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Real contact area plays a critical role in tribology. However, few contact models can give accurate load-area relation at very large contact fraction due to the lack of asperity interactions. In this work, an improved Greenwood–Williamson model is advanced for elastic–plastic rough surfaces. To include the asperity interactions in a simple way, the random distribution of contacting spots is treated as hexagonal distribution, which possesses the same overall isotropic property on surface. A representative asperity is then simulated by finite element method, and the load-area relation is calculated up to almost full contact. It is found that surface morphology and material properties govern the initial linear relationship at small contact fraction, and the interactions among asperities are closely related to contact fraction and lead to a larger load than that neglecting asperity interactions. The load-area relation is further formulated for general rough surfaces by comparing with an incremental equivalent contact model at initial contact. Direct simulations on typical rough surfaces verify the accuracy of the advanced load-area relation within a large range of contact fraction. This result provides a foundation for further analysis of friction, wear and sealing in tribology. [ABSTRACT FROM AUTHOR]

Indentation has become a promising tool to characterize the mechanical parameters of materials. In this work, herein, spherical indentations on elastic‐perfectly plastic materials and strain‐hardening materials are investigated through finite‐element method (FEM) simulations. A new method to extract the yield strength and strain‐hardening exponent from spherical indentation is proposed. The deviation of nominal contact pressure from Hertzian prediction is used as an indicator to determine the plastic parameters. The difference between Hertzian contact pressure and real contact pressure is also investigated for the estimation of contact radius during indentation process. Herein, a more convenient approach is provided in this analysis to directly determine the mechanical parameters of elastic–plastic materials from spherical indentation tests. Validation of this method is presented in the form of a comparison between analytical stress–strain relationships and corresponding curves obtained via iterative FEM simulations of the indentation process. No experimental data are involved in this validation. [ABSTRACT FROM AUTHOR]

Abstract Many soft biological tissues possess a considerable surface stress, which plays a significant role in their biophysical functions, but most previous methods for characterizing their mechanical properties have neglected the effects of surface stress. In this work, we investigate the micropipette aspiration method to measure the mechanical properties of soft tissues and cells with surface effects. The neo-Hookean constitutive model is adopted to describe the hyperelasticity of the measured biological material, and the surface effect is taken into account by the finite element method. It is found that when the pipette radius or aspiration length is comparable to the elastocapillary length, surface energy may distinctly alter the aspiration response. Generally, both the aspiration length and the bulk normal stress decrease with increasing surface energy, and thus neglecting the surface energy would lead to an overestimation of elastic modulus. Through dimensional analysis and numerical simulations, we provide an explicit relation between the imposed pressure and the aspiration length. This method can be applied to determine the mechanical properties of soft biological tissues and organs, e.g., livers, tumors and embryos. [ABSTRACT FROM AUTHOR]

Soft membranous materials widely exist in engineering and nature, and the determination of their constitutive parameters is of both scientific and engineering significance. In this paper, the bulge test method is extended to determine the hyperelastic parameters of soft membranes with or without initial stresses. Two extensively applied models-neo-Hookean model and Arruda-Boyce model-are employed to characterize the nonlinear behavior of the membrane under test. The hyperelastic parameters are then extracted from the pressure-deflection curve of the membrane recorded in the bulge tests. Our method is finally validated by both finite element simulations and uniaxial tension experiments. The proposed method can be used to evaluate various soft membranes and tissues and hold promise for numerous applications in such fields as biomedical engineering and bionic engineering. [ABSTRACT FROM AUTHOR]