Influence de la géométrie des premiers corps sur les instabilités de contact - cas du crissement

International audienceThe squeal is often studied because it is uncomfortable for the user and the environment of the vehicle although not harming the operation of the brake mechanism. A preceding study showed experimentally that, for a convenient value of the coefficient of friction and an adequate third body, the appearance of the squeal can be directly related to the geometry of the first bodies. The results presented here deal with the evolution of the tendencies (amplitude, frequency...) of the vibrations induced by friction according to the relative angle between the surfaces of the first bodies. This is a first stage to propose modifications of the first bodies to solve the problems of squeal

Multiscale seismic characterization and monitoring of a potentially unstable rock mass: the Madonna del Sasso (NW Italy) rockfall

Active (e.g. surface refraction and cross-hole tomography) and passive (monitoring of microseismic events) seismic methods can provide a proper characterization of the inner structure of the rock mass and are key to the comprehension of the mechanisms enhancing the instability of rock masses.We propose a multiscale approach for the characterization of the potentially unstable granitic cliff of Madonna del Sasso (NW Italian Alps) integrating prospecting surveys, laboratory tests, long-term microseismic monitoring and numerical modeling. The complex 3-D fracture setting, the geometry of the unstable sector was achieved through field observations, photogrammetric geomechanical analysis and interpretation of on-site seismic surveys, which revealed to be fundamental for constraining the fracture geometry and opening at depth within the rock mass. Physical and mechanical properties of the investigated medium were obtained through laboratory tests on granite samples. Continuous monitoring of ambient vibration at the site (October 2013 - present) did not highlight irreversible changes in the rock mass properties precursory to an acceleration to failure. However, a strong thermal control was found to govern the stability of the cliff, with reversible seasonal opening and closing of fractures resulting from thermal contraction and expansion. Moreover, the vibration modes of the unstable sector were found to be strongly controlled by the complex 3-D geometry of the main fracture planes affecting the site. Detection and location of microseismic events within the prone-to-fall rock mass highlighted the concentration of low energy releases close to the major fracture planes. Microseismic monitoring at the laboratory scale of deformation and rupture processes is expected to further highlight the relationships between energy release, seismic signatures and seismic sources. Finally, finite element modeling on the 3-D geometry allowed an experimental validation and interpretation

Seismic noise parameters as indicators of reversible modifications in slope stability: a review

Continuous ambient seismic monitoring of potentially unstable sites is increasingly attracting the attention of researchers for precursor recognition and early warning purposes. Twelve cases of long-term continuous noise monitoring have been reported in the literature between 2012 and 2020. Only in a few cases rupture was achieved and irreversible drops in resonance frequency values or shear wave velocity extracted from noise recordings were documented. On the other hand, all monitored sites showed clear reversible fluctuations of the seismic parameters on a daily and seasonal scale due to changes in external weather conditions (air temperature and precipitation). A quantitative comparison of these reversible modifications is used to gain insight into the mechanisms driving the site seismic response. Six possible mechanisms were identified, including three temperature-driven mechanisms (temperature control on fracture opening/closing, superficial stress conditions and bulk rigidity), one precipitation-driven mechanism (water infiltration effect) and two mechanisms sensitive to both temperature and precipitation (ice formation and clay behavior). The reversible variations in seismic parameters under the meteorological constraints are synthesized and compared to the irreversible changes observed prior to failure in different geological conditions

Neurophysiological oscillatory markers of hypoalgesia in conditioned pain modulation

Introduction:Conditioned pain modulation (CPM) is an experimental procedure that consists of an ongoing noxious stimulus attenuating the pain perception caused by another noxious stimulus. A combination of the CPM paradigm with concurrent electrophysiological recordings can establish whether an association exists between experimentally modified pain perception and modulations of neural oscillations.Objectives:We aimed to characterize how CPM modifies pain perception and underlying neural oscillations. We also interrogated whether these perceptual and/or neurophysiological effects are distinct in patients affected by chronic pain.Methods:We presented noxious electrical stimuli to the right ankle before, during, and after CPM induced by an ice pack placed on the left forearm. Seventeen patients with chronic pain and 17 control participants rated the electrical pain in each experimental condition. We used magnetoencephalography to examine the anatomy-specific effects of CPM on the neural oscillatory responses to the electrical pain.Results:Regardless of the participant groups, CPM induced a reduction in subjective pain ratings and neural responses (beta-band [15-35 Hz] oscillations in the sensorimotor cortex) to electrical pain.Conclusion:Our findings of pain-induced beta-band activity may be associated with top-down modulations of pain, as reported in other perceptual modalities. Therefore, the reduced beta-band responses during CPM may indicate changes in top-down pain modulations.</p

Hearing faces: how the infant brain matches the face it sees with the speech it hears

Speech is not a purely auditory signal. From around 2 months of age, infants are able to correctly match the vowel they hear with the appropriate articulating face. However, there is no behavioral evidence of integrated audiovisual perception until 4 months of age, at the earliest, when an illusory percept can be created by the fusion of the auditory stimulus and of the facial cues (McGurk effect). To understand how infants initially match the articulatory movements they see with the sounds they hear, we recorded high-density ERPs in response to auditory vowels that followed a congruent or incongruent silently articulating face in 10-week-old infants. In a first experiment, we determined that auditory–visual integration occurs during the early stages of perception as in adults. The mismatch response was similar in timing and in topography whether the preceding vowels were presented visually or aurally. In the second experiment, we studied audiovisual integration in the linguistic (vowel perception) and nonlinguistic (gender perception) domain. We observed a mismatch response for both types of change at similar latencies. Their topographies were significantly different demonstrating that cross-modal integration of these features is computed in parallel by two different networks. Indeed, brain source modeling revealed that phoneme and gender computations were lateralized toward the left and toward the right hemisphere, respectively, suggesting that each hemisphere possesses an early processing bias. We also observed repetition suppression in temporal regions and repetition enhancement in frontal regions. These results underscore how complex and structured is the human cortical organization which sustains communication from the first weeks of life on

Heterogeneous Cortical Effects of Spinal Cord Stimulation

Objectives: The understanding of the cortical effects of spinal cord stimulation (SCS) remains limited. Multiple studies have investigated the effects of SCS in resting-state electroencephalography. However, owing to the large variation in reported outcomes, we aimed to describe the differential cortical responses between two types of SCS and between responders and nonresponders using magnetoencephalography (MEG). Materials and Methods: We conducted 5-minute resting-state MEG recordings in 25 patients with chronic pain with active SCS in three sessions, each after a one-week exposure to tonic, burst, or sham SCS. We extracted six spectral features from the measured neurophysiological signals: the alpha peak frequency; alpha power ratio (power 7–9 Hz/power 9–11 Hz); and average power in the theta (4–7.5 Hz), alpha (8–12.5 Hz), beta (13–30 Hz), and low-gamma (30.5–60 Hz) frequency bands. We compared these features (using nonparametric permutation t-tests) for MEG sensor and cortical map effects across stimulation paradigms, between participants who reported low (&lt; 5, responders) vs high (≥ 5, nonresponders) pain scores, and in three representative participants. Results: We found statistically significant (p &lt; 0.05, false discovery rate corrected) increased MEG sensor signal power below 3 Hz in response to burst SCS compared with tonic and sham SCS. We did not find statistically significant differences (all p &gt; 0.05) between the power spectra of responders and nonresponders. Our data did not show statistically significant differences in the spectral features of interest among the three stimulation paradigms or between responders and nonresponders. These results were confirmed by the MEG cortical maps. However, we did identify certain trends in the MEG source maps for all comparisons and several features, with substantial variation across participants. Conclusions: The considerable variation in cortical responses to the various SCS treatment options necessitates studies with sample sizes larger than commonly reported in the field and more personalized treatment plans. Studies with a finer stratification between responders and nonresponders are required to advance the knowledge on SCS treatment effects.</p

Caratterizzazione geofisica e monitoraggio microsismico di un ammasso roccioso instabile

Nella presente nota vengono illustrati i risultati preliminari della caratterizzazione geofisica e del monitoraggio microsismico dell'ammasso roccioso instabile di Madonna del Sasso (Verbania). I dati raccolti hanno permesso di meglio comprendere le ragioni dell'instabilità in atto, distinguendo chiaramente le frequenze fondamentali di vibrazione dell'ammasso instabile e le sue direzioni di oscillazione. Sono state inoltre stabilite utili correlazioni tra gli stessi parametri ed i fattori ambientali che influenzano l'ammasso ed un utile confronto in back analysis dei dati di monitoraggio geotecnico raccolti in passato sul medesimo sito