Extrinsic Measurement of Carbon Black Aggregate Distribution within a Fluoroelastomer Matrix from Nanoindentation Experiments.

A novel extrinsic method for the measurement of particle surface distribution in a carbon black-filled elastomer via nanoindentation is developed. This method is based on the measurement of the contact stiffness obtained from the continuous stiffness measurement mode. The proposed tip-particle model is held by two main hypotheses: the particles do not deform significantly during indentation so that only the elastomer matrix elastically deforms; particles are physically bounded with the surrounding matrix. Therefore, when the tip comes in contact with a particle, the latter becomes a hard extension of the tip, able to deform the elastomer matrix. Finally, the evolution of the measured contact stiffness is directly related to the increase of the contact area between the tip-particles set and the elastomer matrix. The proposed model is validated through a numerical and an experimental study. Moreover, an evaluation of the measurements bias allows to correct the particle surface distribution. A good agreement is found between the distribution measured from transmission electron microscopy observations and nanoindentation measurements.

Effect of water desorption on the rheology and dynamic response of human hair to a non-contact impact.

Human hair is a non-homogeneous complex material made of keratin fibers oriented along the longitudinal axis which offer anisotropic mechanical properties. Nowadays, it is possible to measure the mechanical properties of hairs with the classical tests, but most often, these tests are destructive and make hard to measure the influence of some external factors or treatments on the behavior of a same hair fiber. In the current paper, vibrations induced by a non-contact impact have been utilized as a representative response of the mechanical behavior of hair. The characteristics of the vibratory response allow measuring the variation in the mechanical properties and the instantaneous effect of an external factor on the properties of a same sample. First, load relaxation tests have been performed on hair samples after moisturization and for different times of an air-drying process in order to characterize the change in the visco-elastic behavior of hair during the water desorption. Other hair samples have been tested with our non-contact impact and vibration technique in order to observe the change in the vibratory response during the water desorption. The vibratory response has then been correlated to the mechanical properties of the hair fiber.

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. METHOD: A suction deformation performed on the volar aspect of the forearm of a subject is considered. This test is modeled with the finite element method to compare the experimental and simulated curves using an inverse method that allows the skin mechanical parameters identification. This process is based on two optimization algorithms, Kalman's filter and Gauss-Newton's methods. To account for the nonlinear behavior of the skin, a specific nonlinear elastic law, which is then compared with standard linear elastic and neo-Hookean's mechanical behaviors, was developed. RESULTS: The obtained results first prove that neither linear elasticity nor neo-Hookean's laws can be used to model the skin. On the contrary, the nonlinear elastic model presents a relevant fit of the experimental curves. The skin thickness is also proved to be another key point to be taken into consideration. CONCLUSIONS: The obtained results are successfully compared with literature and the reliability of the proposed method is underlined with the identification of 300 additional experimental curves. The different works we are currently focusing on are finally introduced.