Conductance of rough random profiles

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On the effect of the loading apparatus stiffness on the equilibrium and stability of soft adhesive contacts under shear loads

The interaction between contact area and frictional forces in adhesive soft contacts is receiving much attention in the scientific community due to its implications in many areas of engineering such as surface haptics and bioinspired adhesives. In this work, we consider a soft adhesive sphere that is pressed against a rigid substrate and is sheared by a tangential force where the loads are transferred to the sphere through a normal and a tangential spring, representing the loading apparatus stiffness. We derive a general linear elastic fracture mechanics solution, taking into account also the interaction between modes, by adopting a simple but effective mixed-mode model that has been recently validated against experimental results in similar problems. We discuss how the spring stiffness affects the stability of the equilibrium contact solution, i.e. the transition to separation or to sliding

The interaction of frictional slip and adhesion for a stiff sphere on a compliant substrate

How friction affects adhesion is addressed. The problem is considered in the context of a very stiff sphere adhering to a compliant, isotropic, linear elastic substrate, and experiencing adhesion and frictional slip relative to each other. The adhesion is considered to be driven by very large attractive tractions between the sphere and the substrate that can act only at very small distances between them. As a consequence, the adhesion behavior can be represented by the Johnson-Kendall-Roberts model, and this is assumed to prevail also when frictional slip is occurring. Frictional slip is considered to be resisted by a uniform, constant shear traction at the slipping interface, a model that is considered to be valid for small asperities and for compliant elastomers in contact with stiff material. A model for the interaction of friction and adhesion, known to agree with some experimental data, is utilized. This model is due to Johnson, and its adhesion-friction interaction is assumed to stem, upon shrinkage of the contact area, from a postulated reversible energy release associated with frictional slip. This behavior is considered to arise from surface microstructures generated or eliminated by frictional slip, where these microstructures store some elastic strain energy in a reversible manner. The associated reversible energy release rate is derived from the energy exchanges that occur in the system. The Johnson model, and an asymptotic analysis of it for small amounts of frictional slip, is shown to be consistent with the reversible energy release rate that we identify.Comment: 11 page

DIC analysis of mechanical response of tooth aligners under simulated swallowing acts

In this work, the mechanical and deformation behavior of clear Polyethylene Terephthalate-glycol (PET-g) aligners, under cyclic loading was investigated using a full-field optical technique: the Digital Image Correlation. In particular, the PET-g aligners thermoformed from 0.88 mm thick discs, were subjected to cyclic compression tests for 13000 load cycles from 0 to 50 N in the atmospheric environment (~25°C). This number of cycles was chosen because it simulates, on average, the intraoral load associated with the swallowing acts that an aligner is subjected to during the time of use of 1 week. At the same time, the results from the analysis of hysteresis loops obtained by the DIC technique were compared with those obtained by the testing machine. The mechanical response of clear aligners was evaluated in terms of maximum displacement, energy loss and relative stiffness along the load direction to seven different stages of the 13000 load cycles. A comparable trend was found between the measurements obtained by Digital Image Correlation analysis and the analysis of the hysteresis loops obtained from the cyclic compression tests. Furthermore, the morphological features of the PET-g aligner at the end of the tests were analyzed by optical microscopy (OM). The OM analyses showed that thesurface of PET-g aligner was affected by morphological variations such as high depressions and cracks

Shakedown analysis for rolling and sliding contact problems

There is a range of problems where repeated rolling or sliding contact occurs. For such problems shakedown and limit analyses provides significant advantages over other forms of analysis when a global understanding of deformation behaviour is required. In this paper, a recently developed numerical method. Ponter and Engelhardt (2000) and Chen and Ponter (2001), for 3-D shakedown analyses is used to solve the rolling and sliding point contact problem previously considered by Ponter, Hearle and Johnson (1985) for a moving Herzian contact, with friction, over a half space. The method is an upper bound programming method, the Linear Matching Method, which provides a sequence of reducing upper bounds that converges to the least upper bound associated with a finite element mesh and may be implemented within a standard commercial finite element code. The solutions given in Ponter, Hearle and Johnson (1985) for circular contacts are reproduced and extended to the case when the frictional contact stresses are at an angle to the direction of travel. Solutions for the case where the contact region is elliptic are also given

Sella turcica and craniofacial morphology in patients with palatally displaced canines: a retrospective study

Background: The aim of the study was to evaluate the sella and craniofacial morphological features in growing patients with palatally displaced canines compared to controls. Materials and methods: Twenty-two subjects with palatally displaced canines were retrospectively selected and compared to 22 controls matched for age and gender. Lateral cephalograms were collected and sagittal and vertical cephalometric variables were measured, together with sella interclinoid distance, sella depth, and sella diameter. The independent samples T-test or Mann-Whitney U-test were used to compare all the variables between the two groups. A Pearson correlation was computed for the craniofacial and sella variables that differed significantly (p < 0.05) between the groups. Results: Patients with palatally displaced canines showed a smaller interclinoid distance and a greater SNA angle than control subjects. The interclinoid distance and the SNA angle were negatively correlated (–0.52, p = 0.017) in the experimental group. Conclusions: Growing patients with palatally displaced canines had smaller sella interclinoid distances and a greater SNA angle than control subjects