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A Bayesian regularization-backpropagation neural network (BR-BPNN) model is employed to predict some aspects of the gecko spatula peeling, viz. the variation of the maximum normal and tangential pull-off forces and the resultant force angle at detachment with the peeling angle. K -fold cross validation is used to improve the effectiveness of the model. The input data is taken from finite element (FE) peeling results. The neural network is trained with 75 % of the FE dataset. The remaining 25 % are utilized to predict the peeling behavior. The training performance is evaluated for every change in the number of hidden layer neurons to determine the optimal network structure. The relative error is calculated to draw a clear comparison between predicted and FE results. It is shown that the BR-BPNN model in conjunction with the k -fold technique has significant potential to estimate the peeling behavior. [ABSTRACT FROM AUTHOR]

We develop two new continuum contact models for coupled adhesion and friction, and discuss them in the context of existing models proposed in the literature. Our new models are able to describe sliding friction even under tensile normal forces, which seems reasonable for certain adhesion mechanisms. In contrast, existing continuum models for combined adhesion and friction typically include sliding friction only if local contact stresses are compressive. Although such models work well for structures with sufficiently strong local compression, they fail to capture sliding friction for soft and compliant systems (like adhesive pads), for which the resistance to bending is low. This can be overcome with our new models. For further motivation, we additionally present experimental results for the onset of sliding of a smooth glass plate on a smooth elastomer cap under low normal loads. As shown, the findings from these experiments agree well with the results from our models. In this paper we focus on the motivation and derivation of our continuum contact models, and provide a corresponding literature survey. Their implementation in a nonlinear finite element framework as well as the algorithmic treatment of adhesion and friction will be discussed in future work. [ABSTRACT FROM AUTHOR]

While cementless implants are now widely used clinically, implant debonding still occur and is difficult to anticipate. Assessing the biomechanical strength of the bone–implant interface can help improving the understanding of osseointegration phenomena and thus preventing surgical failures. A dedicated and standardized implant model was considered. The samples were tested using a mode III cleavage device to assess the mechanical strength of the bone-implant interface by combining experimental and numerical approaches. Four rough (Sa = 24.5 μm) osseointegrated coin-shaped implants were left in sheep cortical bone during 15 weeks of healing time. Each sample was experimentally rotated at 0.03°/sec until complete rupture of the interface. The maximum values of the torque were comprised between 0.48 and 0.72 N m, while a significant increase of the normal force from 7-12 N to 31–43 N was observed during the bone-implant interface debonding, suggesting the generation of bone debris at the bone-implant interface. The experimental results were compared to an isogeometric finite element model describing the adhesion and debonding phenomena through a modified Coulomb's law, based on a varying friction coefficient to represent the transition from an unbroken to a broken bone-implant interface. A good agreement was found between numerical and experimental torques, with numerical friction coefficients decreasing from 8.93 to 1.23 during the bone-implant interface rupture, which constitutes a validation of this model to simulate the debonding of an osseointegrated bone-implant interface subjected to torsion. [ABSTRACT FROM AUTHOR]

This paper provides an overview of recent advances in the computational modeling of the gecko adhesion mechanism. Several efficient modeling approaches are introduced that address the adhesion mechanism at various length scales, ranging from the molecular interaction at the Ångstrom scale to the toe description at the centimeter scale. In particular, detailed three-dimensional models for the gecko spatulae and setae are discussed. The emphasis here is placed on the computational efficiency of the presented contact formulations using coarse-graining and model order reduction. Different finite element methods, based on nonlinear structural mechanics, are considered to analyze the resulting models. Both continuum and beam formulations are discussed. In order to obtain an accurate description of peeling, a local finite element surface enrichment technique is presented. This paper also provides some ideas on future modeling approaches. [ABSTRACT FROM PUBLISHER]

This paper presents a detailed finite element analysis of the adhesion of a gecko spatula. The gecko spatulae form the tips of the gecko foot hairs that transfer the adhesional and frictional forces between substrate and foot. The analysis is based on a parameterised description of the 3D geometry of the spatula that only requires 12 parameters. The adhesion is described by a nonlinear computational contact formulation that accounts for the van der Waals interaction between spatula and substrate. The spatula adhesion model is implemented using an enriched contact finite element formulation recently developed by the first author. The finite element model is then used to simulate the peeling behaviour of the gecko spatula under applied vertical and rotational loading for various model parameters. Variations of the material stiffness, adhesional strength and range, stiction, spatula size and spatula inclination are considered to account for the natural variation of spatula properties. The study demonstrates that the spatula can function over a wide range of conditions. The computed pull-off forces are in agreement with experimental results reported in the literature. The study also examines the energy required for the spatula pull-off. The proposed model is ideal to study the influence of substrate roughness on the spatula adhesion, as is finally demonstrated. [ABSTRACT FROM AUTHOR]

SUMMARY Numerical solution of dynamic problems requires accurate temporal discretization schemes. So far, to the best of the authors' knowledge, none have been proposed for adhesive contact problems. In this work, an energy-momentum-conserving temporal discretization scheme for adhesive contact problems is proposed. A contact criterion is also proposed to distinguish between adhesion-dominated and impact-dominated contact behaviors. An adhesion formulation is considered, which is suitable to describe a large class of interaction mechanisms including van der Waals adhesion and cohesive zone modeling. The current formulation is frictionless, and no dissipation is considered. Performance of the proposed scheme is compared with other schemes. The proposed scheme involves very little extra computational overhead. It is shown that the proposed new temporal discretization scheme leads to major accuracy gains both for single-degree-of-freedom and multi-degree-of-freedom systems. The single-degree-of-freedom system is critically analyzed for various parameters affecting the response. For the multi-degree-of-freedom system, the effect of the time step and mesh discretization on the solution is also studied using the proposed scheme. It is further shown that a temporal discretization scheme based on the principle of energy conservation is not sufficient to obtain a convergent solution. Results with higher order contact finite elements for discretizing the contact area are also discussed. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: This work presents the theory and numerical implementation of a contact formulation based on surface potentials. The new theory formulates contact based on distance-dependent surface interaction potentials and distinguishes between three interaction classes: point interaction, short-range surface interaction and long-range surface interaction. Here the focus is placed on frictionless contact, although the first class readily admits frictional sticking contact as is also shown. The proposed contact theory provides a unified framework for various contact formulations, ranging from numerical constraint formulations, like penalty, barrier, cross-constrained and augmented Lagrange multiplier methods, to physical interaction formulations, like cohesive zone models, as well as electrostatic, gravitational and van-der-Waals interaction. Apart from recovering classical penalty and barrier formulations, the new theory also naturally leads to a modified penalty and barrier method. The formulation also recovers classical one-pass contact algorithms, however the real advantages lie in a novel two-pass contact algorithm, denoted the two-half-pass contact algorithm, since each pass only accounts for the contact forces acting on the slave body. This implies that traction continuity is only satisfied in theory, but not imposed a priori in the algorithm. Instead, it is obtained naturally to high accuracy as is demonstrated by several 2D and 3D numerical examples. These include sliding contact, peeling contact and electrostatic attraction between deformable solids. Among the examples is a detailed comparison between the new formulation and classical one-pass approaches. It is further shown that the new contact formulation passes the contact patch test. [Copyright &y& Elsevier]

A computational contact formulation is presented that is suitable for simulating contact interaction problems at very small length scales. The contact model is based on the coarse-graining of the intermolecular forces between neighboring bodies, like van der Waals attraction, into an effective continuum contact description. The model is cast into a nonlinear 3D finite element implementation that is capable of integrating the challenges encountered in the modeling of adhesive systems. The contact model is then applied to the dynamic modeling and simulation of the adhesion and deformation of a gecko seta based on a 3D multiscale approach. The approach spans six orders of magnitude and combines three distinct modeling levels, that describe the effective adhesion behavior at the seta scale, the spatula scale and the molecular scale. The rate-dependent pull-off behavior of adhering setae and spatulae is computed and it is shown that the model is successful in capturing pull-off forces that have been observed experimentally. [ABSTRACT FROM AUTHOR]

A 3D multiscale model is presented which describes the adhesion and deformation of a gecko seta. The multiscale approach combines three models at different length scales: at the top level, on the order of several micrometers, a nonlinear finite element beam model is chosen to capture the branched microstructure of the gecko seta. At the intermediate level, on the order of several nanometers, a second finite element model is used to capture the detailed behaviour of the seta tips, the so-called spatulae. At the lowest level, on the order of a few angstroms, a molecular interaction potential is used to describe the van der Waals adhesion forces between spatulae and substrate. Coarse-graining techiques are used to bridge the scale between the model levels. To illustrate and validate the proposed gecko seta model, numerical pull-off simulations are shown and compared to experimental data from the literature. [ABSTRACT FROM AUTHOR]