51 research outputs found
Effect of fingerprints orientation on skin vibrations during tactile exploration of textured surfaces
In humans, the tactile perception of fine textures is mediated by skin
vibrations when scanning the surface with the fingertip. These vibrations are
encoded by specific mechanoreceptors, Pacinian corpuscules (PCs), located about
2 mm below the skin surface. In a recent article, we performed experiments
using a biomimetic sensor which suggest that fingerprints (epidermal ridges)
may play an important role in shaping the subcutaneous stress vibrations in a
way which facilitates their processing by the PC channel. Here we further test
this hypothesis by directly recording the modulations of the
fingerpad/substrate friction force induced by scanning an actual fingertip
across a textured surface. When the fingerprints are oriented perpendicular to
the scanning direction, the spectrum of these modulations shows a pronounced
maximum around the frequency v/lambda, where v is the scanning velocity and
lambda the fingerprints period. This simple biomechanical result confirms the
relevance of our previous finding for human touch.Comment: Addendum to: Scheibert J, Leurent S, Prevost A, Debr\'egeas G. The
role of fingerprints in the coding of tactile information probed with a
biomimetic sensor. Science 2009; 323:1503?6 3 pages, 1 figur
Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces
Recent years have witnessed the development of so-called relaxation
tribometers, the free oscillation of which is altered by the presence of
frictional stresses within the contact. So far, analysis of such oscillations
has been restricted to the shape of their decaying envelope, to identify in
particular solid or viscous friction components. Here, we present a more
general expression of the forces possibly acting within the contact , and
retain six possible, physically relevant terms. Two of them, which had never
been proposed in the context of relaxation tribometry, only affect the
oscillation frequency, not the amplitude of the signal. We demonstrate that
each of those six terms has a unique signature in the time-evolution of the
oscillation, which allows efficient identification of their respective weights
in any experimental signal. We illustrate our methodology on a PDMS
sphere/glass plate torsional contact
MĂ©canique du contact rugueux et perception tactile
5 pages, 2 figuresNational audienceDans un contact entre solides rugueux, l'interface constitue la partie la plus déformable. Son comportement mécanique détermine les contraintes s'établissant dans les deux solides, ainsi que la dynamique de frottement. Deux nouvelles méthodes expérimentales, fondées respectivement sur une observation optique directe et sur l'utilisation d'un microcapteur de force MEMS, permettent de sonder la mécanique locale de ces interfaces. Le dispositif MEMS, qui est un analogue très rudimentaire de l'extrémité du doigt humain, nous a permis de proposer un rôle possible des empreintes digitales dans la transduction de l'information tactile
Probing the micromechanics of a multi-contact interface at the onset of frictional sliding
Digital Image Correlation is used to study the micromechanics of a
multi-contact interface formed between a rough elastomer and a smooth glass
surface. The in-plane elastomer deformation is monitored during the incipient
sliding regime, i.e. the transition between static and sliding contact. As the
shear load is increased, an annular slip region, in coexistence with a central
stick region, is found to progressively invade the contact. From the
interfacial displacement field, the tangential stress field can be further
computed using a numerical inversion procedure. These local mechanical
measurements are found to be correctly captured by Cattaneo and Mindlin (CM)'s
model. However, close comparison reveals significant discrepancies in both the
displacements and stress fields that reflect the oversimplifying hypothesis
underlying CM's scenario. In particular, our optical measurements allow us to
exhibit an elasto-plastic like friction constitutive equation that differs from
the rigid-plastic behavior assumed in CM's model. This local constitutive law,
which involves a roughness-related length scale, is consistent with the model
of Bureau \textit{et al.} [Proc. R. Soc. London A \textbf{459}, 2787 (2003)]
derived for homogeneously loaded macroscopic multi-contact interfaces, thus
extending its validity to mesoscopic scales.measurements allow for the first
quantitative test of Cattaneo and Mindlin (CM) classical model of the incipient
sliding of a smooth interface. Small deviations are observed and interpreted as
a result of the finite compliance of the rough interface, a behavior which
contrasts with Amontons' law of friction assumed to be valid locally in CM's
model. We illustrate how these measurements actually provide a method for
probing the rheology of the rough interface, which we find to be of the
elasto-plastic type.Comment: 11 page
Direct numerical simulation of the dynamics of sliding rough surfaces
The noise generated by the friction of two rough surfaces under weak contact
pressure is usually called roughness noise. The underlying vibration which
produces the noise stems from numerous instantaneous shocks (in the microsecond
range) between surface micro-asperities. The numerical simulation of this
problem using classical mechanics requires a fine discretization in both space
and time. This is why the finite element method takes much CPU time. In this
study, we propose an alternative numerical approach which is based on a
truncated modal decomposition of the vibration, a central difference
integration scheme and two algorithms for contact: The penalty algorithm and
the Lagrange multiplier algorithm. Not only does it reproduce the empirical
laws of vibration level versus roughness and sliding speed found experimentally
but it also provides the statistical properties of local events which are not
accessible by experiment. The CPU time reduction is typically a factor of 10.Comment: 16 pages, 16 figures, accepted versio
Statistics of the separation between sliding rigid rough surfaces: Simulations and extreme value theory approach
When a rigid rough solid slides on a rigid rough surface, it experiences a
random motion in the direction normal to the average contact plane. Here,
through simulations of the separation at single-point contact between
self-affine topographies, we characterize the statistical and spectral
properties of this normal motion. In particular, its rms amplitude is much
smaller than that of the equivalent roughness of the two topographies, and
depends on the ratio of the slider's lateral size over a characteristic
wavelength of the topography. In addition, due to the non-linearity of the
sliding contact process, the normal motion's spectrum contains wavelengths
smaller than the smallest wavelength present in the underlying topographies. We
show that the statistical properties of the normal motion's amplitude are well
captured by a simple analytic model based on the extreme value theory
framework, extending its applicability to sliding-contact-related topics
History-dependent friction and slow slip from time-dependent microscopic junction laws studied in a statistical framework
To study the microscopic origins of friction, we build a framework to
describe the collective behaviour of a large number of individual
micro-junctions forming a macroscopic frictional interface. Each micro-junction
can switch in time between two states: A pinned state characterized by a
displacement-dependent force, and a slipping state characterized by a
time-dependent force. Instead of tracking each micro-junction individually, the
state of the interface is described by two coupled distributions for (i) the
stretching of pinned junctions and (ii) the time spent in the slipping state.
We show how this framework represents an overarching structure for important
models existing in the friction literature. We then use it to study
systematically the effect of the time-scale that controls the duration of the
slipping state. We first find the steady-state friction force as a function of
the sliding velocity. As the framework allows for a whole family of
micro-junction behaviour laws, we show how these laws can be chosen to obtain
monotonic (strengthening or weakening) or non-monotonic velocity dependence at
the macroscale. By then considering transient situations, we predict that the
macroscopic static friction coefficient is strongly influenced by the way the
interface was prepared, in particular by the slip dynamics of the previous
sliding event. We also show that slow slip spontaneously occurs in the
framework for a wide range of behaviour laws.Comment: 20 pages, 10 figure
On the speed of fast and slow rupture fronts along frictional interfaces
The transition from stick to slip at a dry frictional interface occurs
through the breaking of the junctions between the two contacting surfaces.
Typically, interactions between the junctions through the bulk lead to rupture
fronts propagating from weak and/or highly stressed regions, whose junctions
break first. Experiments find rupture fronts ranging from quasi-static fronts
with speeds proportional to external loading rates, via fronts much slower than
the Rayleigh wave speed, and fronts that propagate near the Rayleigh wave
speed, to fronts that travel faster than the shear wave speed. The mechanisms
behind and selection between these fronts are still imperfectly understood.
Here we perform simulations in an elastic 2D spring--block model where the
frictional interaction between each interfacial block and the substrate arises
from a set of junctions modeled explicitly. We find that a proportionality
between material slip speed and rupture front speed, previously reported for
slow fronts, actually holds across the full range of front speeds we observe.
We revisit a mechanism for slow slip in the model and demonstrate that fast
slip and fast fronts have a different, inertial origin. We highlight the long
transients in front speed even in homogeneous interfaces, and we study how both
the local shear to normal stress ratio and the local strength are involved in
the selection of front type and front speed. Lastly, we introduce an
experimentally accessible integrated measure of block slip history, the Gini
coefficient, and demonstrate that in the model it is a good predictor of the
history-dependent local static friction coefficient of the interface. These
results will contribute both to building a physically-based classification of
the various types of fronts and to identifying the important mechanisms
involved in the selection of their propagation speed.Comment: 29 pages, 21 figure
Damage mechanisms in the dynamic fracture of nominally brittle polymers
Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to
evaluate quantitatively the energy flux released to the tip of a growing crack.
Still, the way in which the crack selects its velocity in response to this
energy flux remains far from completely understood. To uncover the underlying
mechanisms, we experimentally studied damage and dissipation processes that
develop during the dynamic failure of polymethylmethacrylate (PMMA),
classically considered as the archetype of brittle amorphous materials. We
evidenced a well-defined critical velocity along which failure switches from
nominally-brittle to quasi-brittle, where crack propagation goes hand in hand
with the nucleation and growth of microcracks. Via post-mortem analysis of the
fracture surfaces, we were able to reconstruct the complete spatiotemporal
microcracking dynamics with micrometer/nanosecond resolution. We demonstrated
that the true local propagation speed of individual crack fronts is limited to
a fairly low value, which can be much smaller than the apparent speed measured
at the continuum-level scale. By coalescing with the main front, microcracks
boost the macroscale velocity through an acceleration factor of geometrical
origin. We discuss the key role of damage-related internal variables in the
selection of macroscale fracture dynamics.Comment: 18 pages, 21 figures, to appear in International Journal of Fractur
Contact with coupled adhesion and friction: Computational framework, applications, and new insights
Contact involving soft materials often combines dry adhesion, sliding
friction, and large deformations. At the local level, these three aspects are
rarely captured simultaneously, but included in the theoretical models by
Mergel et al. (2019). We here develop a corresponding finite element framework
that captures 3D finite-strain contact of two deformable bodies. This framework
is suitable to investigate sliding friction even under tensile normal loads.
First, we demonstrate the capabilities of our finite element model using both
2D and 3D test cases, which range from compliant tapes to structures with high
stiffness, and include deformable-rigid and deformable-deformable contact. We
then provide new results on the onset of sliding of smooth elastomer-glass
interfaces, a setup that couples nonlinear material behavior, adhesion, and
large frictional stresses. Our simulations not only agree well with both
experimental and theoretical findings, they also provide new insights into the
current debate on the shear-induced reduction of the contact area in
elastomeric contact
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