Shear-Induced Anisotropy in Rough Elastomer Contact.

True contact between randomly rough solids consists of myriad individual microjunctions. While their total area controls the adhesive friction force of the interface, other macroscopic features, including viscoelastic friction, wear, stiffness, and electric resistance, also strongly depend on the size and shape of individual microjunctions. We show that, in rough elastomer contacts, the shape of microjunctions significantly varies as a function of the shear force applied to the interface. This process leads to a growth of anisotropy of the overall contact interface, which saturates in the macroscopic sliding regime. We show that smooth sphere-plane contacts have the same shear-induced anisotropic behavior as individual microjunctions, with a common scaling law over 4 orders of magnitude in the initial area. We discuss the physical origin of the observations in light of a fracture-based adhesive contact mechanics model, described in the companion article, which captures the smooth sphere-plane measurements. Our results shed light on a generic, overlooked source of anisotropy in rough elastic contacts, not taken into account in current rough contact mechanics models.

Shear-induced contact area anisotropy explained by a fracture mechanics model.

This paper gives a theoretical analysis for the fundamental problem of anisotropy induced by shear forces on an adhesive contact, discussing the experimental data of the companion Letter. We present a fracture mechanics model where two phenomenological mode-mixity functions are introduced to describe the weak coupling between modes I and II or I and III, which changes the effective toughness of the interface. The mode-mixity functions have been interpolated using the data of a single experiment and then used to predict the behavior of the whole set of experimental observations. The model extends an idea by Johnson and Greenwood, to solve purely mode I problems of adhesion in the presence of a nonaxisymmetric Hertzian geometry, to the case of elliptical contacts sheared along their major or minor axis. Equality between the stress intensity factors and their critical values is imposed solely at the major and minor axes. We successfully validate our model against experimental data. The model predicts that the punch geometry will affect both the shape and the overall decay of the sheared contact area.

Evolution of real contact area under shear and the value of static friction of soft materials.

The frictional properties of a rough contact interface are controlled by its area of real contact, the dynamical variations of which underlie our modern understanding of the ubiquitous rate-and-state friction law. In particular, the real contact area is proportional to the normal load, slowly increases at rest through aging, and drops at slip inception. Here, through direct measurements on various contacts involving elastomers or human fingertips, we show that the real contact area also decreases under shear, with reductions as large as 30[Formula: see text], starting well before macroscopic sliding. All data are captured by a single reduction law enabling excellent predictions of the static friction force. In elastomers, the area-reduction rate of individual contacts obeys a scaling law valid from micrometer-sized junctions in rough contacts to millimeter-sized smooth sphere/plane contacts. For the class of soft materials used here, our results should motivate first-order improvements of current contact mechanics models and prompt reinterpretation of the rate-and-state parameters.

Effects of finite probe size on self-affine roughness measurements.

The roughness of fracture surfaces exhibits self-affinity for a wide variety of materials and loading conditions. The universality and the range of scales over which this regime extends are still debated. The topography of these surfaces is however often investigated with a finite contact probe. In this case, we show that the correlation function of the roughness can only be measured down to a length scale Deltax{c} which depends on the probe size R, the Hurst exponent zeta of the surface and its topothesy l, and exhibits spurious behavior at smaller scales. First, we derive the dependence of Deltax{c} on these parameters from a simple scaling argument. Then, we verify this dependence numerically. Finally, we establish the relevance of this analysis from AFM measurements on an experimental glass fracture surface and provide a metrological procedure for roughness measurements.

Efectos del estiramiento isquiotibial sobre el patrón de activación muscular del recto del abdomen durante la flexoextensión del tronco / Effects of hamstring stretching on rectus abdominis muscle activation pattern during trunk flexion and extension

ObjetivoDeterminar los efectos del estiramiento de las estructuras isquiotibiales mediante técnicas de facilitación neuromuscular propioceptiva sobre el patrón de activación del recto del abdomen durante los movimientos de flexoextensión del tronco.HipótesisEl estiramiento repetido de las estructuras isquiotibiales podría retrasar la activación del recto del abdomen durante la flexión del tronco y adelantarla durante la extensión.MetodologíaSe registraron en 11 sujetos voluntarios la activación electromiográfica del recto del abdomen y los grados de flexión de la cadera durante movimientos de flexoextensión del tronco antes y después de realizar los estiramientos. Las variables de estudio fueron los porcentajes de flexión de la cadera al inicio y al final de la contracción del recto del abdomen y la actividad electromiográfica media del músculo.ResultadosEl t-test mostró que hay diferencias significativas en el inicio y el final de la contracción del recto del abdomen (onset y offset) antes y después del estiramiento (p<0,05).ConclusiónTras un estiramiento prolongado de la musculatura isquiotibial, el recto del abdomen tarda más tiempo en activarse durante la flexión del tronco, y la contracción es de menor duración. El efecto combinado de la laxitud aumentada de las estructuras isquiotibiales y la alteración de la respuesta del recto del abdomen podría alterar los sistemas sensoriomotores de control del movimiento de la región lumbopélvica, aumentando el riesgo de lesiones(AU)

ObjectiveTo determine the effects of hamstring stretching (propioceptive neuromuscular propioception) on the muscle activation pattern of rectus abdominis (RA) during trunk flexion and extension.HypothesisRepeated hamstring stretching techniques could either delay or anticipate RA activation during trunk flexion or extension, respectively.MethodologyElectromyography activity of the rectus abdominis and grades of hip flexion during flexion-extension movements of the trunk before and after stretching hamstring muscles was recorded in 11 volunteer subjects. Study variables: RA onset, RA offset (expressed as a percentage of the maximum hip flexion value) and RA average EMG.ResultsThe paired T-test showed significant differences in RA onset and offset before and after stretching (p<0.05).ConclusionAfter prolonged hamstring muscle stretching, the rectus abdominis takes a long time to become activated during trunk flexion and the contraction is shorter. The combined effect of increased laxity of hamstring and altered response of RA after stretching might alter the sensorimotor systems of lumbopelvic region motor control, increasing the risk of injuries(AU)

The crack tip: a nanolab for studying confined liquids.

We study the equilibrium properties of a liquid phase condensed at the nanoscale between the surfaces of a sharp crack in fused silica in a moist controlled atmosphere. The extension of the condensed phase along the fracture is measured by in situ atomic force microscopy phase imaging and it is shown to be determined by a critical distance between the opposite crack surfaces, which is an increasing function of humidity. The present technique is very promising for measuring the properties of confined liquids at the nanoscale as well as for modeling the physics and chemistry of slow crack propagation in glasses.