concepts.