Perturbation damage indicators based on complex modes

The papers focusing on dynamic identification of structural damages usually rely on the comparison of two or more responses of the structure; the measure of damage is related to the differences of the vibration signals. Almost all literature methods assume damping proportionality to mass and stiffness; however, this is acceptable for new, undamaged structures, but not for existing, potentially damaged structures, especially when localised damages occur. It is well-known that in non-proportionally damped systems the modes are no longer the same of the undamped system: thus, some authors proposed to use modal complexity as a damage indicator. This contribution presents a perturbation approach that can easily reveal such a modal complexity

Perturbation methods for bifurcation analysis from multiple nonresonant complex eigenvalues

It is shown that the logical bases of the static perturbation method, which is currently used in static bifurcation analysis, can also be applied to dynamic bifurcations. A two-time version of the Lindstedt–Poincare ́ Method and the Multiple Scale Method are employed to analyze a bifurcation problem of codimension two. It is found that the Multiple Scale Method furnishes, in a straightforward way, amplitude modulation equations equal to normal form equations available in literature. With a remarkable computational improvement, the description of the central manifold is avoided. The Lindstedt–Poincare ́ Method can also be employed if only steady-state solutions have to be determined. An application is illustrated for a mechanical system subjected to aerodynamic excitation

Phonon-Metamorphosis in Ferromagnetic Manganite Films: Probing the Evolution of an Inhomogeneous State

The analysis of phonon anomalies provides valuable information about the cooperative dynamics of lattice, spin and charge degrees of freedom. Significant is the anomalous temperature dependence of the external modes observed in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) films. The two external modes merge close to the ferromagnetic to paramagnetic transition at $T_C$ and, moreover, two new modes evolve in this temperature range with strong resonances at slightly higher frequencies. We propose that this observed phonon metamorphosis probes the inhomogeneous Jahn-Teller distortion, manifest on the temperature scale $T_C$. The analysis is based on the first observation of all eight phonon modes in the metallic phase of LSMO and on susceptibility measurements which identify a Griffiths-like phase.Comment: 4 pages, 4 figure

Hydrogen absorption properties of amorphous (Ni0.6Nb0.4−yTay)100−xZrx membranes

Ni based amorphous materials have great potential as hydrogen purification membranes. In the present work the melt spun (Ni0.6Nb0.4−yTay)100−xZrx with y=0, 0.1 and x=20, 30 was studied. The result of X-ray diffraction spectra of the ribbons showed an amorphous nature of the alloys. Heating these ribbons below T < 400 °C, even in a hydrogen atmosphere (1−10 bar), the amorphous structure was retained. The crystallization process was characterized by differential thermal analysis and the activation energy of such process was obtained. The hydrogen absorption properties of the samples in their amorphous state were studied by the volumetric method, and the results showed that the addition of Ta did not significantly influence the absorption properties, a clear change of the hydrogen solubility was observed with the variation of the Zr content. The values of the hydrogenation enthalpy changed from ~37 kJ/mol for x=30 to ~9 kJ/mol for x=20. The analysis of the volumetric data provides the indications about the hydrogen occupation sites during hydrogenation, suggesting that at the beginning of the absorption process the deepest energy levels are occupied, while only shallower energy levels are available at higher hydrogen content, with the available interstitial sites forming a continuum of energy levels

Observation of the cluster spin-glass phase in La_{2-x}Sr_{x}CuO_{4} by anelastic spectroscopy

An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x = 0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin fluctuations in antiferromagnetic-correlated clusters is expected to occur. The acoustic absorption is attributed to changes of the sizes of the quasi-frozen clusters induced by the vibration stress through magnetoelastic coupling.Comment: LaTeX, 2 PostScript figures, submitted to Phys. Rev.

Optimal sensors placement in dynamic damage detection of beams using a statistical approach

Structural monitoring plays a central role in civil engineering; in particular, optimal sensor positioning is essential for correct monitoring both in terms of usable data and for optimizing the cost of the setup sensors. In this context, we focus our attention on the identification of the dynamic response of beam-like structures with uncertain damages. In particular, the non-localized damage is described using a Gaussian distributed random damage parameter. Furthermore, a procedure for selecting an optimal number of sensor placements has been presented based on the comparison among the probability of damage occurrence and the probability to detect the damage, where the former can be evaluated from the known distribution of the random parameter, whereas the latter is evaluated exploiting the closed-form asymptotic solution provided by a perturbation approach. The presented case study shows the capability and reliability of the proposed procedure for detecting the minimum number of sensors such that the monitoring accuracy (estimated by an error function measuring the differences among the two probabilities) is not greater than a control small value

Positive impact of mindfulness meditation on mental health of female teachers during the COVID-19 outbreak in Italy

The Covid-19 pandemic and subsequent public health measures were shown to impact negatively on people\u2019s mental health. In particular, women were reported to be at higher risk than men of developing symptoms of stress/anxiety/depression, and resilience was considered a key factor for positive mental health outcomes. In the present study, a sample of Italian female teachers (n = 66, age: 51.5 \ub1 7.9 years) was assessed with self-report instruments one month before and one month after the start of the Covid-19 lockdown: mindfulness skills, empathy, personality profiles, interoceptive awareness, psychological well-being, emotional distress and burnout levels were measured. Meanwhile, they received an 8-week Mindfulness-Oriented Meditation (MOM) course, through two group meetings and six individual video-lessons. Based on baseline personality profiles, analyses of variance were performed in a low-resilience (LR, n = 32) and a high-resilience (HR, n = 26) group. The LR and HR groups differed at baseline in most of the self-report measures. Pre\u2013post MOM significant improvements were found in both groups in anxiety, depression, affective empathy, emotional exhaustion, psychological well-being, interoceptive awareness, character traits and mindfulness levels. Improvements in depression and psychological well-being were higher in the LR vs. HR group. We conclude that mindfulness-based training can effectively mitigate the psychological negative consequences of the Covid-19 outbreak, helping in particular to restore well-being in the most vulnerable individuals