1,721,019 research outputs found
Multiscale seismic characterization and monitoring of a potentially unstable rock mass: the Madonna del Sasso (NW Italy) rockfall
Full text linkActive (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
Numerical and experimental analysis of nonlinear vibrational response due to pressure-dependent interface stiffness
Full text linkModelling interface interaction with wave propagation in a medium is a fundamental requirement for several types of application, such as structural diagnostic and quality control. In order to study the influence of a pressure-dependent interface stiffness on the nonlinear response of contact interfaces, two nonlinear contact laws are investigated. The study consists of a complementary numerical and experimental analysis of nonlinear vibrational responses due to the contact interface. The laws investigated here are based on an interface stiffness model, where the stiffness property is described as a nonlinear function of the nominal contact pressure. The results obtained by the proposed laws are compared with experimental results. The nonlinearity introduced by the interface is highlighted by analysing the second harmonic contribution and the vibrational time response. The analysis emphasizes the dependence of the system response, i.e., fundamental and second harmonic amplitudes and frequencies, on the contact parameters and in particular on contact stiffness. The study shows that the stiffness-pressure trend at lower pressures has a major effect on the nonlinear response of systems with contact interfaces
Interaction between contact behaviour and vibrational response for dry contact system
No full textThis work wants to provide insights on the coupling between contact behaviour (local scale) and vibrational response (global scale) which brings to different contact scenarios arising in dry frictional systems. A newer setup, named TriboWave, has been developed in order to reproduce and investigate the system response to frictional contact, under well-controlled boundary conditions. The experimental results highlighted how a simple frictional system can switch from stable friction-induced vibrations to unstable vibrations, i.e. either macroscopic stick–slip instabilities or mode coupling instabilities. The effect of the contact surface roughness on the reproduced frictional scenario has been investigated too
Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions
Full text linkModeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding
Active and passive seismic methods for characterization and monitoring of unstable rock masses: field surveys, laboratory tests and modeling
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Application of the Modal Absorption Index (MAI) to reduce the CEA instability over-prediction on a complex frictional system
No full textSteady state of modal coupling instabilities as a dynamic energy equilibrium
No full textContact interaction between different parts of mechanical system is at the origin of phenomena known in literature as friction induced vibrations. In this paper friction induced unstable vibrations are analyzed by an energy point of view. The analysis is here developed on a polycarbonate disc, which is constrained on the external circumference and in frictional contact with a steel cylinder rotating at its inner circumference. In a classical approach the stability of the system can be evaluated by a complex eigenvalue analysis, performed with the commercial finite element code ANSYS. Then, the steady state of the system can be evaluated by a transient non linear analysis, performed with the explicit finite element code PLAST2D. Power flows are analyzed during the transient response to observe the effect of the local friction coefficient and the boundary conditions on the different terms of the energy balance
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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