Marc Arthur Masen

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.

A review of fingerpad contact mechanics and friction and how this affects tactile perception

In the sliding contact between the fingerpad and a rough surface when touching a product’s surface, friction plays a role in the perception of roughness, slipperiness and warmth. For product engineers who aim to control and optimize the sensorial properties of a product surface interacting with the skin, it is essential to understand this frictional behaviour. However, the friction of skin is yet poorly understood. The variation that is observed within or between skin friction studies can be assigned to gender, age and orientation of the finger. Analysing data collected from literature shows some consistent trends. The coefficient of friction increases considerably with increasing hydration level of the skin, due to softening of the top layer of the skin. The coefficient of friction of the fingerpad decreases with normal load to a constant value, which can be attributed to effects of normal adhesion and the deformation behaviour of the fingerpad. There is no consistent effect of velocity on the coefficient of friction. Friction decreases with increasing Ra roughness. When the Ra roughness increases further, the contribution of deformation causes an increase in the friction after which it remains constant. Some influence of the finishing method is reported. The type of material has a smaller influence than the surface roughness of the sample or the condition of the skin. Even though the coefficient of friction of the fingerpad shows some consistent trends, examining the friction behaviour at a more detailed level might explain the contribution of friction to tactile perception. The measuring signal contains relevant information and should be analysed thoroughly as opposed to taking the average coefficient of friction of the steady state part of the signal. Future work should involve the study of local friction behaviour at the scale of the surface roughness.

A multivariable model for predicting the frictional behaviour and hydration of the human skin

The frictional characteristics of skin‐object interactions are important when handling objects, in the assessment of perception and comfort of products and materials and in the origins and prevention of skin injuries. In this study, based on statistical methods, a quantitative model is developed that describes the friction behaviour of human skin as a function of the subject characteristics, contact conditions, the properties of the counter material as well as environmental conditions.

Tribologically modified surfaces on elastomeric materials

As the result of tribological loading, the properties of the surface of elastomeric materials will alter. This effect has been observed using SEM and EDS analysis of the tread of a used car tyre, where differences in structure between the substrate and the area near the surface were found. To study the effects of tribological loading on the surface properties of elastomers in a more reproducible manner, a pin-on-disk tribometer was employed, using EPDM disks and spherical pins made of 100Cr6 steel. In the tribologically loaded area, a modification of the surface with a thickness ranging between 4 and 8 µm was observed, whilst in the unloaded area such a surface modification was not observed. The mechanical and tribological properties of the modified and unmodified surfaces were analysed using micro-indentations and friction experiments on a nano-tribometer, showing a reduction of the stiffness of the tribo-modified surface as well as altered time-dependent behaviour and a 60% reduction in the coefficient of friction when sliding against steel.

A Finite Element Approach to Modeling Abrasive Wear Modes

ABSTRACT Machine components operating in sandy environments will wear because of the abrasive interaction with sand particles. In this work, a method is derived to predict the amount of wear caused by such abrasive action, in order to improve the maintenance concept of the components. A finite element model is used to simulate various tips scratching a smooth surface. The model is verified by comparing the obtained results with a set of experiments performed earlier (M. Woldman, et al., 2013, Wear, 301(1–2), pp 76–81).

Knowledge Based Cloud FE simulation – data-driven material characterization guidelines for the hot stamping of aluminium alloys

The Knowledge Based Cloud FEA (KBC-FEA) simulation technique allows multiobjective FE simulations to be conducted on a cloud-computing environment, which effectively reduces computation time and expands the capability of FE simulation software. In this paper, a novel functional module was developed for the data mining of experimentally verified FE simulation results for metal forming processes obtained from KBC-FE. Through this functional module, the thermo-mechanical characteristics of a metal forming process were deduced, enabling a systematic and data-driven guideline for mechanical property characterization to be developed, which will directly guide the material tests for a metal forming process towards the most efficient and effective scheme. Successful application of this data-driven guideline would reduce the efforts for material characterization, leading to the development of more accurate material models, which in turn enhance the accuracy of FE simulations.

The dynamic contact area of elastomers at different velocities

The friction in tribo-systems that contain viscoelastic materials, such as elastomers, is relevant for a large number of applications. Examples include tyres, hoses, transmission and conveyor belts. To quantify the friction in these applications, one must first understand the contact behaviour of such viscoelastic materials, both in static and in dynamic situations. This work discusses an experimental study into the change of the contact area with the sliding velocity and relates the change in contact area with the mechanical properties of the elastomer. The results show that for a tribo-system containing an elastomer, there is a threshold velocity, above which the size of the contact area significantly reduces.

Tool-life prediction under multi-cycle loading during metal forming: a feasibility study

In the present research, the friction and wear behaviour of a hard coating were studied by using ball-on-disc tests to simulate the wear process of the coated tools for sheet metal forming process. The evolution of the friction coefficient followed a typical dual-plateau pattern, i.e. at the initial stage of sliding, the friction coefficient was relatively low, followed by a sharp increase due to the breakdown of the coatings after a certain number of cyclic dynamic loadings. This phenomenon was caused by the interactive response between the friction and wear from a coating tribo-system, which is often neglected by metal forming researchers, and constant friction coefficient values are normally used in the finite element (FE) simulations to represent the complex tribological nature at the contact interfaces. Meanwhile, most of the current FE simulations consider single-cycle loading processes, whereas many metal-forming operations are conducted in a form of multi-cycle loading. Therefore, a novel friction/wear interactive friction model was developed to, simultaneously, characterise the evolutions of friction coefficient and the remaining thickness of the coating layer, to enable the wear life of coated tooling to be predicted. The friction model was then implemented into the FE simulation of a sheet metal forming process for feasibility study.

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

An experimental investigation into dental wear: tooth-tooth contact.

Abstract This work discusses some experiments with respect to the sliding contact between human teeth. The objective is to study whether in-depth investigations into the dominant wear modes in dental wear can be made using a standard tribotester with samples manufactured from natural teeth. A tribological pair of two test samples was prepared from each single incisor by cutting it into halves and machining one half to be flat and the other to be hemispherically shaped. Sphere-on-flat reciprocating sliding contact experiments were conducted. The resulting wear track was analysed, and the wear volumes were quantified. The observed variation in wear volumes was larger than what would be expected as inherent to the wear process, which is attributed to biological variations and the resulting variations in mechanical properties of the specimens used. For in-depth studies of the basic mechanisms involved in dental wear, the samples should receive special care and attention, particularly with respect to enamel thickness and prism orientation. The obtained results indicate that two wear regimes are predominant and that their occurrence might be related to the thickness of the enamel layer that covers the teeth.