A mechanistic approach to predicting the friction behaviour of human skin

In this work, analytical models available from contact mechanics theory having a proven record in mechanical engineering were used to develop a model predicting the friction behavior of human skin. A multi-scale contact model was developed in which the contact parameters are calculated at three levels, each level characterized by its elastic behavior and geometry. For a product part in contact with the so-called hairy skin the skin topography can be described as being composed of spherical contacts, whereas for the finger in contact with a product surface the fingerprint ridges are modeled as annulus shaped line contacts. Sliding friction was measured in vivo between the skin and different surface textures produced using ultra-short pulsed laser technology. The results observed during in vivo experiments are very well explained by the developed model, which predicts the friction as a function of product geometry, asperity geometry and normal load. Copyright © 2012 by ASME

Experimental Analysis and Wear Prediction Model for Unfilled Polymer–Polymer Sliding Contacts

Lifetime prediction of polymer–polymer contacts is a major challenge. Current design methods stemming from metal contact surfaces lack accuracy because polymers behave differently, especially regarding temperature variations. Experiments were performed on a pin-on-disk setup alternating static and rotating elements. Common unfilled engineering polymers, viz. polyoxymethylene (POM), polypropylene (PP), polyamide 6.6 (PA6.6), and polycarbonate (PC) were tested at ambient and elevated temperatures. Material combinations were analyzed regarding the effects of load, velocity, temperature, and the product of contact pressure and sliding velocity (PV limit). The experimental results show that the PV limit is not predictive for polymer–polymer contacts; rather, each material combination has a critical factor that determines the wear and frictional values and thus the contact’s durability and lifetime. The critical factor is the value of contact pressure or sliding velocity or temperature at which there is sudden increase in wear rate. The experimental results also demonstrate that the application temperature in operation has an important influence on the lifetime. A temperature increase can either be beneficial or have a negative impact depending on the material combination. Resulting from the extensive experimental analysis, a new design method, based on the principle of deformation energy, is proposed. The new model is different from existing models because it includes thermal properties of the materials in contact and it makes use of the Péclet number. Because the proposed model requires only data sheet values and design parameters to predict wear volume, the model improves the support of engineers in designing durable polymer–polymer sliding contacts

Contact modelling of human skin: what value to use for the modulus of elasticity?

In modelling and understanding the contact and friction behaviour of human skin, the elastic modulus of the skin is an important input parameter. For the development of design rules for the engineering of surfaces in contact with the skin an expression that describes the relation between the elastic modulus of the skin and the size of the contact is essential. Although an exact description of the mechanical behaviour of the skin requires an anisotropic, nonlinear, viscoelastic model, in this study it was found that for contact modelling involving relatively small deformations, the mechanical behaviour seems to be accurately described by a single parameter: the effective elastic modulus. The effective elastic modulus is shown to decrease several orders of magnitude when the length scale increases, which is the consequence of the rather complex anatomy of the skin. At an indentation depth of 10 µm, the effective elastic modulus was shown to decrease from 0.15 to 0.015 MPa when the radius of curvature of the indenter increases from 10 µm to 10 mm. The variation of the elasticity is explained by the variation in the composition and properties of the different skin layers. This study shows that for the contact modelling of human skin, a closed-form expression based on the anatomy of the skin exists, which yields the magnitude of the effective elastic modulus of the skin as a function of the length scale of the contact depending on variables such as age, gender and environmental conditions

An experimental study on the relation between surface texture and tactile friction

The main obstacle to calculate the ‘feel’ of a product from its surface properties is the ill-defined surface topography that is encountered after the most surface finishing processes. In this work this obstacle was avoided by producing well-defined surface topographies by laser texturing. The friction of textures having surface features with varying radii and spacings was investigated by measuring friction against the fingerpad. Within the range of conditions tested the coefficient of friction decreased with increasing normal load. The relation between the surface texture parameters and the coefficient of friction is influenced by the scale-dependent elastic behaviour of the skin top layer