Friction analysis of anisotropic surface for viscoelastic material

Purpose The anisotropic surfaces of viscoelastic materials play a role in sliding friction; the purpose of this paper is to study the effect of the anisotropic surfaces on contact area and the friction coefficient. Design/methodology/approach A complex elastic modulus and an anisotropic power spectrum are used to compute the coefficient of friction based on the extension Persson theory which considers the partial contact and the variation in the roughness slopes. Findings The ratios of the relative contact area that varies with velocity are obtained with different angles and eccentricities, and the effect of the elastic modulus needs to be considered. The coefficients of the friction parallel to the direction of motion decrease as the angle increases, or as the eccentricity decreases. The friction coefficients in the vertical direction change irregularly when the angles or eccentricities increase. Originality/value An extension of Persson’s work considering the partial contact and the effective mean square slope of the roughness is applied to study sliding friction, and the effect of the elastic modulus on contact area is considered.

On the Effect of a Rate-Dependent Work of Adhesion in the Detachment of a Dimpled Surface

Patterned surfaces have proven to be a valuable design to enhance adhesion, increasing hysteresis and the detachment stress at pull-off. To obtain high adhesive performance, soft materials are commonly, used, which easily conform to the countersurface, such as soft polymers and elastomers. Such materials are viscoelastic; i.e., they show rate-dependent properties. Here, the detachment of two half spaces is studied, one being flat and the other having a dimple in the limit of short range adhesion and a power law rate-dependent work of adhesion, as observed by several authors. Literature results have suggested that the dimpled surface would show pressure-sensitive adhesion, showing two possible adhered states, one weak, in partial contact, and one strong when full contact is achieved. By accounting for a power law rate-dependent work of adhesion, the “weak state” may be much stronger than it was in the purely elastic case, and hence the interface may be much more tough to separate. We study the pull-off detachment stress of the dimpled surface, showing that it weakly depends on the preload, but it is strongly affected by the dimensionless unloading rate. Finally, possible implications of the presented results in the detachment of soft materials from rough substrates are discussed.

Evaluation of Oxidative Stress Parameters in Healthy Saddle Horses in Relation to Housing Conditions, Presence of Stereotypies, Age, Sex and Breed

Oxidative stress plays an important role in the development of many horse diseases and it has been shown that housing has important implications for the psychophysical well-being of horses. The aim of this study is to determine if there are any differences between the redox status in horses in relation to housing conditions. The four housing conditions analyzed were: single box, without external access and without contact (Cat A), single box with external access and possibility of partial contact (Cat B), group housing with box and large paddock (Cat C), pasture with more than 7 horses and the possibility of green forage for the whole year (Cat D). A group of 117 healthy horses were selected in several private stables in Northern Italy. All subjects treated with any type of drug were excluded. At the end of the enrollment, the 117 selected horses were divided into the four housing categories. Stereotypies were highest in the group of horses in single box, without external access and without contact (Cat A). Oxidative stress was evaluated by testing plasma or serum samples for the following parameters: superoxide anion (WST), nitric oxide (NO), reactive oxygen species (d-ROMs), ferric reducing ability of plasma (FRAP), and the activity of superoxide dismutase (SOD). Simultaneously with the blood sampling, the owners completed a questionnaire with all the management aspects of the horse (signaling, feeding, equestrian activity, vaccinations, foot management etc.). The statistical evaluation was carried out based on the categories previously described, on the presence and absence of stereotypies and on some signaling data obtained from the questionnaire. There were no significant differences in the parameters analyzed between the categories. No significant redox status differences were detected based on the presence or absence of stereotypies. Interestingly, when the age was introduced as selection (<14 and >14 years old) parameter inside the categories, statistical significance was observed for some of the stress markers considered. Finally, independently of the housing conditions, the horses of the most two represented breeds exhibited different values of FRAP. All these aspects are commented in the discussion.

Ice shelf rift propagation: stability, three-dimensional effects, and the role of marginal weakening

Understanding the processes that govern ice shelf extent is important to improving estimates of future sea-level rise. In present-day Antarctica, ice shelf extent is most commonly determined by the propagation of through-cutting fractures called ice shelf rifts. Here, I present the first three-dimensional analysis of ice shelf rift propagation. I model rifts using the assumptions of linear elastic fracture mechanics (LEFM). The model predicts that rifts may be stabilized (i.e., stop propagating) when buoyant flexure results in the partial contact of rift walls. This stabilizing tendency may be overcome, however, by processes that act in the ice shelf margins. In particular, loss of marginal strength, modeled as a transition from zero tangential displacement to zero tangential shear stress, is shown to favor rift propagation. Rift propagation may also be triggered if a rift is carried with the ice flow (i.e., advected) out of an embayment and into a floating ice tongue. I show that rift stability is closely related to the transition from uniaxial to biaxial extension known as the compressive arch. Although the partial contact of rift walls is fundamentally a three-dimensional process, I demonstrate that it may be parameterized within more numerically efficient two-dimensional calculations. This study constitutes a step towards a first-principle description of iceberg calving due to ice shelf rift propagation.

Explanation of the onset of bouncing cycles in isotropic rotor dynamics; a grazing bifurcation analysis

The dynamics associated with bouncing-type partial contact cycles are considered for a 2 degree-of-freedom unbalanced rotor in the rigid-stator limit. Specifically, analytical explanation is provided for a previously proposed criterion for the onset upon increasing the rotor speed Ω of single-bounce-per-period periodic motion, namely internal resonance between forward and backward whirling modes. Focusing on the cases of 2 : 1 and 3 : 2 resonances, detailed numerical results for small rotor damping reveal that stable bouncing periodic orbits, which coexist with non-contacting motion, arise just beyond the resonance speed Ω p : q . The theory of discontinuity maps is used to analyse the problem as a codimension-two degenerate grazing bifurcation in the limit of zero rotor damping and Ω = Ω p : q . An analytic unfolding of the map explains all the features of the bouncing orbits locally. In particular, for non-zero damping ζ , stable bouncing motion bifurcates in the direction of increasing Ω speed in a smooth fold bifurcation point that is at rotor speed O ( ζ ) beyond Ω p : q . The results provide the first analytic explanation of partial-contact bouncing orbits and has implications for prediction and avoidance of unwanted machine vibrations in a number of different industrial settings.

Tactile Sensor with High-Density Microcantilever and Multiple PDMS Bumps for Contact Detection

In the study, we investigated a detection method of partial contact of an object owing to curved or uneven surface of the contact object by a tactile sensor. The sensor is developed using three microcantilevers embedded in a polydimethylsiloxane (PDMS) bump. First, three bumps were employed to place a bump for each cantilever. It was possible to detect a contact position because the resistance change in the strain gauge on the cantilever under each bump significantly depended on the contact/non-contact state of each bump. Second, a tactile sensor with high-density arrangement of microcantilevers was used to detect partial or tilted contact situations. The results indicated that the output of a tactile sensor with high-density arrangement of microcantilevers reflected partial or tilted contact. It is suggested that a tactile sensor with multiple bumps and high-density microcantilevers allows for more dexterous gripping control based on the shape of the object and contact angle.

Ice shelf rift propagation: stability, three dimensional effects, and the role of marginal weakening

Understanding the processes that govern ice shelf extent are of fundamental importance to improved estimates of future sea level rise. In present-day Antarctica, ice shelf extent is most commonly determined by the propagation of through-cutting fractures called ice shelf rifts. Here, I present the first three-dimensional analysis of ice shelf rift propagation. I present a linear elastic fracture mechanical (LEFM) description of rift propagation. The model predicts that rifts may be stabilized when buoyant flexure results in contact at the tops of the near-tip rift walls. This stabilizing tendency may be overcome, however, by processes that act in the ice shelf margins. In particular, both marginal weakening and the advection of rifts into an ice tongue are shown to be processes that may trigger rift propagation. Marginal shear stress is shown to be the determining factor that governs these types of rift instability. I furthermore show that rift stability is closely related to the transition from uniaxial to biaxial extension known as the compressive arch. Although the partial contact of rift walls is fundamentally a three-dimensional process, I demonstrate that it may be parameterized within more numerically efficient two-dimensional calculations. This study provides a step towards a description of calving physics that is based in fracture mechanics.