Numerical analysis of squeal instability

International audienceThe name “brake squeal” groups a large set of high frequency sound emissions from brake systems, generated during the braking phase and characterized by a periodic or harmonic spectrum. This paper presents two different numerical approaches to identify the mechanism bringing to the dynamic instability, producing squeal noise. The first approach performs a finite element modal analysis on the brake system, to identify the eigenfrequencies of the components and to relate them to the squeal frequencies. The second one uses a specific finite element program, Plast3, appropriate for non-linear dynamic analysis in the time domain and particularly addressed to contact problems with friction between deformable bodies. The use of this program allows to verify the behavior of the components and to link results obtained from the time analysis and the modal one. The study presented is carried on a simple model, composed by a disc, a small pad and a beam supporting the pad, to simplify the behavior of its components

The role of biomechanics in the assessment of carotid atherosclerosis severity: a numerical approach

Numerical fluid biomechanics has been proved to be an efficient tool for understanding vascular diseases including atherosclerosis. There are many evidences that atherosclerosis plaque formation and rupture are associated with blood flow behavior. In fact, zones of low wall shear stress are vivid areas of proliferation of atherosclerosis, and in particular, in the carotid artery. In this paper a model is presented for investigating how the presence of the plaque influences the distribution of the wall shear stress. In complement to a first approach with rigid walls, an FSI model is developed as well to simulate the coupling between the blood flow and the carotid artery deformation. The results show that the presence of the plaque causes an attenuation of the WSS in the after-plaque region as well as the emergence of recirculation areas

Experimental analysis on squeal modal instability

International audienceIn this paper, an experimental analysis performed on a simplified brake apparatus is presented. Brake squeal is a major concern in braking design. During past years a common approach for squeal prediction was the complex eigenvalues analysis. Squeal phenomenon is treated like a dynamic instability. When two modes of the brake system couple at the same frequency, one of them becomes unstable leading to increasing vibration. The presented experimental analysis is focused on correlating squeal characteristics with the dynamic behavior of the system. The experimental modal identification of the set-up is performed and different squeal conditions and frequencies are reproduced and analyzed. Particular attention is addressed to the system dynamics in function of the driving parameters on squeal occurrence. Squeal events are correlated with the modal behavior of the system in function of the main parameters, like contact pressure, friction material properties and system geometry. The robustness of the obtained squeal events permits a further analysis on the triggering of the squeal instability during braking, including the values of parameters that bring to instability. The obtained results agree with the modal coupling approach for squeal prediction, and confirm the characterization of squeal as dynamic instabilit

Frictional response of reinforced polymers under quasistatic and fast-transient dry contact conditions

Reinforced polymers have recently gained interest because of their high stiffness associated with the classical features and cost-effectiveness of polymers. A further characteristic, suitable for several applications, is the possibility to provide high frictional and wear resistance. The frictional response of commercially available reinforced materials was here investigated in a wide range of contact boundary conditions. Experimental tests were performed on different test benches, to investigate the material frictional response under either quasistatic or fast-dynamic contact solicitations. While carbon-fiber-reinforced material exhibits a stable but low friction coefficient, the glass-fiber-reinforced material leads to the suitable combination of high friction and low wear. The PPS material, 40% (wt) glass-reinforced polymer, sliding against the Ti6Al4V titanium alloy, provided high static friction coefficients (>0.4). The same material pair was then tested in endurance under fast-dynamic contact solicitations, highlighting their resistance to wear

Numerical investigation on the mode coupling contact dynamic instabilities.

When dealing with complex mechanical systems, the frictional contact is at the origin of significant changes in the dynamic behavior of systems. The presence of frictional contact can give rise to mode-coupling instabilities that produce harmonic "friction induced vibrations". Unstable vibrations can reach large amplitude that could compromise the structural integrity of the system and are often associated with annoying noise emission. The study of this kind of dynamic instability has been object of many studies ranging from both theoretical and numerical study of simple lumped models to numerical and experimental study on real mechanical systems, such as automotive brakes, typically affected by such issue. In this paper the numerical analysis of a lumped system constituted by several degrees of freedom in frictional contact with a slider is presented, where the introduction of friction gives rise to an unstable dynamic behavior. Two different approaches are used to investigate the effects of friction forces. The linear Complex Eigenvalue Analysis (CEA) allows for calculating of the complex eigenvalues of the system that can be characterized by a positive real part (i.e. negative apparent modal damping). The effects of the main parameters on the system stability are investigated. In the second approach a non linear model has been developed that takes into account the stick slip behavior at the interface to solve the time-history solution and analyze the unstable vibration. The mode selection mechanism occurring in transient nonlinear analysis, when several unstable modes are predicted by the linear CEA, and driving the selection of the frequency of the unstable vibrations, is investigated. Furthermore, by means of the transient analysis, the influence of the type of perturbation at the equilibrium position on the time history of the system vibrations is analyzed. Results comparison between the two different approaches highlights how nonlinearities affect the time-history solution and how stable and unstable behavior can be predicted by the linear CEA. The obtained results have been extended to the finite element model of a simple mechanical system

Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and frictioninduced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature

The BLAST Survey of the Vela Molecular Cloud: Dynamical Properties of the Dense Cores in Vela-D

The Vela-D region, according to the nomenclature given by Murphy & May (1991), of the star forming complex known as the Vela Molecular Ridge (VMR), has been recently analyzed in details by Olmi et al. (2009), who studied the physical properties of 141 pre- and proto-stellar cold dust cores, detected by the ``Balloon-borne Large-Aperture Submillimeter Telescope'' (BLAST) during a much larger (55 sq. degree) Galactic Plane survey encompassing the whole VMR. This survey's primary goal was to identify the coldest, dense dust cores possibly associated with the earliest phases of star formation. In this work, the dynamical state of the Vela-D cores is analyzed. Comparison to dynamical masses of a sub-sample of the Vela-D cores estimated from the 13CO survey of Elia et al. (2007), is complicated by the fact that the 13CO linewidths are likely to trace the lower density intercore material, in addition to the dense gas associated with the compact cores observed by BLAST. In fact, the total internal pressure of these cores, if estimated using the 13CO linewidths, appears to be higher than the cloud ambient pressure. If this were the case, then self-gravity and surface pressure would be insufficient to bind these cores and an additional source of external confinement (e.g., magnetic field pressure) would be required. However, if one attempts to scale down the 13CO linewidths, according to the observations of high-density tracers in a small sample of sources, then most proto-stellar cores would result effectively gravitationally bound.Comment: This paper has 12 pages and 6 figures. Accepted for publication by the Astrophysical Journal on July 19, 201

Multidimensional non-linear laser imaging of Basal Cell Carcinoma

We have used a multidimensional non-linear laser imaging approach to visualize ex-vivo samples of basal cell carcinoma (BCC). A combination of several non-linear laser imaging techniques involving fluorescence lifetime, multispectral two-photon and second-harmonic generation imaging has been used to image different skin layers. This approach has elucidated some morphological (supported by histopathological images), biochemical, and physiochemical differences of the healthy samples with respect to BCC ones. In particular, in comparison with normal skin, BCC showed a blue-shifted fluorescence emission, a higher fluorescence response at 800 nm excitation wavelength and a slightly longer mean fluorescence lifetime. Finally, the use of aminolevulinic acid as a contrast agent has been demonstrated to increase the constrast in tumor border detection. The results obtained provide further support for in-vivo non-invasive imaging of Basal Cell Carcinoma

Spectral morphological analysis of skin lesions with a polarization multispectral dermoscope.

Dermoscopy is the conventional technique used for the clinical inspection of human skin lesions. However, the identification of diagnostically relevant morphologies can become a complex task. We report on the development of a polarization multispectral dermoscope for the in vivo imaging of skin lesions. Linearly polarized illumination at three distinct spectral regions (470, 530 and 625 nm), is performed by high luminance LEDs. Processing of the acquired images, by means of spectral and polarization filtering, produces new contrast images, each one specific for melanin absorption, hemoglobin absorption, and single scattering. Analysis of such images could facilitate the identification of pathological morphologies. (C) 2013 Optical Society of Americ