Hassan Zahouani

Dynamic indentation on human skin in vivo: ageing effects

Background/purpose: Knowledge of the mechanical properties of the human skin is very important for cosmetic and clinical research. Objective and quantitative measurements are essential to compare studies performed by different experimenters in different centres. The aim of this paper is to present a method to measure the viscoelastic properties of human skin in vivo using dynamic indentation.

A nonlinear elastic behavior to identify the mechanical parameters of human skin in vivo

Background/purpose: Various analyses have been performed to identify the mechanical properties of the human skin tissue in vivo. They generally use different approaches and hypotheses (behavior laws as well as mechanical tests) and the obtained results are consequently difficult to analyze and compare. In this paper, an inverse method that can be adapted to any kind of mechanical tests and behavior laws is presented.

Characterization of abrasive grain’s behavior and wear mechanisms

Abstract Grinding is a finishing process largely used in motor industry, aeronautics, space industry and precision cutting tool manufacturers. The grinding process can be summarized by the action of a grinding wheel on a workpiece. The wheel is constituted by abrasive grains. Thus grinding is in fact the action of grains on the workpiece. The grain behavior changes according to numerous parameters (geometry, mechanical characteristics, wear mechanisms). In some cases abrasive wear is observed while micro-cutting is obtained in some other cases. In this paper two useful and complementary experimental approaches for the interface physics understanding is presented. The study of the cutting power is carried out using a high-speed scratch test device in order to understand the grain behavior and the wear mechanisms for several wheel surface speeds. In this paper an approach for the specific abrasion energy computation is also presented.

Angular Parameter for Characterizing Road Surface Microtexture

Development of a device for measuring road surface microtexture and the determination of a microtexture parameter related to wet road/tire friction are discussed. A laser autofocus system was selected from existing devices. The required characteristics were defined from consideration of the contact between wet roads and tires. The autofocus principle is presented briefly, and comparison is made with a tactile system. The microtexture parameter, called the theta parameter, is defined from the useful parts of the texture profiles, that is, those in contact with the tire. It characterizes relative positions of profile peaks. A simple method was proposed to define the peaks. The profile analysis program is presented briefly. Relevance of the theta parameter was validated on surfaces composed of coarse aggregates with planar exposed faces. The specimens were polished using a projection method to emphasize the microtexture contrast between surfaces. Friction was measured using a skid resistance tester (SRT) pendulum. Profiles were measured using both laser and tactile systems. Characteristics of profiles are given. The statistical distribution of theta values obtained on a profile may be described by an exponential function. Close agreement was found between theta values obtained from laser and tactile profiles, values from laser profiles being somewhat lower. The difference is attributed to profile length. Fair correlation was found (r2 > 0.80) between theta values and SRT friction, and similar tendencies were observed from laser and tactile data.

Wear assessments of prehistoric instruments

Abstract Analysis of wear mechanisms of prehistoric materials are fundamental in many aspects for archaeology to determine their use within a given socio-economic context. Wear test methods have brought significant contributions to the identification of prehistoric tools implemented for the manufacture of stone objects such as vessels, figurines or pendants during the VIIth millennium bc , in Cyprus. Manual experiments conducted by archaeologists have been completed by tribological experiments—scratch test and friction; 3D images and 3D statistical parameters. Quantitative and qualitative analyses of surface states contribute to the characterisation of the wear mechanisms. Our purpose is to characterize contacts between chert, the tool raw material, and different kind of stones. Two case studies will be presented here: a contact between chert and diabase (an igneous rock) and a contact between chert and picrolite (a kind of serpentinite). They illustrate two opposed contact phenomena and wears, on the one hand, between two fragile materials and on the other hand, between a fragile and a ductile material. The development of distinct third bodies during the dynamic contacts have been observed. Lubrication, contact phenomena and surface characterization constitute the basis of our work and the obtained results have brought decisive elements to understand the social organization of a craft activity in a Neolithic Village of Cyprus, Khirokitia, a craft activity which is predominant in the identity of insular communities at that time.

Energetic analysis of cutting mechanisms in belt finishing of hard materials

Belt finishing has been tested successfully as a complementary process to hard turning. This technology improves the surface texture and generates compressive residual stress. However, the mechanisms and characteristics of this new process have not yet been fully explained. This article provides a comprehensive characterization of cutting mechanisms generated by belt finishing. First, an analytical analysis based on cutting forces is developed. Then, the macroscopic specific energy is dissociated into a cutting specific energy responsible of shearing and ploughing mechanisms and a sliding specific energy due to adhesion. It has been demonstrated that cutting is more predominant than sliding in belt finishing process. The omnipresence of cutting demonstrates the effectiveness and the profitability of belt finishing operation.

Contribution of human skin topography to the characterization of dynamic skin tension during senescence: morpho-mechanical approach

The structuring of the dermis with a network of collagen and elastic fibres gives a three-dimensional structure to the skin network with directions perpendicular and parallel to the skin surface. This three-dimensional morphology prints on the surface of the stratum corneum a three dimensional network of lines which express the mechanical tension of the skin at rest. To evaluate the changes of skin morphology, we used a three-dimensional confocal microscopy and characterization of skin imaging of volar forearm microrelief. We have accurately characterize the role of skin line network during chronological aging with the identification of depth scales on the network of lines (z ≤ 60μm) and the network of lines covering Langers lines (z > 60 microns). During aging has been highlighted lower rows for elastic fibres, the decrease weakened the tension and results in enlargement of the plates of the microrelief, which gives us a geometric pertinent indicator to quantify the loss of skin tension and assess the stage of aging. The study of 120 Caucasian women shows that ageing in the volar forearm zone results in changes in the morphology of the line network organisation. The decrease in secondary lines (z ≤ 60 μm) is counterbalanced by an increase in the depth of the primary lines (z > 60 μm) and an accentuation of the anisotropy index.

Assessment of the in-plane biomechanical properties of human skin using a finite element model updating approach combined with an optical full-field measurement on a new tensile device.

Human skin is one of the most important organ of the body. The assessment and knowledge of its properties are very useful for clinical or cosmetic research. Many techniques are used to measure the mechanical properties of this organ, like suction, indentation, torsion or tension tests. The aim of this paper is to present a new device based on tension technique and combining mechanical and optical measurements. The whole procedure used to assess the displacement field as described, and first results of tests performed in vivo are shown.

Use of the Kalman filters for the analysis of the mechanical properties of human skin in vivo

In this article, we present an inverse method for the identification of the mechanical properties of human skin, a complex multi-layered organ which has been studied in vivo using a suction deformation technique. To identify the required properties, experimental results were compared to finite element solutions of the test, with the assumption that skin behaves as a single isotropic hyperelastic layer. The inverse method used in this article is based on that of the extended Kalman filters principle, with two modifications of this standard formulation for use in skin analysis. The modified formulation was then tested using Finite Element Method (FEM) simulated mechanical data and also with a study of linear and nonlinear theoretical problems. The results of the new formulation were also compared with the Gauss–Newton, recursive least square and Kalman smoother approaches. Finally, the reliability of the method was tested on a case study.

Contribution to the Determination of In Vivo Mechanical Characteristics of Human Skin by Indentation Test

This paper proposes a triphasic model of intact skin in vivo based on a general phenomenological thermohydromechanical and physicochemical (THMPC) approach of heterogeneous media. The skin is seen here as a deforming stratified medium composed of four layers and made out of different fluid-saturated materials which contain also an ionic component. All the layers are treated as linear, isotropic materials described by their own behaviour law. The numerical simulations of in vivo indentation test performed on human skin are given. The numerical results correlate reasonably well with the typical observations of indented human skin. The discussion shows the versatility of this approach to obtain a better understanding on the mechanical behaviour of human skin layers separately.